Vibrational spectroscopy in the electron microscope.

Vibrational spectroscopies using infrared radiation, Raman scattering, neutrons, low-energy electrons and inelastic electron tunnelling are powerful techniques that can analyse bonding arrangements, identify chemical compounds and probe many other important properties of materials. The spatial resolution of these spectroscopies is typically one micrometre or more, although it can reach a few tens of nanometres or even a few ångströms when enhanced by the presence of a sharp metallic tip. If vibrational spectroscopy could be combined with the spatial resolution and flexibility of the transmission electron microscope, it would open up the study of vibrational modes in many different types of nanostructures. Unfortunately, the energy resolution of electron energy loss spectroscopy performed in the electron microscope has until now been too poor to allow such a combination. Recent developments that have improved the attainable energy resolution of electron energy loss spectroscopy in a scanning transmission electron microscope to around ten millielectronvolts now allow vibrational spectroscopy to be carried out in the electron microscope. Here we describe the innovations responsible for the progress, and present examples of applications in inorganic and organic materials, including the detection of hydrogen. We also demonstrate that the vibrational signal has both high- and low-spatial-resolution components, that the first component can be used to map vibrational features at nanometre-level resolution, and that the second component can be used for analysis carried out with the beam positioned just outside the sample--that is, for 'aloof' spectroscopy that largely avoids radiation damage.

Properties of Dipole-Mode Vibrational Energy Losses Recorded From a TEM Specimen.

The authors discuss the dipole vibrational modes that predominate in the energy-loss spectra of ionic materials below 1 eV, concentrating on thin-film specimens of typical transmission electron microscopy (TEM) thickness. The thickness dependence of the intensity is shown to be a useful guide to the bulk or surface character of vibrational peaks. The lateral and depth resolution of the energy-loss signal is investigated with the aid of finite-element calculations.

Phase plates in the transmission electron microscope: operating principles and applications.

In this paper, we review the current state of phase plate imaging in a transmission electron microscope. We focus especially on the hole-free phase plate design, also referred to as the Volta phase plate. We discuss the implementation, operating principles and applications of phase plate imaging. We provide an imaging theory that accounts for inelastic scattering in both the sample and in the hole-free phase plate.

Low-dose performance of parallel-beam nanodiffraction.

We evaluate the low-dose performance of parallel nano-beam diffraction (NBD) in the transmission electron microscope as a method for characterizing radiation sensitive materials at low electron irradiation dose. A criterion, analogous to Rose's, is established for detecting a diffraction spot with desired signal-to-noise ratio. Our experimental data show that a dose substantially lower than in high-resolution bright-field imaging is sufficient to determine structure and orientation of individual nanoscale objects embedded in amorphous matrix. In an instrument equipped with a cold field-emission gun it is possible to form a probe with sub-3 nm diameter and sub-0.3 mrad convergence angle with sufficient beam current to record a diffraction pattern with less than 0.2 s acquisition time. The interpretation of NBD patterns is identical to that of selected area diffraction patterns. We illustrate the physical principles underlying good low-dose performance of NBD by means of a phase grating. The electron irradiation dose needed to detect a diffraction peak in NBD is found proportional to 1/N2, where N is the number of lattice planes contributing to the peak.

Alternative methods of identifying the oxidation of metallic nanoparticles embedded in a matrix.

We compare methods for valence-state analysis based on energy-loss near-edge structure (ELNES), including the white-line ratio (WLR), pre-edge peak (prepeak) and post-edge peak (postpeak) techniques. Starting from multiple-scattering calculations, we correlate the appearance of a prepeak in the O-K edge and postpeak in the L-edge with oxidation of a transition metal (TM). The ability to use more than one technique is especially advantageous for a nanocomposite of metallic nanoparticles embedded in a matrix, as we show for the case of iron nanoparticles in a silica matrix.

Interpretation of the postpeak in iron fluorides and oxides.

A broad post-edge peak (postpeak) in the core-loss spectrum of a relatively thick TEM specimen is readily accounted for by plural scattering of the transmitted electrons, involving a core-loss and a bulk plasmon. However, we observe a prominent postpeak, about 40 eV above iron L3 edge, even in very thin films of iron fluoride. The peak gradually disappears as fluorine is removed by electron irradiation, as indicated by decay of the fluorine K-edge and change in white-line ratio, but appears when an iron film is oxidized, therefore it appears to be characteristic of iron in an oxidized state. To study its origin, we performed real-space multiple-scattering (RSMS) calculations of the near-edge fine structure of the Fe L-edge, allowing us to discuss the origin of the postpeak within the framework of electron-scattering theory. In the belief that oxygen and fluorine anions in transition-metal compounds are strong backscatters, we propose that the postpeak is a common feature of transition metals in an oxidized state and can be used as an additional verification of such oxidization in an unknown sample.

Energy-loss near-edge fine structures of iron nanoparticles.

