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2 Apr 2007

Volume 90, Issue 14, Articles (14xxxx)

Issue Cover Spotlight Figure

Appl. Phys. Lett. 90, 142110 (2007); http://dx.doi.org/10.1063/1.2720640 (3 pages)

M. T. Björk, O. Hayden, H. Schmid, H. Riel, and W. Riess
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Impact of optical in-plane anisotropy on near-field phonon polariton spectroscopy

S. Schneider, J. Seidel, S. Grafström, L. M. Eng, S. Winnerl, D. Stehr, and M. Helm

Appl. Phys. Lett. 90, 143101 (2007); http://dx.doi.org/10.1063/1.2718489 (3 pages) | Cited 10 times

Online Publication Date: 2 April 2007

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The authors report a spectroscopic near-field investigation using a tunable free-electron laser in combination with a scattering near-field optical microscope. They excite optically uniaxial LiNbO3 close to a phonon resonance in the infrared regime, thereby exciting a phonon polariton resonance in the coupled tip-sample system. They find that the resonance shows a clear dependence on the orientation of the optical axis of the birefringent crystal within the surface plane. This provides evidence that in addition to the dominant contribution of the dipole moment parallel to the tip axis, also the component along the surface is sensed in such a scattering experiment.
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63.20.D- Phonon states and bands, normal modes, and phonon dispersion
71.36.+c Polaritons (including photon-phonon and photon-magnon interactions)
78.20.Fm Birefringence

Anomalous microphotoluminescence of high-aspect-ratio Si nanopillars formatted by dry-etching Si substrate with self-aggregated Ni nanodot mask

Gong-Ru Lin, Chun-Jung Lin, Hao-Chung Kuo, Huang-Sheng Lin, and Chi-Chiang Kao

Appl. Phys. Lett. 90, 143102 (2007); http://dx.doi.org/10.1063/1.2719152 (3 pages) | Cited 13 times

Online Publication Date: 2 April 2007

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Microphotoluminescence (μ-PL) of high-aspect-ratio Si nanopillars fabricated by etching Ni-nanodot/SiO2 masked Si substrate is investigated. The 320-nm-tall Si nanopillars obtained by CF4/Ar mixed inductively coupled-plasma reactive ion etching process with density of 2.8×1010 cm−2 further shrink size from 30 to 6 nm by oxidation and etching. Blue-green μ-PL with two decomposed wavelengths at 425 and 475 nm is attributed to oxygen-related defects on the oxidized Si nanopillar surface. Defect-related near-infrared PL at 703 and 740 nm remains unchanged, while a quantum-confinement-effect-dependent PL blueshifted from 874 to 826 nm as the Si nanopillar size reduces from 7.2 to 6.0 nm is preliminarily observed.
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78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
78.55.Ap Elemental semiconductors
78.30.Am Elemental semiconductors and insulators
81.16.Nd Micro- and nanolithography
81.65.Mq Oxidation
81.65.Cf Surface cleaning, etching, patterning

Reliable and flexible carbon-nanofiber-based all-plastic field emission devices

H. S. Sim, S. P. Lau, H. Y. Yang, L. K. Ang, M. Tanemura, and K. Yamaguchi

Appl. Phys. Lett. 90, 143103 (2007); http://dx.doi.org/10.1063/1.2719239 (3 pages) | Cited 19 times

Online Publication Date: 2 April 2007

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The authors present the fabrication and electrical characterization of carbon nanofiber-based flexible field emitters prepared by an ion beam technique. The flexible emitters are extremely robust under various stress conditions and show no sign of degradation after 16 h long lifetime test. An all-plastic flexible field emission device with excellent emission properties has also been demonstrated using phosphor-coated polyester as an anode.
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85.45.Db Field emitters and arrays, cold electron emitters
85.45.Fd Field emission displays (FEDs)

Subwavelength lasers

Nabil M. Lawandy

Appl. Phys. Lett. 90, 143104 (2007); http://dx.doi.org/10.1063/1.2719035 (3 pages) | Cited 3 times

Online Publication Date: 3 April 2007

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Composite media comprised of a host with amplification and high aspect ratio metallic spheroids exhibit a dielectric resonance which is accompanied by both a gigantic index of refraction and large collective gain. Expressions for the complex index of refraction near this dielectric anomaly are derived for the case of oriented spheroids in an amplifying host. The large index of refraction and gain can be used to satisfy the laser cavity resonance condition in nanoscale subwavelength structures with EH11 transverse modes allowing for the future miniaturization of laser devices.
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42.55.-f Lasers
42.60.Da Resonators, cavities, amplifiers, arrays, and rings

