Low-energy Electron Transmission Research Articles

Coherent imaging with low-energy electrons, quantitative analysis

Low-energy electrons (20–300eV) hold the promise for low-dose, non-destructive, high-resolution imaging, but at the price of challenging data analysis. This study provides theoretical considerations and models for the quantitative analysis of experimental data observed in low-energy electron transmission microscopy and in-line holography. The scattering of low-energy electrons and the imaging parameters, such as the inelastic mean free path, point spread function, depth of focus, and resolution, are quantitatively described. It is shown that unlike high-energy electrons (20–300 keV), low-energy electrons (20–300eV) introduce a large phase shift into the probing electron waves. Using the projected potentials formalism, the maximal phase shift acquired by a 120eV electron wave scattered by a carbon atom is theoretically estimated to be 5.03 radian and experimentally measured to be between 3 and 7.5 radian. The point spread function evaluated for low-energy electrons shows that they diffract much stronger than high-energy electrons, and that only very thin objects of up to 3Å in thickness can be imaged in focus. Thus, when imaging an object of finite thickness, such as a macromolecule, the obtained image will always be blurred due to the out-of-focus signal. This can provide an explanation for a long-standing problem of limited resolution in low-energy electron holography of macromolecules. As for imaging of a macromolecule’s structure, it is shown that the amplitude of the wavefront reconstructed from the sample’s hologram provides the best match to the projected potential distribution of the macromolecule. To evaluate the absorption properties, the inelastic mean free path (IMFP) is considered. The IMFP values calculated from theoretical models agree with the measured values. The IMFP of about 5Å was measured by transmission through graphene of 50–200eV electrons. This result implies that the internal structure of only very thin samples can be imaged in transmission mode. A simple method to quantitatively evaluate the absorption of a specimen from its in-line hologram without the need to reconstruct the hologram is presented.

The influence of micro/nano-structure on transmission ability of low-energy Electron Transmission Window

It is a extremely difficult to make the Electron Transmission Window (ETW) have the transmission ability for low-energy electrons, excellent mechanical strength, and thicker thickness at a large barometric pressure gradient, i.e., from vacuum to atmosphere. We present a new physical model of the ETW, which has a periodic array microstructure to restrain the temperature increase and nanostructures to provide transmission channels for electron sources. Two different periodic array micro-pattern ETWs with nanostructures have been prepared to testify the physical mechanisms, which are a thicker periodic array triangle micro-pattern unit with a nested triangle pillar type (NETW) and a thinner one without nested part type (OETW). A nanosecond-pulse power source and an array field emission electrode are adopted to focus the energy and reduce the energy dissipation of the electron source. The transmission efficiencies of the NETW and the OETW are 5.86% and 2.82% at −3.5 kV, 8.90% and 5.77% at −5.5 kV, and 12.69% and 11.97% at −8.5 kV pulsed voltages, respectively. In addition, the transmission abilities of the two ETWs have been compared by visualization diagnoses in transient and steady states, which match well with the results of transmission efficiencies. It is found that the results are consistent with those of this novel model mechanism.

Low-energy transmission electron diffraction and imaging of large-area graphene.

Two-dimensional (2D) materials have attracted interest because of their excellent properties and potential applications. A key step in realizing industrial applications is to synthesize wafer-scale single-crystal samples. Until now, single-crystal samples, such as graphene domains up to the centimeter scale, have been synthesized. However, a new challenge is to efficiently characterize large-area samples. Currently, the crystalline characterization of these samples still relies on selected-area electron diffraction (SAED) or low-energy electron diffraction (LEED), which is more suitable for characterizing very small local regions. This paper presents a highly efficient characterization technique that adopts a low-energy electrostatically focused electron gun and a super-aligned carbon nanotube (SACNT) film sample support. It allows rapid crystalline characterization of large-area graphene through a single photograph of a transmission-diffracted image at a large beam size. Additionally, the low-energy electron beam enables the observation of a unique diffraction pattern of adsorbates on the suspended graphene at room temperature. This work presents a simple and convenient method for characterizing the macroscopic structures of 2D materials, and the instrument we constructed allows the study of the weak interaction with 2D materials.

