THE interest in carbon nanotubes has been greatly stimulated by theoretical predictions that their electronic properties are strongly modulated by small structural variations1–8. In particular, the diameter and the helicity of carbon atoms in the nanotube shell are believed to determine whether the nanotube is metallic or a semiconductor. Because of the enormous technical challenge of making measurements on individual nanotubes, however, experimental studies have been limited mainly to bulk measurements9, which indicate only that a fraction of the nanotubes are metallic or narrow-band semiconductors10. Recently, measurements of the magneto-conductance of a single multi-shell nanotube in a two-probe configuration showed that the transport is characterized by disorder and localization phenomena11. To avoid possible ambiguities due to poor sample contacts, four-probe measurements are needed. Here we report four-probe measurements on single nanotubes made by lithographic deposition of tungsten leads across the tubes. We find that each multi-shell nanotube has unique conductivity properties. Both metallic and non-metallic behaviour are observed, as well as abrupt jumps in conductivity as the temperature is varied. The differences between the electrical properties of different nanotubes are far greater than expected. Our results suggest that differences in geometry play a profound part in determining the electronic behaviour.

Electrical-Conductivity of Individual Carbon Nanotubes

Myoglobin rebinding of carbon monoxide and dioxygen after photodissociation has been observed in the temperature range between 40 and 350 K. A system was constructed that records the change in optical absorption at 436 nm smoothly and without break between 2 musec and 1 ksec. Four different rebinding processes have been found. Between 40 and 160 K, a single process is observed. It is not exponential in time, but approximately given by N(t) = (1 + t/to)-n, where to and n are temperature-dependent, ligand-concentration independent, parameters. At about 170 K, a second and at 200 K, a third concentration-independent process emerge. At 210 K, a concentration-dependent process sets in. If myoglobin is embedded in a solid, only the first three can be seen, and they are all nonexponential. In a liquid glycerol-water solvent, rebinding is exponential. To interpret the data, a model is proposed in which the ligand molecule, on its way from the solvent to the binding site at the ferrous heme iron, encounters four barriers in succession. The barriers are tentatively identified with known features of myoglobin. By computer-solving the differential equation for the motion of a ligand molecule over four barriers, the rates for all important steps are obtained. The temperature dependences of the rates yield enthalpy, entropy, and free-energy changes at all barriers. The free-energy barriers at 310 K indicate how myoglobin achieves specificity and order. For carbon monoxide, the heights of these barriers increase toward the inside; carbon monoxide consequently is partially rejected at each of the four barriers. Dioxygen, in contrast, sees barriers of about equal height and moves smoothly toward the binding site. The entropy increases over the first two barriers, indicating a rupturing of bonds or displacement of residues, and then smoothly decreases, reaching a minimum at the binding site. The magnitude of the decrease over the innermost barrier implies participation of heme and/or protein. The nonexponential rebinding observed at low temperatures and in solid samples implies that the innermost barrier has a spectrum of activation energies. The shape of the spectrum has been determined; its existence can be explained by assuming the presence of many conformational states for myoglobin. In a liquid at temperatures above about 230 K, relaxation among conformational states occurs and rebinding becomes exponential.

Dynamics of ligand binding to myoglobin

https://iopscience.iop.org/article/10.1088/0034-4885/18/1/301/pdf

The Displacement of Atoms in Solids by Radiation

An updated tabulation is presented of the optical properties of intrinsic silicon relevant to solar cell calculations. the absorption coeficient, refractive index and extinction coeficient at 300 K are tabulated over the 0.25-1.45 μm wavelength range at 0.01 μm intervals.

Optical properties of intrinsic silicon at 300 K

Ion implantation is being applied extensively to silicon device technology. Two principle features are utilized- 1) charge control in MOS structures for threshold shift, autoregistration, and complementary wells and 2) distribution control in microwave and bipolar structures. Another feature that has not been extensively exploited is to combine the advantages of the high resolution capabilities of electric beam pattern delineation with the low lateral spread inherent in the implantation process. This talk reviews some of the general features of the characteristics of implanted layers in terms of depth distribution, radiation damage and electron activity. Implantation processes in silicon are reasonably well understood. There remain areas which require further clarification. For compound semiconductors, particularly GaAs, implantation techniques offer attractive possibilities for the fabrication of high frequency devices. In these materials, the substrate temperature during implantation and the dielectric coating required to prevent dissociation during thermal anneal play major roles.

Ion implantation in semiconductors

The kinetic behavior of systems in which processes occur which are distributed over a range of activation energies is considered. The effects produced by the initial distribution, the order of reaction, and the frequency factor are discussed. Imaginary and actual experimental situations are used to illustrate the large errors which can result when the distribution of the processes in activation energy is neglected. Some of the complications which can result from successive reactions and varying frequency factors are mentioned.

Kinetics of Processes Distributed in Activation Energy

The photoelastic constants and their dispersion for visible light were determined for two samples of vitreous silica, Corning and Herasil No. 1. The results for the mercury green line, 546 mμ, are quoted here. The photoelastic constants were 4.22×10−13 (d/cm2)−1 (ordinary ray) and 3.56×10−13 (d/cm2)−1 (extraordinary ray). From the ratio of Poisson's ratio to Young's modulus 0.216×10−12 (d/cm2)−1 and Young's modulus 0.76×1012 (d/cm2), which were also determined, there were calculated the pressure coefficient of refractive index 0.909×10−12 (d/cm2)−1 and the elasticity volume coefficient of refractive index V dN/dV=0.34. Comparison of these results with those reported for other glasses indicates that the oxygen ions of vitreous silica are more deformable then the oxygen ions of other siliceous glasses. In vitreous silica about ⅓ of the volume change accompanying an elastic dilatation arises from the dilatation of the oxygen ions. A comparison is also made with the results reported for corresponding thermally induced effects. It indicates that (a) during a thermal dilatation of a siliceous glass, the oxygen ion undergoes a dilatation which in the case of most siliceous glasses causes an equivalent dilatation of the body, but which in the case of vitreous occurs internally instead, and (b) in crystal quartz no appreciable dilatation of the oxygen ion occurs during a thermal dilatation. The thermal dilatation of the oxygen ion in the glasses would thus seem related to its strained bond configuration.

