arXiv subjects

Jani Kotakoski

Publications and source records attributed to Jani Kotakoski.

At least 37 records · Page 2Linked to original sources

arXiv2022

Indirect measurement of the carbon adatom migration barrier on graphene

Although surface diffusion is critical for many physical and chemical processes, including the epitaxial growth of crystals and heterogeneous catalysis, it is particularly challenging to directly study. Here, we estimate the carbon adatom migration barrier on freestanding monolayer graphene by quantifying its temperature-dependent electron knock-on damage. Due to the fast healing of vacancies by diffusing adatoms, the damage rate decreases with increasing temperature. By analyzing the observed damage rates at 300-1073 K using a model describing our finite scanning probe, we find a barrier of (0.33 \pm 0.03) eV.

cond-mat.mtrl-sci↗

arXiv2021

Towards Exotic Layered Materials: 2D Cuprous Iodide

Heterostructures composed of two-dimensional (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 is 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 in other temperatures and pressures. Here, we demonstrate how these structures can be stabilized in 2D van der Waals stacks under room temperature via growing them directly in graphene encapsulation by using graphene oxide as the template material. Specifically, we produce 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. Our results establish a route to the production of more exotic phases of materials that would otherwise be difficult or impossible to stabilize for experiments in ambient.

cond-mat.mtrl-sci↗

arXiv2021

Three-dimensional description of vibration-assisted electron knock-on damage

Elastic knock-on is the main electron irradiation damage mechanism in metals including graphene. Atomic vibrations influence its cross-section, but only the out-of-plane direction has been considered so far in the literature. Here, we present a full three-dimensional theory of knock-on damage including the effect of temperature and vibrations to describe ejection into arbitrary directions. We thus establish a general quantitative description of electron irradiation effects through elastic scattering. Applying our methodology to in-plane jumps of pyridinic nitrogen atoms, we show their observed rates imply much stronger inelastic effects than in pristine graphene.

cond-mat.mtrl-sci↗

arXiv2021

Highly Efficient Bilateral Doping of Single-Walled Carbon Nanotubes

A boost in the development of flexible and wearable electronics facilitates the design of new materials to be applied as transparent conducting films (TCFs). Although single-walled carbon nanotube (SWCNT) films are the most promising candidates for flexible TCFs, they still do not meet optoelectronic requirements demanded their successful industrial integration. In this study, we proposed and thoroughly investigated a new approach that comprises simultaneous bilateral (outer and inner surfaces) SWCNT doping after their opening by thermal treatment at 400 C under an ambient air atmosphere. Doping by a chloroauric acid (HAuCl$_{4}$) ethanol solution allowed us to achieve the record value of sheet resistance of 31 $\pm$ 4 $Ω$/sq at a transmittance of 90% in the middle of visible spectra (550 nm). The strong p-doping was examined by open-circuit potential (OCP) measurements and confirmed by ab initio calculations demonstrating a downshift of Fermi level around 1 eV for the case of bilateral doping.

cond-mat.mtrl-sci↗

arXiv2020

Chemistry at graphene edges in the electron microscope

Transmission electron microscopy (TEM) and scanning TEM (STEM) are indispensable tools for materials characterization. However, during a typical (S)TEM experiment, the sample is subject to a number of effects that can change its atomic structure. Of these, perhaps the least discussed are chemical modifications due to the non-ideal vacuum around the sample. With single-layer graphene, we show that even at relatively low pressures typical for many instruments, these processes can have a significant impact on the sample structure. For example, pore growth becomes up to two orders of magnitude faster at a pressure of ca. 10^{-6} mbar as compared to ultra-high vacuum (UHV; 10^{-10} mbar). Even more remarkably,the presence of oxygen at the sample also changes the observed atomic structure: When imaged in UHV, nearly 90% of the identifiable graphene edge configurations have the armchair structure, whereas armchair and zigzag structures are nearly equally likely to occur when the oxygen partial pressure in the column is higher. Our results both bring attention to the role of the often neglected vacuum composition of the microscope column, and show that control over it can allow atomic-scale tailoring of the specimen structure.

cond-mat.mtrl-sci↗

arXiv2020

Single indium atoms and few-atom indium clusters anchored onto graphene via silicon heteroatoms

