Porous Metal-Organic Polyhedra: Morphology, Porosity, and Guest Binding.

Designing porous materials which can selectively adsorb CO2 or CH4 is an important environmental and industrial goal which requires an understanding of the host-guest interactions involved at the atomic scale. Metal-organic polyhedra (MOPs) showing permanent porosity upon desolvation are rarely observed. We report a family of MOPs (Cu-1a, Cu-1b, Cu-2), which derive their permanent porosity from cavities between packed cages rather than from within the polyhedra. Thus, for Cu-1a, the void fraction outside the cages totals 56% with only 2% within. The relative stabilities of these MOP structures are rationalized by considering their weak nondirectional packing interactions using Hirshfeld surface analyses. The exceptional stability of Cu-1a enables a detailed structural investigation into the adsorption of CO2 and CH4 using in situ X-ray and neutron diffraction, coupled with DFT calculations. The primary binding sites for adsorbed CO2 and CH4 in Cu-1a are found to be the open metal sites and pockets defined by the faces of phenyl rings. More importantly, the structural analysis of a hydrated sample of Cu-1a reveals a strong hydrogen bond between the adsorbed CO2 molecule and the Cu(II)-bound water molecule, shedding light on previous empirical and theoretical observations that partial hydration of metal-organic framework (MOF) materials containing open metal sites increases their uptake of CO2. The results of the crystallographic study on MOP-gas binding have been rationalized using DFT calculations, yielding individual binding energies for the various pore environments of Cu-1a.

Toward Exotic Layered Materials: 2D Cuprous Iodide.

Heterostructures composed of 2D materials are already opening many new possibilities in such fields of technology as electronics and magnonics, but far more could be achieved if the number and diversity of 2D materials were increased. So far, only a few dozen 2D crystals have been extracted from materials that exhibit a layered phase in ambient conditions, omitting entirely the large number of layered materials that may exist at other temperatures and pressures. This work demonstrates how such structures can be stabilized in 2D van der Waals (vdw) stacks under room temperature via growing them directly in graphene encapsulation by using graphene oxide as the template material. Specifically, an ambient stable 2D structure of copper and iodine, a material that normally only occurs in layered form at elevated temperatures between 645 and 675 K, is produced. The results establish a simple route to the production of more exotic phases of materials that would otherwise be difficult or impossible to stabilize for experiments in ambient.

Beam-driven Dynamics of Aluminium Dopants in Graphene.

Substituting heteroatoms into graphene can tune its properties for applications ranging from catalysis to spintronics. The further recent discovery that covalent impurities in graphene can be manipulated at atomic precision using a focused electron beam may open avenues towards sub-nanometer device architectures. However, the preparation of clean samples with a high density of dopants is still very challenging. Here, we report vacancy-mediated substitution of aluminium into laser-cleaned graphene, and without removal from our ultra-high vacuum apparatus, study their dynamics under 60 keV electron irradiation using aberration-corrected scanning transmission electron microscopy and spectroscopy. Three- and four-coordinated Al sites are identified, showing excellent agreement with ab initio predictions including binding energies and electron energy-loss spectrum simulations. We show that the direct exchange of carbon and aluminium atoms predicted earlier occurs under electron irradiation, although unexpectedly it is less probable than the same process for silicon. We also observe a previously unknown nitrogen-aluminium exchange that occurs at Al─N double-dopant sites at graphene divacancies created by our plasma treatment.

Mechanism of Electron-Beam Manipulation of Single-Dopant Atoms in Silicon.

The precise positioning of dopant atoms within bulk crystal lattices could enable novel applications in areas including solid-state sensing and quantum computation. Established scanning probe techniques are capable tools for the manipulation of surface atoms, but at a disadvantage due to their need to bring a physical tip into contact with the sample. This has prompted interest in electron-beam techniques, followed by the first proof-of-principle experiment of bismuth dopant manipulation in crystalline silicon. Here, we use first-principles modeling to discover a novel indirect exchange mechanism that allows electron impacts to non-destructively move dopants with atomic precision within the silicon lattice. However, this mechanism only works for the two heaviest group V donors with split-vacancy configurations, Bi and Sb. We verify our model by directly imaging these configurations for Bi and by demonstrating that the promising nuclear spin qubit Sb can be manipulated using a focused electron beam.

