Electronic and magnetic properties of CoPc and FePc molecules on graphene: the substrate, defect, and hydrogen adsorption effects

Iron(II) Phthalocyanine Adsorbed on Defective Graphenes: A Density Functional Study

The adsorptions of iron(II) phthalocyanine (FePc) on graphene and defective graphene were investigated systematically using density functional theory. Three types of graphene defects covering stone-wales (SW), single vacancy (SV), and double vacancy (DV) were taken into account, in which DV defects included DV(5-8-5), DV(555-777), and DV(5555-6-7777). The calculations of formation energies of defects showed that the SW defect has the lowest formation energy, and it was easier for DV defects to form compared with the SV defect. It is more difficult to rotate or move FePc on the surface of defective graphenes than on the surface of graphene due to bigger energy differences at different sites. Although the charge analysis indicated the charge transfers from graphene or defective graphene to FePc for all studied systems, the electron distributions of FePc on various defective graphenes were different. Especially for FePc@SV, the d _xy orbital of Fe in the conduction band moved toward the Fermi level about 1 eV, and the d _xz of Fe in the valence band for FePc@SV also moved toward the Fermi level compared with FePc@graphene and other FePc@defective graphenes. Between the planes of FePc and defective graphene, the electron accumulation occurs majorly in the position of the FePc molecular plane for FePc@SW, FePc@DV(5-8-5), and FePc@DV(5555-6-7777) as well as FePc@graphene. However, electrons were accumulated on the upper and lower surfaces of the FePc molecular plane for FePc@SV and FePc@DV(555-777). Thus, the electron distribution of FePc can be modulated by introducing the interfaces of different defective graphenes.

Unravelling the robustness of magnetic anisotropy of a nickelocene molecule in different environments: a first-principles-based study

Recent scanning tunneling spectroscopy with single metallocene molecule-functionalized tips have proved to be a powerful tool to probe and control individual spins and spin-spin exchange interactions due to the robustness of the magnetic properties of the metallocene molecule in different surroundings. However, accurate prediction of such robustness at a first-principles-based level by the conventional density functional theory (DFT) has remained challenging. In this paper, we have performed a detailed investigation of the evolution of electronic and magnetic properties of a nickelocene molecule (NiCp₂) in different environments, i.e., free-standing, adsorbed on Cu(100) and as a functionalized tip apex. Using an embedding method, which combines DFT and the complete active space self-consistent field (CASSCF) method recently developed, we demonstrate that the nickelocene molecule almost preserves its spin and magnetic anisotropy upon adsorption on Cu(100), and also in the position of the tip apex. In particular, the cyclic π* orbital of the Cp rings could hybridize with the singly occupied d_π orbitals of the Ni center of the molecule, protecting these orbitals from external states. Hence the molecular spin maintains S = 1, the same as in the free-standing case, and its magnetic anisotropy is also robust with energies of 3.56, 3.34, and 3.51 meV in free-standing, adsorbed on Cu(100), and functionalized tip apex states, respectively, in good agreement with previous theoretical and experimental results. This work thus provides a first-principles-based understanding of the relevant experiments. Such agreement between theoretical simulations and experimental measurements highlights the potential usefulness of the method for investigating the local electronic and spin states of organometallic molecule-surface composite systems.

Magnetic Moment Preservation and Emergent Kondo Resonance of Co-Phthalocyanine on Semimetallic Sb(111)

Magnetic molecules on surfaces have been widely investigated to reveal delicate interfacial couplings and for potential technological applications. In these endeavors, one prevailing challenge is how to preserve or recover the molecular spins, especially on highly metallic substrates that can readily quench the magnetic moments of the admolecules. Here, we use scanning tunneling microscopy and spectroscopy to exploit the semimetallic nature of antimony and observe, surprisingly yet pleasantly, that the spin of Co-phthalocyanine is well preserved on Sb(111), as unambiguously evidenced by the emergent strong Kondo resonance across the molecule. Our first-principles calculations further confirm that the optimal density of states near the Fermi level of the semimetal is a decisive factor, weakening the overall interfacial coupling, while still ensuring sufficiently effective electron-spin scattering in the many-body system. Beyond isolated admolecules, we discover that each of the magnetic moments in a molecular dimer or a densely packed island is distinctly preserved as well, rendering such molecular magnets immense potentials for ultrahigh density memory devices.

