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The novel technique of liquid-phase pulsed laser ablation has been used to obtain unique, highly ordered nanostructures of crystalline carbon nitride (C3N4) via ablation of a graphite target submerged in aqueous ammonia solution. Transmission electron microscopy (TEM) analysis shows that the morphology of the carbon nitride material changes at different length scales, depending upon the synthesis conditions. The initial ablation product comprises spherical nanoparticles of carbon nitride, which then elongate to nanorods (10 nm by ∼200 nm) with further ablation. Given sufficient concentration, the nanorods aggregate to form multilayered structures, and then ultimately larger, ordered “leaf-shaped” structures 30−50 nm by ∼200 nm in size. With even higher concentration, these leaflike structures can themselves aggregate to create interconnected networks of large micrometer scale clusters, which ultimately rearrange to micrometer-sized flowerlike structures. The various nanostructures were characterized using a number of analysis techniques, and their composition was found to be consistent with that of crystalline α- or β-phase carbon nitride. A formation mechanism for these structures is proposed that rationalizes the observed morphologies.

Two series of multibranched compounds with vinylenes attaching to the s-triazine core have been synthesized. The two-photon absorption cross section (σ) increases significantly as the branch number increases, with the ratios of σ between one-, two-, and three-branched compounds being 1.0:3.3:7.0 for series A and 1.0:3.7:7.0 for series B respectively, indicating remarkable cooperative enhancement.

We report an experimental implementation of free-space quantum secure direct communication based on single photons. The quantum communication scheme uses phase encoding, and the asymmetric Mach–Zehnder interferometer is optimized so as to automatically compensate phase drift of the photons during their transitions over the free-space medium. At a 16 MHz pulse repetition frequency, an information transmission rate of 500 bps over a 10 m free space with a mean quantum bit error rate of 0.49 % ± 0.27 % is achieved. The security is analyzed under the scenario that Eve performs the collective attack for single-photon state and the photon number splitting attack for multi-photon state in the depolarizing channel. Our results show that quantum secure direct communication is feasible in free space.