Body centered cubic (bcc) Fe nanoparticles were fabricated by in situ decomposition of iron fluoride films in a transmission electron microscope. Electron energy-loss near edge structure (ELNES) was used to characterize this exposure process. In particular, the L(3)/L(2) white-line intensity ratio (WLR) was used to monitor the iron valence state during exposure, and as an indicator of other properties of the iron nanoparticles. Iron nanoparticles with sizes between 2 and 20nm exhibit a constant WLR, whose value is same as that for a continuous bcc iron film, suggesting little or no dependence of the local magnetic moment or structure on the particle size. A broad but prominent peak which occurs 40eV after the L(3)-ionization threshold in the iron fluoride, is absent for a metallic iron film but reappears when the iron is converted to an oxide. Long-range ferromagnetic coupling was observed in samples densely populated with iron nanoparticles. Because there is little interaction between particles and the supporting carbon substrate, these samples provide an ideal model system for studying the influence of particle size and interparticle distance on magnetic properties.

Accurate measurement of relative tilt and azimuth angles in electron tomography: a comparison of fiducial marker method with electron diffraction.

Electron tomography is a method whereby a three-dimensional reconstruction of a nanoscale object is obtained from a series of projected images measured in a transmission electron microscope. We developed an electron-diffraction method to measure the tilt and azimuth angles, with Kikuchi lines used to align a series of diffraction patterns obtained with each image of the tilt series. Since it is based on electron diffraction, the method is not affected by sample drift and is not sensitive to sample thickness, whereas tilt angle measurement and alignment using fiducial-marker methods are affected by both sample drift and thickness. The accuracy of the diffraction method benefits reconstructions with a large number of voxels, where both high spatial resolution and a large field of view are desired. The diffraction method allows both the tilt and azimuth angle to be measured, while fiducial marker methods typically treat the tilt and azimuth angle as an unknown parameter. The diffraction method can be also used to estimate the accuracy of the fiducial marker method, and the sample-stage accuracy. A nano-dot fiducial marker measurement differs from a diffraction measurement by no more than ±1°.

Local thickness measurement through scattering contrast and electron energy-loss spectroscopy.

Scattering contrast measurements were performed on thin films of amorphous carbon and polycrystalline Au, as well as single-crystal MgO nanocubes. Based on the exponential absorption law, mass-thickness can be obtained within 10% accuracy by measuring the incident and transmitted intensities in the same image. For mass-thickness measurement of a thin amorphous specimen, a small collection semiangle improves the measurement sensitivity, whereas for the measurement of polycrystalline or single-crystal specimens, a large collection semiangle should be used to reduce diffraction-contrast effects. EELS thickness measurements on MgO nanocubes suggest that the Kramers-Kronig sum-rule method (with correction for plural and surface scattering) gives 10% accuracy at medium collection semiangles but overestimates the thickness at small collection semiangles, due to underestimation of the surface-mode scattering. The log-ratio method, with a formula for inelastic mean free path proposed by Malis et al. (1988), provides 10% accuracy at small collection semiangle, while that proposed by Iakoubovskii et al. (2008a) is preferable for medium and large collection semiangles. As a result of this work, we provide recommendations of preferred methods and conditions for local-thickness measurement in the TEM.

Convenient contrast enhancement by a hole-free phase plate.

Decrease of the irradiation dose needed to obtain a desired signal-to-noise ratio can be achieved by Zernike phase-plate imaging. Here we present results on a hole-free phase plate (HFPP) design that uses the incident electron beam to define the center of the plate, thereby eliminating the need for high precision alignment and with advantages in terms of ease of fabrication. The Zernike-like phase shift is provided by a charge distribution induced by the primary beam, rather than by a hole in the film. Compared to bright-field Fresnel-mode imaging, the hole-free phase plate (HFPP) results in two- to four-fold increase in contrast, leading to a corresponding decrease in the irradiation dose required to obtain a desired signal-to-noise ratio. A local potential distribution, developed due to electron beam-induced secondary-electron emission, is the most likely mechanism responsible for the contrast-transfer properties of the HFPP.

Heteroepitaxial growth of gold nanostructures on silicon by galvanic displacement.

This work focuses on the synthesis and interfacial characterization of gold nanostructures on silicon surfaces, including Si(111), Si(100), and Si nanowires. The synthetic approach uses galvanic displacement, a type of electroless deposition that takes place in an efficient manner under aqueous, room-temperature conditions. The case of gold-on-silicon has been widely studied and used for several applications and yet, a number of important, fundamental questions remain as to the nature of the interface. Some studies are suggestive of heteroepitaxial growth of gold on the silicon surface, whereas others point to the existence of a silicon-gold intermetallic sandwiched between the metallic gold and the underlying silicon substrate. Through detailed high resolution transmission electron microscopy (TEM), combined with selected area electron diffraction (SAED) and nanobeam diffraction (NBD), heteroepitaxial gold that is grown by galvanic displacement is confirmed on both Si(100) and Si(111), as well as silicon nanowires. The coincident site lattice (CSL) of gold-on-silicon results in a very small 0.2% lattice mismatch due to the coincidence of four gold lattices to three of silicon. The presence of gold-silicon intermetallics is suggested by the appearance of additional spots in the electron diffraction data. The gold-silicon interfaces appear heterogeneous with distinct areas of heteroepitaxial gold on silicon, and others, less well-defined, where intermetallics may reside. The high resolution cross-sectional TEM images reveal a roughened silicon interface under these aqueous galvanic displacement conditions, which most likely promotes nucleation of metallic gold islands that merge over time: a Volmer-Weber growth mechanism in the initial stages.