In nanoscale materials, the Coulombic interaction between electrons are stronger than in bulk materials. These stronger interactions, caused by confinement and reduced dielectric screening, have interesting consequences for light-matter interactions. In carbon nanotubes (CNTs), strong interactions can enhance the impact ionization process, and thus assist photocurrent generation in CNTs. Conversely, the strength of the attraction between photo-excited electrons and holes can hinder photocurrent generation by binding electrons and holes to each other. An understanding of both impact ionization and electron-hole binding is needed to determine which will dominate the system and under what circumstances. The central question of this research is whether high efficiency photocurrent can be generated in a material that has strong Coulombic interactions between charge carriers. In this thesis I explore the possibility of using electric fields to ionize excitons and thereby access a regime where highly efficient photocurrent generation can be achieved.
I begin by introducing simple theoretical models that explain the general features of CNT optical properties. Increasing the complexity of the models reveals finer details of the system and allows us to estimate the effect of electric field on the exciton energy state and ionization rate.
To maximize Coulmbic interactions between charge carriers, I made fully suspended CNTs by growing the nanotubes over a trench using a fast-heat chemical vapor deposition process. Split-gate electrodes at the bottom of the trench are used create a pn junction in the nanotube by applying opposite voltages to each gate, populating each half of the nanotube opposite-sign charge. The devices form near ideal diodes, and the samples are remarkably clean. The same design of CNT devices that I used are also used by our collaborators at University of Utah to study strongly interacting transport phenomena at low temperature.
A number of computational models were used to quantify the CNT device parameters. Working with Dr. Andrea Bertoni, we created and experimentally verified an electrostatics model that calculates the electric field in the CNT pn junction and determines the intrinsic region length. Experimental verification of the simulations was achieved using scanning photocurrent microscopy. Simulations of the optical interference patterns near the nanotube were performed using finite-difference-time-domain software to determine the number of photons absorbed by the CNT. Dr. Vasili Perebeinos computed the effect of electric field on the excited states in carbon nanotubes by solving the Bethe-Salpeter equation for a CNT exciton in a static electric field.
Photocurrent spectra were measured around the S22 resonance on eight different CNTs over a range of electric fields up to ~10 V/μm. The spectra are processed to extract the photocurrent quantum yield (PCQY) from the photocurrent peak. The PCQY increases with field in all cases, and increases by a factor of 35 to a value of 1.85 electrons per photon in the largest diameter CNT (with D = 2.8 nm). The same procedure was performed for the S33 resonance which shows a weaker field dependence, but larger PCQY ~ 0.3 at low field.
The results show that the photocurrent quantum yield can exceed 100% at the S22 exciton resonance sufficiently large axial electric field and CNT diameter. This suggests that impact ionization can coexist with efficient exciton dissociation when the electric field is ~10 V/"μm" and the CNT diameter is ~ 2.8 nm. We observed a different PCQY field dependence for S33 excitons. A large fraction of S33 excitons autoionize at low fields, resulting in a high PQCY at low field. We observe a gradual increase in PCQY with respect to field. The gradual increase is attributed to reduced recombination of the free carriers by sweeping them out of the device more quickly. To interpret our data we compared our experimental results with theoretical calculations of the decay products of S22 and S33 excitons. This research serves as a framework that can be extended to other systems with strong interactions, and motivates future work tuning the carrier interaction strength to achieve high-efficiency photocurrent generation.