Photoelastic Constants of Vitreous Silica and Its Elastic Coefficient of Refractive Index

The course of the changes produced by exposure in nuclear reactors and the subsequent alteration of the radiation-induced property changes caused by heating were followed by measuring the dilatation, refractive index, rotatory power, and birefringence. The fastneutron dosages determined by a comparison of radiation dannage in several substances are used to give a consistent scale for presenting the effects seen from the initial changes to nearly the saturation state, despite the many irradiation facilities used for the work. The property changes resulting from heating specimens irradiated to various extents are given. The damaging phenomena are explained in terms of the interaction of the scattered atoms with the silica structure, and the annealing phenomena in terms of the behavior of the anticipated structural changes. On the basis of the effect of displaced atoms and thermal spikes on solid structures there are delineated a number of classes of radiation damage among; insulators. For one of these, silica is taken to be the prototype. Thermal spikes cause the displaced atoms to be accommodated by a local defornnation, which is depicted as analogous to the rapid inversions in which a minor reorientaiion of the Si-O tetrahedra occurs without extensive disruption of the structure. Themore » resulting gradually accumulating disorder is considered the basic formulation of a metamictization for which the damaging of quartz thus becomes the simplest example. The same basic processes are considered to take place in vitreous silica; but since the substance is already disordered, the changes which take place are minor, the slight compacting of the structure evidently representing nnainly the partial freezing-in of a high-temperature state produced by the thermal spike. (auth)« less

Fast-Neutron-Induced Changes in Quartz and Vitreous Silica

The dilatations (negative) caused by neutrons, 40–600 keV electrons, 140 keV H+, D+, He+, and gamma rays are given as a function of dose and are compared. The presaturation dependence is linear for neutrons, He+, and D+, and depends on the 0.5–0.7 power of dose for H+, electrons, and gamma rays. The dilatations are not temperature dependent from about 0°–100°C, hence are not thermally activated. The dilatations are explained as compaction of the silica structure resulting from oxygens moving into some of the ``free volume,'' the structural change being similar to that occurring on pressure, shock, or thermal compaction of vitreous silica. The mechanism given for the neutron‐induced compaction involves the state of high vibrational excitation developed in the slowing down of scattered atoms. The compaction by gamma rays, electrons, protons, or deuterons is explained as caused by transient ionization relaxing Si–O binding to permit oxygens to move into the ``free volume'' where they may be locked‐in on reco...

The Radiation Compaction of Vitreous Silica

The principal electrical conductivities (${\ensuremath{\sigma}}_{a}$ in the basal plane and ${\ensuremath{\sigma}}_{c}$ perpendicular to the basal plane) of graphite crystals isolated from several North American marbles were studied. By means of measurements of potential distributions on the surfaces of crystals carrying electric current, it was conclusively shown that $\frac{{\ensuremath{\sigma}}_{a}}{{\ensuremath{\sigma}}_{c}}$ for these crystals is about several hundred at room temperature. By direct measurement ${\ensuremath{\sigma}}_{a}$ was found to be (2.6\ifmmode\pm\else\textpm\fi{}0.2)\ifmmode\times\else\texttimes\fi{}${10}^{4}$ (ohm ${\mathrm{c}\mathrm{m})}^{\ensuremath{-}1}$ and ${\ensuremath{\sigma}}_{c}$ to vary from 150 to 230 (ohm ${\mathrm{c}\mathrm{m})}^{\ensuremath{-}1}$ (\ifmmode\pm\else\textpm\fi{}20 percent) at room temperature. Below room temperature it was found for two crystals that ${\ensuremath{\sigma}}_{a}$ increased slowly to liquid nitrogen temperatures, then followed a ${T}^{\ensuremath{-}1}$ law to about 15\ifmmode^\circ\else\textdegree\fi{}K, and again slowly increased in the region 15\ifmmode^\circ\else\textdegree\fi{}K to 4.2\ifmmode^\circ\else\textdegree\fi{}K. Two other crystals for which ${\ensuremath{\sigma}}_{c}$ was measured at low temperatures did not behave alike. For both, ${\ensuremath{\sigma}}_{c}$ decreased as the temperature was lowered, reached a minimum above liquid nitrogen temperatures, and then started increasing, the one showing a ${T}^{\ensuremath{-}1}$ behavior, the other increasing more slowly. For both, the rate of increase declined markedly in the region 15\ifmmode^\circ\else\textdegree\fi{}K to 4.2\ifmmode^\circ\else\textdegree\fi{}K.

William Primak

Papers

Kinetics of Processes Distributed in Activation Energy

Photoelastic Constants of Vitreous Silica and Its Elastic Coefficient of Refractive Index

Fast-Neutron-Induced Changes in Quartz and Vitreous Silica

The Radiation Compaction of Vitreous Silica

Electrical Conductivities of Natural Graphite Crystals