Scientists are increasingly exploring the potential of combining metal-organic frameworks (MOFs) with topological insulators to create materials exhibiting novel electronic and magnetic behaviours. Anna Kurowská, Jakub Planer, and Pavel Procházka, all from CEITEC, Brno University of Technology, alongside Stará et al., report the successful self-assembly of iron and dicyanoanthracene molecules into two-dimensional MOFs directly on the surface of bismuth selenide. This research is significant because it overcomes challenges in growing ordered MOFs on topological insulator surfaces and reveals two distinct Fe-DCA phases, one of which presents an unexpectedly complex structure not previously observed or predicted by theoretical calculations. These findings represent a crucial step towards engineering MOF/TI interfaces with specifically tailored properties for future spintronic devices.
Room-temperature growth of novel iron-dicyanoanthracene phases on a bismuth selenide surface reveals unique magnetic properties
Scientists have successfully assembled a two-dimensional metal-organic framework directly on the surface of a topological insulator at room temperature. This breakthrough utilizes iron atoms and dicyanoanthracene molecules to create a novel material with potentially exotic electronic and magnetic properties.
The research demonstrates a pathway to engineer quantum materials by carefully controlling the self-assembly of these components on the Bi2Se3(0001) surface. Investigations employing low-energy electron microscopy and diffraction, scanning tunneling microscopy, and theoretical calculations reveal the formation of two distinct phases of the iron-dicyanoanthracene framework.
The first observed phase corresponds to a close-packed Fe1DCA3 structure, exhibiting an expected arrangement of atoms and molecules. However, the second phase presents a surprising discovery. This phase displays a larger unit cell that does not align with any previously known or theoretically predicted configurations, suggesting a unique and complex bonding environment.
Detailed analysis indicates that the observed structure deviates from the anticipated mixed honeycomb-kagomé lattice commonly found in similar systems on other substrates. This work advances the understanding of how metal-organic frameworks grow on topological insulator surfaces, addressing a significant challenge in the field.
By employing a combination of experimental techniques and computational modeling, researchers have gained insights into the factors governing self-assembly on these substrates. The ability to create tailored interfaces between MOFs and topological insulators opens possibilities for designing materials with specific electronic and magnetic characteristics. These findings pave the way for exploring quantum phenomena, such as the quantum anomalous Hall effect, and potentially realizing advanced spintronic devices and topological quantum computation architectures.
Bi₂Se₃ crystal growth, sample preparation and MOF deposition parameters were carefully optimized
Low-energy electron microscopy and diffraction (LEEM/LEED) served as a primary tool for investigating the self-assembly of metal-organic frameworks (MOFs) on the Bi2Se3(0001) surface. Samples were examined within a UHV system maintained at a base pressure of 2×10-10 mbar, allowing for surface characterisation without contamination.
Diffraction patterns were collected from a 15×10 μm2 surface area, with microdiffraction analysis focused on a 3.7μm spot size using a mechanical aperture. Resulting crystals, measuring 4, 8mm in length, 3, 6mm in width, and up to 3mm thick, underwent in situ exfoliation within the UHV cluster using a custom sample holder.
Dicyanoanthracene (DCA) molecules were deposited using a near-ambient effusion cell at a process temperature of 55, 75°C onto the room-temperature substrate. Deposition rates, calibrated against Ag(100) substrates, ranged from 0.02 to 0.2 monolayers per minute. Iron atoms were deposited using a high-temperature cell at 1030°C, following thorough UHV degassing.
A quartz crystal microbalance calibrated the Fe deposition rate to 0.01pm/s, corresponding to approximately 0.002 monolayers per minute, with calculations suggesting 0.02 ML of Fe is required for complete MOF layer coverage. The formation of the Fe-DCA MOF involved simultaneous deposition of both components, typically for 10, 20 minutes, with potential pre-deposition of DCA and post-deposition of Fe to optimise growth.