Multiple plasmon resonances from gold nanostructures

A. K. Sheridan, A. W. Clark, A. Glidle, J. M. Cooper, and D. R. S. Cumming

Appl. Phys. Lett. 90, 143105 (2007); http://dx.doi.org/10.1063/1.2719161 (3 pages) | Cited 28 times

Online Publication Date: 3 April 2007

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Understanding and controlling plasmon resonances from metallic nanoscale structures have been the focus of much attention recently, with applications including local surface plasmon resonance sensing, surface enhanced Raman spectroscopy, and negative refractive index materials. In this letter the authors demonstrate the fabrication of uniform arrays of split rings from gold and show that such structures are capable of supporting multiple plasmon resonances. The authors show that up to five plasmon resonances can be identified and use finite difference time domain modeling and absorption spectroscopy to fully characterize and identify each resonance. The implications of higher order surface plasmon resonances for sensing are discussed.
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73.22.Lp Collective excitations
73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)
78.68.+m Optical properties of surfaces
78.30.Er Solid metals and alloys

Nanometer-scale material contrast imaging with a near-field microwave microscope

Atif Imtiaz, Steven M. Anlage, John D. Barry, and John Melngailis

Appl. Phys. Lett. 90, 143106 (2007); http://dx.doi.org/10.1063/1.2719164 (3 pages) | Cited 17 times

Online Publication Date: 3 April 2007

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The authors report topography-free material contrast imaging on a nanofabricated boron-doped silicon sample measured with a near-field scanning microwave microscope over a broad frequency range. The boron doping was performed using the focus ion beam technique on a silicon wafer with nominal resistivity of 61 Ω cm. A topography-free doped region varies in sheet resistance from 1000 Ω/◻ to about 400 kΩ/◻ within a lateral distance of 4 μm. The qualitative spatial resolution in sheet resistance imaging contrast is no worse than 100 nm as estimated from the frequency shift signal.
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72.20.Fr Low-field transport and mobility; piezoresistance
61.72.uf Ge and Si
72.80.Cw Elemental semiconductors

Porous, self-supported Ni3S2/Ni nanoarchitectured electrode operating through efficient lithium-driven conversion reactions

Qiang Wang, Rui Gao, and Jinghong Li

Appl. Phys. Lett. 90, 143107 (2007); http://dx.doi.org/10.1063/1.2716308 (3 pages) | Cited 8 times

Online Publication Date: 4 April 2007

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To benefit from the large capacity gain advantages offered by lithium-driven conversion reactions and to overcome intrinsic kinetic limitations, a porous, self-supported Ni3S2/Ni nanoarchitectured electrode concept is reported. The originality of this electrode configuration is rooted in low-temperature hydrothermal techniques, which provide an efficient way to enable the realization of a better interface contact in order to easily fuel electrons to the active material and to add mechanical cohesion to the electrode. A preliminary result shows that the nanoarchitectured electrode exhibits a sustained reversible capacity at 2C rate with a loss of only 18% after 20 cycles and outstanding rate capability even at a rate of 16C.
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82.45.Fk Electrodes
82.45.Yz Nanostructured materials in electrochemistry
68.37.Hk Scanning electron microscopy (SEM) (including EBIC)
79.60.Jv Interfaces; heterostructures; nanostructures

Quantum mechanical effects in nanometer field effect transistors

Jun-Wei Luo, Shu-Shen Li, Jian-Bai Xia, and Lin-Wang Wang

Appl. Phys. Lett. 90, 143108 (2007); http://dx.doi.org/10.1063/1.2719151 (3 pages) | Cited 8 times

Online Publication Date: 4 April 2007

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The atomistic pseudopotential quantum mechanical calculations for million atom nanosized metal-oxide-semiconductor field-effect transistors (MOSFETs) are presented. When compared with semiclassical Thomas-Fermi simulation results, there are significant differences in I-V curve, electron threshold voltage, and gate capacitance. In many aspects, the quantum mechanical effects exacerbate the problems encountered during device minimization, and it also presents different mechanisms in controlling the behaviors of a nanometer device than the classical one.
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85.30.Tv Field effect devices
85.35.-p Nanoelectronic devices
85.30.De Semiconductor-device characterization, design, and modeling