Porous materials: Look but don't touch.

By combining an electron-counting camera with low-energy transmission electron microscopy, it is possible to directly image the surface structure of delicate metal–organic framework crystals and their coherent interfaces.

Mapping unoccupied electronic states of freestanding graphene by angle-resolved low-energy electron transmission

We report angle-resolved electron transmission measurements through freestanding graphene sheets in the energy range of 18 to 30 eV above the Fermi level. The measurements are carried out in a low-energy electron point source microscope, which allows simultaneously probing the transmission for a large angular range. The characteristics of low-energy electron transmission through graphene depend on its electronic structure above the vacuum level. The experimental technique described here allows mapping the unoccupied band structure of freestanding two-dimensional materials as a function of energy and probing angle, respectively in-plane momentum. Our experimental findings are consistent with theoretical predictions of a resonance in the band structure of graphene above the vacuum level [V. U. Nazarov, E. E. Krasovskii, and V. M. Silkin, Physical Review B 87, 041405 (2013)].

Low-energy electron transmission for the analysis of the interface barrier formation and the density of the unoccupied electronic states in the ultra-thin layers of fluorinated copper-phthalocyanine

AbstractThe interfacial structure made from the thermally deposited 5 – 7 nm thick layers of hexadecafluoro copper phthalocyanine (F

Validities of three multislice algorithms for quantitative low-energy transmission electron microscopy

Three different types of multislice algorithms, namely the conventional multislice (CMS) algorithm, the propagator-corrected multislice (PCMS) algorithm and the fully-corrected multislice (FCMS) algorithm, have been evaluated in comparison with respect to the accelerating voltages in transmission electron microscopy. Detailed numerical calculations have been performed to test their validities. The results show that the three algorithms are equivalent for accelerating voltage above 100kV. However, below 100 kV, the CMS algorithm will introduce significant errors, not only for higher-order Laue zone (HOLZ) reflections but also for zero-order Laue zone (ZOLZ) reflections. The differences between the PCMS and FCMS algorithms are negligible and mainly appear in HOLZ reflections. Nonetheless, when the accelerating voltage is further lowered to 20 kV or below, the PCMS algorithm will also yield results deviating from the FCMS results. The present study demonstrates that the propagation of the electron wave from one slice to the next slice is actually cross-correlated with the crystal potential in a complex manner, such that when the accelerating voltage is lowered to 10 kV, the accuracy of the algorithms is dependent of the scattering power of the specimen.

Orientation-Dependent Electronic Structures and Charge Transport Mechanisms in Ultrathin Polymeric n-Channel Field-Effect Transistors

We investigated the role of metal/organic semiconductor interface morphology on the charge transport mechanisms and energy level alignment of the n-channel semiconductor poly{[N,N'-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5'-(2,2'-bithiophene)} (P(NDI2ODT2)). Variable-temperature study of well-ordered edge-on-oriented P(NDI2OD-T2) monolayer and multilayer field-effect transistors fabricated via Langmuir-Schäfer (LS) method reveals a higher activation energy for the edge-on morphology when compared to that extracted for the face-on oriented P(NDI2OD-T2) spin-coated films, which showed a weaker temperature dependence. Near-ultraviolet inverse photoemission and low-energy electron transmission spectroscopies are utilized to study these microstructurally defined polymeric films. The cross correlations of these techniques with the device characterization reveals the role of the molecular orientation at the semiconductor/contact interface in shifting the charge injection barrier. Finally, we demonstrate that the injection barrier for electrons is higher for the LS/edge-on than in the spin-coated/face-on films.

