Single atom imaging and spectroscopy in nanostructured carbon materials

Since the invention of the transmission electron microscope (TEM) in 1931 by Max Knoll and Ernst Ruska, modern TEMs underwent remarkable developments in its overall stability and electron optics. In particular, correction of higher-order lens aberrations has facilitated structural characterization of single molecules at atomic resolution. This improvement in resolution has greatly assisted the design and development of new nano-objects in the field of materials and biological sciences. However, taking full advantage of these developments requires high chemical purity in the sample preparation technique and an electron transparent sample support that not only ensures the reliability in structural characterization, but also boosts the signal-to-noise ratio of TEM images. Moreover, the electron beam in TEMs, in addition to image formation, can be used for structural transformation due to the strong interaction of electrons with the investigated material. In this thesis, the structure of mass-selected molecular species, deposited on graphene via electro-spray ion beam deposition (ES-IBD), is investigated by aberration-corrected high-resolution transmission electron microscopy (AC-HRTEM) at acceleration voltages between 60 and 80 kV. Due to its impressive electronic and mechanical properties, single- layer graphene (SLG) is used as TEM substrate throughout this work. The transfer methodology of the SLG, obtained by mechanical exfoliation or chemical vapor deposition (CVD), to the TEM grids is presented, followed by thorough structural characterization to analyze the morphology and crystallinity of the transferred SLG using HRTEM. First, beam-sensitive organic molecules such as cytochrome-class hemeprotein (cytochrome-c), chaperonin complex protein molecule (GroEL), copper-pthalocyanine (CuPc) and com- plex molecular magnetic systems in the form of single molecule magnets (SMMs) were investigated. The results showed that despite the presence of the highly conductive substrate and the use of low electron acceleration voltages, small organic molecules did not survive. However, large globular proteins showed some level of electron resistance before suffering radiation damage. Next, the molecular structure of single phosphotungstic acid (PTA) anion molecules deposited by either ES-IBD or the drop-cast method was analyzed and compared. The results showed homogeneous morphology of the samples prepared by ES-IBD. Further- more, by varying coverage and landing energy, it was possible to modify the required morphology. This allowed better control of deposition as compared to the drop cast ap- proach. A main advantage of ES-IBD is the chemical purity during deposition. This is not guaranteed in the drop-cast technique which is performed under ambient conditions. The main part of this thesis deals with the electron-beam-induced transformation of alkali-iodide molecules (CsI, RbI, KI and NaI). The deposited alkali iodide nanoparticles were intentionally exposed to the electron beam to disintegrate them into single ions. It is found that during electron exposure these ions form regular clusters consisting of 4 to 7 atoms, or 2D-crystals. Notably, in the case of CsI, 2D-crystals form only on BLG, whereas on SLG only atomic clusters were formed. HRTEM in combination with electron energy-loss spectroscopy (EELS) is employed to identify the atomic species of the 2D crystals on the atomic scale. In the case of NaI, 2D-crystals form on SLG. Their crystal structure corresponds to the CsCl-type, which has not been reported before. The etching behavior of graphene in the presence of alkali-iodide species is observed in HRTEM experiments for all the deposited species (CsI, RbI, KI, and NaI). By mea- suring the increase in the perimeter of etched holes in SLG, the rate of etching has been calculated. We found that etching was faster in the presence of both alkali and halide ions, as compared to the presence of only one ionic species. First-principles DFT simulation studies were performed to understand the interaction of alkali-iodide dimers on SLG. These calculations yielded quantitative estimates on the charge transfer between the dimer species and the underlying SLG. It was found that adsorption of alkali-iodide on SLG reduces the bonding electron density between the carbon atoms, thus reducing the C-C binding energy. This effect makes the SLG susceptible to damage when exposed to 80-keV electrons, which establishes the catalytic behavior of alkali-iodide species as etchants of SLG.

We perform fluorescence imaging of a single 87Rb atom after its release from an optical dipole trap. The time-of-flight expansion of the atomic spatial density distribution is observed by accumulating many single atom images. The position of the atom is revealed with a spatial resolution close to 1 μm by a single-photon event, induced by a short resonant probe. The expansion yields a measure of the temperature of a single atom, which is in very good agreement with the value obtained by an independent measurement based on a release-and-recapture method. The analysis presented in this paper provides a way of calibrating an imaging system useful for experimental studies involving a few atoms confined in a dipole trap.