Relaxation of photoinduced spins and carriers in ferromagnetic InMnSb films

K. Nontapot, R. N. Kini, A. Gifford, T. R. Merritt, G. A. Khodaparast, T. Wojtowicz, X. Liu, and J. K. Furdyna

Appl. Phys. Lett. 90, 143109 (2007); http://dx.doi.org/10.1063/1.2719173 (3 pages) | Cited 7 times

Online Publication Date: 4 April 2007

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The authors report time resolved measurements and control of photoinduced spin and carrier relaxations in InMnSb ferromagnetic films with 2% Mn content (grown by low-temperature molecular beam epitaxy) using femtosecond laser pulses, and compare them to analogous measurements on InBeSb and InSb films. In this work, magneto-optical Kerr effect and standard pump-probe techniques provided a direct measure of the photoexcited spin and carrier lifetimes, respectively. They observe decrease in relaxations times in the high laser fluence regime and an absence of temperature dependence of the relaxation times.
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75.70.Ak Magnetic properties of monolayers and thin films
73.61.Ey III-V semiconductors
75.50.Dd Nonmetallic ferromagnetic materials
75.50.Pp Magnetic semiconductors
78.66.Fd III-V semiconductors
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy

Thermal conductance of single-walled carbon nanotube embedded in an elastic half-space

Ravi Prasher

Appl. Phys. Lett. 90, 143110 (2007); http://dx.doi.org/10.1063/1.2719240 (3 pages) | Cited 12 times

Online Publication Date: 4 April 2007

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Calculations of ballistic thermal conductance of single-walled carbon nanotube (CNT) embedded in an elastic half-space are presented. The conductance is very different than the conductance of freestanding CNT due to change in phonon velocity and maximum cutoff frequency. Depending on the temperature, the conductance of embedded CNT can be higher or lower than that of freestanding CNT. Applications of the formulation are in CNT-based nanocomposites or CNT in contact with an elastic substrate. Good agreement with experimental thermal conductivity of nanocomposites made from single-walled CNT and epoxy is found.
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65.80.-g Thermal properties of small particles, nanocrystals, nanotubes, and other related systems
63.22.-m Phonons or vibrational states in low-dimensional structures and nanoscale materials

Negative differential resistance of electrons in graphene barrier

D. Dragoman and M. Dragoman

Appl. Phys. Lett. 90, 143111 (2007); http://dx.doi.org/10.1063/1.2719670 (3 pages) | Cited 15 times

Online Publication Date: 4 April 2007

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The graphene is a native two-dimensional crystal material consisting of a single sheet of carbon atoms. In this unique one-atom-thick material, the electron transport is ballistic and is described by a quantum relativisticlike Dirac equation rather than by the Schrödinger equation. As a result, a graphene barrier behaves very differently compared to a common semiconductor barrier. The authors show that a single graphene barrier acts as a switch with a very high on-off ratio and displays a significant differential negative resistance, which promotes graphene as a key material in nanoelectronics.
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73.23.Ad Ballistic transport
72.80.-r Conductivity of specific materials

Molecular dynamics simulation of the dynamical behaviors of an aromatic carboxylic acid molecule with different conformations on a Au (111) surface

Shin-Pon Ju, Wen-Jay Lee, Hui-Chuan Chen, and Jee-Gong Chang

Appl. Phys. Lett. 90, 143112 (2007); http://dx.doi.org/10.1063/1.2718485 (3 pages) | Cited 3 times

Online Publication Date: 5 April 2007

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In this study, the behavior of a single tricarboxylic acid derivative, 1,3,5-tris(carboxymethoxy) benzene [TCMB, C6H3(OCH2COOH)3] on a Au (111) substrate at 50 K has been investigated by molecular dynamics simulation. Four possible conformations of the TCMB molecule adsorbed on the Au (111) substrate are found and which display different trajectories of movement and dynamical behaviors. The lock-and-key geometry between the TCMB molecule of different conformations and atomic arrangement of the Au (111) surface was also observed.
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68.43.Fg Adsorbate structure (binding sites, geometry)
68.43.Mn Adsorption kinetics
68.47.De Metallic surfaces

Technique for tilting GaAs photonic crystal nanocavities out of plane

Cedrik Meier and Kevin Hennessy

Appl. Phys. Lett. 90, 143113 (2007); http://dx.doi.org/10.1063/1.2719612 (3 pages) | Cited 1 time

