Effects of Thermal Annealing on the Properties of Mechanically Exfoliated Suspended and On-Substrate Few-Layer Graphene

Graphene’s novel electrical, optical, and mechanical properties are affected both by substrate interaction and processing steps required to fabricate contacts and devices. Annealing is used to clean graphene devices, but this can lead to doping and defect changes and strain effects. There is often disagreement about which of these effects are occurring and which result in observed changes in Raman spectra. The effects of vacuum annealing on mechanically exfoliated pristine, suspended, and attached thin and thick few-layer graphene on SiO2/Si are investigated here using scanning electron microscopy (SEM), Raman spectroscopy, and atomic force microscopy (AFM). Before annealing, Raman shows that the differences in 2D and G band positions and the appearance of a disorder-induced D band of all regions were mainly because of compressive or tensile structural deformations emerging through mechanical exfoliation instead of charge doping. Annealing at low temperature is sufficient to eliminate most of the defects. However, compressive strain is induced in the sheet by annealing at high temperature, and for thin regions increased substrate conformation leads to the apparent disappearance of the sheets. The intensity ratio of the 2D and G bands also reduces with induced compressive strain, and thus should not be used to detect doping

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

Nondestructive Method for Mapping Metal Contact Diffusion in In2O3 Thin-Film Transistors

The channel width-to-length ratio is an important transistor parameter for integrated circuit design. Contact diffusion into the channel during fabrication or operation alters the channel width and this important parameter. A novel methodology combining atomic force microscopy and scanning Kelvin probe microscopy (SKPM) with self-consistent modeling is developed for the nondestructive detection of contact diffusion on active devices. Scans of the surface potential are modeled using physically based Technology Computer Aided Design (TCAD) simulations when the transistor terminals are grounded and under biased conditions. The simulations also incorporate the tip geometry to investigate its effect on the measurements due to electrostatic tip–sample interactions. The method is particularly useful for semiconductor– and metal–semiconductor interfaces where the potential contrast resulting from dopant diffusion is below that usually detectable with scanning probe microscopy

Enhancement of Multiwalled Carbon Nanotubes’ Electrical Conductivity Using Metal Nanoscale Copper Contacts and Its Implications for Carbon Nanotube-Enhanced Copper Conductivity

Herein, we present an experimental/computational approach for probing the interaction between metal contacts and carbon nanotubes (CNTs) with regard to creating the most efficient, low resistance junction. Tungsten probes have been coated with copper or chromium and the efficiency of nanocontact transport into multiwalled carbon nanotubes (MWCNTs) has been investigated experimentally, using scanning tunneling spectroscopy and nanoscale two-point probe I-V measurements, and in silico, employing DFT calculations. Experimental I-V measurements suggest the relative conductivity of the metal-CNT interaction to be Cu > W > Cr. It has been found that copper when in contact with MWCNTs results in a high density of states at the Fermi level, which contributes states to the conduction band. It was observed that the density of states also increased when chromium and tungsten probes were in contact with CNTs; however, in these cases the density of states increase would only occur under high voltage/high temperature situations. This is demonstrated by an increase in the experimental electrical resistance when compared to the copper probe. These results suggest that in future copper tips should be used when carrying out all intrinsic conduction measurements on CNTs, and they also provide a rationale for the ultraconductivity of Cu-CNT and Cu-graphene composites

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

Effects of Vacuum Annealing on the Conduction Characteristics of ZnO Nanosheets

This paper is open acess and available in full at http://www.nanoscalereslett.com/content/10/1/368 .ZnO nanosheets are a relatively new form of nanostructure and have demonstrated potential as gas-sensing devices and dye sensitised solar cells. For integration into other devices, and when used as gas sensors, the nanosheets are often heated. Here we study the effect of vacuum annealing on the electrical transport properties of ZnO nanosheets in order to understand the role of heating in device fabrication. A low cost, mass production method has been used for synthesis and characterisation is achieved using scanning electron microscopy (SEM), photoluminescence (PL), auger electron spectroscopy (AES) and nanoscale two-point probe. Before annealing, the measured nanosheet resistance displayed a non-linear increase with probe separation, attributed to surface contamination. Annealing to 300 °C removed this contamination giving a resistance drop, linear probe spacing dependence, increased grain size and a reduction in the number of n-type defects. Further annealing to 500 °C caused the n-type defect concentration to reduce further with a corresponding increase in nanosheet resistance not compensated by any further sintering. At 700 °C, the nanosheets partially disintegrated and the resistance increased and became less linear with probe separation. These effects need to be taken into account when using ZnO nanosheets in devices that require an annealing stage during fabrication or heating during use

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

Finishing the euchromatic sequence of the human genome

The sequence of the human genome encodes the genetic instructions for human physiology, as well as rich information about human evolution. In 2001, the International Human Genome Sequencing Consortium reported a draft sequence of the euchromatic portion of the human genome. Since then, the international collaboration has worked to convert this draft into a genome sequence with high accuracy and nearly complete coverage. Here, we report the result of this finishing process. The current genome sequence (Build 35) contains 2.85 billion nucleotides interrupted by only 341 gaps. It covers ∼99% of the euchromatic genome and is accurate to an error rate of ∼1 event per 100,000 bases. Many of the remaining euchromatic gaps are associated with segmental duplications and will require focused work with new methods. The near-complete sequence, the first for a vertebrate, greatly improves the precision of biological analyses of the human genome including studies of gene number, birth and death. Notably, the human enome seems to encode only 20,000-25,000 protein-coding genes. The genome sequence reported here should serve as a firm foundation for biomedical research in the decades ahead

The shifting sands of time : maturation and athlete development

In the first two decades of life, notably the years associated with childhood and adolescence, the human body grows and changes substantially. Physical growth is considered a continuous process with a predictable sequence of changes occurring until full adult maturity at approximately 18–20 years of age for males, and 16–18 years for females. The most observable changes are associated with physical (somatic) body proportions (e.g., height and body mass), although growth encompasses multiple other components, including bone, nerve, and neural network, as well as muscle and fat tissue development (Malina, Bouchard, & Bar-Or, 2004). Considering such changes, this chapter focuses on: (i) highlighting the potential for growth and maturational variability between individuals at similar chronological ages; (ii) outlining the relationships between physical maturation and facets of athletic performance; (iii) providing a conceptual model to demonstrate normative growth and maturational related change over time; and (iv) noting the resultant systematic biases in athlete participation, evaluation and selection. In a final section, maturation measurement methods and issues are discussed. These topics are addressed to help establish a clear message – that sports organisations and practitioners need to better understand and account for variability in growth and maturation within their sport systems and athlete development programs