Single atoms and few-atom nanoclusters are of high interest in catalysis and plasmonics, but pathways for their fabrication and stable placement remain scarce. We report here the self-assembly of room-temperature-stable single indium (In) atoms and few-atom In clusters (2-6 atoms) that are anchored to substitutional silicon (Si) impurity atoms in suspended monolayer graphene membranes. Using atomically resolved scanning transmission electron microscopy (STEM), we find that the exact atomic arrangements of the In atoms depend strongly on the original coordination of the Si anchors in the graphene lattice: Single In atoms and In clusters with 3-fold symmetry readily form on 3-fold coordinated Si atoms, whereas 4-fold symmetric clusters are found attached to 4-fold coordinated Si atoms. All structures are produced by our fabrication route without the requirement for electron-beam induced materials modification. In turn, when activated by electron beam irradiation in the STEM, we observe in situ the formation, restructuring and translation dynamics of the Si-anchored In structures: Hexagon-centered 4-fold symmetric In clusters can (reversibly) transform into In chains or In dimers, whereas C-centered 3-fold symmetric In clusters can move along the zig-zag direction of the graphene lattice due to the migration of Si atoms during electron-beam irradiation, or transform to Si-anchored single In atoms. Our results provide a novel framework for the controlled self-assembly and heteroatomic anchoring of single atoms and few-atom clusters on graphene.

cond-mat.mtrl-sci↗

arXiv2020

CuAu, a hexagonal two-dimensional metal

Growth of two-dimensional metals has eluded materials scientists since the discovery of the atomically thin graphene and other covalently bound 2D materials. Here, we report a two-atom-thick hexagonal copper-gold alloy, grown through thermal evaporation on freestanding graphene and hexagonal boron nitride. The structures are imaged at atomic resolution with scanning transmission electron microscopy and further characterized with spectroscopic techniques. Electron irradiation in the microscope provides sufficient energy for a phase transformation of the 2D structure--atoms are released from their lattice sites with the gold atoms eventually forming face-centered cubic nanoclusters on top of 2D regions during observation. The presence of copper in the alloy enhances sticking of gold to the substrate, which has clear implications for creating atomically thin electrodes for applications utilizing 2D materials. Its practically infinite surface-to-bulk ratio also makes the 2D CuAu particularly interesting for catalysis applications.

cond-mat.mtrl-sci↗

arXiv2019

Quantifying transmission electron microscopy irradiation effects using two-dimensional materials

Important recent advances in transmission electron microscopy instrumentation and capabilities have made it indispensable for atomic-scale materials characterization. At the same time, the availability of two-dimensional materials has provided ideal samples where each atom or vacancy can be resolved. Recent studies have also revealed new possibilities for a different application of focused electron irradiation: the controlled manipulation of structures and even individual atoms. Evaluating the full range of future possibilities for this method requires a precise physical understanding of the interactions of electrons with energies as low as 15 keV now used in (scanning) transmission electron microscopy, becoming feasible due to advances both in experimental techniques and in theoretical models. We summarize the state of current knowledge of the underlying physical processes based on the latest results on two-dimensional materials, with a focus on the physical principles of the electron-matter interaction, rather than the material-specific irradiation-induced defects it causes.

cond-mat.mtrl-sci↗

arXiv2019

Influence of temperature on the displacement threshold energy in graphene

The atomic structure of nanomaterials is often studied using transmission electron microscopy. In addition to image formation, the energetic electrons may also cause damage while impinging on the sample. In a good conductor such as graphene the damage is limited to the knock-on process caused by elastic electron-nucleus collisions. This process is determined by the kinetic energy an atom needs to be sputtered, ie, its displacement threshold energy. This is typically assumed to have a fixed value for all electron impacts on equivalent atoms within a crystal. Here we show using density functional tight-binding simulations that the displacement threshold energy is affected by the thermal perturbation of the atoms from their equilibrium positions. We show that this can be accounted for in the estimation of the displacement cross section by replacing the constant threshold value with a distribution. The improved model better describes previous precision measurements of graphene knock-on damage, and should be considered also for other low-dimensional materials.

