July 31, 2019 feature
Atomically precise bottom-up synthesis of π-extended  triangulene
Chemists have predicted zigzag-edged triangular graphene molecules (ZTGMs) to host ferromagnetically coupled edge states, with net spin scaling with the molecular size. Such molecules can afford large spin tunability, which is crucial to engineer next-generation molecular spintronics. However, the scalable synthesis of large ZTGMs and the direct observation of their edge states are a long-standing challenge due to the high chemical instability of the molecule.
In a recent report on Science Advances, Jie Su and colleagues at the interdisciplinary departments of chemistry, advanced 2-D materials, physics and engineering developed bottom-up synthesis of π-extended triangulene with atomic precision using surface-assisted cyclodehydrogenation of a molecular precursor on metallic surfaces. Using atomic force microscopy (AFM) measurements, Su et al. resolved the ZTGM-like skeleton containing 15 fused benzene rings. Then, using scanning tunneling spectroscopy (STM) measurements they revealed the edge-localized electronic states. Coupled with supporting density functional theory calculations, Su et al. showed that triangulenes synthesized on gold [Au (111)] retained an open-shell π-conjugated character with magnetic ground states.
In synthetic organic chemistry, when triangular motifs are clipped along the zigzag orientation of graphene, scientists can create an entire family of zigzag-edged triangular graphene molecules. Such molecules are predicted to have multiple, unpaired π-electrons (Pi-electrons) and high-spin ground states with large net spin that scaled linearly with the number of carbon atoms of the zigzag edges. Scientists therefore consider ZTGMs as promising candidates for molecular spintronic devices.
The direct chemical synthesis of unsubstituted ZTGMs is a long-standing challenge due to their high chemical instability. Researchers had recently adopted a tip-assisted approach to synthesize unsubstituted triangulene with detailed structural and electrical properties, but the method could only manipulate a single target molecule at a time. The strategy was therefore only useful for specific applications due to a lack of scalability.
In comparison, a bottom-up, on-surface synthetic approach has great potential to fabricate atomically precise graphene-based nanostructures. The method typically involves cyclodehydrogenation of precursor monomers or polymerized monomers via intramolecular or intermolecular aryl-aryl coupling to predominate along the armchair direction, instead of the zigzag direction. In the present work, Su et al. therefore addressed the existing challenge of designing appropriate molecular precursors to synthesize large homologs of zigzag-edged triangulenes with predicted large net spin.
The scientists first designed a unique molecular precursor to synthesize π-extended triangulene. The precursor contained a central triangular core with six hexagonal rings and three 2,6-dimethylphenyl substituents attached at meso-positions of the core. The precursor design underwent cyclodehydrogenation and ring closure reactions on a catalytic metal surface at elevated temperatures.
To produce the well-separated target molecules of interest, the scientists deposited a low amount of precursor on the substrates and imaged them using low-temperature scanning tunneling microscopy (LT-STM) at 4.5 K. They found that annealing the precursor-decorated copper [Cu(111)] substrate induced a cyclodehydrogenation reaction at ~500 K to form flat triangle-shaped molecules. In contrast, the scientists could conduct the synthesis of triangulene on the inert Au (111) substrate at a higher temperature (~600 K) to obtain a much lower yield (~5%) of the product (compared to ~60% yield on the Cu substrate).
Su et al. used large-scale STM images to reveal well-separated triangle-shaped molecules after annealing to the precursor-decorated Cu (111) and Au (111) surfaces. They recorded the magnified STM images with a metallic tip to show that individual molecules adopted triangular/planar configurations on both substrates. At the edge of these molecules, the research team observed characteristic nodal features resembling the zigzag edges or termini of graphene nanoribbons (GNRs). When they conducted noncontact AFM (nc-AFM) measurements to accurately determine the chemistry of reaction products, the bright areas represented a high-frequency shift with higher electron density. As a result, they clearly resolved the zigzag-edged topology of 15 fused benzene rings, where the experimental results were in excellent agreement with those simulated using a numerical model in a previous study . The observed molecular morphology therefore corresponded to the expected triangulene.
The freestanding triangulene contained four unpaired π-electrons as theoretically predicted. To unveil the peculiar electronic properties of the molecule, Su et al. performed scanning tunneling spectroscopy (STS) measurements of single triangulene grown on the weakly interacting Au (111) substrates using a metallic tip. To capture the spatial distribution of the observed electron states, the scientists completed differential conductance (dI/dV) mapping on a single triangulene molecule at different sample biases. On examination, the differential conductance map revealed five bright lobes located at the edge of the triangulene, represented by a characteristic nodal map. The observed characteristic feature was similar to the nodal pattern of spin-polarized electronic states seen with zigzag termini and zigzag edge of GNRs.
To gain further insights into the triangulene electronic structure, Su et al. performed spin-polarized density functional theory (DFT) calculations. The energy ordering of these electron states were consistent with previous calculations of similar graphene molecular systems. Additionally, the calculations also revealed a total magnetic moment of 3.58 μb for triangulene on the Au substrate, suggesting that its magnetic ground state could be retained on the Au (111) surface. The DFT (density functional theory) provided reliable information on the ground-state energy ordering and spatial shape of molecular orbitals. Su et al. observed the frontier molecular orbitals (highest-energy occupied and lowest-energy unoccupied molecular orbitals) to contain four pairs of orbitals with corresponding wave function plots.
Su et al also used the GW method of many-body perturbation to calculate the quasiparticle energies of a free triangulene, where the quasiparticle gap was predicted to be 2.81 eV. They then experimentally determined the energy gap of Au-supported triangulene to be ~1.7 eV consistent with previous studies of GNRs and other molecular systems of comparable size. All observations indicated a magnetic ground state of triangulene on Au (111), which the scientists also validated with the DFT calculations.
In this way, Jie Su and colleagues demonstrated a feasible bottom-up approach to synthesize atomically precise unsubstituted triangulene on metallic surfaces. They used nc-AFM imaging to ambiguously confirm the zigzag edge topology of the molecule and used STM measurements to resolve the edge localized electronic states. The successful synthesis of π-extended triangulenes will allow scientists to investigate magnetism and spin transport properties at the level of the single-molecule.
The scientists envision that the synthetic process will open a new avenue to engineer larger, triangular zigzag edged graphene quantum dots with atomic precision for spin and quantum transport applications. It is therefore of great interest to continue generating similar systems with diverse sizes and spin numbers to uncover their properties on a variety of substrates using spin-polarized STM studies.
Manuel Melle-Franco. When 1 + 1 is odd, Nature Nanotechnology (2017). DOI: 10.1038/nnano.2017.9
Yasushi Morita et al. Synthetic organic spin chemistry for structurally well-defined open-shell graphene fragments, Nature Chemistry (2011). DOI: 10.1038/nchem.985
Pascal Ruffieux et al. On-surface synthesis of graphene nanoribbons with zigzag edge topology, Nature (2016). DOI: 10.1038/nature17151
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