Online Publication Date: 5 April 2007

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The authors demonstrate a technique to mechanically tilt GaAs membranes out of the substrate plane by an arbitrary angle. Tilting is achieved by dosing a thin line of a perforated membrane with Ga ions using a focused ion beam. The change in material properties at the dosing site initiates a tilt about this axis that is monitored in situ to achieve the desired angle. The authors apply this technique to modify the emission direction of photonic crystal cavity, quantum dot light sources.
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42.70.Qs Photonic bandgap materials
78.66.Fd III-V semiconductors
78.67.Bf Nanocrystals, nanoparticles, and nanoclusters

Barium strontium oxide coated carbon nanotubes as field emitters

Feng Jin, Yan Liu, and Christopher M. Day

Appl. Phys. Lett. 90, 143114 (2007); http://dx.doi.org/10.1063/1.2719645 (3 pages) | Cited 8 times

Online Publication Date: 5 April 2007

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The authors report a field emitter structure based on oxide coated carbon nanotubes (CNTs). This emitter consists of a thin tungsten ribbon with CNTs on the surface and a thin layer of low work function barium strontium oxide coating on the CNTs. This oxide coated CNT emitter was designed to combine the benefits of the high field enhancement factor from CNTs and the low work function from the emissive oxide coating. The field emission properties of the emitters were characterized. A field enhancement factor of 467 and a work function of 1.9 eV were obtained for the oxide coated CNTs. Compared to the uncoated CNTs, the field emission from the oxide coated CNTs increased by a factor of 2–3. At 4.4 V/μm, the field emission current of 23.6 μA was obtained from an emitting surface of 0.012 cm2.
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79.70.+q Field emission, ionization, evaporation, and desorption
73.30.+y Surface double layers, Schottky barriers, and work functions

Electrospun dye-doped polymer nanofibers emitting in the near infrared

A. Camposeo, F. Di Benedetto, R. Stabile, R. Cingolani, and D. Pisignano

Appl. Phys. Lett. 90, 143115 (2007); http://dx.doi.org/10.1063/1.2720262 (3 pages) | Cited 15 times

Online Publication Date: 5 April 2007

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The authors report on the fabrication and characterization of near infrared fluorescent nanofibers. The nanofibers are composed by an organic dye dispersed in a poly(methylmethacrylate) inert matrix and realized by electrospinning. They exhibit diameters down to 70 nm, with average values in the range of 170–480 nm, depending on the process parameters, and photoluminescence emission peaked at 865 nm. The temporal behavior of the emission under ultraviolet excitation in air can be described by an oxygen diffusion model with a characteristic time τ in the range of 400–1200 s, depending on the fiber size, which correspond to a photostability longer than (0.4–1.2)×105 excitation laser pulses. These results open the way for large volume and cost-effective realization of infrared-emitting nanofibers, which are promising candidates as nanoscale infrared light sources.
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42.81.Bm Fabrication, cladding, and splicing
42.81.Cn Fiber testing and measurement of fiber parameters
42.81.Wg Other fiber-optical devices
78.55.Kz Solid organic materials

ZnO nanowires and nanobelts: Shape selection and thermodynamic modeling

Hong Jin Fan, Amanda S. Barnard, and Margit Zacharias

Appl. Phys. Lett. 90, 143116 (2007); http://dx.doi.org/10.1063/1.2720715 (3 pages) | Cited 26 times

Online Publication Date: 6 April 2007

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The authors show that, during a steady-state vapor phase growth of ZnO nanomaterials, indium doping causes the structural change from usual [0001]-axial short nanowires to [11math0]-axial nanobelts of much larger aspect ratio. They used an analytical thermodynamic model based on geometric summation of the Gibbs free energy to predict the dimension dependence of the nanowires and nanobelts for both pure and In-doped ZnO. The calculation result agrees with the experiment observation that in situ indium doping influences the nucleation and supports the dominating growth of a-axial nanobelts over c-axial nanowires.
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68.65.La Quantum wires (patterned in quantum wells)
81.10.Bk Growth from vapor
61.72.uj III-V and II-VI semiconductors
65.40.G- Other thermodynamical quantities

Combined micro-/nanoscale surface roughness for enhanced hydrophobic stability in carbon nanotube arrays

Z. Wang, N. Koratkar, L. Ci, and P. M. Ajayan

Appl. Phys. Lett. 90, 143117 (2007); http://dx.doi.org/10.1063/1.2720761 (3 pages) | Cited 29 times