Low-energy electron transmission imaging of clusters on free-standing graphene

We investigated the utility of free-standing graphene as a transparent sample carrier for imaging nanometer-sized objects by means of low-energy electron holography. The sample preparation for obtaining contamination-free graphene as well as the experimental setup and findings are discussed. For incoming electrons with 66 eV kinetic energy, graphene exhibits 27% opacity per layer. Hence, electron holograms of nanometer-sized objects adsorbed on free-standing graphene can be recorded and numerically reconstructed to reveal the object's shapes and distribution. Furthermore, a Moiré effect has been observed with free-standing graphene multi-layers.

Low energy charged particles interacting with amorphous solid water layers

The interaction of charged particles with condensed water films has been studied extensively in recent years due to its importance in biological systems, ecology as well as interstellar processes. We have studied low energy electrons (3-25 eV) and positive argon ions (55 eV) charging effects on amorphous solid water (ASW) and ice films, 120-1080 ML thick, deposited on ruthenium single crystal under ultrahigh vacuum conditions. Charging the ASW films by both electrons and positive argon ions has been measured using a Kelvin probe for contact potential difference (CPD) detection and found to obey plate capacitor physics. The incoming electrons kinetic energy has defined the maximum measurable CPD values by retarding further impinging electrons. L-defects (shallow traps) are suggested to be populated by the penetrating electrons and stabilize them. Low energy electron transmission measurements (currents of 0.4-1.5 μA) have shown that the maximal and stable CPD values were obtained only after a relatively slow change has been completed within the ASW structure. Once the film has been stabilized, the spontaneous discharge was measured over a period of several hours at 103 ± 2 K. Finally, UV laser photo-emission study of the charged films has suggested that the negative charges tend to reside primarily at the ASW-vacuum interface, in good agreement with the known behavior of charged water clusters.

An accurate multislice method for low-energy transmission electron microscopy

The conventional multislice method (CMS), originally proposed by Cowley and Moodie (1957), is an important algorithm for image and electron diffraction calculations in transmission electron microscopy. Nevertheless, this method is based on the so-called high-energy approximation, which neglects the second differential term ∂ 2 φ( r)/∂ z 2 to greatly simplify the calculation without severe loss of accuracy. In the current study, we show that for low-energy transmission electron microscopy (LE-TEM) (<100 kV), the high-energy approximation error becomes large and the accurate multislice method, proposed by Chen and Van Dyck (1997), can be used as an alternative method to obtain more accurate calculations. The accurate multislice method, called the revised real space method (RRS) in this paper, can be realized by treating the propagation and scattering as an entirety in real space. A detailed comparison of the numerical results of the RRS and CMS at different accelerating voltages, Debye–Waller factors, and beam tilts is performed. Results show that for image and diffraction simulations in LE-TEM, CMS is no longer sufficiently accurate and the RRS procedure can be used as an alternative method with reasonable computing time.

Optimal design of a semiconductor heterostructure tunnel diode with linear current–voltage characteristic

Atomic layer precision in design of heterostructure tunnel diodes can be used to mimic an Ohmic (linear) current–voltage characteristic over a range of voltage bias. This is achieved by manipulating low energy electron transmission resonances. Calculations demonstrate how greater than 65 dB dynamic range can be obtained in an optimized heterostructure tunnel device that is only 17 nm thick.

Low-energy electron transmission in a partially unzipped zigzag nanotube

Based on the nearest-neighbor tight-binding approximation, we present exact analytical expressions for electron transmission in nanotube/ribbon junctions, generated by incomplete unzipping of zigzag nanotubes. By assuming one-dimer-line difference in the widths of the leads, it is demonstrated that such a contact exhibits zero backscattering of low-energy electrons entering from the graphene side of the junction. We also show that a zigzag nanotube section sandwiched between two armchair graphene ribbons is completely transparent for incident low-energy electrons. Possible application of the results to nanosensor engineering is also included.

Low-Energy Scanning Electron Transmission Measurements of Thin Polymer Films Employed in Organic Solar Cells

Extended abstract of a paper presented at Microscopy and Microanalysis 2010 in Portland, Oregon, USA, August 1 – August 5, 2010.

