Selective Imaging of Discrete Polyoxometalate Ions on Graphene Oxide under Variable Voltage Conditions

Atomic-number (Z)-correlated atomic sizes for deciphering electron microscopic molecular images

Atomic resolution transmission electron microscopy (TEM) has opened up a new era of molecular science by providing atomic video images of dynamic motions of single organic and inorganic molecules. However, the images often look different from the images of molecular models, because these models are designed to visualize the electronic properties of the molecule instead of nuclear electrostatic potentials that are felt by the e-beam in TEM imaging. Here, we propose a molecular model that reproduces TEM images using atomic radii correlated to atomic number (Z). The model serves to provide a priori a useful idea of how a single molecule, molecular assemblies, and thin crystals of organic or inorganic materials look in TEM.

With the advent of atomic resolution transmission electron microscopy (AR-TEM) achieving sub-Ångstrom image resolution and submillisecond time resolution, an era of cinematic molecular science where chemists can visually study the time evolution of molecular motions and reactions at atomistic precision has arrived. However, the appearance of experimental TEM images often differs greatly from that of conventional molecular models, and the images are difficult to decipher unless we know in advance the structure of the specimen molecules. The difference arises from the fundamental design of the molecular models that represent atomic connectivity and/or the electronic properties of molecules rather than the nuclear charge of atoms and electrostatic potentials that are felt by the e-beam in TEM imaging. We found a good correlation between the atomic number (Z) and the atomic size seen in TEM images when we consider shot noise in digital images. We propose Z-correlated (ZC) atomic radii for modeling AR-TEM images of single molecules and ultrathin crystals with which we can develop a good estimate of the molecular structure from the TEM image much more easily than with conventional molecular models. Two parameter sets were developed for TEM images recorded under high-noise (ZC_HN) and low-noise (ZC_LN) conditions. The molecular models will stimulate the imaginations of chemists planning to use AR-TEM for their research.

Single-molecule imaging and kinetic analysis of intermolecular polyoxometalate reactions

We induce and study reactions of polyoxometalate (POM) molecules, [PW12O40]3- (Keggin) and [P2W18O62]6- (Wells-Dawson), at the single-molecule level. Several identical carbon nanotubes aligned side by side within a bundle provided a platform for spatiotemporally resolved imaging of ca. 100 molecules encapsulated within the nanotubes by transmission electron microscopy (TEM). Due to the entrapment of POM molecules their proximity to one another is effectively controlled, limiting molecular motion in two dimensions but leaving the third dimension available for intermolecular reactions between pairs of neighbouring molecules. By coupling the information gained from high resolution structural and kinetics experiments via the variation of key imaging parameters in the TEM, we shed light on the reaction mechanism. The dissociation of W-O bonds, a key initial step of POM reactions, is revealed to be reversible by the kinetic analysis, followed by an irreversible bonding of POM molecules to their nearest neighbours, leading to a continuous tungsten oxide nanowire, which subsequently transforms into amorphous tungsten-rich clusters due to progressive loss of oxygen atoms. The overall intermolecular reaction can therefore be described as a step-wise reductive polycondensation of POM molecules, via an intermediate state of an oxide nanowire. Kinetic analysis enabled by controlled variation of the electron flux in TEM revealed the reaction to be highly flux-dependent, which leads to reaction rates too fast to follow under the standard TEM imaging conditions. Although this presents a challenge for traditional structural characterisation of POM molecules, we harness this effect by controlling the conditions around the molecules and tuning the imaging parameters in TEM, which combined with theoretical modelling and image simulation, can shed light on the atomistic mechanisms of the reactions of POMs. This approach, based on the direct space and real time chemical reaction analysis by TEM, adds a new method to the arsenal of single-molecule kinetics techniques.

In Situ Atomic-Level Studies of Gd Atom Release and Migration on Graphene from a Metallofullerene Precursor

We show how gadolinium (Gd)-based metallofullerene (Gd₃N@C₈₀) molecules can be used to create single adatoms and nanoclusters on a graphene surface. An in situ heating holder within an aberration-corrected scanning transmission electron microscope is used to track the adhesion of endohedral metallofullerenes (MFs) to the surface of graphene, followed by Gd metal ejection and diffusion across the surface. Heating to 900 °C is used to promote adatom migration and metal nanocluster formation, enabling direct imaging of the assembly of nanoclusters of Gd. We show that hydrogen can be used to reduce the temperature of MF fragmentation and metal ejection, enabling Gd nanocluster formation on graphene surfaces at temperatures as low as 300 °C. The process of MF fragmentation and metal ejection is captured in situ and reveals that after metal release, the C₈₀ cage opens further and fuses with the surface monolayer carbon glass on graphene, creating a highly stable carbon layer for further Gd adatom adhesion. Small voids and defects (∼1 nm) in the surface carbon glass act as trapping sites for Gd atoms, leading to atomic self-assembly of 2D monolayer Gd clusters. These results show that MFs can adhere to graphene surfaces at temperatures well above their bulk sublimation point, indicating that the surface bound MFs have strong adhesion to dangling bonds on graphene surfaces. The ability to create dispersed single Gd adatoms and Gd nanoclusters on graphene may have impact in spintronics and magnetism.

Electron microscopy of polyoxometalate ions on graphene by electrospray ion beam deposition

Aberration-corrected high-resolution transmission electron microscopy (AC-HRTEM) has enabled atomically resolved imaging of molecules adsorbed on low-dimensional materials like carbon nanotubes, graphene oxide and few-layer-graphene. However, conventional methods for depositing molecules onto such supports lack selectivity and specificity. Here, we describe the chemically selective preparation and deposition of molecules-like polyoxometalate (POM) anions [PW₁₂O₄₀]^3- using electrospray ion-beam deposition (ES-IBD) along with high-resolution TEM imaging. This approach provides access to sub-monolayer coatings of intact molecules on freestanding graphene, which enables their atomically resolved ex situ characterization by low-voltage AC-HRTEM. The capability to tune the deposition parameters in either soft or reactive landing mode, combined with the well-defined high-vacuum deposition conditions, renders the ES-IBD based method advantageous over alternative methods such as drop-casting. Furthermore, it might be expanded towards depositing and imaging large and nonvolatile molecules with complex structures.

Keggin-type polyoxometalate nanosheets: synthesis and characterization via scanning transmission electron microscopy

Polyoxometalate nanosheets were synthesized and their structure was elucidated at the atomic and molecular levels.