Tunable spin and conductance in porphyrin-graphene nanoribbon hybrids

Graphene nanoribbons (GNRs), with their unique electronic properties like width-dependent band gaps, edge states, and long spin relaxation times, are promising candidates for nanoelectronic and spintronic devices. Porphyrins, known for their tunable spin properties depending on the central metal ion and ligand field, offer an attractive approach for enhancing GNR functionality. The creation of well-ordered porphyrin-GNR (Por-GNR) hybrids holds significant promise for tailoring band gaps, inducing topological phases, and enabling magnetic signal detection in transport. On-surface synthesis, a bottom-up approach, facilitates the creation of atomically precise nanostructures, enabling the fabrication of such hybrids without the challenges of random molecular placement or metal cluster formation. Previous research has focused primarily on porphyrin oligomers/polymers or porphyrin nanotapes, lacking the GNR backbone. This study addresses the gap in understanding electronic and particularly spin transport in GNRs incorporating porphyrins, especially those with magnetic centers. The research focuses on two proposed Por-GNR hybrids, aiming to demonstrate their feasibility and explore their potential for spintronics and sensing applications.

Extensive research has explored the properties of GNRs, highlighting their potential in nanoelectronics and spintronics due to their unique electronic structure. Studies have demonstrated the ability to engineer robust topological quantum phases in GNRs and to tune their properties with electric fields. The potential for high magnetoresistance in GNR devices has also been investigated. Similarly, the tunable spin properties of porphyrins have been extensively studied, with various synthetic methods used to create functional porphyrin-based materials. The use of porphyrins as molecular electronic components in functional devices has also been explored. On-surface synthesis techniques have proven successful in creating atomically precise nanostructures, including graphene nanoribbons and other complex molecules. However, previous work on porphyrin-GNR hybrids has primarily focused on limited structures, lacking the comprehensive exploration of spin transport and tunability presented in this study. This work builds upon the established understanding of GNRs and porphyrins to create novel hybrid systems with enhanced functionality.

The study employed first-principles density functional theory (DFT) calculations, utilizing the SIESTA/TransSIESTA code with the GGA-PBE exchange-correlation functional and a DZP basis set. A Hubbard U correction (3 eV) was applied to the Fe 3d orbitals. Calculations were performed with a low electronic temperature (50 K) and included spin polarization. A k-point mesh of 1 x 6 x 1 and a 25 Å vacuum layer were used. All atoms were relaxed until forces were below 0.01 eV/Å. The accuracy of the computational parameters was verified by comparing results with the Vienna ab initio simulation package (VASP) using PBE and HSE06 functionals. The nonequilibrium Green's function (NEGF) formalism was used to investigate transport properties. The Z2 topological invariant was calculated to determine the topological phase of the metal-free hybrid structures. Two-terminal device setups were created to study spin-dependent transport, using pristine H2-hybrid2 as electrodes. The effects of mechanical strain and CO adsorption were investigated by modifying the atomic structure and performing subsequent DFT-NEGF calculations. Analysis included examining band structures, density of states (DOS), transmission functions, and eigenchannel scattering states to understand the electronic structure and transport mechanisms.

The study identified two feasible Por-GNR hybrid structures, hybrid1 (straight) and hybrid2 (S-shaped), synthesized from existing carbon-based precursors and a porphyrin unit. DFT calculations revealed that hybrid2 possesses a small band gap (0.1-0.2 eV, depending on the functional used), a crucial feature for device applications. The introduction of an iron atom in the porphyrin center in hybrid2 led to a spin-polarized ground state (S=1) with a 3E electronic configuration due to the coupling between GNR and Fe-porphyrin states. A two-terminal device setup with Fe-hybrid2 sandwiched between H2-hybrid2 electrodes showed a spin-dependent Fano anti-resonance in the spin-down channel near the Fermi level. This Fano resonance is directly related to the Fe 3dxz orbital and is highly sensitive to the Fe spin state. Applying 3% uniaxial strain induced a spin crossover to S=2, drastically changing the transmission characteristics and demonstrating potential for mechanically driven spin filtering. CO adsorption on the Fe atom resulted in a spin crossover to S=0, completely quenching the magnetism and suggesting potential for chemical sensing applications. The Z2 topological invariant was calculated to be 1 for both hybrid1 and hybrid2, indicating a topologically non-trivial phase.

The findings demonstrate the significant potential of Por-GNR hybrids for spintronics and chemical sensing. The tunability of the spin state through mechanical strain and chemical adsorption opens up avenues for creating novel spintronic devices, such as mechanically driven spin filters. The clear correlation between the Fe spin state and the conductance, evident in the Fano anti-resonance, establishes the feasibility of chemical sensors based on these hybrids. The small band gap of hybrid2 makes it a particularly attractive candidate for device electrodes. The observed spin crossover behavior is consistent with previously reported findings on Fe-porphyrin systems. The successful demonstration of spin state manipulation through external stimuli underscores the versatility and controllability of these hybrid nanostructures. The research contributes to the advancement of carbon-based spintronics by providing a detailed understanding of the electronic and transport properties of these novel materials.

This study successfully demonstrated the tunable spin and conductance properties of novel porphyrin-graphene nanoribbon hybrids. The small band gap of hybrid2, the spin polarization induced by Fe incorporation, the Fano anti-resonance in spin transport, and the controllable spin crossover via strain and chemical adsorption, highlight their potential for both spintronics and chemical sensing. Future work should explore surface effects (Kondo resonance), topological quantum phases, spin-spin interactions, and the potential of extending these findings to two-dimensional structures.

The study's computational approach relies on DFT, which has inherent limitations in accurately describing strongly correlated systems. The consideration of only two specific hybrid structures may not fully encompass the range of possible structures and their properties. The experimental realization of these structures and their integration into functional devices require further investigation. The effects of temperature and environmental factors on the spin state and transport properties were not explicitly considered in this theoretical study.