Direct Observation of Locally Modified Excitonic Effects within a Moiré Unit Cell in Twisted Bilayer Graphene.

Bilayer graphene, which forms moiré superlattices, possesses distinct electronic and optical properties owing to its hybridized energy band and the emergence of van Hove singularities depending on its twist angle. Extensive research has been conducted on the global characteristics of moiré superlattices induced by their long-range periodicity. However, the local properties, which differ owing to the variations in the three-dimensional atomic arrangement, within a moiré unit cell have been rarely explored. In this study, we demonstrate the highly localized excitation of carbon 1s electrons to unoccupied van Hove singularities in twisted bilayer graphene by electron energy loss spectroscopy using a monochromated transmission electron microscope. The core-level excitations associated with the van Hove singularities exhibit a systematic twist-angle dependence analogous to optical excitations. Furthermore, local variations in the core-level van Hove singularity peaks, which can originate from the core-exciton lifetimes and band modifications corresponding to the local stacking geometry within a moiré unit cell, are unambiguously corroborated.

Momentum-Dependent Oscillator Strength Crossover of Excitons and Plasmons in Two-Dimensional PtSe2.

The 1T-phase layered PtX2 chalcogenide has attracted widespread interest due to its thickness dependent metal-semiconductor transition driven by strong interlayer coupling. While the ground state properties of this paradigmatic material system have been widely explored, its fundamental excitation spectrum remains poorly understood. Here we combine first-principles calculations with momentum (q) resolved electron energy loss spectroscopy (q-EELS) to study the collective excitations in 1T-PtSe2 from the monolayer limit to the bulk. At finite momentum transfer, all the spectra are dominated by two distinct interband plasmons that disperse to higher energy with increasing q. Interestingly, the absence of long-range screening in the two-dimensional (2D) limit inhibits the formation of long wavelength plasmons. Consequently, in the small-q limit, excitations in monolayer PtSe2 are exclusively of excitonic nature, and the loss spectrum coincides with the optical spectrum. The qualitatively different momentum dependence of excitons and plasmons enables us to unambiguously disentangle their spectral fingerprints in the excited state spectrum of layered 1T-PtSe2. This will help to discern the charge carrier plasmon and locally map the optical conductivity and trace the layer-dependent semiconductor to metal transition in 1T-PtSe2 and other 2D materials.

Multiple 2D Phase Transformations in Monolayer Transition Metal Chalcogenides.

Phase transformation lies at the heart of materials science because it allows for the control of structural phases of solids with desired properties. It has long been a challenge to manipulate phase transformations in crystals at the nanoscale with designed interfaces and compositions. Here in situ electron microscopy is employed to fabricate novel 2D phases with different stoichiometries in monolayer MoS2 and MoSe2 . The multiphase transformations: MoS2  â†’ Mo4 S6 and MoSe2  â†’ Mo6 Se6 which are highly localized with atomically sharp boundaries are observed. Their atomic mechanisms are determined as chalcogen 2H ↔ 1T sliding, cation shift, and commensurate lattice reconstructions, resulting in decreasing direct bandgaps and even a semiconductor-metal transition. These results will be a paradigm for the manipulation of multiphase heterostructures with controlled compositions and sharp interfaces, which will guide the future phase engineered electronics and optoelectronics of metal chalcogenides.

Deciphering the Intense Postgap Absorptions of Monolayer Transition Metal Dichalcogenides.

Rich valleytronics and diverse defect-induced or interlayer pre-bandgap excitonics have been extensively studied in transition metal dichalcogenides (TMDCs), a system with fascinating optical physics. However, more intense high-energy absorption peaks (∼3 eV) above the bandgaps used to be long ignored and their underlying physical origin remains to be unveiled. Here, we employ momentum resolved electron energy loss spectroscopy to measure the dispersive behaviors of the valley excitons and intense higher-energy peaks at finite momenta. Combined with accurate Bethe-Salpeter equation calculations, non-band-nesting transitions at the Q valley and at the midpoint of KM are found to be responsible for the high-energy broad absorption peaks in tungsten dichalcogenides and present spin polarizations similar to A excitons, in contrast with the band-nesting mechanism in molybdenum dichalcogenides. Our experiment-theory joint research will offer insights into the physical origins and manipulation of the intense high-energy excitons in TMDC-based optoelectronic devices.

Unexpected Huge Dimerization Ratio in One-Dimensional Carbon Atomic Chains.

