A Compact Low Energy Electron Microscope for Surface Analysis

The description and function characterization of a flange-on type low energy electron mi- croscope are given. In this microscope a magnetic beam separator with 10° deflection angle is used in order to facilitate compacting the instrument on a single 10 in. flange. Mean- while some correcting elements in the electron optical system are simplified to reduce the complexities of construction and operation. The sample is set close to ground potential, so that all the electrostatic lenses are easily to float at high voltages. The performance of the microscope in typical low energy electron microscopy, low energy electron diffraction and photoemission electron microscopy modes is demonstrated through several experiments. A lateral resolution of 51 nm is estimated for low energy electron microscopy imaging. With femtosecond laser as light source, the consequent nonlinear photoemission makes this micro-scope also suitable for the observation of optical near field phenomena and a lateral resolution of 110 nm is obtained.

Obtain martian magnetic pileup boundary by spectrum of low energy electrons

The electron spectrometer of Mars Express (MEX) provides the flux of low energy electrons (<20 keV) near Mars. 96 pieces of continuously measured data are analyzed, and the crossings of the magnetic pileup boundary (MPB) can be determined by fitting the energy spectrum of the low energy electrons. The shape and position of the MPB can be gained from these crossings, and they are in good agreement with the results obtained by the Mars Global Surveyor (MGS) and Phobos-2. In addition, we classify these crossings based on the crustal magnetic field nearby. It turns out that the position of MPB near the strong (>50 nT) crustal magnetic field is higher than the position of the MPB near the weak (<10 nT) crustal magnetic field. This result reflects the effect of the crustal magnetic field on the interaction between the Martian atmosphere and solar wind.

The Stability of Anionic States of Thymine-Glycine Dimers with Excess Electron

It has been demonstrated that low energy electrons(LEEs)can induce serious DNA damages including bases loss and even single and double strand breaks.Experiments also showed that LEE induced DNA damages will be reduced with the presence of amino acids.For understanding of the protection of amino acids to DNA,the stability of 6 kinds of thymine and glycine(T-g)dimers with planar configurations with an excess electron were studied with density functional theory(DFT)method.The results show that,when the excess electron is vertically attached,all the dimers become more active with higher energy.After re-optimization,4 kinds(66.7%)of T-g dimers become more stable than the corresponding neutral states.For the most stable anionic dimer noted as[34-A]-,the excess electron is localized on the thymine,while one proton transfers from glycine to thymine.The proton transformation decreases the activities and prevents further reactions of the excess electron.For other three dimers,there is no chemical topology change.The glycine attracts the excess electron with hydrogen-bonding to the thymine.

Benchmark Integral Cross Sections for Electron Impact Excitation of the n=2 States in Helium

In this paper, we present integral cross sections （ICS） for electron impact excitation of the n = 2 levels in helium in the impact energy range of 23.5 eV to 35 eV. The ICS of each final state, 2^3S, 2^1S, 2^3P and 2^1P, has been determined by integration of the angular differential cross sections （DCS） over all of 0° to 180°, where those DCS were obtained from both our previous experiments and the extrapolation using the convergent close coupling calculation. The present experimental ICS for the optically allowed 21P transition state are also compared with those obtained from the BE f-scaling method. Very good agreement between the experimental and BE f-scaled 21P ICSs is generally found in the measured impact energy region.

Segregation growth of epitaxial graphene overlayers on Ni（111）

The orientation control of graphene overlayers on metal surface is an important issue which remains as a challenge in graphene growth on Ni surface. Here we have demonstrated that epitaxial graphene overlayers can be obtained by annealing a nickel carbide covered Ni（111） surface using in situ surface imaging techniques. Epitaxial graphene islands nucleate and grow via segregation of dissolved carbon atoms to the top surface at about 400 ℃. This is in contrast to a mixture of epitaxial and non-epi- taxial graphene domains grown directly on Ni（111） at 540 ℃. The different growth behaviors are related to the nucleation dynamics which is controlled by local carbon densities in the near surface region.

Segregation growth of epitaxial graphene overlayers on Ni(111)

The orientation control of graphene overlayers on metal surface is an important issue which remains as a challenge in graphene growth on Ni surface. Here we have demonstrated that epitaxial graphene overlayers can be obtained by annealing a nickel carbide covered Ni(111)surface using in situ surface imaging techniques. Epitaxial graphene islands nucleate and grow via segregation of dissolved carbon atoms to the top surface at about 400 °C.This is in contrast to a mixture of epitaxial and non-epitaxial graphene domains grown directly on Ni(111) at540 °C. The different growth behaviors are related to the nucleation dynamics which is controlled by local carbon densities in the near surface region.

Development and study of an imaging detector based on high-position resolution THGEM

Introduction THickGaseous Electron Multiplier(THGEM)has many advantages while the moderate position resolution is regarded as the main inferiority comparing with the traditional GEM,so this limits the applications of THGEM,such as X-ray imaging and charge particle tracking.Materials and methods By improving the production techniques,THGEMs with smaller pitch and hole diameter can be made,i.e.,0.4 and 0.15 mm by mechanical drilling,and 0.3 and 0.1 mm by laser etching,respectively.Based on the new THGEMs,a two-dimensional imaging detector with 50×50mm sensitive area was developed for 0.1∼50 MeVlow-energy electrons detection and reaching better than 100μm position resolution(sigma).At the same time,a set of front-end electronics was developed based on homemade ASIC chips,i.e.,Charge Amplifier and Shaping Amplifier for GEM(CASAGEM),and applied successfully to the detector.Conclusion The X-ray and beam tests results indicate that both detector and Front End Electronics(FEE)worked well,and the position resolution achieved 74.9μm by using the charge center-of-gravity method.This indicates that the high-position resolution THGEM is promising for imaging and tracking application.

Dynamical evolution of Ge quantum dots on Si(111):From island formation to high temperature decay

Heteroepitaxial growth is a process of profound fundamental importance as well as an avenue to realize nanostructures such as Ge/Si quantum dots(QDs),with appealing properties for applications in opto-and nanoelectronics.However,controlling the Ge/Si QD size,shape,and composition remains a major obstacle to their practical implementation.Here,Ge nanostructures on Si(111)were investigated in situ and in real-time by low energy electron microscopy(LEEM),enabling the observation of the transition from wetting layer formation to 3D island growth and decay.The island size,shape,and distribution depend strongly on the growth temperature.As the deposition temperature increases,the islands become larger and sparser,consistent with Brownian nucleation and capture dynamics.At 550◦C,two distinct Ge/Si nanostructures are formed with bright and dark appearances that correspond to flat,atoll-like and tall,faceted islands,respectively.During annealing,the faceted islands increase in size at the expense of the flat ones,indicating that the faceted islands are thermodynamically more stable.In contrast,triangular islands with uniform morphology are obtained from deposition at 600◦C,suggesting that the growth more closely follows the ideal shape.During annealing,the islands formed at 600◦C initially show no change in morphology and size and then rupture simultaneously,signaling a homogeneous chemical potential of the islands.These observations reveal the role of dynamics and energetics in the evolution of Ge/Si QDs,which can serve as a step towards the precise control over the Ge nanostructure size,shape,composition,and distribution on Si(111).