Proximity-Induced Superconductivity in Atomically Precise Nanographene on Ag/Nb(110)

Obtaining a robust superconducting state in atomically precise nanographene (NG) structures by proximity to a superconductor could foster the discovery of topological superconductivity in graphene. On-surface synthesis of such NGs has been achieved on noble metals and metal oxides; however, it is still absent on superconductors. Here, we present a synthetic method to induce superconductivity of polymeric chains and NGs adsorbed on the superconducting Nb(110) substrate covered by thin Ag films. Using atomic force microscopy at low temperature, we characterize the chemical structure of each subproduct formed on the superconducting Ag layer. Scanning tunneling spectroscopy further allows us to elucidate the electronic properties of these nanostructures, which consistently show a superconducting gap.


Pristine Nb(110) substrate
The Nb(110) substrate was prepared by Ar + sputtering and followed by cycles of heat flash and cooling. As shown in Fig. S1a, bright protrusions and textures of short segments observed in the inset imply that there are still oxides remaining on the surface. The superconductivity of Nb(110) was examined on the sample with less than 1 monolayer (ML) of Ag (red dot in Fig. S1b). Despite of the oxidized surface, the superconducting gap still shows BCS lineshape as in Fig. S1c. The gap was fitted with the thermally broadened S1 Figure S1: The STM image and the superconductivity measurement of the pristine Nb(110) substrate. a, The pristine Nb(110) surface after the cleaning process. The inset is a zoom-in of the surface. The texture of short segments indicates the Nb-O reconstruction (I t = 1 pA, V= -100 mV). b, The Nb(110) surface with Ag coverage of 0.11 ML. c, The superconducting gap of Nb measured at the position of the red dot in b (I t = 100 pA, V= 10 mV, A mod = 80 µV). The shaded area marks the width 2∆, which is also the width of the superconducting gap.
BSC function using the least square method. The fitted ∆= 1.5 meV±7.3×10 −7 meV, which is align to the ideal superconducting gap of Nb.

Ag film growth and characterization
To characterize the two-dimensional (2D) growth of Ag films on Nb(110), we keep the Ag evaporation flux and the subsequent annealing process unchanged, and adjust the Ag deposition duration. The Ag coverage is calculated by the software Gwyddion, which shows 21% and 42% of coverage for

Zigzag and armchair intermediate compounds
After the deposition of DBBA on the Ag/Nb substrate, we found coexisting zigzag and armchair chains (Fig. S4). As discussed in the main text, both of the configurations are organometallic (OM) intermediate compounds towards graphene nanoribbons (GNRs).

S4
The spontaneous conjugation indicates that the energy barrier for forming intermediates is very low, and it can be overcome by the surface-assisted process on this Ag/Nb (110) substrate. Despite of the uniqueness of this certain substrate, OM intermediates are still able to transform to bisanthene-Ag chains by annealing at a much lower temperature estimated to be 150 • C.

dI/dV spectra on the bisanthene-Ag chain
A series of dI/dV spectra were measured along the central axis of a 3-unit bisanthene-Ag chain and across a bisanthene unit as marked in Fig. S5a. For spectra along the central chain axis (Fig. S5b), we find that the onset of the conduction band (CB) is roughly fixed at +0.50 eV while the onset of the valence band (VB) fluctuates between -1.13 eV and -0.60 eV.
On the contrary, the measurement across a bisanthene monomer (Fig. S5c) shows an almost identical VB edge while the CB edge oscillates between +0.13 eV and +0.50 eV. As the result, the smallest molecular gap arises at the position of Ag atoms, which has the value of 1.10 eV. In addition, we also find that the resonance state at +0.76 eV is pronounced only along the chain, while it is hardly observed on spectra measured at the bisanthene armchair edge (purple and blue dots in Fig. S5a).

DFT calculations of the bisanthene-Ag chain
Structural calculation. To assist the observation of the novel bisanthene-Ag chain, DFT calculation was performed using Ag(111) as the substrate. Considering different bisanthene-Ag adsorption orientations, the (4×4) orthogonal supercell yields the shortest C-C distance between two adjacent middle peripheral rings (Table S1). The discrepancy in the C-C distance between simulated and measured values is discussed in the main text. Table S1: Bisanthene-Ag/Ag(111) supercells used for the on-surface models. The C-C distance on the (4×4) orthogonal supercell is closest to the experimental value (2.51Å), thus this structure is used.

On-surface reactions on the thick Ag film
On-surface reactions of DBBA were also performed on thick Ag layer (≥ 5 ML) as comparison with thin Ag layer. We occasionally found stripe structures as shown in Fig. S9 by annealing the sample to 330 • C over 30 minutes. These structures differs from the ones reported in the main manuscript and can be attributed to GNRs. We wish to explore them by AFM imaging in future works. Figure S8: Characterization of the thick Ag layer grown on Nb(110). a, STM topographic image of the Ag layer showing a Stranki-Krastanov growth mode (I t = 100 pA, V= 800 mV). b, Surface state is probed by dI/dV at different positions of the surface (≈ -10 mV) below the Fermi level (I t = 100 pA, V= -500 mV, A mod = 10 mV). As reported by Tomanic et al. 1 , the surface state is found to become fully superconducting for such coverage. We thus speculate that GNRs synthesized on such Ag films might show similar superconducting gap as reported in the main manuscript. Figure S9: STM images of the products synthesized on the thick Ag layer. Stripe-like structures are synthesized on the thick Ag after the slow annealing process that could be attributed to GNRs (I t = 100 pA, V= 1 V).