Leveraging Molecular Properties to Tailor Mixed-Dimensional Heterostructures beyond Energy Level Alignment

The surface sensitivity and lack of dielectric screening in two-dimensional (2D) materials provide numerous intriguing opportunities to tailor their properties using adsorbed π-electron organic molecules. These organic-2D mixed-dimensional heterojunctions are often considered solely in terms of their energy level alignment, i.e., the relative energies of the frontier molecular orbitals versus the 2D material conduction and valence band edges. While this simple model is frequently adequate to describe doping and photoinduced charge transfer, the tools of molecular chemistry enable additional manipulation of properties in organic-2D heterojunctions that are not accessible in other solid-state systems. Fully exploiting these possibilities requires consideration of the details of the organic adlayer beyond its energy level alignment, including hybridization and electrostatics, molecular orientation and thin-film morphology, nonfrontier orbitals and defects, excitonic states, spin, and chirality. This Perspective explores how these relatively overlooked molecular properties offer unique opportunities for tuning optical and electronic characteristics, thereby guiding the rational design of organic-2D mixed-dimensional heterojunctions with emergent properties.

Multiple magnetic states of CoPc molecule on a two-dimensional layer of NbSe2

Molecular spintronics hinges on the detailed understanding of electronic and magnetic properties of molecules interfaced with various materials. Here we demonstrate withab initiosimulations that the prototypical Co-phthalocyanine (CoPc) molecule can surprisingly develop multi-spin states once deposited on the two-dimensional 2H-NbSe₂layer. Conventional calculations based on density functional theory (DFT) show the existence of low, regular and high spin states, which reduce to regular and high spins states once correlations are incorporated with a DFT +Uapproach. Depending onU, the ground state is either the low spin or high spin state with energy differences affected by the molecular orientation on top of the substrate. Our results are compared to recent scanning probe measurements and motivate further theoretical and experimental studies on the unveiled rich multi-magnetic behavior of CoPc molecule.

Precise Spin Manipulation of Single Molecule Positioning on Graphene by Coordination Chemistry

Precise spin manipulation of single molecules is crucial for future molecular spintronics. However, it has been a formidable challenge due to the complexities of the strong molecule-substrate coupling as well as the response of the molecule to external stimulus. Here we demonstrate by density functional theory calculations that precise spin manipulation can be achieved by extra CO and NO molecules coordination to transition metal phthalocyanine (TMPc) (TM = Co, Fe, Mn) molecules deposited on metal-supported graphene; the spins of TMPc molecules are switched from S to S - ¹/₂ (|S - 1|) after NO (CO) coordination. With the aid of a combination of molecular orbitals (MO) theory and recently developed principal interacting spin-orbital (PISO) analysis, the impacts of NO and CO coordinations on both adsorption configuration and spin polarization of TMPc are well elucidated. We reveal the different coordination geometries that CO always coordinates axially to the TM center with a linear geometry, while NO prefers a bent geometry, which can be attributed to the competition between the σ- and π-type interactions according to the PISO analysis. Particularly, the NO-MnPc complex adopts a bent geometry deviating from the prediction by the existing Enemark-Feltham formalism. In addition, MO analysis suggests that during the CO coordination, the simultaneous existence of σ-donation and π-back-donation promotes electrons flowing from the d_z² to partially occupied d_π (d_xz and d_xz) orbitals with subsequent reordering of the TM d-orbitals, resulting in the spin transition of S → |S - 1|. In comparison, given that NO is regarded as NO^- when it adopts a bent geometry coordinating to the TM center, the complete (CoPc) or partial (FePc and MnPc) quenching of the molecular spins caused by NO coordination is attributed to the electron transfer from TM to NO. These theoretical findings provide important insights into relevant experiments and offer an effective design strategy to realize underlying single-molecular spintronics devices integrated with two-dimensional materials.

Interface hybridization and spin filter effect in metal-free phthalocyanine spin valves

Spin–orbit coupling has been regarded as the core interaction to determine the efficiency of spin conserved transport in semiconductor spintronics. Here, we show the spin filter effect should be responsible for the magnetoresistance of H₂Pc device.