An environmental transmission electron microscope (ETEM) at Brookhaven National Laboratory (BNL) is used to record videos of carbon nanotube synthesis, to explore the dynamics of catalyst particle development and CNT nucleation. Image processing of recorded growth videos enables a statistical understanding of the evolution of the size, density, and CNT nucleation efficiency of catalyst nanoparticle populations under different annealing atmospheres.
Equipment - ETEM at BNL
I operated Brookhaven's ETEM for in situ experimentation, which allows for gas flows and temperatures up to 1000 oC to obtain atomic resolution video capture of CNT synthesis.
Methods - 1) In situ video capture and automated particle identification
Custom scripts in Python were written to use computer vision to automatically identify catalyst particles in the ETEM videos and track them across frames.
Methods - 2) automated particle analysis
Python scripts using machine learning techniques (e.g. convolutional neural networks, Bayesian inference) were also used to determine the size, CNT nucleation, and phase of each particle.
Results - Particle and CNT density in different annealing atmospheres
Statistical analysis of the particles in each video for 4 experiments of different annealing conditions reveals the ability to achieve a 3x increase in nanoparticle formation, 3x increase in catalyst activation, and ~10x increase in CNT nucleation using the "C preload" process conditions I developed.
Publication - in preparation
with Martin Schneider, Dmitri Zakharov, and John Hart.
Residual carbon species in the CVD reactors have been found to strongly influence CNT forest synthesis. A simple procedure is shown to take advantage of these effects to increase the density and height of the synthesized CNT assemblies.
Equipment - Hot-wall CVD reactor
Results - CNT forest density vs height
The empirical limit of CNT forest density vs height is achieved with the straightforward "carbon preload" procedure that avoids complicated material preparation and processing steps required by other studies.
Publication - Dee et al. "Carbon-Assisted Catalyst Pretreatment Enables Straightforward Synthesis of High-Density Carbon Nanotube Forests," Carbon, 2019.
A custom CVD reactor with a rapid heating stage and integrated Raman probe is used to acquire chemical signatures of the CNTs during growth. A software platform analyzes the Raman spectra to quantitatively characterize the growing CNTs towards real-time tuning and optimization of CVD process parameters.
Equipment - Cold-wall CVD reactor
Methods - Real-time Raman spectra acquisition
A time series of the normalized Raman spectra shows the emergence of the D and G peaks, which reveal the nucleation of CNTs and can be used to determine CNT quality and properties throughout growth.
A custom CVD reactor with a micromanipulator allows for controlled application of compressive loads to growing CNTs and measurement of the CNT forest height to ascertain the influence of mechanical forces on the CNT growth.
Equipment - CVD reactor with micromanipulator
Results - 1) CNT forest morphology
Electron microscope images reveal that increasing compressive loads increase the tortuosity of the CNTs and cause buckling in the CNT forest structure, yet the CNTs are still able to resist the load and continue growing.
Results - 2) CNT growth kinetics
Combining the real-time height measurements with ex situ small-angle x-ray scattering analysis, the average CNT length vs time for different applied loads is determined, revealing that the CNT growth rate decreases with increasing mechanical compression -- i.e. CNT growth is mechanochemically modulated.
Publication - Dee et al. "In Situ Mechanochemical Modulation of Carbon Nanotube Forest Growth," Chemistry of Materials, 2019.
A custom roll-to-roll (R2R) CVD reactor is used to create continuous production of high-quality graphene for the scalable production of nanoporous membranes.
Equipment - R2R CVD reactor
Graphene is synthesized on copper foil that passes through an annealing zone and a growth zone (each with independent gas compositions and temperature) of a furnace, and then wound on a roller in the tower downstream of the furnace.
Results - Graphene production rate vs quality
The independent annealing and growth zones allows for decoupling of the graphene nucleation and growth, and our study shows the trade-off between production rate and graphene quality.
Publication - Kidambi, Mariappan, Dee, et al. "A Scalable Route to Nanoporous Large-Area Atomically Thin Graphene Membranes by Roll-to-Roll Chemical Vapor Deposition and Polymer Support Casting," Appl. Mater. Interfaces, 2018.