Engineering the magnetism of nanographene via co-depositing hetero precursors


 The magnetism of carbon nanomaterials is dominated by the structure of its carbon skeleton. However, the magnetism engineering is hindered due to finite precursors. Here, we develop a new strategy to engineer the magnetism of nanographene through hetero-coupling two different precursors on Au(111) surface by using low-temperature bond-resolved scanning tunneling microscopy and scanning tunneling spectroscopy, combined with spin-polarized density functional theory calculations. Our results demonstrate that two homo-coupled products host close shell structure along with defects inducing magnetic one with the total spin number S = 1/2. Upon simultaneous depositing with another precursor, two hetero-coupled products switch to magnetic structure with the S = 1/2 and S = 1 resulting from carbon skeleton transformation. Our results provide a valid way via inducing different molecular precursors to engineer the magnetism of carbon nanomaterials, which could be extended in other magnetic materials instruction.

The total spin quantum number (S) of NGs might be engineered in three ways.
Firstly, the radical positions of NGs could be passivated by H atoms released from cyclodehydrogenation, which will modulate S from 0 to 1/2 28 and 1 to 1/2 23 . In this situation, the S could be recovered by applying bias to remove the redundant H atoms.
Bond-resolved scanning tunneling microscope (BR-STM), scanning tunneling spectroscopy (STS), together with spin-polarized density functional theory (SP-DFT) calculations are combined to determine the atomic structures, electronic, and magnetic properties of two homo-coupled products and two hetero-coupled products.
Design synthesis routes of magnetic NGs. As illustrated in route 1 of figure 1, surface-catalyzed Ullmann homo-coupling and afterwards cyclodehydrogenation reactions (300 °C, 6 °C/min) of precursor 1 lead to the formation of NG 1a (defect-free). Simultaneously, the NG 1b endowed with a pentagonal terminus resulting from losing a methyl group in precursor 1 could be formed as well during the annealing process. According to chemical structures and Lerb's theorem 24 , the total spin of 1a and 1b is 0 and 1/2, respectively. NG 1a can be described as either an open shell non-Kekulé structure with 5 Clar sextets or a close shell Kekulé structure with 4 Clar sextets, respectively. The aromaticity of 5 Clar sextets is higher than 4 Clar sextets one, resulting in the higher kinetically stability 38 , whereas the open shell structure is more reactive than the close shell. In addition, DFT calculation show that the real structure of NG 1a is still ambiguous (Supplementary table 1

Conclusion
In the final analysis, we demonstrate a new strategy to engineer the magnetism of NGs by using combined bond-resolved scanning tunneling microscopy, scanning tunneling spectroscopy as well as spin-polarized density functional theory. Two homo-coupled products (NG 1a and NG 1b) acquired by Ullmann coupling and cyclodehydrogenation reactions of precursor 1 and two hetero-coupled products (NG 2a and NG 2b) co-deposited by precursor 1 and precursor 2 have been successively fabricated. Even though the calculated energy of open shell structure and the close shell one is comparable, the defect-free NG 1a performs as the non-magnetic close shell structure as no enhanced state and inelastic spin excitation signal was detected.
Defective NG 1b host spins of S = 1/2 resulting from a sublattice imbalanced carbon skeleton. Co-deposition induced magnetism switching for defect-free NG 2a and defective NG 2b host spins of S = 1/2 and S = 1, respectively. Our work successfully realizes the magnetism engineering by inducing another precursor and offers a valid opportunity via the co-deposition to extend the magnetism engineering of NG on metal substrate.

Sample/tip preparation and STM/STS measurements
All the STM measurements were operated with a commercial LT-STM from Scienta Omicron under ultra-high vacuum (base pressure better than 1 x 10 -10 mbar) at a temperature of 4.2 K. Au(111) single crystal was prepared with several cycles Ar + ions sputtering and following annealing at 800 K. Precursor 1 and 2 were deposited at 348 K and 318 K from a commercial 4-cell evaporator on clean Au(111) held at 453K, respectively and post-annealing at 573 K for cyclodehydrogenation and oxidative cyclization (6 °C/min). A tungsten tip was fabricated via electrochemical corrosion.
To fabricate gold decorated tungsten tip for the STM imaging and spectroscopy, tungsten tip was slightly indent to the Au(111) single crystal. All STM and STS measurements were performed in constant current mode, except for noting otherwise.
The tunneling bias voltages are applied with respect to the sample. dI/dV spectra and maps were obtained with a lock-in amplifier (Zurich Instruments) operating at a frequency of 600 Hz. Lock-in modulation voltages (Vrms) are given in the figure captions. Bond resolving STM images were acquired in constant-height mode with a CO-functionalized tip, and the current signal was recorded. The preparation of CO-functionalized tips is prepared as the papers guided before 43 . The data were processed and analyzed with WSxM software 44 .

Data availability
The authors declare that all the relevant data of this study are available.