Advanced nano-structured materials have emerged as key elements in enhancing the acting of strength storage devices in the way that fuel cells, solar cells, Li-ion batteries, super-capacitors, and capacitors. These matters offer superior characteristics including high surface area, enhanced electrical conductivity, and improved mechanical strength, enabling efficient energy storage and conversion processes. In fuel cells and solar cells, nano-structured materials like graphene, quantum dots, and alloy oxides facilitate rapid charge transfer and increase design efficiency. In Li-ion batteries, nano-structured electrodes reinforce ion diffusion and provide higher energy densities. Super-capacitors benefit from nano-structured carbon materials with increased capacitance and faster charge/discharge rates. Capacitors with nano-structured dielectrics exhibit enhanced energy storage capabilities and prolonged lifespan.

A Study of Certified Diabetes Educators: A Descriptive Exploration of the Delivery of Diabetes Self-Management Support in Community-based Settings in Ontario

Until very recently a single atom interacting with an electromagnetic field could only be a topic of theoretical consideration because experimentally only ensembles of atoms were accessible. Atomic spectra and the probabilities of transitions in atoms had been successfully studied in atomic ensembles for a long time. Quantum mechanics enables one to calculate both spectra and probabilities. However, quantum mechanics predicts that any transition happens at a random moment of time. In accordance with this prediction a single atom irradiated by light will make random jumps between its ground and excited electronic states. However, these jumps cannot be observed if we detect emission from the whole atomic ensemble, because a statistical average hides the random character of the processes. Therefore they cannot be observed in ensembles. This is only one example of new aspects which emerge when we move from atomic ensembles to a single atom. In this book we intend to discuss not only problems of probabilistic and spectral calculations, but also various random characteristics of atoms and molecules, the so-called quantum trajectories, which are typical in the spectroscopy of single atoms or molecules. In order to prepare for the discussion of quantum trajectories, we have to discuss the basic principles underlying the spectroscopy of a single two-level atom interacting with a transverse electromagnetic field.

We report experimental measurements of the dynamical polarizability of dysprosium, at a wavelength of 532 nm. We measure all three components (scalar, vector, and tensor) of the anisotropic polarizability for the ground and the excited manifolds of the intercombination transition of Dy at 626 nm. The apparatus on which the measurements are performed is presented. We obtain with this setup imaging of single Dy atoms with fidelity above 99% and losses below 2.5% induced by imaging. We then describe the methods used to extract the polarizability. In particular, we combine a measurement of trap frequency and trap depth on single atoms in optical tweezers, allowing us to obtain a measurement of the ground-state polarizability free of errors in trap waist calibration. The obtained values give a magic condition between two Zeeman states in the ground and excited manifolds, which is used to image single atoms in optical tweezer arrays. The scalar polarizability of the ground state is in disagreement with theoretical expectations, calling for future investigations to resolve the discrepancy.

9 - Challenges and emerging direction of carbon nanostructure materials in magnetism and spintronics

Formation of large-grain crystalline germanium on single layer graphene on insulator by rapid melting growth

Single-atom catalysts are attracting attention due to their superior catalytic activity and cost. Nitrogen (N) enhances the stability of single platinum (Pt) atoms on graphene because Pt atoms dispersed by plasma sputtering in an N2 atmosphere are less likely to aggregate. However, the atomic structure of Pt and N on graphene has not been clarified. Here, we experimentally revealed the atomic arrangement of Pt, N, and carbon (C) by scanning transmission electron microscopy and electron energy loss spectroscopy. Pt and N atoms were adsorbed near the step edge of nanographene stacked on single-layer pristine graphene rather than on the terrace. The density functional theory (DFT) calculations using the experimental structure confirmed that the single Pt atom has high stability because N strengthens the bond between Pt and C at the step edge. In addition to a large decrease in the population of Pt 5dxy-orbital, an increase in the population of 5dyz-orbital was observed.

Metal, nitrogen‐doped carbon materials have attracted interest as heterogenous catalysts that contain MNx active sites that are analogous to molecular catalysts. Of particular interest is Ni,N‐doped carbon, a catalyst that is active for the electrochemical reduction of CO2 to CO. Critical to the understanding of these materials is proof of single atomic sites and characterization of the environment surrounding the metal atom; however, directly probing this coordination remains challenging. This challenge is addressed by combining scanning transmission electron microscopy (STEM), single atom electron energy loss spectroscopy (EELS), and time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS). Through STEM imaging, atomic dispersion of Ni in the carbon framework is confirmed and image analyses are utilized to give semiquantitative estimates of neighbor distance distributions and site densities of Ni atoms. Atomic resolution EELS demonstrates that N and Ni are colocated at the single Ni atom sites suggesting Ni–N coordination. ToF‐SIMS reveals a distribution of NiNxCy− fragments that reflect the Ni–N bonding environments within Ni,N‐doped carbon. The fragmentation from Ni,N‐doped carbon is similar to Ni phthalocyanine, suggesting the existence of heterogenized, molecular‐like NiN4 active sites which motivates future studies that leverage insight from molecular catalysis design to develop next‐generation heterogeneous catalysts.

