Date of Award
Master of Science (MS)
Charge transport in single crystal, p type copper oxide (CuO) nanowires (NWs) was studied through temperature based (120 K – 400 K) current-voltage (I-V) measurements of 2-terminal single NW devices. Individual CuO NWs with an average diameter of 220 nm were attached to Au electrodes 2.5 µm apart, using a dielectrophoresis method. I-V curves showed a transition from linear behavior at low bias to strong power law dependence (I ~ Va) at high bias, which can be attributed to space charge limited current (SCLC) mechanism. At low electrical fields (< 0.89 × 103 V·cm-1), the number of injected charges was smaller than the number of free charge carriers inside the intrinsic p type CuO NW, resulting in Ohmic conduction. The thermal conductivity activation energy was calculated from the linear I-V curves to be 272 meV, and a thermal equilibrium hole concentration (peff) of 1.5 × 1016 cm-3 was obtained from the Ohmic regime. At higher electrical fields (> 0.89 × 103 V·cm-1), the number of injected charges exceeded the number of free charge carriers, and the localized space charge moved from one site to another under the electrical field in discrete jumps, resulting in space charge limited current. Our results showed that there were exponentially distributed trap states in the CuO NWs (a>2). As the free holes were trapped, current caused by the flow of holes was dictated by the density and energy distribution of the trap states. Study of the SCLC regime allowed further understanding of the trap states in CuO nanowires, which has not been reported until now. We obtained an average trap energy (ET) = 26.6 meV and trap density, NT = 3.4 × 1015cm-3. The charge carrier mobility was very low (< 0.01 cm2/V·sec), as expected for the small polaron type reported for CuO. Furthermore, the small polaron mobility was found to be dominated by phonon scattering at low temperature and thermally activated hopping mechanism at high temperature. At temperatures higher than 210 K, small polarons started hopping, and the activation energy for hopping mobility was estimated to be 44 meV.
The second part of the thesis focused on the photoelectrochemical water splitting application of electrodes fabricated using CuO nanowires. The valence band edge of CuO nanowires was found to be 4.93 eV below the vacuum level through the Mott Schottky test. Combining the experimental results from single nanowire 2-terminal devices and the Mott Schottky test, the band structure of the CuO nanowires was calculated. A photocurrent density of 0.75 mA·cm-2 was observed at a bias of -0.3 V vs. Ag/AgCl. The transient chopping light measurement indicated CuO nanowires had a strong resistance to photocorrosion under the testing conditions. Incident photon-to-current efficiency spectrum showed a peak value of 5 % at 430 nm, with a broad band photoresponse from the UV to near IR region due to the low bandgap (1.4 eV) of CuO. Based on our study, CuO nanowires are one of the promising water splitting photocathode materials despite the low intrinsic carrier concentration and mobility, mainly due to its one dimensional geometry and single crystal nature.