Plasmon spectroscopy answers key questions for future electronics
The outstanding electrical, thermal, chemical and mechanical properties of carbon nanotubes (CNTs) have seen them used – and proposed for use – in a wide range of applications. Their high current carrying capacity is of particular interest to scientists exploring the next generation of small-scale electronic devices, who see CNTs as a suitable substitute for more traditional bulk metallic interconnect materials, like copper and gold. A key step towards achieving this goal is to establish how damage-tolerant CNTs are under the high current densities and temperatures typically experienced in an electronic device.
A group of Japanese researchers has found a novel way to investigate the robustness of CNT interconnects. Using plasmon spectroscopy, they’ve measured the temperature distribution of Joule-heated, suspended nanotubes under an extremely high current density. Writing in a forthcoming issue of Carbon [DOI: 10.1016/j.carbon.2022.10.006], they say that their approach produces nanometre-resolution temperature maps, and allows them to gain insights into the poorly-understood damage mechanism that affects CNTs.
The basis of their technique is electron energy-loss spectroscopy (EELS), which can measure phonon scattering, excitonic absorption, and plasmon excitations. All materials experience minute shifts in their plasmon energy spectrum in response to temperature changes. The energy (E) has a square-root dependency on a material’s electron density (n), so as a material expands, n and E decrease. This leads to a simple relationship between the temperature difference and the relative plasmon shift divided by that material’s thermal expansion coefficient.
Therefore, accurate measurement of this coefficient is critical in determining the temperature. The team started with a series of arc-discharge multi-wall carbon nanotubes (MWCNTs), which were deposited on a grid and uniformly heated to known temperatures using an in-situ heating holder inside a commercial tunnelling electron microscope (TEM). Plasmon shifts were measured over the entire CNT surface at a range of temperatures. This allowed them to calculate the all-important thermal expansion coefficients, and calibrate their results.
Next, inside the TEM, individual nanotubes were joined to a sharp gold probe without using an adhesive. The other end of the CNT contacted a bulk gold wire. The entire assembly operated in high vacuum. A voltage was applied across each nanotube (~4 V to 0 V) to induce Joule heating, and the resulting I-V curves indicated the contacts on both sides of the CNT were ohmic. EELS was then used to characterise the plasmon spectra of these suspended CNTs, and the position of its two main peaks were measured as the temperature of the nanotube increased.
The authors say that, “Under the highest applied voltage, the nanotube reaches a maximum temperature of more than 2000 K while maintaining its room-temperature structure.” In addition, the nanotubes were seen to withstand current densities of over 5 × 107 A/cm2 without experiencing any damage. As a final step, the team compared their experimental results to those predicted by a series of simulations, and found that they were in “good agreement”.
They conclude, “These results verify the expected robustness of these structures and confirm them as ideal candidates for applications as interconnects under extremely high current densities and temperatures.”
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Ovidiu Cretu, Dai-Ming Tang, Da-Bao Lu, Bo Da, Yoshihiro Nemoto, Naoyuki Kawamoto, Masanori Mitome, Zejun Ding, Koji Kimoto. “Nanometer-level temperature mapping of Joule-heated carbon nanotubes by plasmon spectroscopy,” Carbon 201 (2023) 1025–1029. DOI: 10.1016/j.carbon.2022.10.006