cond-mat.mtrl-sci↗

arXiv2019

Substitutional Si impurities in monolayer hexagonal boron nitride

We report the first observation of substitutional silicon atoms in single-layer hexagonal boron nitride (h-BN) using aberration corrected scanning transmission electron microscopy (STEM). The medium angle annular dark field (MAADF) images reveal silicon atoms exclusively filling boron vacancies. This structure is stable enough under electron beam for repeated imaging. Density functional theory (DFT) is used to study the energetics, structure and properties of the experimentally observed structure. The formation energies of all possible charge states of the different silicon substitutions (Si$_\mathrm{B}$, Si$_\mathrm{N}$ and Si$_\mathrm{BN}$) are calculated. The results reveal Si$_\mathrm{B}^{+1}$ as the most stable substitutional configuration. In this case, silicon atom elevates by 0.66Å out of the lattice with unoccupied defect levels in the electronic band gap above the Fermi level. The formation energy shows a slightly exothermic process. Our results unequivocally show that heteroatoms can be incorporated into the h-BN lattice opening way for applications ranging from single-atom catalysis to atomically precise magnetic structures.

cond-mat.mtrl-sci↗

arXiv2019

Perforating freestanding molybdenum disulfide monolayers with highly charged ions

Porous single layer molybdenum disulfide (MoS$_2$) is a promising material for applications such as DNA sequencing and water desalination. In this work, we introduce irradiation with highly charged ions (HCIs) as a new technique to fabricate well-defined pores in MoS$_2$. Surprisingly, we find a linear increase of the pore creation efficiency over a broad range of potential energies. Comparison to atomistic simulations reveals the critical role of energy deposition from the ion to the material through electronic excitation in the defect creation process, and suggests an enrichment in molybdenum in the vicinity of the pore edges at least for ions with low potential energies. Analysis of the irradiated samples with atomic resolution scanning transmission electron microscopy reveals a clear dependence of the pore size on the potential energy of the projectiles, establishing irradiation with highly charged ions as an effective method to create pores with narrow size distributions and radii between ca. 0.3 and 3 nm.

cond-mat.mtrl-sci↗

arXiv2019

Gas-phase synthesis of carbon nanotube-graphene heterostructures

Graphene and carbon nanotubes (CNTs) share the same atomic structure of hexagonal carbon lattice. Yet, their synthesis differs in many aspects, including the shape and size of the catalyst. Here, we demonstrate a floating-catalyst chemical vapor deposition (FCCVD) technique for substrate-free, single-step growth of CNT-graphene heterostructures (CNT-G-H) using ethylene as a carbon source. The formation of CNT-G-H is directly evidenced by lattice-resolved (scanning) transmission electron microscopy (STEM) and electron diffraction experiments, corroborated by atomic force microscopy (AFM). Our experiments show the relative number density of graphene-nanoflakes can be tuned by optimizing the synthesis conditions. Since in the applied process the formation of the structures take place in gas-suspension, the as-synthesized CNT-G-H films can be deposited on any surface in ambient temperature with an arbitrary thickness. Moreover, this process of CNT-G-H synthesis with strong universality has also been realized in multiple systems of ethylene-based FCCVD with various catalysts and set-ups.

physics.app-ph↗

arXiv2019

Enhanced Tunnelling in a Hybrid of Single-Walled Carbon Nanotubes and Graphene

Transparent and conductive films (TCFs) are of great technological importance. The high transmittance, electrical conductivity and mechanical strength make single-walled carbon nanotubes (SWCNTs) a good candidate for their raw material. Despite the ballistic transport in individual SWCNTs, however, the electrical conductivity of their networks is limited by low efficiency of charge tunneling between the tube elements. Here, we demonstrate that the nanotube network sheet resistance at high optical transmittance is decreased by more than 50% when fabricated on graphene and thus provides a comparable improvement as widely adopted gold chloride ($\mathrm{AuCl_3}$) doping. However, while Raman spectroscopy reveals substantial changes in spectral features of doped nanotubes, no similar effect is observed in presence of graphene. Instead, temperature dependent transport measurements indicate that graphene substrate reduces the tunneling barrier heights while its parallel conductivity contribution is almost negligible. Finally, we show that combining the graphene substrate and $\mathrm{AuCl_3}$ doping, the SWCNT thin films can exhibit sheet resistance as low as 36 $Ω$/sq. at 90% transmittance.

physics.app-ph↗

arXiv2019

Electron-Beam Manipulation of Silicon Impurities in Single-Walled Carbon Nanotubes