Peierls theory predicted atomic distortion in one-dimensional (1D) crystal due to its intrinsic instability in 1930. Free-standing carbon atomic chains created in situ in transmission electron microscope (TEM)1-3 are an ideal example to experimentally observe the dimerization behavior of carbon atomic chain within a finite length. We report here a surprisingly huge distortion found in the free-standing carbon atomic chains at 773 K, which is 10 times larger than the value expected in the system. Such an abnormally distorted phase only dominates at the elevated temperatures, while two distinct phases, distorted and undistorted, coexist at lower or ambient temperatures. Atom-by-atom spectroscopy indeed shows considerable variations in the carbon 1s spectra at each atomic site but commonly observes a slightly downshifted π* peak, which proves its sp1 bonding feature. These results suggest that the simple model, relaxed and straight, is not fully adequate to describe the realistic 1D structure, which is extremely sensitive to perturbations such as external force or boundary conditions.

Application of preparative disk gel electrophoresis for antigen purification from inclusion bodies.

Specific antibodies are a reliable tool to examine protein expression patterns and to determine the protein localizations within cells. Generally, recombinant proteins are used as antigens for specific antibody production. However, recombinant proteins from mammals and plants are often overexpressed as insoluble inclusion bodies in Escherichia coli. Solubilization of these inclusion bodies is desirable because soluble antigens are more suitable for injection into animals to be immunized. Furthermore, highly purified proteins are also required for specific antibody production. Plastidic acetyl-CoA carboxylase (ACCase: EC 6.4.1.2) from Arabidopsis thaliana, which catalyzes the formation of malonyl-CoA from acetyl-CoA in chloroplasts, formed inclusion bodies when the recombinant protein was overexpressed in E. coli. To obtain the purified protein to use as an antigen, we applied preparative disk gel electrophoresis for protein purification from inclusion bodies. This method is suitable for antigen preparation from inclusion bodies because the purified protein is recovered as a soluble fraction in electrode running buffer containing 0.1% sodium dodecyl sulfate that can be directly injected into immune animals, and it can be used for large-scale antigen preparation (several tens of milligrams).

Structural and Chemical Dynamics of Pyridinic-Nitrogen Defects in Graphene.

High density and controllable nitrogen doping in graphene is a critical issue to realize high performance graphene-based devices. In this paper, we demonstrate an efficient method to selectively produce graphitic-N and pyridinic-N defects in graphene by using the mixture plasma of ozone and nitrogen. The atomic structure, electronic structure, and dynamic behavior of these nitrogen defects are systematically studied at the atomic level by using a scanning transmission electron microscopy. The pyridinic-N exhibits higher chemical activity and tends to trap a series of transition metal atoms (Mg, Al, Ca, Ti, Cr, Mn, and Fe) as individual atoms.

Structure and local chemical properties of boron-terminated tetravacancies in hexagonal boron nitride.

Imaging and spectroscopy performed in a low-voltage scanning transmission electron microscope are used to characterize the structure and chemical properties of boron-terminated tetravacancies in hexagonal boron nitride. We confirm earlier theoretical predictions about the structure of these defects and identify new features in the electron energy-loss spectra of B atoms using high resolution chemical maps, highlighting differences between these areas and pristine sample regions. We correlate our experimental data with calculations which help explain our observations.

Stability and spectroscopy of single nitrogen dopants in graphene at elevated temperatures.

Single nitrogen (N) dopants in graphene are investigated using atomic-resolution scanning transmission electron microscopy (STEM) combined with electron energy loss spectroscopy (EELS). Using an in situ heating holder at 500 °C provided us with clean graphene surfaces, and we demonstrate that isolated N substitutional atoms remain localized and stable in the graphene lattice even during local sp(2) bond reconstruction. The high stability of isolated N dopants enabled us to acquire 2D EELS maps with simultaneous ADF-STEM images to map out the local bonding variations. We show that a substitutional N dopant causes changes in the EELS of the carbon (C) atoms it is directly bonded to. An upshift in the π* peak of the C K-edge EELS of ∼0.5 eV is resolved and supported by density functional theory simulations.

Single molecular spectroscopy: identification of individual fullerene molecules.

We report the molecule-by-molecule spectroscopy of individual fullerenes by means of electron spectroscopy based on scanning transmission electron microscopy. Electron energy-loss fine structure analysis of carbon 1s absorption spectra is used to discriminate carbon allotropes with known symmetries. C(60) and C(70) molecules randomly stored inside carbon nanotubes are successfully identified at a single-molecular basis. We show that a single molecule impurity is detectable, allowing the recognition of an unexpected contaminant molecule with a different symmetry. Molecules inside carbon nanotubes thus preserve their intact molecular symmetry. In contrast, molecules anchored at or sandwiched between atomic BN layers show spectral modifications possibly due to a largely degraded structural symmetry. Moreover, by comparing the spectrum from a single C(60) molecule and its molecular crystal, we find hints of the influence of solid-state effects on its electronic structure.