Low-energy electron transmission through dielectric capillaries

Transmission of electrons with energies from 2 to 10 keV through glass polycapillaries, tubes, and cones was experimentally studied. The dependence of the transmittance on the electron energy, beam current, capillary diameter, and irradiation time was measured. The electron transmittance through polycapillaries decreases with the beam current and measurement time. The results of measurements of the electron and X-ray spectrum at the quartz tube output suggest that a fraction of electrons incident on the wall charge it and ionize wall atoms; other electrons pass through the channel without collision with the wall, since their energy is almost unchanged. It follows from the measurement results that the electron transmission through capillaries is controlled by the potential at the inner channel surface induced by the charge injected during irradiation.

Local conductance measurements of double-layer graphene on SiC substrate

The microscopic structural and electrical properties of few-layer graphene grown on an SiCsubstrate were characterized by low-energy electron microscopy, transmission electronmicroscopy and scanning probe microscopy measurements of local conductance. Thedouble-layer graphene sheet was confirmed to be continuous across the atomic steps on theburied SiC substrate surface, and the measured local conductance was clearly modified inthe vicinity of the steps. The conductance decreased (slightly increased) at the lower(upper) side of the steps, suggesting deformation-induced strain is the origin of theconductance modification. From the contact force dependence of the conductance images,the effective contact areas for both nanogap-probe and point-probe measurements wereestimated.

Surface effects in α-Fe2O3 nanoparticles studied by ILEEMS and TMS

To understand the particular properties of thin α-Fe2O3 films, hematite nanoparticles with sizes between 20 and 140 nm have been examined by integral low-energy electron Mossbauer spectroscopy (ILEEMS) and transmission Mossbauer spectroscopy (TMS) at room temperature and 80 K. By comparing the results of both Mossbauer variants, surface effects of the Morin transition are studied. The results clearly indicate that the Morin transition takes place at higher temperature in the interior of the particles as compared to the outer layers. It is found that the lowering of the Morin transition is much stronger at the surface of smaller particles.

Fano resonances in low-energy electron transmission through crystalline films

It is shown that in crystalline films with a definite lattice symmetry there existone-dimensional bands of bound electron states above the boundary of the continuousspectrum. The intensity of electron reflection near the state of complete transmission oflow-energy electrons is found to have the form of a Fano resonance. We obtain therelationship between the Fano function parameters and physical characteristics of thesystem. The conditions for the electron transparency to arise near the bound state bands inthe continuum are determined.

Sample size effects on the transport characteristics of mesoscopic graphite samples

AbstractIn this work we investigated correlations between the internal microstructure and sample size (lateral as well as thickness) of mesoscopic, tens of nanometer thick graphite (multigraphene) samples and the temperature (T) and field (B) dependence of their electrical resistivity ρ (T, B). Low energy transmission electron microscopy reveals that the original highly oriented pyrolytic graphite material – from which the multigraphene samples were obtained by exfoliation – is composed of a stack of ∼50 nm thick and micrometer long crystalline regions separated by interfaces running parallel to the graphene planes. We found a qualitative and quantitative change in the behavior of ρ (T, B) upon thickness of the multigraphene samples, indicating that their internal microstructure is important. The overall results indicate that the metallic‐like behavior of ρ (T) at zero field measured for bulk graphite samples is not intrinsic of ideal graphite. The results suggest that the interfaces between crystalline regions may be responsible for the superconducting‐like properties observed in graphite. Our transport measurements also show that reducing the sample lateral size as well as the length between voltage electrodes decreases the magnetoresistance, in agreement with recently published results. The magnetoresistance of the multigraphene samples shows a scaling of the form [(R (B) – R (0))/R (0)]/Tα = f (B /T) with a sample dependent exponent α ∼ 1, which applies in the whole temperature 2 K ≤ T ≤ 270 K and magnetic field range B ≤ 8 T. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Low-Energy Electron Transmission through Thin-Film Molecular and Biomolecular Solids

Low-Energy Electron Transmission through Thin-Film Molecular and Biomolecular Solids