Propane dehydrogenation (PDH) is an effective approach to produce propylene. Downsizing the Pt species to single atom catalysts (SACs) has become a hotspot, owing to the maximum utilization and excellent catalytic behavior. However, the agglomeration of SACs is the decisive limitation for high temperature PDH. Herein, single Pt atoms were anchored on graphene with different types of vacancies, and their catalytic performances on PDH were explored based on density functional theory (DFT). As the vacancy size increased, the catalytic activity decreased. It was because the combined site of the detached H atom in propane would transfer from the Pt atom to the C atom around vacancies, thus increasing the migration distance and lowering the activity. However, with the increase of vacancy size, the selectivity to propylene was improved, owing to the enhanced repulsion between C atoms in graphene and propylene. Therefore, instead of stabilizing the single atom, vacancies in carbon materials can also tailor the catalytic performance by geometric disturbance. This fundamental work opens up the possibility for purposeful SAC design in PDH.

Single atom imaging in high-resolution UHV electron microscopy: Bi on Si(111) surface

The C‐scorpionate iron(II) complex [FeCl2(Tpm)] [Tpm=κ3‐HC(C3H3N2)3] (1) was immobilized on five different nanostructured carbon materials (nanodiamonds, graphene nanoplatelets, graphene oxide, reduced graphene oxide, and nanohorns) to produce active, selective, and recyclable catalysts for alkane and alcohol oxidations. The heterogenized systems (including the first ever reported complexes supported on carbon nanohorns) exhibited good activity concomitant with rather high selectivity to the formation of ketone alcohol (KA) oil (cyclohexanol and cyclohexanone mixture, yields up to 29 %) from microwave‐assisted oxidation of cyclohexane, and allowed their easy recovery and reuse, at least for five consecutive cycles maintaining 90.3 % of the initial activity. Moreover, the functionalized nanodiamond supports (used for the first time as supports for iron complexes) were also able to effectively (yields up to 97 %) catalyze the microwave‐induced oxidation of 1‐ and 2‐phenylethanol to acetophenone and 2‐phenylacetaldehyde, respectively, and could be reused for seven consecutive cycles without losing catalytic activity.

Nanostructured carbon materials were successfully synthesized on a tin thin film using a commercially available paper phenolic (PP) board, which is composed mainly of phenol formaldehyde (PF), as the carbon source. The heating source for thermal decomposition of PF is a current-stressed Cu trace which was patterned on the PP board near the tin thin film. The tin thin film was deposited on the PP board near the Cu trace using evaporation. During thermal treatment, nanostructured carbon materials were formed on the tin thin film. The synthesized products contain carbon nanosheets and tubular carbon nanofibers (CNFs) in which the former was formed near the heating Cu trace and the latter was found at a more distant location. A potential mechanism for the formation of nanostructured carbon materials is also discussed.

Quinoline (QN) is highly toxic and carcinogenic and has been detected in soil, groundwater, and biological tissues. Advanced oxidation processes (AOPs) have shown promise to address its degradation in wastewater treatment, with catalytic wet peroxide oxidation (CWPO) being highlighted due to its cost-effectiveness and mild operation. However, developing active and inexpensive catalysts is crucial for CWPO’s effectiveness. Another pressing issue is the accumulation of mixed, dirty plastic solid waste (PSW), particularly polyolefins used in packaging. Although recycling rates have increased, much plastic packaging remains in landfills. However, polyolefins can be converted into carbon-based nanostructured materials (CNMs), such as carbon nanotubes (CNTs), through chemical vapor deposition (CVD) using PSW as a carbon precursor. While many studies focus on CNT preparation, their application is often overlooked. In this context, this work proposes the preparation of CNMs, particularly CNTs, through CVD using a single-stage pyrolysis reactor. Polyolefins (LDPE, HDPE, and PP), both individually and in a mixture simulating PSW, were used as carbon sources. Given a sufficiently high temperature, the desired CNT architecture was successfully synthesized regardless of the starting polymer. These CNMs were then tested as catalysts for CWPO in simulated wastewater containing QN. The results showed a rapid degradation of QN (30–120 min) and high removals of total organic carbon (TOC) and aromatic compounds (75% and >90%, respectively), demonstrating the applicability of PSW-derived CNTs in the CWPO process for QN abatement.