Tuning the spin-related transport properties of FePc on Au(111) through single-molecule chemistry

Manipulation of the Magnetic State of a Porphyrin-Based Molecule on Gold: From Kondo to Quantum Nanomagnet via the Charge Fluctuation Regime

By moving individual Fe-porphyrin-based molecules with the tip of a scanning tunneling microscope in the vicinity of the elbow of the herringbone-reconstructed Au(111) containing a Br atom, we reversibly and continuously control their magnetic state. Several regimes are obtained experimentally and explored theoretically: from the integer spin limit, through intermediate magnetic states with renormalized magnetic anisotropy, until the Kondo-screened regime, corresponding to a progressive increase of charge fluctuations and mixed valency due to an increase in the interaction of the molecular Fe states with the substrate Fermi sea. Our study demonstrates the potential of utilizing charge fluctuations to generate and tune quantum magnetic states in molecule-surface hybrids.

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.

Origin of Asymmetric Splitting of Kondo Peak in Spin-Polarized Scanning Tunneling Spectroscopy: Insights from First-Principles-Based Simulations

The spin-polarized scanning tunneling microscope (SP-STM) has served as a versatile tool for probing and manipulating the spintronic properties of atomic and molecular devices with high precision. The interplay between the local spin state and its surrounding magnetic environment significantly affects the transport behavior of the device. Particularly, in the contact regime, the strong hybridization between the SP-STM tip and the magnetic atom or molecule could give rise to unconventional Kondo resonance signatures in the differential conductance (dI/dV) spectra. This poses challenges for the simulation of a realistic tip control process. By combining the density functional theory and the hierarchical equations of motion methods, we achieve first-principles-based simulation of the control of a Ni-tip/Co/Cu(100) junction in both the tunneling and contact regimes. The calculated dI/dV spectra reproduce faithfully the experimental data. A cotunneling mechanism is proposed to elucidate the physical origin of the observed unconventional Kondo signatures.

Atomic-scale intermolecular interaction of hydrogen with a single VOPc molecule on the Au(111) surface

Molecular dynamics of hydrogen molecules (H2) on surfaces and their interactions with other molecules have been studied with the goal of improvement of hydrogen storage devices for energy applications. Recently, the dynamic behavior of a H2 at low temperature has been utilized in scanning tunnelling microscopy (STM) for sub-atomic resolution imaging within a single molecule. In this work, we have investigated the intermolecular interaction between H2 and individual vanadyl phthalocyanine (VOPc) molecules on Au(111) substrates by using STM and non-contact atomic force microscopy (NC-AFM). We measured tunnelling spectra and random telegraphic noise (RTN) on VOPc molecules to reveal the origin of the dynamic behavior of the H2. The tunnelling spectra show switching between two states with different tunnelling conductance as a function of sample bias voltage and RTN is measured near transition voltage between the two states. The spatial variation of the RTN indicates that the two-state fluctuation is dependent on the atomic-scale interaction of H2 with the VOPc molecule. Density functional theory calculations show that a H2 molecule can be trapped by a combination of a tip-induced electrostatic potential well and the potential formed by a VOPc underneath. We suggest the origin of the two-state noise as transition of H2 between minima in these potentials with barrier height of 20-30 meV. In addition, the bias dependent AFM images verify that H2 can be trapped and released at the tip-sample junction.

Manipulation of Molecular Spin State on Surfaces Studied by Scanning Tunneling Microscopy

The adsorbed magnetic molecules with tunable spin states have drawn wide attention for their immense potential in the emerging fields of molecular spintronics and quantum computing. One of the key issues toward their application is the efficient controlling of their spin state. This review briefly summarizes the recent progress in the field of molecular spin state manipulation on surfaces. We focus on the molecular spins originated from the unpaired electrons of which the Kondo effect and spin excitation can be detected by scanning tunneling microscopy and spectroscopy (STM and STS). Studies of the molecular spin-carriers in three categories are overviewed, i.e., the ones solely composed of main group elements, the ones comprising 3d-metals, and the ones comprising 4f-metals. Several frequently used strategies for tuning molecular spin state are exemplified, including chemical reactions, reversible atomic/molecular chemisorption, and STM-tip manipulations. The summary of the successful case studies of molecular spin state manipulation may not only facilitate the fundamental understanding of molecular magnetism and spintronics but also inspire the design of the molecule-based spintronic devices and materials.

