Chris J. Barnett

Quantitative analysis of annealed scanning probe tips using energy dispersive x-ray spectroscopy

A quantitative method to measure the reduction in oxide species on the surface of electrochemically etched tungsten tips during direct current annealing is developed using energy dispersive x-ray spectroscopy. Oxide species are found to decrease with annealing current, with the trend repeatable over many tips and along the length of the tip apex. A linear resistivity approximation finds significant oxide sublimation occurs at 1714 K, but surface melting and tip broadening at 2215 K. This method can be applied to calibrate any similar annealing stage, and to identify the tradeoff regime between required morphological and chemical properties.

Forming reproducible non-lithographic nanocontacts to assess the effect of contact compressive strain in nanomaterials

The application of electrical nanoprobes to measure and characterize nanomaterials has become widely spread. However, the formation of quality electrical contacts using metallic probes on nanostructures has not been directly assessed. We investigate here the electrical behaviour of non-lithographically formed contacts to ZnO nanowires (NWs) and develop a method to reproducibly form Ohmic contacts for accurate electrical measurement of the nanostructures. The contacting method used in this work relies on an electrical feedback mechanism to determine the point of contact to the individual NWs, ensuring minimal compressive strain at the contact. This developed method is compared with the standard tip deflection contacting technique and shows a significant improvement in reproducibility. The effect of excessive compressive strain at the contact was investigated, with a change from rectifying to ohmic I–V behaviour observed as compressive strain at the contact was increased, leading to irreversible changes to the electrical properties of the NW. This work provides an ideal method for forming reproducible non-lithographic nanocontacts to a multitude of nanomaterials.

The role of probe oxide in local surface conductivity measurements

Local probe methods can be used to measure nanoscale surface conductivity, but some techniques including nanoscale four point probe rely on at least two of the probes forming the same low resistivity non-rectifying contact to the sample. Here, the role of probe shank oxide has been examined by carrying out contact and non-contact I V measurements on GaAs when the probe oxide has been controllably reduced, both experimentally and in simulation. In contact, the barrier height is pinned but the barrier shape changes with probe shank oxide dimensions. In non-contact measurements, the oxide modifies the electrostatic interaction inducing a quantum dot that alters the tunneling behavior. For both, the contact resistance change is dependent on polarity, which violates the assumption required for four point probe to remove probe contact resistance from the measured conductivity. This has implications for all nanoscale surface probe measurements and macroscopic four point probe, both in air and vacuum, where the role of probe oxide contamination is not well understood.

The effects of surface stripping ZnO nanorods with argon bombardment

ZnO nanorods are used in devices including field effects transistors, piezoelectric transducers, optoelectronics and gas sensors. However, for efficient and reproducible device operation and contact behaviour, surface contaminants must be removed or controlled. Here we use low doses of argon bombardment to remove surface contamination and make reproducible lower resistance contacts. Higher doses strip the surface of the nanorods allowing intrinsic surface measurements through a cross section of the material. Photoluminescence finds that the defect distribution is higher at the near-surface, falling away in to the bulk. Contacts to the n-type defect-rich surface are near-Ohmic, whereas stripping away the surface layers allows more rectifying Schottky contacts to be formed. The ability to select the contact type to ZnO nanorods offers a new way to customize device behaviour.

Investigation into the initial growth parameters of hydrothermally grown zinc oxide nanowires

Zinc oxide nanowires have been synthesized hydrothermally using varying initial growth parameters, namely concentrations of precursors and temperature. SEM was then used to characterize the shape and size of the nanowires. PL was also implemented to examine the effects of altering the growth conditions on the defect band of the zinc oxide nanowires. It was found that growing zinc oxide nanowires using higher concentrations of zinc nitrate synthesized nanowires with larger diameters and that they were more tapered. Growing at higher temperatures resulted in nanowires that were longer but also had a larger defect band. Synthesis does not occur when the growth solution is lower than 55 °C.