Online Publication Date: 6 April 2007

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Extreme water repellency is greatly desired for anticontamination and self-cleaning applications. Aligned multiwalled carbon nanotube arrays exhibit superhydrophobic behavior but suffer from poor hydrophobic stability and contact angle hysteresis. In this work the authors selectively grow multiwalled nanotubes onto a patterned substrate and engineer a novel high aspect ratio architecture which combines a micro- and a nano-scale roughness structure. While there is no significant difference in the static contact angle of the patterned and uniform nanotube arrays, dynamic measurements indicate a dramatic increase in hydrophobic stability for the patterned array caused by entrapped air pockets which prevent Cassie to Wenzel state transition.
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68.35.B- Structure of clean surfaces (and surface reconstruction)
61.46.Fg Nanotubes
81.07.De Nanotubes
81.16.-c Methods of micro- and nanofabrication and processing

Room temperature observation of negative differential resistance effect using ZnO nanocrystal structure with double Schottky barriers

Daisuke Ito, Takashi Tomita, and Tsuyonobu Hatazawa

Appl. Phys. Lett. 90, 143118 (2007); http://dx.doi.org/10.1063/1.2721117 (3 pages) | Cited 3 times

Online Publication Date: 6 April 2007

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A varistic nonlinear I-V characteristic caused by a tunneling effect was observed in two-dimensional nanopolycrystal ZnO (NPC-ZnO) with double Schottky barriers (DSBs). Id-Vd characteristic of a NPC-ZnO field effect transistor showed a negative differential resistance characteristic at room temperature. The Id-Vg showed clear current peaks and valleys although this characteristic has an asymmetric hysteresis. An ultraviolet irradiation on the Id-Vg showed the increase of current peaks and disappearance of the hysteresis. These results could be related to the tunneling effect via DSBs and quasibound states that were caused by the internal defect of ZnO dots or the grain boundary.
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85.30.Tv Field effect devices
85.35.Be Quantum well devices (quantum dots, quantum wires, etc.)
85.30.Mn Junction breakdown and tunneling devices (including resonance tunneling devices)

Vacancy mediated desorption of hydrogen from a sodium alanate surface: An ab initio spin-polarized study

A. J. Du, Sean C. Smith, and G. Q. Lu

Appl. Phys. Lett. 90, 143119 (2007); http://dx.doi.org/10.1063/1.2721127 (3 pages) | Cited 4 times

Online Publication Date: 6 April 2007

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Ab initio spin-polarized density functional theory calculations are performed to explore the effect of single Na vacancy on NaAlH4(001) surface on the initial dehydrogenation kinetics. The authors found that two Al–H bond lengths become elongated and weakened due to the presence of a Na vacancy on the NaAlH4(001) surface. Spontaneous recombination from the surface to form molecular hydrogen is observed in the spin-polarized ab initio molecular dynamics simulation. The authors’ results indicate that surface Na vacancies play a critical role in accelerating the dehydrogenation kinetics in sodium alanate. The understanding gained here will aid in the rational design and development of complex hydride materials for hydrogen storage.
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61.72.J- Point defects and defect clusters
68.43.Mn Adsorption kinetics
68.43.Bc Ab initio calculations of adsorbate structure and reactions
72.25.-b Spin polarized transport

Highly efficient electron gun with a single-atom electron source

Tsuyoshi Ishikawa, Tomohiro Urata, Boklae Cho, Eiji Rokuta, Chuhei Oshima, Yoshinori Terui, Hidekazu Saito, Akira Yonezawa, and Tien T. Tsong

Appl. Phys. Lett. 90, 143120 (2007); http://dx.doi.org/10.1063/1.2720348 (3 pages) | Cited 13 times

Online Publication Date: 6 April 2007

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The authors have demonstrated highly collimated electron-beam emission from a practical electron gun with a single-atom electron source; ∼ 80% of the total emission current entered the electron optics. This ratio was two or three orders of magnitude higher than those of the conventional electron sources such as a cold field emission gun and a Zr/O/W Schottky gun. At the pressure of less than 1×10−9 Pa, the authors observed stable emission of 20 nA, which generates the specimen current of 5 pA required for scanning electron microscopes.
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29.25.Bx Electron sources
07.77.Ka Charged-particle beam sources and detectors
41.75.Fr Electron and positron beams
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