Silicon Substitution in Nanotubes and Graphene via Intermittent Vacancies.

The chemical and electrical properties of single-walled carbon nanotubes (SWCNTs) and graphene can be modified by the presence of covalently bound impurities. Although this can be achieved by introducing chemical additives during synthesis, it often hinders growth and leads to limited crystallite size and quality. Here, through the simultaneous formation of vacancies with low-energy argon plasma and the thermal activation of adatom diffusion by laser irradiation, silicon impurities are incorporated into the lattice of both materials. After an exposure of ∼1 ion/nm2, we find Si-substitution densities of 0.15 nm-2 in graphene and 0.05 nm-2 in nanotubes, as revealed by atomically resolved scanning transmission electron microscopy. In good agreement with predictions of Ar irradiation effects in SWCNTs, we find Si incorporated in both mono- and divacancies, with ∼2/3 being of the first type. Controlled inclusion of impurities in the quasi-1D and -2D carbon lattices may prove useful for applications such as gas sensing, and a similar approach might also be used to substitute other elements with migration barriers lower than that of carbon.

Implanting Germanium into Graphene.

Incorporating heteroatoms into the graphene lattice may be used to tailor its electronic, mechanical and chemical properties, although directly observed substitutions have thus far been limited to incidental Si impurities and P, N and B dopants introduced using low-energy ion implantation. We present here the heaviest impurity to date, namely 74Ge+ ions implanted into monolayer graphene. Although sample contamination remains an issue, atomic resolution scanning transmission electron microscopy imaging and quantitative image simulations show that Ge can either directly substitute single atoms, bonding to three carbon neighbors in a buckled out-of-plane configuration, or occupy an in-plane position in a divacancy. First-principles molecular dynamics provides further atomistic insight into the implantation process, revealing a strong chemical effect that enables implantation below the graphene displacement threshold energy. Our results demonstrate that heavy atoms can be implanted into the graphene lattice, pointing a way toward advanced applications such as single-atom catalysis with graphene as the template.

Comparison of atomic scale dynamics for the middle and late transition metal nanocatalysts.

Catalysis of chemical reactions by nanosized clusters of transition metals holds the key to the provision of sustainable energy and materials. However, the atomistic behaviour of nanocatalysts still remains largely unknown due to uncertainties associated with the highly labile metal nanoclusters changing their structure during the reaction. In this study, we reveal and explore reactions of nm-sized clusters of 14 technologically important metals in carbon nano test tubes using time-series imaging by atomically-resolved transmission electron microscopy (TEM), employing the electron beam simultaneously as an imaging tool and stimulus of the reactions. Defect formation in nanotubes and growth of new structures promoted by metal nanoclusters enable the ranking of the different metals both in order of their bonding with carbon and their catalytic activity, showing significant variation across the Periodic Table of Elements. Metal nanoclusters exhibit complex dynamics shedding light on atomistic workings of nanocatalysts, with key features mirroring heterogeneous catalysis.

Stop-Frame Filming and Discovery of Reactions at the Single-Molecule Level by Transmission Electron Microscopy.

