Supporting Information Mechanistic Insights of Seeded Diamond Growth from Molecular Precursors

We report mechanistic insights of the bottom-up synthesis of nanodiamonds (NDs) from adamantane derivatives as molecular precursors. Thermal cracking of tetracosane below the decomposition temperature of adamantane, 1-adamantylamine or 2-azaadamantane revealed the initial steps of seeded diamond growth mechanism based on pyrolysis studies.

S3 thermocouple in contact with the diamond anvil. The Inconel gaskets were drilled with 150 µm diameters using an electrode drilling machine (EDM) (Almax-easyLab) with a depth typically of 100 µm, the needles for drilling were made of WC. After drilling, the gaskets were ultra-sonicated for 5 min to remove any residue from the EDM needle or the gasket. The diamond anvils (Almax-easyLabs) were Type 1a (100)-oriented 16-sided with 500 µm cullet diameter. The pressure inside the sample chamber was monitored in situ using the fluorescence from ruby chips placed inside the sample chamber. The pressure was increased stepwise to around 14.0 GPa at room temperature. Then the temperature was increased to 450 °C and held for 2.5 h. After letting the DAC cooled to room temperature, the pressure was released and the product was extracted directly on a carbon film grid (Plano GmbH).
After cooling to room temperature, the ampoules were opened. Before analysis, amine containing samples were separated using TLC. Samples without amine function were filtered over silica (0.5 × 3 cm, CHCl3).

Nuclear magnetic resonance (NMR) spectroscopy
NMR spectra were recorded on a BRUKER AVANCE 300 spectrometer using the remaining CHCl3 signal (7.26 ppm) in CDCl3 solvent as reference at 298 K. The data was analyzed with MestReNova.

Transmission electron microscopy (TEM)
The high resolution TEM images and electron diffraction were obtained with Field-emission transmission electron microscopy (FE-TEM, Tecnai G2 F20 U-TWIN). The grid was treated at 200 °C in air for 2 h before TEM measurement.

In situ Raman characterization
The in situ Raman imaging is conducted using a home-built confocal microscope with 532 nm excitation laser (LaserQuantum Tau532) typically operating at 100mW output power focused onto the sample using a 10x Mitutoyo air objective with long working distance. The laser power into the sample was typically 5 mW (measured using PM100D ThorLabs Digital Optical and Energy Power Meter and ThorLabs S121C Standard Photodiode Power Sensor) and we note here that 5 mW power is not strong enough to cause photothermal damage to the TC or to the synthesized nanodiamonds. Raman signal from the sample was analyzed using a grating spectrometer S5 (Princeton Instruments, Acton Standard Series SP-2556 Imaging Spectrograph and PyLoN 400BR_eXcelon Digital CCD).

Thin-layer chromatography-mass spectrometry (TLC-MS)
Selected spots of the TLC plate were directly analyzed by APCI-MS (atmospheric pressure chemical ionization-mass spectrometry) using the TLC plate reader "Plate Express" and the "expression L" mass spectrometer from Advion.

Pyrolysis experiment of 1-adamantylamine
The glass ampoule was loaded with 1-adamantylamine (1.90 mg, 12.56 µmol) and treated according to the general procedure (section 1.4, 2.5 h). The pyrolysate was obtained as a colorless solid. The product was subjected to NMR analysis with tetracosane (2.30 mg, 6.78 μmol) as an internal standard ( Figure S1). The terminal C-H of 1-adamantylamine were integrated with the internal standard. NMR analysis showed, that 93% of the 1-adamantylamine stayed intact. Figure S1: 1 H-NMR spectrum of 1-adamantylamine after thermal treatment. The product was subjected to NMR analysis with tetracosane (2.30 mg, 6.78 μmol) as an internal standard at 298 K. The stability was calculated referring to the integrals of the signals of CH3 (TC, 0.89 ppm) and bridgehead CH (AdNH2, 2.07 ppm).

Pyrolysis experiment of adamantane
The glass ampoule was loaded with adamantane (12.1 mg) and treated according to the general procedure (section 1.4, 2.5 h). The pyrolysate was obtained as a colorless solid and subjected to NMR analysis with an internal anisole standard (9.6 mg, 88.86 µmol). Integration of the terminal C-H of adamantane showed a stability of 100% during pyrolysis ( Figure S2). Figure S2: 1 H-NMR spectrum of adamantane after thermal treatment. The product (12.1 mg, 88.82 µmol) was subjected to NMR analysis with anisole (9.6 mg, 88.82 μmol) as an internal standard at 298 K. The stability was calculated referring to the integrals of the signals of CH3 (anisole, 3.84 ppm) and bridgehead CH (Ad, 1.90 ppm).

Pyrolysis experiment of 2-azaadamantane
The glass ampoule was loaded with 2-azaadamantane (4.30 mg, 31.1 µmol) and treated according to the general procedure (section 1.4, 2.5 h). The pyrolysate was subjected to NMR analysis with an internal anisole standard (5.76 mg, 53.3 µmol). Integration of the N-C-H of 2-azaadamantane showed a stability of 62% during pyrolysis ( Figure S3).

Pyrolysis experiment of tetracosane
The glass ampoule was loaded with tetracosane (36.5 mg, 108 µmol) and treated according to the general procedure (section 1.4, 2.5, or 5 h). The pyrolysate was obtained as a brown volatile liquid and subjected to GC-MS analysis. The main fraction in the chromatogram is still unmodified tetracosane ( Figure S4-6). Nevertheless, smaller and more volatile hydrocarbon species were also observed.