Controlled growth of a single carbon nanotube on an AFM probe

The rapid advancement of three-dimensional (3D) nanodevices necessitates efficient and nondestructive morphological characterization techniques. Atomic force microscopy (AFM) is a prime candidate, but conventional AFM probes struggle with high-aspect-ratio 3D nanostructures due to their limitations in resolution and wear resistance. Carbon nanotubes (CNTs), with their small diameter, high aspect ratio, and superior wear resistance, offer a promising solution. Existing methods for fabricating CNT-AFM probes, including manual assembly and direct growth, suffer from drawbacks such as inconsistent CNT attachment, bundle formation, length control issues, and time-consuming processes. This research aims to address these challenges by developing a facile and controlled method for growing a single CNT on an AFM probe tip.

Previous studies have explored two primary methods for creating CNT-AFM probes: manual assembly and direct growth. Manual assembly, while simple, often results in bundles of CNTs rather than single, precisely positioned CNTs. The control of CNT length and reliability are significant limitations. Multi-step assembly methods improve control but are time-consuming and rely on SEM monitoring. Other approaches, such as solvent evaporation or dielectrophoresis, lead to variations in CNT length and straightness, impacting scanning performance. Direct growth methods, while offering higher bonding strength, also struggle with single CNT placement and tip length control, often requiring post-growth cutting. The existing methods lack the precision and efficiency needed for widespread application in high-resolution nanostructure imaging.

The authors present a novel two-step pick-up process. The key is controlling the trigger threshold of a standard AFM, which precisely adjusts the cantilever beam bending and consequently, the immersion depth of the AFM probe tip into a growth solution. This immersion depth directly controls the amount of growth solution adhered to the tip. The growth solution comprises inorganic chlorides (AlCl3·6H2O, SiCl4, and FeCl3·6H2O), a triblock copolymer, and an alcohol solution. After picking up the solution, the AFM probe is placed in a CVD furnace for CNT synthesis. The force curves obtained at different trigger thresholds (0.25 V and 0.55 V) were analyzed to quantify the immersion depth and understand its effect on the amount of catalyst adhered to the tip. The immersion depths were measured to be approximately 56 nm and 98 nm, respectively. The SEM images and energy spectrum analysis characterized the growth solution droplets. The resultant CNT probes were then characterized using SEM to determine the number of CNTs grown, the length of the CNT, and the diameter of the solidified growth solution on the tip. The yield was determined based on the perpendicularity of the CNT tip.

The controlled pick-up process yielded a high success rate of over 93% for single CNT growth on the AFM tips. The length of the grown CNTs was precisely controlled by adjusting the trigger threshold; an increase of 0.05 V resulted in a 16 ± 2.0 nm increase in CNT length. The average length of the CNT probes was 589.17 nm with a standard deviation of 27.60 nm. The average diameter of the CNTs was 50 ± 1.53 nm. The diameter of the solidified growth solution on the tip correlated directly with the trigger threshold, allowing for predictable control of CNT growth. Crucially, the length of the CNT probes did not require subsequent cutting, unlike previous methods. Testing against standard grating samples and high-aspect-ratio nanoholes fabricated in bulk fused silica demonstrated significantly improved depth resolution compared to conventional AFM probes. The CNT probes showed superior accuracy in measuring the height of the standard grating sample (568 ± 0.8 nm vs 344 ± 2.09 nm) and provided clearer edge definition for the nanoholes. The measurements obtained using CNT probes were closer to the real morphology of the nanoholes.

The results demonstrate a significant improvement in the fabrication of CNT-AFM probes. The precise control over CNT growth achieved through the manipulation of the AFM trigger threshold allows for the reliable production of single, appropriately sized CNT probes. This method significantly simplifies the fabrication process while simultaneously enhancing its precision and yield, eliminating the need for time-consuming post-processing steps like cutting. The superior imaging performance of the CNT probes compared to conventional probes highlights their potential for high-resolution 3D nanostructure characterization. The improved accuracy and clarity in measuring depth profiles of complex structures are highly relevant to various fields, including nanometrology and the characterization of advanced nanomaterials.

This study presents a facile and highly effective method for growing single CNTs on AFM probes. By controlling the trigger threshold of the AFM, the amount of growth solution adhered to the probe tip can be precisely controlled, leading to the selective growth of a single, suitably sized CNT. This method offers a significant advancement in the fabrication of CNT-AFM probes, yielding high-quality probes with high efficiency and eliminating the need for post-growth processing. The enhanced resolution and accuracy demonstrated in the imaging of complex nanostructures highlight the potential of this method for various applications in nanometrology and nanoscience. Future work could focus on exploring different growth solutions and optimization of the process for even greater precision and yield.

While the yield is high (93.33%), the stochastic nature of the catalyst particle distribution in the growth solution means complete control over the positioning of a single CNT remains a challenge. Although the length deviation is small under the same trigger threshold, further investigation is needed to investigate the long-term stability of the CNT tips. The study focused on a specific type of AFM probe and growth solution; future studies should examine the generalizability of the method to other types of probes and growth conditions.