Collective Spin Manipulation in Antiferroelastic Spin-Crossover Metallo-Supramolecular Chains

Coupled spin-crossover complexes in supramolecular systems feature rich spin phases that can exhibit collective behaviors. Here, we report on a molecular-level exploration of the spin phase and collective spin-crossover dynamics in metallo-supramolecular chains. Using scanning tunneling microscopy, spectroscopy, and density functional theory calculations, we identify an antiferroelastic phase in the metal-organic chains, where the Ni atoms coordinated by deprotonated tetrahydroxybenzene linkers on Au(111) are at a low-spin (S = 0) or a high-spin (S = 1) state alternately along the chains. We demonstrate that the spin phase is stabilized by the combined effects of intrachain interactions and substrate commensurability. The stability of the antiferroelastic structure drives the collective spin-state switching of multiple Ni atoms in the same chain in response to electron/hole tunneling to a Ni atom via a domino-like magnetostructural relaxation process. These results provide insights into the magnetostructural dynamics of the supramolecular structures, offering a route toward their spintronic manipulations.

Switching the Spin on a Ni Trimer within a Metal-Organic Motif by Controlling the On-Top Bromine Atom

Controlling the spin of metal atoms embedded in molecular systems is a key step toward the realization of molecular electronics and spintronics. Many efforts have been devoted to explore the influencing factors dictating the survival or quenching of a magnetic moment in a metal-organic molecule, and among others, the spin control by axial ligand attachments is the most promising. Herein, from the interplay of high-resolution scanning tunneling microscopy imaging/manipulation and scanning tunneling spectroscopy measurements together with density functional theory calculations, we successfully demonstrate that a Ni trimer within a metal-organic motif acquires a net spin promoted by the adsorption of an on-top Br atom. The spin localization in the trimetal centers bonded to Br was monitored via the Kondo effect. The removal of the Br ligand resulted in the switch from a Kondo ON to a Kondo OFF state. The magnetic state induced by the Br ligand is theoretically attributed to the enhanced Br 4p_z and Ni 3d_z² states due to the charge redistribution. The manipulation strategy reported here provides the possibility to explore potential applications of spin-tunable structures in spintronic devices.

Endohedral Fullerene Fe@C28 Adsorbed on Au(111) Surface as a High-Efficiency Spin Filter: A Theoretical Study

We present a theoretical study on the adsorption and spin transport properties of magnetic Fe@C₂₈ using Ab initio calculations based on spin density functional theory and non-equilibrium Green’s function techniques. Fe@C₂₈ tends to adsorb on the bridge sites in the manner of C–C bonds, and the spin-resolved transmission spectra of Fe@C₂₈ molecular junctions exhibit robust transport spin polarization (TSP). Under small bias voltage, the transport properties of Fe@C₂₈ are mainly determined by the spin-down channel and exhibit a large spin polarization. When compressing the right electrode, the TSP is decreased, but high spin filter efficiency (SFE) is still maintained. These theoretical results indicate that Fe@C₂₈ with a large magnetic moment has potential applications in molecular spintronics.

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

Transition metal phthalocyanines (TMPcs) are particularly appealing for spintronic processing and data storage devices due to their structural simplicity and functional flexibility. To realize effective control of the spins in TMPc-based systems, it is necessary to quantify how the structural and chemical environment of the molecule affects its spin center. Herein we perform a detailed investigation of the electronic and spintronic properties of vertically stacked heterostructures formed by CoPc or FePc adsorbed on a monolayer of graphene under the influences of the gold substrate, vacancies in graphene, and extra atomic hydrogen coordination on the TMPc. By using density functional theory (DFT), we reveal that both the TMPc molecules prefer the carbon-top position on graphene, and the existence of the Au substrate enhances the stability of the adsorption, while this enhanced adsorption will not modify the molecular magnetism, keeping it the same value as in the free standing case. Moreover, with the aid of a combination of DFT and ab initio wavefunction-based calculations, our results indicate that the magnetic anisotropy of the FePc-graphene complex can be actively tuned by the Au substrate. Our calculations also show that defects in graphene including single and double vacancies can modify the magnetism of these heterostructures. In particular, the spin state of FePc can be tuned from S = 1 to S = 2 with such defect engineering. Further spin state tunability can be achieved from a hydrogenation process, with the coordination of one extra hydrogen on the Co-top site for CoPc and the pyridinic N site for FePc, respectively, tuning their spin states from S = 1/2 to S = 0 and from S = 1 to S = 2. These findings may prove to be instrumental for rational design of future molecular spintronics devices integrated with two-dimensional materials.