Electrical conduction of ion tracks in tetrahedral amorphous carbon: temperature, field and doping dependence and comparison with matrix data

This paper gives an extended overview of the electrical properties of ion tracks in hydrogen-free tetrahedral amorphous carbon (ta-C) with a sp(3) bond fraction of about 80%. The films were grown by mass selected ion beam deposition of 100 eV C-12(+) ions. The ion tracks are generated by irradiation of ta-C films with uranium ions of 1 GeV kinetic energy. Along the ion path a conversion from diamondlike (sp(3)) carbon to graphite-like (sp(2)) carbon takes place. Topography and current measurements of individual ion tracks were performed by atomic force microscopy at ambient temperature. The temperature dependence of the electric conductivity was studied between 15 and 390 K by means of 0.28 mm(2) large contact pads averaging over about 10 7 tracks. For each sample and at each temperature the conductivity as a function of the applied electrical field (non-ohmic behaviour) was measured separately and the data were extrapolated to field zero. In this way, the zero-field conductivity was determined independent from the field dependence. In spite of large differences in the absolute values, the temperature dependence of the zero-field conductivities is found to be very similar in shape for all samples. The conductivities follow a T-1/4 law up to temperatures slightly below room temperature. At higher temperatures a transport mechanism based on over-barrier hopping dominates with an activation energy of about 220 meV for tracks and 260 meV for the ta-C matrix. The field dependence measurements show that the deviation of the I-V characteristics from ohmic behaviour decreases with increasing zero-field conductivity. We also tested Cu-doped ta-C samples and found that they conduct significantly better than pure ta-C. However, the doping also increases the zero-field conductivity resulting in a weaker contrast between the track and matrix. The data are interpreted within the so-called 'barrier model' where the electrons are assumed to move fairly freely in well-conducting sp(2) regions but encounter barriers in track sections consisting of more sp(3)-like bonds

Amorphization of ZnSe by ion implantation at low temperatures

Radioactive Cd and Se ions were implanted into high-resistivity ZnSe single crystals around 60 K and 300 K. Their lattice sites were determined by measuring the channelling and blocking effects of the emitted conversion electrons or positrons directly after implantation and after annealing at different temperatures up to 600 K. Implantation doses were in the range of 3$\times 10^{12}$ - 3$\times10^{13}$/cm$^2$. The experimental results of this emission channelling technique yield a high substitutional fraction of the implanted ions directly after implantation at room temperature. At 60 K the substitutional fraction of implanted ions is highly sensitive to the ion dose. Above a critical dose of around 1.4$\times10^{13}$ Cd/cm$^2$ or 2.1$\times10^{13}$ Se/cm$^2$ the substitutional fraction completely disappears indicating an amorphous surrounding of the probe atom. Damage recovery was observed below room temperature and at an annealing temperature around 500 K. A quantitative analysis of measured channelling yields will be given by comparison with calculated electron channelling profiles based on the dynamical theory of electron diffraction

Cryogenic Buffer Gas beams of AlF, CaF, MgF, YbF, Al, Ca, Yb and NO -- a comparison

Cryogenic buffer gas beams are central to many cold molecule experiments. Here, we use absorption and fluorescence spectroscopy to directly compare molecular beams of AlF, CaF, MgF, and YbF molecules, produced by chemical reaction of laser ablated atoms with fluorine rich reagents. The beam brightness for AlF is measured as 2 X 1012 molecules per steradian per pulse in a single rotational state, comparable to an Al atomic beam produced in the same setup. The CaF, MgF and YbF beams show an order of magnitude lower brightness than AlF, and far below the brightness of Ca and Yb beams. The addition of either NF3 or SF6 to the cell extinguishes the Al atomic beam, but has a minimal effect on the Ca and Yb beams. NF3 reacts more efficiently than SF6, as a significantly lower flow rate is required to maximise the molecule production, which is particularly beneficial for long-term stability of the AlF beam. We use NO as a proxy for the reactant gas as it can be optically detected. We demonstrate that a cold, rotationally pure NO beam can be generated by laser desorption, thereby gaining insight into the dynamics of the reactant gas inside the buffer gas cell

Lattice sites of ion-implanted Li in diamond

Published in: Appl. Phys. Lett. 66 (1995) 2733-2735 citations recorded in [Science Citation Index] Abstract: Radioactive Li ions were implanted into natural IIa diamonds at temperatures between 100 K and 900 K. Emission channelling patterns of a-particles emitted in the nuclear decay of 8Li (t1/2 = 838 ms) were measured and, from a comparison with calculated emission channelling and blocking effects from Monte Carlo simulations, the lattice sites taken up by the Li ions were quantitatively determined. A fraction of 40(5)% of the implanted Li ions were found to be located on tetrahedral interstitial lattice sites, and 17(5)% on substitutional sites. The fractions of implanted Li on the two lattice sites showed no change with temperature, indicating that Li diffusion does not take place within the time window of our measurements.

α\alpha-emission channeling investigations of the lattice location of Li in Ge

The $\alpha$-emission channeling and blocking technique is a direct method for lattice site determination of radioactive atoms in single crystals. Position-sensitive detection of emitted $\alpha$ -particles provides an efficient means of carrying out such experiments at very low doses (10$^{10}$-10$^{11}$ implanted probe atoms per spectrum). Comparison of the experimental data to Monte Carlo simulations of complete two-dimensional channeling patterns (e.g. ±2°C around , and axes, which also includes all relevant planar directions) allows for straight-forward identification and rather accurate quantitative determination of occupied lattice sites, while at the same time the energy spectrum of emitted a particles gives information on the probe atom depth distribution. We illustrate this for the case of ion implanted $^8$Li (t_=0.8 s) in Ge, where we identify mainly tetrahedral Li at room temperature, and bond-centered Li at slightly elevated temperature

Single-atom spectroscopy of phosphorus dopants implanted into graphene

One of the keys behind the success of modern semiconductor technology has been the ion implantation of silicon, which allows its electronic properties to be tailored. For similar purposes, heteroatoms have been introduced into carbon nanomaterials both during growth and using post-growth methods. However, due to the nature of the samples, it has been challenging to determine whether the heteroatoms have been incorporated into the lattice as intended. Direct observations have so far been limited to N and B dopants, and incidental Si impurities. Furthermore, ion implantation of these materials is challenging due to the requirement of very low ion energies and atomically clean surfaces. Here, we provide the first atomic-resolution imaging and electron energy loss spectroscopy (EELS) evidence of phosphorus atoms in the graphene lattice, implanted by low-energy ion irradiation. The measured P L ₂‚₃-edge shows excellent agreement with an ab initio spectrum simulation, conclusively identifying the P in a buckled substitutional configuration. While advancing the use of EELS for single-atom spectroscopy, our results demonstrate the viability of phosphorus as a lattice dopant in sp ²-bonded carbon structures and provide its unmistakable fingerprint for further studies

Nonperturbative QCD Vacuum Effects in Nonlocal Quark Dynamics

A straightforward calculation reveals the essentially nonlocal character of the leading heavy $Q\bar{Q}$ interaction arising from nonperturbative gluon field correlations in the model of a fluctuating QCD vacuum. In light of this quarkonium spin splitting ratio predictions which have supported the scalar confinement ansatz are reconsidered as a specific example of possible consequences for spectroscopy.Comment: Latex, 9 page