The recent discovery that impurity atoms in crystals can be manipulated with focused electron irradiation has opened novel perspectives for top-down atomic engineering. These achievements have been enabled by advances in electron optics and microscope stability, but also in the preparation of suitable materials with impurity elements incorporated via ion and electron-beam irradiation or chemical means. Here it is shown that silicon heteroatoms introduced via plasma irradiation into the lattice of single-walled carbon nanotubes (SWCNTs) can be manipulated using a focused 55-60 keV electron probe aimed at neighboring carbon sites. Moving the silicon atom mainly along the longitudinal axis of large 2.7 nm diameter tubes, more than 90 controlled lattice jumps were recorded and the relevant displacement cross sections estimated. Molecular dynamics simulations show that even in 2 nm SWCNTs the threshold energies for out-of-plane dynamics are different than in graphene, and depend on the orientation of the silicon-carbon bond with respect to the electron beam as well as the local bonding of the displaced carbon atom and its neighbors. Atomic-level engineering of SWCNTs where the electron wave functions are more strictly confined than in two-dimensional materials may enable the fabrication of tunable electronic resonators and other devices.

cond-mat.mtrl-sci↗

arXiv2019

Si Substitution in Nanotubes and Graphene via Intermittent Vacancies

The 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, that 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 $\sim$1 ion/nm$^{2}$, we find Si substitution densities of 0.15 nm$^{-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 $\sim$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.

cond-mat.mtrl-sci↗

arXiv2019

Scanning transmission electron microscopy under controlled low-pressure atmospheres

Transmission electron microscopy (TEM) is carried out in vacuum to minimize the interaction of the imaging electrons with gas molecules while passing through the microscope column. Nevertheless, in typical devices, the pressure remains at 10^-7 mbar or above, providing a large number of gas molecules for the electron beam to crack, which can lead to structural changes in the sample. Here, we describe experiments carried out in a modified scanning TEM (STEM) instrument, based on the Nion UltraSTEM 100. In this instrument, the base pressure at the sample is around 2x10^-10 mbar, and can be varied up to 10^-6 mbar through introduction of gases directly into the objective area while maintaining atomic resolution imaging conditions. We show that air leaked into the microscope column during the experiment is efficient in cleaning graphene samples from contamination, but ineffective in damaging the pristine lattice. Our experiments also show that exposure to O2 and H2O lead to a similar result, oxygen providing an etching effect nearly twice as efficient as water, presumably due to the two O atoms per molecule. H2 and N2 environments have no influence on etching. These results show that the residual gas environment in typical TEM instruments can have a large influence on the observations, and show that chemical etching of carbon-based structures can be effectively carried out with oxygen.

cond-mat.mes-hall↗

arXiv2018

Computational insights and the observation of SiC nanograin assembly: towards 2D silicon carbide

While an increasing number of two-dimensional (2D) materials, including graphene and silicene, have already been realized, others have only been predicted. An interesting example is the two-dimensional form of silicon carbide (2D-SiC). Here, we present an observation of atomically thin and hexagonally bonded nanosized grains of SiC assembling temporarily in graphene oxide pores during an atomic resolution scanning transmission electron microscopy experiment. Even though these small grains do not fully represent the bulk crystal, simulations indicate that their electronic structure already approaches that of 2D-SiC. This is predicted to be flat, but some doubts have remained regarding the preference of Si for sp$^{3}$ hybridization. Exploring a number of corrugated morphologies, we find completely flat 2D-SiC to have the lowest energy. We further compute its phonon dispersion, with a Raman-active transverse optical mode, and estimate the core level binding energies. Finally, we study the chemical reactivity of 2D-SiC, suggesting it is like silicene unstable against molecular absorption or interlayer linking. Nonetheless, it can form stable van der Waals-bonded bilayers with either graphene or hexagonal boron nitride, promising to further enrich the family of two-dimensional materials once bulk synthesis is achieved.

cond-mat.mtrl-sci↗

arXiv2018

Efficient first principles simulation of electron scattering factors for transmission electron microscopy

Electron microscopy is a powerful tool for studying the properties of materials down to their atomic structure. In many cases, the quantitative interpretation of images requires simulations based on atomistic structure models. These typically use the independent atom approximation that neglects bonding effects, which may, however, be measurable and of physical interest. Since all electrons and the nuclear cores contribute to the scattering potential, simulations that go beyond this approximation have relied on computationally highly demanding all-electron calculations. Here, we describe a new method to generate ab initio electrostatic potentials when describing the core electrons by projector functions. Combined with an interface to quantitative image simulations, this implementation enables an easy and fast means to model electron microscopy images. We compare simulated transmission electron microscopy images and diffraction patterns to experimental data, showing an accuracy equivalent to earlier all-electron calculations at a much lower computational cost.

cond-mat.mtrl-sci↗