The viability of U-2 OS cells on Zinc Oxide nanowires observed via MTS assay in vitro

U-2 OS, an Osteosarcoma cell line was incubated on Zinc Oxide nanowire (ZnO NW) arrays of differing morphology for 72 hours at different seeding densities. MTS assays were conducted to ascertain the viability of the cells and the biocompatibility of ZnO NWs. It was found that the viability of the U-2 OS cell line when subjected to a high concentration of ~1.22μg/ml NWs lies between ~71-96% and ~68-72% for substrates with a precursor chemical ratio of 2:1 and 1:1, respectively. It was observed that the shape of the NWs determined the viability of cells on the NWs; which was attributed to changes in the surface area available for cell adhesion. It was determined that the increased surface area of the 2:1 NW array provides a higher area for interaction and formation of focal adhesions compared to arrays formed from the smaller, pointier 1:1 NW array that inhibit cell proliferation due to non adhesive gaps.

Investigation into the effects of surface stripping ZnO nanosheets

ZnO nanosheets are polycrystalline nanostructures that are used in devices including solar cells and gas sensors. However, for efficient and reproducible device operation and contact behaviour the conductivity characteristics must be controlled and surface contaminants removed. Here we use low doses of argon bombardment to remove surface contamination and make reproducible lower resistance contacts. Higher doses strip the surface of the nanosheets altering the contact type from near-ohmic to rectifying by removing the donor-type defects, which photoluminescence shows to be concentrated in the near-surface. Controlled doses of argon treatments allow nanosheets to be customised for device formation.

Spatial and contamination dependent electrical properties of carbon nanotubes

Two-point probe and Raman spectroscopy have been used to investigate the effects of vacuum annealing and argon bombardment on the conduction characteristics of multiwalled carbon nanotubes (MWCNTs). Surface contamination has a large effect on the two-point probe conductivity measurements which results in inconsistent and nonreproducible contacts. The electric field under the contacts is enhanced which results in overlapping depletion regions when probe separations are small (<4 μm) causing very high resistances. Annealing at 200 and 500 °C reduced the surface contamination on the MWCNT, but high resistance contacts still did not allow intrinsic conductivity measurements of the MWCNT. The high resistance measured due to the overlapping depletion regions was not observed after annealing to 500 °C. Argon bombardment reduced the surface contamination more than vacuum annealing at 500 °C but caused a slight increase in the defects concentration, enabling the resistivity of the MWCNT to be calculated, which is found to be dependent on the CNT diameter. The observations have significant implications for future CNT-based devices.

XPS investigation of titanium contact formation to ZnO nanowires

Ti is often used to form an initial Ohmic interface between ZnO and Au due to its low work function, and the TiO2/ZnO heterojunction is also of great importance for many practical applications of nanoparticles. Here, Ti has been controllably deposited onto hydrothermally grown ZnO nanowires and the formation of metal-semiconductor contact has been investigated using x-ray photoelectron spectroscopy (XPS), photoluminescence (PL) spectroscopy and scanning electron microscopy. XPS results showed that that the Ti initially reacts with surface oxygen species to form TiO2, and further deposition results in the formation of oxides with oxidation state numbers lower than four, and eventually metallic Ti on top of the TiO2. The formation of TiC was also observed. XPS showed that the onset of metallic Ti coincided with a Zn 3p core level shift to lower binding energy, indicating upwards band bending and the formation of a rectifying contact. Annealing caused a near-complete conversion of the metallic Ti to TiO2 and caused the Zn 3p to shift back to its original higher binding energy, resulting in downwards band bending and a more Ohmic contact. PL measurements showed that the optical properties of the nanowires are not affected by the contact formation.

Surface sensitivity of four-probe STM resistivity measurements of bulk ZnO correlated to XPS

Multi-probe instruments based on scanning tunnelling microscopy (STM) are becoming increasingly common for their ability to perform nano- to atomic-scale investigations of nanostructures, surfaces and in situ reactions. A common configuration is the four-probe STM often coupled with in situ scanning electron microscopy (SEM) that allows precise positioning of the probes onto surfaces and nanostructures enabling electrical and scanning experiments to be performed on highly localised regions of the sample. In this paper, we assess the sensitivity of four-probe STM for in-line resistivity measurements of the bulk ZnO surface. The measurements allow comparisons to established models that are used to relate light plasma treatments (O and H) of the surfaces to the resistivity measurements. The results are correlated to x-ray photoelectron spectroscopy (XPS) and show that four-probe STM can detect changes in surface and bulk conduction mechanisms that are beyond conventional monochromatic XPS.