Time Resolved Electrical Conductance of Growing CNT Forests Using In Situ SEM Synthesis

Carbon nanotubes (CNTs) possess exceptional engineering properties that surpass those of conventional materials, making them highly desirable for diverse multi-functional materials, including sensors, flexible electronics, and conductive interfacial materials. Vertically aligned films of CNTs, known as CNT forests or CNT arrays, are frequently synthesized by chemical vapor deposition (CVD) using a thin film catalyst. The ensemble properties of CNT forests are typically degraded when compared to the individual CNTs because of morphological features including waviness, kinks, and inhomogeneities in density. The process-structure relationships for CNT forests are difficult to assess because the CVD synthesis environment is typically enclosed, at high temperature (&gt; 600 ^oC), requires a controlled gaseous environment, and may be conducted at low pressure.<br/><br/>We have recently developed a procedure to directly observe the synthesis and self-assembly of CNT forests within an environmental SEM. The process uses thin-film MEMS heaters as substrates, upon which a typical catalyst film comprised of Al₂O₃ and Fe are deposited. Acetylene gas is introduced to the SEM chamber to initiate CNT forest synthesis. We observe the catalyst evolution, early growth of independent CNTs, CNT assembly, CNT delamination, and self-termination using this technique.<br/><br/>Digital imaging techniques are applied to the in-situ SEM synthesis sequences to estimate the CNT density as a function of time. CNTSegNet, our image segmentation deep learning network is applied to reduce image noise and to estimate CNT density in individual images. Digital image correlation software is used to track the engineering strain within a growing CNT forest. The Meta Co-Tracker algorithm is used to track the growth profile of CNTs.<br/><br/>The evolving transverse electrical conductance within a growing CNT forest is also measured using pre-fabricated electrodes that span the heated zone of the growth substrates. The conductance initially increases as CNT-CNT contacts accumulate between the electrodes. However, the conductance peaks and decreases at later times due to CNT delamination from the electrode surfaces. Finite element simulation is used to model the evolving CNT forest morphology and predict the CNT delamination stress based on the simultaneous CNT density observation and in-situ electrical conductivity. Simulations predict a delamination force of several nano-Newtons is required to delaminate growing CNTs from the growth substrate, in line with prior experiments conducted after CNT forest synthesis and at room temperature.