The controlled coupling of spin centers is essential in the construction of molecular spin-based quantum information processing architectures. A major challenge is to induce the requisite coupling between two adjacent spins, while protecting them from neighboring spins and other environmental interactions. Owing to their native spin properties, endohedral fullerenes are attractive for use as elements in quantum information processing architectures. N@C60 is an endohedral fullerene molecule with a highly reactive nitrogen atom at the center of the carbon cage. The endohedral nitrogen is atomic and not covalently bound to the cage atoms; therefore, the nitrogen atom is chemically inert toward the outer environment. Owing to its remarkably long electron-spin lifetimes and sharp resonances, N@C60 has exceptional properties for quantum computing. The thermal stability and molecular structure of N@C60 make it a useful embodiment of a quantum bit — a fundamental element for a quantum computer. Several future quantum computer architectures based on N@C60 have been proposed, one of which is a two-dimensional, quantum-bit array on specific substrates. However, a challenging yet important task is to understand the effect of various substrates on the spin properties of the endohedral fullerene, since the interaction between the endohedral fullerene and the substrate may largely affect the spin characters of the endohedral N atom. The fabrication of an endohedral fullerene molecular array on substrates is also a challenge because high-temperature methods such as evaporation will cause decomposition of N@C60. Here we report our investigation on the electron spin resonance (ESR) of N@C60 molecules on various substrates such as Au(111), Si(111), and SiO2. In this study, N@C60 was prepared using the ion implantation method, and enrichment was performed using a multistep and recycling high-performance liquid chromatography (HPLC) system with a Cosmosil Buckyprep column. N@C60 molecular films on Au(111) substrates were prepared at room temperature. In addition, scanning tunneling microscope (STM) topography of the N@C60/C60 monolayer on Au(111) was obtained at a sample temperature of 5 K in ultra high vacuum (UHV). We found that the ESR signal of the N@C60 molecules decreases rapidly and disappeared approximately 360 min after the deposition of N@C60 on Au(111). In comparison, the ESR signal was maintained for a longer time on the Si(111) and SiO2 substrates. We propose that coupling between the Au(111) substrate and the endohedral N atoms quenches the ESR signal of the endohedral N atom, while the Si(111) and SiO2 substrates have a smaller effect on the ESR signal. This result offers useful information for the design of basic quantum computer architectures.
Received: 23 January 2018
Published: 01 March 2018
Yeliang ZHAO,Bing WANG. Effect of Substrate on the Electron Spin Resonance Spectra of N@C60 Molecules. Acta Phys. -Chim. Sin., 2018, 34(12): 1312-1320.
Fig 1 (a) HPLC spectrum for the final 6 cycles of N@C60 purification from the mixture N@C60 and C60. (b) The enlarged peaks for the 6th cycle in (a), and the segment Ⅱ denotes the retention time for high concentration N@C60 collection.
Fig 2 MALDI-TOF mass spectroscopy of the N@C60/C60 mixture.
Fig 3 ESR spectra of N@C60 molecular film on (a) Au(111) with 200μL N@C60/C60 toluene solution (sample-1), (b) Au(111) with 400 μL N@C60/C60 toluene solution (sample-2), (c) Si(111) surface with 400 μL N@C60/C60 toluene solution (sample-3) and (d) SiO2 surface with 400 μL N@C60/C60 toluene solution (sample-4). (e) The change of ESR signal intensity as a function of time obtained from the spectra in the figures a–d.
Fig 4 (a) ESR spectra of N@C60/C60 on Au(111) after different treatments, spectrum-1: dried by N2 flow, spectrum-2: pumped to 10 torr, spectrum-3: pumped to 10-7 torr. (b)ESR spectra of N@C60/C60 on SiO2 after different treatments, spectrum-1: dried by N2 flow, spectrum-2: pumped to 10 torr, spectrum-3: pumped to 10?7 torr. (c) ESR spectra of N@C60/C60 in toluene solution obtained by resolving from the Au(111) (in red) and the SiO2 (in black) substrates. The spectra are shifted vertically for clarity. 1 torr = 133.322 Pa.
Fig 5 ESR spectra of N@C60/C60 on SiO2 substrate after annealing at different temperatures. The spectra are shifted vertically for clarity.
Fig 6 (a) STM topography of N@C60 on Au(111). Imaged at 1.0 mV, 50 pA at 5 K, Size: 90 nm × 80 nm; Inset: Imaged at 100 mV, 200 pA at 5 K, Size: 4.2 nm × 4.2 nm. (b) Line profile from the labeled line in the figure a.
Fig 7 Schematic of N@C60/C60 molecular layer on Au surface before and after vacuum procedure. Blue ellipses represent for small molecular impurities and solvent molecules.
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