Study of electromechanical and optoelectronic behavior of carbon nanostructures

Abstract

Electric field induces a large actuation (up to 14-30%) in bulk CNT cellular structure.

The actuation is symmetric, polarity independent, non-instantaneous, and directly proportional to the square of the applied electric field.

In general, the actuation in the direction of the applied electric field is much larger than the direction perpendicular to it. The maximum actuation for the CNT sample is observed in the axial direction (i.e. along the nominal length of the CNT strands) if the electric field is also applied in the same direction.

Loading 1% volume fraction of CuO nanoparticles enhances the electric field induced actuation of the bulk CNT by more than a factor of 2. An electrostrictive mechanism based on the electric field induced polarization of the CNT can aptly explain the observed actuation behavior.

Moreover, CNT-nanoparticles interaction affects the mechanical and ECR properties of the nanoparticle dispersed CNT cellular material.

The mechanical strength of CNT mat enhances compared to that of the CNT mat alone through the impregnation of them with nanoparticles. This, in turn, leads to a nonlinear electrical response, as reflected in the measured I-V characteristics that show a power law dependence of differential conductivity on the applied bias voltage. The extracted power law exponent was observed to be inversely dependent on depth of penetration and varies with the nature of CNT-nanoparticle interaction.

Asymmetric electrical response was observed and it was hypothesized that it may be due to structural asymmetry of the bulk CNT. Amongst the various nanoparticles considered, the CNT mat with Ag nanoparticles exhibits the maximum number of conducting channels for electron tunneling compared to the semiconducting nanoparticles.

Our observations are vital to establish an understanding of the complex nature of the entangled CNT microstructures and to tune electromechanical behavior by tunneling in CNT-CNT and CNT-nanoparticle interactions.

Conclusion

A five times higher actuation of bulk CNT structure was demonstrated by coupling both the electric and magnetic fields.

The actuation was found to be dependent on the polarity of the applied voltage in the presence of a magnetic field, as opposed to the polarity independence with the electric field alone. A charge-separation scheme for the high actuation as well as its polarity dependence is presented to be responsible for the high actuation.

This enhanced actuation of CNT can be used in diverse areas like biomedical, automobiles, electrical engineering, etc.

The electro-optic response of graphene is studied under variable conditions of IR incidence angles as well as incident powers. The electrical response is found to be linearly dependent on the IR power; however, a nonlinear relation is deduced from the angle of optical incidence. A large variation in the angle, from 30° to 120°, was applied in order to explain this interesting behavior from the photoresponse of the graphene.

An enhanced photoresponse in hydrogenated graphene photodetector is demonstrated. The photoresponse of hydrogenated graphene is measured to be nearly 3-4 times higher than that of pristine graphene.

A linear photoresponse with IR power is observed for both saturation and cyclic measurements, whereas a nonlinear response is recorded with the change in the optical angle of the incident radiation.

The photoresponse of hydrogenated graphene is understood from the phenomenon of bandgap opening in graphene due to the hydrogen attachment in the graphene lattice.

These results clearly indicate that the IR response is mainly due to the photon drag effect. The photoresponse of hydrogenated graphene detector could be further enhanced by engineering the bandgap more precisely and adding different functionality to the graphene.

In this chapter, we discussed an enhanced and stable photocurrent generated by inducing defects in few-layer graphene using a very simple yet efficient method. We experimentally studied the effects of wrinkles and the presence of multi-walled carbon nanotubes (MWCNTs) on the photocurrent generation in few-layer graphene.

The presence of MWCNTs on few-layer graphene enhances the stability but lacks photoresponsivity compared to that of the wrinkled graphene. Whereas, the wrinkled graphene was found to be highly efficient in both stability and photoresponsivity, except at higher power.

The exact mechanism of photocurrent generation in wrinkled graphene is still unclear and requires further study. However, future work can be directed towards engineering the wrinkles as well as the density variation of MWCNTs to enhance the photocurrent yield.

Photoresponses of stacked graphene layers are evaluated for different stacking configurations, namely crossed and parallel.

The individual responses are compared with each other as well as multilayer graphene. In the crossed configuration, the electrical conductivity is shown to both increase and decrease depending on the graphene stacking.

Differential conductivity demonstrated the emergence of secondary peaks that could be related to the multi-charge puddles at the graphene interfaces.

This thesis mainly focuses on measuring the actuation and photoresponses of the one- and two-dimensional carbon nanomaterials, i.e., carbon nanotube (CNT) and graphene. Chapter 3 presented an ultrahigh actuation of ~14% in the bulk CNT structure, which was nine orders of magnitude higher than the existing materials. Further, the actuation is increased to ~30% by loading dielectric CuO nanoparticles. The actuation response in CNT demonstrated an electrostriction behavior due to the capacitive behavior upon application of an electric field. Further, the effect of nanoparticles on the coupled electro-mechanical behavior of the bulk CNT structure was studied by the nanoindentation technique. It was evident from the results that the addition of the nanoparticles enhances the mechanical strength of the bulk structure and electrical response depends on the electrical property of the nanoparticle. Chapter 4 presents the enhancement of the actuation response upon coupling electric and magnetic fields. Results demonstrated a three times higher actuation compared to the only electric field actuation. Chapter 5 describes photoresponses of both single-layer and hydrogenated graphene, and the photoresponse enhances upon doping single-layer graphene with hydrogen by opening the bandgap in graphene. The nonlinear sinusoidal photoresponse was observed for the different angles of IR incidence, and the shift in the response was observed for hydrogenated graphene compared to the single-layer graphene. Chapter 6 explains the enhancement of the photoresponse in the few-layer graphene by creating wrinkles in the graphene and dropcasting MWCNTs on the graphene. A four- and two-fold magnitude enhancement in the photoresponse was observed compared to the only few-layer graphene. Hence, the observed photoresponse has mainly morphological effects on the graphene. Chapter 7 presents the stacked photocurrent response, which indicated two different configurations of graphene, namely parallel and crossed configuration. The sample demonstrated both positive and negative photoconductivities. The measured differential conductivity demonstrated different types of responses due to the interface effect.