Atomic level spatial variations of energy states along graphene edges.

The local atomic bonding of carbon atoms around the edge of graphene is examined by aberration-corrected scanning transmission electron microscopy (STEM) combined with electron energy loss spectroscopy (EELS). High-resolution 2D maps of the EELS combined with atomic resolution annular dark field STEM images enables correlations between the carbon K-edge EELS and the atomic structure. We show that energy states of graphene edges vary across individual atoms along the edge according to their specific C-C bonding, as well as perpendicular to the edge. Unique spectroscopic peaks from the EELS are assigned to specific C atoms, which enables unambiguous spectroscopic fingerprint identification for the atomic structure of graphene edges with unprecedented detail.

Multiple reaction pathways of metallofullerenes investigated by transmission electron microscopy.

Recent advances in molecule-by-molecule transmission electron microscopy (TEM) have provided time-series structural information of individual molecules supported by nano-carbon materials, enabling researchers to trace their motions and reactions. In this paper, the chemical reactions of fullerenes and metallofullerene derivatives, focusing on their deformation process, are reviewed and discussed based on the single-molecule-resolved TEM analysis.

Conformational analysis of single perfluoroalkyl chains by single-molecule real-time transmission electron microscopic imaging.

Whereas a statistical average of molecular ensembles has been the conventional source of information on molecular structures, atomic resolution movies of single organic molecules obtained by single-molecule real-time transmission electron microscopy have recently emerged as a new tool to study the time evolution of the structures of individual molecules. The present work describes a proof-of-principle study of the determination of the conformation of each C-C bond in single perfluoroalkyl fullerene molecules encapsulated in a single-walled carbon nanotube (CNT) as well as those attached to the outer surface of a carbon nanohorn (CNH). Analysis of 82 individual molecules in CNTs under a 120 kV electron beam indicated that 6% of the CF2-CF2 bonds and about 20% of the CH2-CH2 bonds in the corresponding hydrocarbon analogue are in the gauche conformation. This comparison qualitatively matches the known conformational data based on time- and molecular-average as determined for ensembles. The transmission electron microscopy images also showed that the molecules entered the CNTs predominantly in one orientation. The molecules attached on a CNH surface moved more freely and exhibited more diverse conformation than those in a CNT, suggesting the potential applicability of this method for the determination of the dynamic shape of flexible molecules and of detailed conformations. We observed little sign of any decomposition of the specimen molecules, at least up to 10(7) e·nm(-2) (electrons/nm(2)) at 120 kV acceleration voltage. Decomposition of CNHs under irradiation with a 300 kV electron beam was suppressed by cooling to 77 K, suggesting that the decomposition is a chemical process. Several lines of evidence suggest that the graphitic substrate and the attached molecules are very cold.

Heterogeneous nucleation of organic crystals mediated by single-molecule templates.

Fundamental understanding of how crystals of organic molecules nucleate on a surface remains limited because of the difficulty of probing rare events at the molecular scale. Here we show that single-molecule templates on the surface of carbon nanohorns can nucleate the crystallization of two organic compounds from a supersaturated solution by mediating the formation of disordered and mobile molecular nanoclusters on the templates. Single-molecule real-time transmission electron microscopy indicates that each nanocluster consists of a maximum of approximately 15 molecules, that there are fewer nanoclusters than crystals in solution, and that in the absence of templates physisorption, but not crystal formation, occurs. Our findings suggest that template-induced heterogeneous nucleation mechanistically resembles two-step homogeneous nucleation.

Atomic imaging and spectroscopy of low-dimensional materials with interrupted periodicities.

Identification of individual atoms and examination of their electronic properties in materials are the ultimate goal of all microscopy-based analytical techniques. Here, we demonstrate successful single-atom imaging and spectroscopy in low-dimensional materials using (scanning) transmission electron microscopy together with electron energy-loss spectroscopy (EELS). Edges and point defects in single-layered materials such as graphene, hexagonal boron nitride and WS(2) nanoribbons are investigated by annular dark-field imaging and EELS fine-structure analysis. Individual dopant atoms are unambiguously identified in nano-peapods. It is noteworthy that irradiation damage and specimen contamination even at the single-atom level are crucial issues in these experiments.

Core-level spectroscopy of point defects in single layer h-BN.

