Extended Data Fig. 4: Spectroscopic and nanomechanical analyses of GO, ND⁺, and GOαND⁺ membranes.

a. FTIR analysis: GO exhibited typical peaks corresponding to C−O (alkoxy/alkoxide, 1046 cm⁻¹), C−O (carboxyl, 1410 cm⁻¹), C=C (aromatic, 1627 cm⁻¹), C=O (carboxyl/carbonyl, 1726 cm⁻¹), and −OH (3300 cm⁻¹). The bands of ND⁺ at 1720 cm⁻¹ and 1000−1350 cm⁻¹ are corresponding to the stretching of C=O and C−O or C−O−C vibrations, respectively, which is in agreement with the literature⁷⁰. The carbonyl band at 1725 cm⁻¹ broadened and shifted to lower frequencies at 1636 cm⁻¹ by incorporating ND⁺ particles, indicating the hydrogen bonding between GO and ND⁺s. b, c, d. High-resolution XPS analysis: The deconvolutions of C−C region indicate the presence of both sp²- and sp³-hybridized species in both GO and ND⁺. Since ND⁺ particles are poor in O functionalities, the relative abundance (intensity) of the total signals of O−C=O, C=O, and O−C−O species is lower in GO30ND⁺ membranes compared to GO membranes. No nitrogen peak was found in the structure of GO membranes, whereas the GO30ND⁺ membrane exhibited 1.5% nitrogen due to the presence of ND⁺, detected at 399.1 eV (C−NH−C) and 401.1 eV (C₃−N) (see Supplementary Table 2). e. Nanoindentation-based mechanical testing of the laminate membranes. (Error bars represent the standard error of 20 indents.) f. Raman spectra of ND⁺, GO, and GOαND⁺ complexes on silicon wafer substrates. The D and G bands at around 1343 cm⁻¹ and 1589 cm⁻¹ are characteristic of defective and sp²-hybridized (graphenic) sites of GO structure. ND⁺ particles show distinct bands of diamond, disordered (amorphous) sp³ carbon, and G-band of graphene-like carbon (GLC) with sp² hybridization⁷¹. The I_D/I_G for GO and GOαND⁺ membranes range from 0.78 to 0.86. D and G bands of GOαND⁺ samples are slightly broader than those of native GO, likely because of the superimpositions of ND⁺s peaks⁷². By adding ND⁺s, the G peak slightly shifted and reached lower ~1585 cm⁻¹ for GO30ND⁺, partly because of the combination of GO’s G-band with the GLC band of ND⁺s⁷³.

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