Systematic Optimization of the Synthesis of Confined Carbyne

Abstract

Confined carbyne, an sp¹-hybridized linear carbon chain inside a carbon nanotube, is a novel material with remarkable properties and potential applications. Among its currently successful synthesis methods, high temperature high vacuum annealing is prevalent. Further optimization could be achieved by tuning the synthesis pathway. Here, a systematic analysis of key synthesis parameters including precursor filling, annealing step sequences, and temperature conditions during high temperature vacuum processing is performed. A novel yield determination model that overcomes previous limitations related to the irregular resonance Raman behavior of carbyne is applied to evaluate bulk yield and realized growth potential. With this refined model, it is possible to make a quantitative assessment of bulk yield optimization potential in multi-step annealing processes. These results provide crucial insights into the fundamental formation mechanisms of confined carbyne, advancing our understanding of this promising hybrid nanomaterial system. It is therefore possible to establish improved protocols for maximizing confined carbyne yields through precise control of synthesis conditions.

Keywords: Raman spectroscopy; carbon nanotubes; carbyne; confined carbyne; synthesis.

Figure 1

Effects of filling carbon nanotube hosts (1.45 nm arc‐discharge) with C₆₀ on CC yield: Raman spectra of CC from an unfilled nanotube host (red) and from peapods (orange), measured with a 568 nm laser. The bulk yield increases from 20% to 31%. A schematic of the synthesis process for the filled versus unfilled host tubes is also shown.

Figure 2

Two‐step synthesis process for CC grown from 1.45 nm arc‐discharge peapods: Raman spectra of 3 spots on the sample after a) annealing once at 1500°C; b) annealing once at 1300°C; c) annealing twice at 1500° C with an oxidation step in‐between; d) annealing first at 1300° C and then at 1500° C with an oxidation step in‐between. All Raman spectra were measured with a 568 nm laser. Below, the process of carbyne growth from peapods is shown schematically.

Figure 3

Multi‐step synthesis of CC inside different CNT hosts: a) bulk yield of CC inside 1.45 nm arc‐discharge (pink square), 1.33 nm eDIPS (green triangle) and 1.41 nm eDIPS (blue circle); b) realized growth potential for the different CNT hosts; c) schematic of the multi‐step annealing process from peapods; d) growth model for CC: two processes compete with each other ‐ formation of the carbyne chain, which is governed by the temperature and availability of carbonaceous feedstock, and etching/destruction of the carbyne chain due to residual gas in the furnace (i.e., pressure).

Figure 4

Temperature optimization for CC synthesis: a) Raman spectra of CC synthesized from 1.33 nm eDIPS nanotube hosts at different temperatures from 1500°C (bottom) to 1600°C (top) in steps of 20°C, measured using a 568 nm laser. The mode of the CC at around 1850 cm⁻¹ was magnified by a factor of five; b) relative area of the LO‐peak of CC at different temperatures between 1500 and 1600°C from multiple spots on each sample; c) bulk yield calculated using the coupled mode evaluation model from the same experiment; d) bulk yield and realized growth potential calculated for all spectra.