We report an approach, named chemTEM, to follow chemical transformations at the single-molecule level with the electron beam of a transmission electron microscope (TEM) applied as both a tunable source of energy and a sub-angstrom imaging probe. Deposited on graphene, disk-shaped perchlorocoronene molecules are precluded from intermolecular interactions. This allows monomolecular transformations to be studied at the single-molecule level in real time and reveals chlorine elimination and reactive aryne formation as a key initial stage of multistep reactions initiated by the 80 keV e-beam. Under the same conditions, perchlorocoronene confined within a nanotube cavity, where the molecules are situated in very close proximity to each other, enables imaging of intermolecular reactions, starting with the Diels-Alder cycloaddition of a generated aryne, followed by rearrangement of the angular adduct to a planar polyaromatic structure and the formation of a perchlorinated zigzag nanoribbon of graphene as the final product. ChemTEM enables the entire process of polycondensation, including the formation of metastable intermediates, to be captured in a one-shot "movie". A molecule with a similar size and shape but with a different chemical composition, octathio[8]circulene, under the same conditions undergoes another type of polycondensation via thiyl biradical generation and subsequent reaction leading to polythiophene nanoribbons with irregular edges incorporating bridging sulfur atoms. Graphene or carbon nanotubes supporting the individual molecules during chemTEM studies ensure that the elastic interactions of the molecules with the e-beam are the dominant forces that initiate and drive the reactions we image. Our ab initio DFT calculations explicitly incorporating the e-beam in the theoretical model correlate with the chemTEM observations and give a mechanism for direct control not only of the type of the reaction but also of the reaction rate. Selection of the appropriate e-beam energy and control of the dose rate in chemTEM enabled imaging of reactions on a time frame commensurate with TEM image capture rates, revealing atomistic mechanisms of previously unknown processes.

Electron beam controlled covalent attachment of small organic molecules to graphene.

The electron beam induced functionalization of graphene through the formation of covalent bonds between free radicals of polyaromatic molecules and C=C bonds of pristine graphene surface has been explored using first principles calculations and high-resolution transmission electron microscopy. We show that the energetically strongest attachment of the radicals occurs along the armchair direction in graphene to carbon atoms residing in different graphene sub-lattices. The radicals tend to assume vertical position on graphene substrate irrespective of direction of the bonding and the initial configuration. The "standing up" molecules, covalently anchored to graphene, exhibit two types of oscillatory motion--bending and twisting--caused by the presence of acoustic phonons in graphene and dispersion attraction to the substrate. The theoretically derived mechanisms are confirmed by near atomic resolution imaging of individual perchlorocoronene (C24Cl12) molecules on graphene. Our results facilitate the understanding of controlled functionalization of graphene employing electron irradiation as well as mechanisms of attachment of impurities via the processing of graphene nanoelectronic devices by electron beam lithography.

Epitaxial Retrieval of a Disappearing Polymorph.

Recrystallization of [PdCl2([9]aneS2O)] ([9]aneS2O = 1-oxa-4,7-dithiacyclononane), 1, and [PtCl2([9]aneS2O)], 2, by diffusion of Et2O vapor into solutions of the complexes in MeNO2 yielded three phases of 1 and two phases of 2. The known phase of 1, herein designated α-1, was obtained under ambient conditions. A second phase, designated ß-1, was initially also obtained by this method; however, following the advent of a third phase, γ-1, all subsequent efforts over a period of a year to crystallize ß-1 yielded either γ-1, obtained by carrying out the recrystallization at elevated temperature, or α-1, commonly found throughout the study. This persistent absence of a phase which could initially be crystallized with ease led us to the conclusion that ß-1 was an example of a "disappearing polymorph". The first phase obtained of 2, designated α-2, was obtained by recrystallization under ambient conditions and is isomorphous and isostructural with α-1. The second phase ß-2 was obtained by slight elevation of the recrystallization temperature and was found to be isomorphous and isostructural with ß-1. Subsequently, ß-2 was used to seed the growth of the disappearing polymorph ß-1. No third phase of 2 (γ-2) has been isolated thus far.

Electronic and electrochemical doping of graphene by surface adsorbates.

Many potential applications of graphene require its precise and controllable doping with charge carriers. Being a two-dimensional material graphene is extremely sensitive to surface adsorbates, so its electronic properties can be effectively modified by deposition of different atoms and molecules. In this paper, we review two mechanisms of graphene doping by surface adsorbates, namely electronic and electrochemical doping. Although, electronic doping has been extensively studied and discussed in the literature, much less attention has been paid to electrochemical doping. This mechanism can, however, explain the doping of graphene by adsorbates for which no charge transfer is expected within the electronic doping model. In addition, electrochemical doping is in the origin of the hysteresis effects often observed in graphene-based field effect transistors when operating in the atmospheric environment.