Xia Yue-Yuan

Four-parameter formulae for the electronic stopping cross section of low energy ions in solids☆

Abstract A set of four parameter formulae of the electronic stopping cross section is provided for use in the prediction of electronic stopping cross sections of ions in various solids in the low energy range, 0.25 keV/amu E / m 1 , ≤ 200 keV/amu. The parameters, a , b , a and E 0 in the formulae are determined. The formulae can be used not only to describe the experimental Z 1 and Z 2 oscillations of electronic stopping powers but also to satisfactorily describe the dependence of the electronic stopping cross section, S e , on ion energy E .

Tunable Adsorption and Desorption of Hydrogen Atoms on Single-Walled Carbon Nanotubes

Chemical adsorption and desorption of hydrogen atoms on single-walled carbon nanotubes (SWNTs) are investigated by using molecular dynamics simulations. It is found that the adsorption and desorption energy of hydrogen atoms depend on the hydrogen coverage and the diameter of the SWNTs. Hydrogen-adsorption geometry at the coverage of 1.0 is more energetically stable. The adsorption energy decreases with the increasing diameter of the armchair tubes. The adsorption and desorption energy of hydrogen atoms can be modified reversibly by externally radial deformation. The averaged C-H bond energy on the high curvature sites of the deformed tube increases with increasing radial deformation, while that on the low curvature sites decreases.

Stopping power of 100–600 keV F+, Ar+, As+, Br+ and Xe+ ions in silicon☆

Abstract The stopping power of 100–600 keV F + , Ar + , As + , Br + and Xe + ions in silicon have been obtained from measured range distribution obtained by NRA and RBS techniques. A deconvolution program was used in order to obtain true range distributions from the measured NRA excitation curves or RBS spectra. The total stopping powers were determined through fitting the projected ranges based on LSS transport theory to the experimentally determined projected ranges. After subtracting calculated nuclear stopping cross sections, the electronic stopping cross sections were obtained. These results indicate that the electronic stopping cross sections at low velocities may be described by the four-parameter formulae proposed previously and that deviation from the velocity-proportional electronic stopping is evident.

Impact-parameter dependence of the electronic energy loss and angle-dependent energy loss for ions traversing thin foils

The impact-parameter dependence of the electronic energy loss has been investigated using the quantal harmonic oscillator model and the shell-wise model of atomic stopping for ions with velocity v >v0 and v < v0, respectively. The electronic energy losses obtained by the impact-parameter dependent formalism have been used in a Monte Carlo simulation code to give the angular dependence of the energy loss of protons, helium ions and heavy ions transmitted through thin foils. The model calculations are compared with available experimental data.

Electronic energy loss of heavy ions colliding with atoms obtained from the harmonic oscillator model and from the shell-wise model☆

The quantal harmonic oscillator model and the shell-wise model in conjunction with a modified BK effective charge are used to calculate the electronic energy loss of heavy ions colliding with atoms. The stopping power near the stopping maximum obtained from the model calculations is compared with experimental data. It is shown that the experimentally well-known Z1- and Z2-oscillations of the electronic stopping cross sections are reproduced by the model calculations.

Single-Walled Carbon Nanotubes Acting as Controllable Transport Channels

The motion and equilibrium distribution of water molecules adsorbed inside neutral and negatively charged single-walled carbon nanotubes (SWNTs) have been studied using molecular dynamics simulations (MDSs) at room temperature based on CHARMM (Chemistry at HARvard Molecular Mechanics) potential parameters. We find that water molecules have a conspicuous electropism phenomenon and regular tubule patterns inside and outside the charged tube wall. The analyses of the motion behaviour of water molecules in the radial and axial directions show that by charging the SWNT, the adsorption efficiency is greatly enhanced, and the electric field produced by the charged SWNTs prevents water molecules from flowing out of the nanotube. However, water molecules can travel through the neutral SWNT in a fluctuating manner. This indicates that by electrically charging and uncharging the SWNTs, one can control the adsorption and transport behaviour of polar molecules in SWNTs for using as a stable storage medium or long transport channels. The transport velocity can be tailored by changing the charge on the SWNTs, which may have a further application as modulatable transport channels.

Range distribution of fluorine in 19F+-implanted LiNbO3

Depth profiles of fluorine in 19F+implanted LiNbO3 have been accurately measured using the 19F(p, αγ)16O resonant nuclear reaction at ER = 872.1 keV, with Γ = 4.2 keV. A proper convolution calculation method was used to extract the true distribution of fluorine from the experimental excitation yield curves. The experimental range distribution parameters, Rp and ΔRp, were compared with those obtained by Monte Carlo simulation using a computer code developed recently in this group and with results obtained using the TRIM90 code. It shows that the experimental Rp values agree with the Monte Carlo simulation values very well, while the experimental ΔRp values are larger than those obtained from the simulations. The simulation with our computer code improves the agreement between the experimental and calculated range straggling, ΔRp.

Comparison of ion range distributions obtained by different Monte Carlo codes with different electronic stopping powers

Abstract A Monte Carlo simulation program was developed to calculate ion range distributions in amorphous targets. In order to test to what degree the electronic stopping power can affect the range distribution, the ZBL universal potential, which is used in TRIM89, and an experimentally fitted four-parameter electronic stopping power are adopted to treat energy losses in our simulation. Results obtained by our simulation are in good agreement with both experimental and theoretical results. Some of these give improved agreement with the experiment as compared with TRIM89 simulation.

ELECTRONIC STOPPING POWERS DERIVED FROM RANGE MEASUREMENTS FOR IONS AT LOW VELOCITY

Abstract Electronic stopping cross sections for 19 F + in Pb 1− x Sn x Te and 19 F + , 40 Ar + , 75 As + , 79 Br + and 132 Xe + in silicon were obtained by range measurements. Depth profiles of F in Pb 1− x Sn x and Si were measured by 19 F(p, αγ) 16 O resonance nuclear reaction and those of Ar, As, Br and Xe were determined by RBS. In order to obtain the true range distribution from the measured NRA excitation curves or RBS spectra, a deconvolution program was developed using a reference function, the Edgeworth distribution function, and parameter optimization process. By forcing a fit between the experimentally determined projected range and that calculated with the range statistics program the total stopping power was obtained. After subtracting the nuclear stopping power the electronic stopping power was derived. The electronic stopping power can be described by a four-parameter formula.

Depth profiles of fluorine in 19F+ ion-implanted Pb1−xSnxTe and CdTe

The 19F(p,αγ)16O resonance nuclear reaction at 872.1 keV, with Γ = 4.2 keV was used to measure the depth profiles of fluorine in 19F+-implanted Pb1−xSnxTe and CdTe samples. A deconvolution procedure was performed to extract the depth profiles from the experimental excitation yield curves. The range parameters. Rp, ΔRp and SK, obtained in this experiment were compared with theoretical calculations.