Elettra | IUVS Shedding light on carbon atomic wires growth with real-time in situ UV Raman spectroscopy Among all carbon allotropes, one of the most elusive and fascinating is carbyne, a linear, one-dimensional chain made of sp-hybridized carbon atoms. Theoretical studies have highlighted its exceptional mechanical strength, unique electron and thermal mobility, and highly tunable optical properties. Despite increasing efforts, bulk carbyne remains unrealized. However, accessible analogues exist, such as carbon atomic wires (CAWs). These nanostructures are short linear chains capped by small functional groups. When capped with hydrogen, they form hydrogen-capped polyynes, featuring alternating single and triple carbon bonds. CAWs are appealing material platforms for various technological applications and serve as experimentally viable models to explore size effects towards carbyne-like properties. Our work aimed at understanding how CAWs form and evolve during laserbased synthesis in liquid, specifically pulsed laser ablation in liquid (PLAL). Prior studies focused on ex situ analysis after synthesis, offering only indirect insights into how polyynes grow. Key synthesis parameters, like solvent, target material, duration, and laser wavelength, affect the yield and chain length, but without real-time, in situ observations, their precise impact remains unclear. To study CAWs formation in real time, we integrated our PLAL setup with the multi-wavelengths, synchrotron-based UV Resonance Raman (UVRR) system at the IUVS beamline of Elettra (Figs. 1 a, b). Resonance Raman proved to be the ideal technique for this purpose, as CAWs exhibit a structure-dependent fingerprint vibrational mode and absorption profile, letting us selectively detect polyynes of specific lengths. The tunable excitation of the IUVS beamline, combined with the strong resonance enhancement and the extraordinary resonance Raman cross section of carbyne-like materials, enabled us to monitor, for the first time, the dynamic formation of wires with well-defined lengths, such as HC₈H, HC₁₀H, HC₁₂H, and HC₁₄H, during PLAL of a graphite target in water, methanol, isopropanol, and acetonitrile (Figs. 1 c, d). By continuously recording Raman spectra during PLAL, capturing one spectrum every 10 seconds, we were able to monitor how CAWs concentrations evolve in real time. We converted the measured Raman intensities into actual polyyne concentrations (in mol/L) through a calibration model developed from complementary ex situ measurements. Thanks to this stateof-the-art experimental approach, we reconstructed the full-time evolution of CAWs formation: each chain followed a distinct trajectory, starting with a rapid growth phase, followed by a plateau as formation and degradation of the wires balanced out, and, in some cases, a gradual decline due to degradation (Fig. 1 d). From this, we extracted key kinetic parameters, such as the production rate, saturation time, and maximum yield for each solvent and chain length. The solvent role turned out to be critical. In water, for instance, wire formation was significantly limited. Wires concentrations were low, and saturation occurred quickly, often within a few minutes. This can be attributed to water’s high polarity and its tendency to generate reactive species, both promoting degradation of CAWs. Moreover, water offers no extra carbon source beyond the ablated graphite, limiting the overall CAWs yield. As a result, only short chains, such as HC6H and HC8H, are typically formed. By contrast, methanol and isopropanol provide a more favorable environment. In both alcohols, the wire concentration grew more steadily, and longer chains, such as HC12H and HC14H, were easily detected. These organic solvents not only supply additional carbon but also slowed down the degradation processes due to their intermediate polarity. Acetonitrile offered the most promising results. In this solvent, polyynes reached higher concentrations, even for longer chains, than in any other solvent tested. Signals for HC₁₂H and HC₁₄H were particularly strong, and all CAWs exhibited slower saturation kinetics, indicating that formation outpaced degradation for a longer period. Acetonitrile’s high carbon-tohydrogen ratio promotes the formation of long wires against their termination with hydrogen. On the other hand, its low polarity improves the solubility of CAWs, helping preserve the wires once formed. These findings mark an important step forward in the realization of carbyneinspired technologies, with implications spanning from fundamental to applied science. For the first time, we obtained quantitative, time-resolved data on how specific wire lengths form and degrade during synthesis. This allows fine-tuning of synthesis protocols, for example, stopping at the optimal time to get HC₁₂H at its maximum concentration for a photonic application or choosing solvents that favor its stability. Knowing the degradation rates helps identify the time window for wire extraction or stabilization via encapsulation in protective structures (e.g., polymeric matrices or carbon nanotubes). More broadly, our study sheds light on the delicate interplay between wire formation and degradation, highlighting how even minor changes in the chemical environment can affect this equilibrium. This insight is crucial for any practical application involving CAWs, from molecular electronics to nanoscale sensors, where material stability is critical. Finally, our real-time, in situ UVRR approach provides a general strategy to probe the formation of other lowdimensional nanomaterials, including heteroatomic chains or conjugated organic systems. Thanks to its selectivity and resonance enhancement, this method opens new avenues for tracking fast and complex growth processes that were previously inaccessible. Elettra Highlights 2024–2025 78
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