[“Source” series] Keithley’s application in carbon nanotube forest coated fiber composites

The carbon nanotube forest is composed of many vertically growing carbon nanotubes, which looks like a "forest", hence the name. Each carbon nanotube (CNT) is a cylindrical structure formed by curling up a single or multi-layer graphene sheet, with a diameter at the nanometer level and a length of several microns or even millimeters. These nanotubes in the carbon nanotube forest are densely arranged and grow vertically on the substrate.

Characteristics of carbon nanotube coated fibers

High specific surface area: Carbon nanotube forests have a huge surface area, which helps to improve performance in catalysis, sensing, energy storage, etc.

Excellent mechanical properties: Carbon nanotubes have extremely high strength and toughness.

Electrical and Thermal Properties: They exhibit excellent electrical and thermal conductivity.

Applications of CNT-coated fibers

Energy field: used in lithium-ion batteries, capacitors, solar cells, etc.

Sensors: Made into highly sensitive chemical sensors, pressure sensors, etc.

Electronic devices: used as field effect transistors, conductive materials, etc.

Composite materials: Enhance the mechanical properties of materials such as polymers and metals.

Key parameters of CNT coated fibers

Piezoresistance

The piezoresistive properties of CNT can be applied to high-sensitivity sensors, and used to manufacture high-sensitivity pressure sensors and strain sensors. These sensors can detect tiny changes in pressure or deformation; they can be made into flexible films, and combined with their piezoresistive properties, they can be used to develop flexible electronic products such as flexible touch screens and wearable devices; in energy storage devices such as lithium batteries, carbon nanotubes can be used as conductive agents, and their piezoresistive properties can be used to monitor changes in pressure inside the battery and improve battery safety.

The piezoresistive properties are affected by parameters such as the diameter, density, internal connectivity and uniformity of CNTs. For example, CNTs (5-30 nm) have higher flexibility and more CNT-CNT contacts than CNTs with larger diameters (50-70 nm), which is beneficial for improving piezoresistive sensitivity. The higher the CNT density, the more CNT-CNT contacts, which is beneficial for forming better conductive paths, thereby improving piezoresistive sensitivity.

Contact Resistance

Contact resistance is the main factor affecting the sensitivity of CNT sensors. Simulation results show that increasing the number of CNT-electrode contact points significantly reduces the overall resistance, which is the main reason for the higher sensitivity. In contrast, increasing the number of CNT-CNT internal contact points has a much smaller effect on the overall resistance.

For relatively short CNT forests, the changes in contact resistance and intrinsic resistance are balanced, while for taller CNT forests, the response is dominated by the CNT electrode contact resistance. These results are expected to guide the design of piezoresistive flow sensors and tactile sensors.

Photothermoelectric (PTE)

Photothermal voltage refers to the thermovoltage generated when a material is exposed to light. Carbon nanotube film (CNT Film) may exhibit significant photothermal effect due to its unique one-dimensional structure and excellent optical, thermal and electrical properties, and can produce photothermal effect, that is, when exposed to light, a temperature difference will be generated, thereby generating thermovoltage. This photothermal conversion property can be used to develop self-powered mid-infrared detectors. This detector based on photothermal effect has the advantages of wide bandwidth and self-power, providing new opportunities for applications such as industrial monitoring and wearable sensors.

Case 1: How to test the contact resistance and intrinsic resistance of carbon nanotube forests

The CNT-coated fiber was placed on a substrate with staggered electrodes and a maximum pressure of 10 mN was applied using a nanoindenter. The displacement, force, and resistance of the fiber were recorded simultaneously using a digital multimeter. The total resistance was obtained by measuring the voltage difference and current between the electrodes. The total resistance is a combination of the contact resistance and the intrinsic resistance. The total resistance increases almost linearly with the increase in the number of CNT-electrode contact points, indicating that the contact resistance is the dominant factor.

The contact resistance can be calculated by setting the voltage of the CNT node in contact with the electrode equal to the voltage of the electrode. This calculation takes into account the effect of the CNT-electrode contact. The internal resistance is determined by the number of CNT-CNT contacts. The intrinsic resistance can be calculated by setting the voltage of the CNT node in contact with the electrode equal to the electrode voltage. The intrinsic resistance of a short CNT forest will drop rapidly during compression, and this rate of change may be greater than the change in contact resistance.

Experimental and simulation results show that CNT-electrode contact is the mechanism that causes the largest resistance change. Compared with CNT-CNT contact, CNT-electrode contact has a more significant impact on the total resistance.

Case 2: Design of a large-area vertical photothermoelectric detector system for carbon nanotube forests based on MXene electronics

How to conduct system design of large-area vertical photothermoelectric (PTE) detectors based on carbon nanotube forests and MXene electrodes? PTE detectors combine photothermal and thermoelectric conversion to achieve effective infrared detection and overcome bandgap limitations. This study proposes a vertical PTE detector using carbon nanotube forests and MXene as top electrodes, demonstrating sensitive infrared detection and fast response.

This test uses Keithley 6487 and Keithley 6500 to measure IV curves, and Keithley 6500 to measure resistance. Measuring IV curves can be used to characterize the electrical performance of the device; measuring resistance can be used to evaluate the conductive performance of the device. These measurements help understand the working mechanism of the device and optimize the performance of the device. Measuring resistance can also be used to monitor the stability of the device during use.

Case 3: Conductive 3D nanobiohybrid system based on densified carbon nanotube forest and organisms

This paper describes the development of a conductive 3D nanobiohybrid system using dense carbon nanotube (CNT) forests integrated with living cells for applications in bioelectronics and biorobotics. Conductive biohybrid cell-material systems are important for various applications such as organ-on-a-chip systems and muscle actuators. Current conductive scaffolds are limited in conductivity or structure, highlighting the need for 3D biohybrid systems. Carbon nanotube (CNT) forests were chosen because of their unique properties, including high conductivity and good mechanical adaptability. This study developed CNT forest scaffolds with enhanced cell compatibility and demonstrated cell survival and functionality. Gelatin coating on the CNT forests led to densification and formed a 3D structure, providing a suitable scaffold for cell growth and proliferation. This study contributes to the development of advanced biohybrid systems and has broad application prospects.

In this paper, the conductivity of CNT forest is measured by four-probe test method using Keithley 4200A-SCS parameter analyzer. The resistance and conductivity of the sample are calculated by measuring the voltage drop between the two inner probes and combining the linear scan of the current. This four-probe test method can accurately measure the conductivity of the material without being affected by the electrode contact resistance, so as to evaluate the electrical properties of CNT forest, and ensure the improvement of the electron and ion transmission capacity between cells and CNT with high precision and reliability.

Four-probe test function and data

Test plan

As a testing expert in the small signal field, Tektronix Keithley provides a wide range of products to assist in the research of carbon nanotube composite materials.

It has a graphic sampling multimeter that combines a high-precision, high-resolution digital multimeter, a graphic touch screen display, and a high-speed, high-resolution digitizer. And a picoammeter with a current resolution as low as 1fA, which supports the measurement of currents up to 20mA, such as measuring 4-20mA sensor loops. It also supports high-throughput production test needs, up to 1,000 readings per second. There is also the professional performance-leading 4200A-SCS electrical characteristic parameter analyzer, which provides synchronous current-voltage curve testing (IV curve testing), capacitance-voltage curve testing (CV curve testing) and ultra-fast pulse IV curve measurement.