Scanning tunneling microscopy (STM) was performed using an Aarhus 150 system with Kolibri sensors or tungsten tips in constant-current mode at room temperature, with imaging parameters detailed in figure captions. Spin-polarized density functional theory (DFT) calculations were conducted using the VASP package, employing the PBE functional and Grimme’s D3 dispersion corrections.
A Hubbard-like U-J correction of 4 eV was applied to describe Fe 3d orbitals, with a plane-wave basis set energy cut-off of 500 eV. Structural optimizations continued until residual forces on atoms were less than 0.01 eV/Ã…, and calculations accounted for dipole corrections to energy and forces. Bi2Se3(0001) interface models comprised a single quintuple layer, demonstrating negligible differences in calculated surface energy compared to thicker slabs.
Structural characteristics and rotational alignment of coexisting metal-organic framework phases significantly impact material properties
Two distinct phases, denoted A and B, formed during the self-assembly of iron atoms and dicyanoanthracene molecules into two-dimensional metal-organic frameworks on the Bi2Se3(0001) surface at room temperature. Phase B consistently appeared as the dominant phase at low DCA deposition rates, forming both sub-monolayer and full monolayer coverages.
Conversely, phase A only emerged as a coexisting phase with phase B at or above full monolayer coverage, requiring a high deposition rate of both DCA and iron. Analysis of the diffraction patterns revealed that the real-space unit cell size of phase B is 19.0 Ã…, representing a 5% increase compared to the 18.1 Ã… unit cell size of phase A.
Phase A exhibits a 23.4° rotation relative to the principal Bi2Se3 substrate directions, while phase B is rotated by 10.9°. These lattices are commensurate with the substrate, described in matrix notation as 3 −2 2 5 for phase A and 4 −1 1 5 for phase B. Scanning tunneling microscopy revealed a prevalent clover-leaf motif within both phases, attributed to an iron atom coordinated with three DCA molecules.
These motifs arrange themselves in a hexagonal lattice, maintaining symmetrical orientation within single domains. Measurements of angles between clover-leaf motifs in different rotational domains identified two distinct sets of angular differences: approximately 38.2° corresponding to phase B domains and approximately 13.2° corresponding to phase A or interphase domains.
The 5% difference in unit cell size between the two phases remains within the resolution uncertainty of room-temperature STM measurements, limiting precise phase distinction. Upon heating to 80°C, both phases lost long-range order, indicating decomposition and DCA desorption, with phase A exhibiting lower thermal stability than phase B.
Novel structural motifs in iron-dicyanoanthracene metal-organic frameworks on bismuth selenide enhance charge transport properties
Scientists have successfully demonstrated the self-assembly of two-dimensional metal-organic frameworks (MOFs) composed of iron atoms and dicyanoanthracene (DCA) molecules directly on the surface of bismuth selenide, a topological insulator. This achievement was confirmed using a combination of low-energy electron microscopy, diffraction techniques, scanning tunneling microscopy, and theoretical calculations.
The research reveals the formation of two distinct phases, designated A and B, each characterized by a unique arrangement of the iron and DCA components. Phase A corresponds to a well-known close-packed structure previously observed in similar metal-DCA MOFs grown on other substrates. However, phase B exhibits a larger unit cell size and a structural arrangement that does not align with any previously reported or theoretically predicted configurations, suggesting a more intricate bonding environment.
Analysis indicates that the observed structures do not conform to the Mackay-Hermann (MHK) lattice, a common arrangement in MOFs, but instead arise from kinetic effects, substrate interactions, and the positioning of individual atoms. Although the researchers acknowledge that the close-packed structure may still exhibit magnetic coupling, as seen in other systems, the formation of the expected MHK lattice was not observed under the experimental conditions.
These findings contribute to a greater understanding of how MOFs grow on topological insulator surfaces and offer guidance for designing interfaces with specific electronic and magnetic characteristics. The authors note the weak interaction between the substrate and adsorbed organic molecules, suggesting that intermolecular forces and metal-substrate interactions play a crucial role in stabilizing the MOF structures. Future research could explore methods to promote the formation of the MHK lattice or investigate the magnetic properties of the observed phases to fully characterize their potential for applications in spintronics and other fields.