Maureen J. Lagos

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.

The role of electronic coupling between substrate and 2D MoS2 nanosheets in electrocatalytic production of hydrogen

The excellent catalytic activity of metallic MoS2 edges for the hydrogen evolution reaction (HER) has led to substantial efforts towards increasing the edge concentration. The 2H basal plane is less active for the HER because it is less conducting and therefore possesses less efficient charge transfer kinetics. Here we show that the activity of the 2H basal planes of monolayer MoS2 nanosheets can be made comparable to state-of-the-art catalytic properties of metallic edges and the 1T phase by improving the electrical coupling between the substrate and the catalyst so that electron injection from the electrode and transport to the catalyst active site is facilitated. Phase-engineered low-resistance contacts on monolayer 2H-phase MoS2 basal plane lead to higher efficiency of charge injection in the nanosheets so that its intrinsic activity towards the HER can be measured. We demonstrate that onset potentials and Tafel slopes of ∼-0.1 V and ∼50 mV per decade can be achieved from 2H-phase catalysts where only the basal plane is exposed. We show that efficient charge injection and the presence of naturally occurring sulfur vacancies are responsible for the observed increase in catalytic activity of the 2H basal plane. Our results provide new insights into the role of contact resistance and charge transport on the performance of two-dimensional MoS2 nanosheet catalysts for the HER.

Observation of the smallest metal nanotube with a square cross-section

Understanding the mechanical properties of nanoscale systems requires a range of measurement techniques and theoretical approaches to gather the relevant physical and chemical information. The arrangements of atoms in nanostructures and macroscopic matter can be different, principally due to the role of surface energy, but the interplay between atomic and electronic structure in association with applied mechanical stress can also lead to surprising differences. For example, metastable structures such as suspended chains of atoms and helical wires have been produced by stretching metal junctions. Here, we report the spontaneous formation of the smallest possible metal nanotube with a square cross-section during the elongation of silver nanocontacts. Ab initio calculations and molecular simulations indicate that the hollow wire forms because this configuration allows the surface energy to be minimized, and also generates a soft structure capable of absorbing a huge tensile deformation.

Mapping vibrational surface and bulk modes in a single nanocube

Imaging of vibrational excitations in and near nanostructures is essential for developing low-loss infrared nanophotonics, controlling heat transport in thermal nanodevices, inventing new thermoelectric materials and understanding nanoscale energy transport. Spatially resolved electron energy loss spectroscopy has previously been used to image plasmonic behaviour in nanostructures in an electron microscope, but hitherto it has not been possible to map vibrational modes directly in a single nanostructure, limiting our understanding of phonon coupling with photons and plasmons. Here we present spatial mapping of optical and acoustic, bulk and surface vibrational modes in magnesium oxide nanocubes using an atom-wide electron beam. We find that the energy and the symmetry of the surface polariton phonon modes depend on the size of the nanocubes, and that they are localized to the surfaces of the nanocube. We also observe a limiting of bulk phonon scattering in the presence of surface phonon modes. Most phonon spectroscopies are selectively sensitive to either surface or bulk excitations; therefore, by demonstrating the excitation of both bulk and surface vibrational modes using a single probe, our work represents advances in the detection and visualization of spatially confined surface and bulk phonons in nanostructures.

Temperature effects on the occurrence of long interatomic distances in atomic chains formed from stretched gold nanowires

The origin of long interatomic distances in suspended gold atomic chains formed from stretched nanowires remains the object of debate despite the large amount of theoretical and experimental work. Here, we report new atomic resolution electron microscopy observations acquired at room and liquid-nitrogen temperatures and theoretical results from ab initio quantum molecular dynamics on chain formation and stability. These new data are suggestive that the long distances are due to contamination by carbon atoms originating from the decomposition of adsorbed hydrocarbon molecules.

Correlation between quantum conductance and atomic arrangement of atomic-size silver nanowires

We have studied the effect of thermal effects on the structural and transport response of Ag atomic-size nanowires (NWs) generated by mechanical elongation. Our study involves both time-resolved atomic resolution transmission electron microscopy imaging and quantum conductance measurement using an ultra-high-vacuum mechanically controllable break junction. We have observed drastic changes in conductance and structural properties of Ag nanowires generated at different temperatures (150 and 300 K). By combining electron microscopy images, electronic transport measurements, and quantum transport calculations, we have been able to obtain a consistent correlation between the conductance and structural properties of Ag NWs. In particular, our study has revealed the formation of metastable rectangular rod-like Ag wire (3/3) along the [001] crystallographic direction, whose formation is enhanced. These results illustrate the high complexity of analyzing structural and quantum conductance behaviour of metal atomic...

Characterization of misfit dislocations in Si quantum well structures enabled by STEM based aberration correction

The success of aberration correction techniques at the end of the 20th century came at a time of increasing need for atomic resolution imaging to better understand known structural defects that influence semiconductor device operation, and to advance the search for new structures and behavior that will form the basis for devices in the future. With this in mind, it is a pleasure to recognize the contributions of Ondrej Krivanek to the success of aberration correction techniques, and his extension of aberration techniques to EELS equipment that further promises to unite structural studies with characterization of behavior from meV to keV energies in the STEM.

Excitation of long-wavelength surface optical vibrational modes in films, cubes and film/cube composite system using an atom-sized electron beam

Using spatially resolved Electron Energy-Loss Spectroscopy, we investigate the excitation of long-wavelength surface optical vibrational modes in elementary types of nanostructures: an amorphous SiO2 slab, an MgO cube, and in the composite cube/slab system. We find rich sets of optical vibrational modes strongly constrained by the nanoscale size and geometry. For slabs, we find two surface resonances resulting from the excitation of surface phonon polariton modes. For cubes, we obtain three main highly localized corner, edge, and face resonances. The response of those surface phonon resonances can be described in terms of eigenmodes of the cube and we show that the corresponding mode pattern is recovered in the spatially resolved EELS maps. For the composite cube/substrate system we find that interactions between the two basic structures are weak, producing minor spectral shifts and intensity variations (transparency behaviour), particularly for the MgO-derived modes.

Surface effects on the mechanical elongation of AuCu nanowires: De-alloying and the formation of mixed suspended atomic chains

We report here an atomistic study of the mechanical deformation of AuxCu(1−x) atomic-size wires (nanowires (NWs)) by means of high resolution transmission electron microscopy experiments. Molecular dynamics simulations were also carried out in order to obtain deeper insights on the dynamical properties of stretched NWs. The mechanical properties are significantly dependent on the chemical composition that evolves in time at the junction; some structures exhibit a remarkable de-alloying behavior. Also, our results represent the first experimental realization of mixed linear atomic chains (LACs) among transition and noble metals; in particular, surface energies induce chemical gradients on NW surfaces that can be exploited to control the relative LAC compositions (different number of gold and copper atoms). The implications of these results for nanocatalysis and spin transport of one-atom-thick metal wires are addressed.

Low temperature (LN2) and UHV mechanically controllable break junction setup to study quantum electrical transport of atomic-size metal nanowire

Reliable metal nanowire studies requires experimental stringent conditions, as clean samples and environment. In this sense, we have designed and built a dedicated instrument to study electrical transport properties of atomic-size metal contacts based on the mechanically controlled break junction technique, operating at ultra-high-vacuum conditions. Here we describe the chosen setup, its implementation and performance.