Electron energy-loss spectroscopy (EELS) is used to analyze single-layered hexagonal boron-nitride with or without point defects. EELS profiles using a 0.1 nm probe clearly discriminate the chemical species of single atoms but show different delocalization of the boron and nitrogen K edges. A monovacancy at the boron site is unambiguously identified and the electronic state of its nearest neighboring nitrogen atoms is examined by energy-loss near edge fine structure analysis, which demonstrates a prominent defect state. Theoretical calculations suggest that the observed prepeak originates from the 1s to lowest unoccupied molecular orbital excitation of dangling nitrogen bonds, which is substantially lowered in energy with respect to the three coordinated nitrogen atoms.

Electron microscopic imaging of a single Group 8 metal atom catalyzing C-C bond reorganization of fullerenes.

Heating a bulk sample of [60]fullerene complexes, (η(5)-C(5)H(5))MC(60)R(5) (M = Fe, Ru, R = Me, Ph), produces small hydrocarbons because of coupling of R and C(5)H(5) via C-C and C-H bond activation. Upon observation by transmission electron microscopy, these complexes, encapsulated in single-walled carbon nanotubes, underwent C-C bond reorganization reactions to form new C-C bond networks, including a structure reminiscent of [70]fullerene. Quantitative comparison of the electron dose required to effect the C-C bond reorganization of fullerenes and organofullerenes in the presence of a single atom of Ru, Fe, or Ln and in the the absence of metal atoms indicated high catalytic activity of Ru and Fe atoms, as opposed to no catalytic activity of Ln. Organic molecules such as hydrocarbons and amides as well as pristine [60]fullerene maintain their structural integrity upon irradiation by ca. 100 times higher electron dose compared to the Ru and Fe organometallics. The results not only represent a rare example of direct observation of a single-metal catalysis but also have implications for the use of single metal atom catalysis in Group 8 metal heterogeneous catalysis.

Atomically resolved images of I(h) ice single crystals in the solid phase.

The morphology and crystal structure of nanoparticles of ice were examined by high-resolution transmission electron microscopy. Two different crystal structures were found and unambiguously assigned to hexagonal (I(h)) and cubic (I(c)) ice crystals. Direct observation of oxygen columns clearly revealed the hexagonal packing of water molecules. Electron energy-loss spectroscopy was used to monitor the electronic excitation in ice, suggesting possible dissociation of water molecules. Dynamic process of phase transition between I(h) and I(c) phases of individual ice nanoparticles under electron beam irradiation was also monitored by in situ transmission electron diffractometry.

Atom-by-atom spectroscopy at graphene edge.

The properties of many nanoscale devices are sensitive to local atomic configurations, and so elemental identification and electronic state analysis at the scale of individual atoms is becoming increasingly important. For example, graphene is regarded as a promising candidate for future devices, and the electronic properties of nanodevices constructed from this material are in large part governed by the edge structures. The atomic configurations at graphene boundaries have been investigated by transmission electron microscopy and scanning tunnelling microscopy, but the electronic properties of these edge states have not yet been determined with atomic resolution. Whereas simple elemental analysis at the level of single atoms can now be achieved by means of annular dark field imaging or electron energy-loss spectroscopy, obtaining fine-structure spectroscopic information about individual light atoms such as those of carbon has been hampered by a combination of extremely weak signals and specimen damage by the electron beam. Here we overcome these difficulties to demonstrate site-specific single-atom spectroscopy at a graphene boundary, enabling direct investigation of the electronic and bonding structures of the edge atoms-in particular, discrimination of single-, double- and triple-coordinated carbon atoms is achieved with atomic resolution. By demonstrating how rich chemical information can be obtained from single atoms through energy-loss near-edge fine-structure analysis, our results should open the way to exploring the local electronic structures of various nanodevices and individual molecules.

Analysis of the reactivity and selectivity of fullerene dimerization reactions at the atomic level.

High-resolution transmission electron microscopy has proved useful for its ability to provide time-resolved images of small molecules and their movements. One of the next challenges in this area is to visualize chemical reactions by monitoring time-dependent changes in the atomic positions of reacting molecules. Such images may provide information that is not available with other experimental methods. Here we report a study on bimolecular reactions of fullerene and metallofullerene molecules inside carbon nanotubes as a function of electron dose. Images of how the fullerenes move during the dimerization process reveal the specific orientations in which two molecules interact, as well as how bond reorganization occurs after their initial contact. Studies on the concentration, specimen temperature, effect of catalyst and accelerating voltage indicate that the reactions can be imaged under a variety of conditions.