Nanoindentation induced formation of diamond and/or lonsdaleite phases from multigraphene

The paper was published in JETP Letters.

In this paper we employed computer simulation to study how diamond forms in multilayer graphene under pressure induced by indentation. We utilized machine-learning potentials to describe the interactions between atoms in the system.

Our simulations revealed that the initially formed diamond nucleus originates near the indenter and does not penetrate the entire thickness of the graphene film. The structure of the diamond seed is highly sensitive to the stacking pattern of the graphene layers and the size of the indenter.

We examined two specific graphene stacking patterns: ABC and AA', which could potentially lead to diamond formation. We hypothesized that these stacking patterns would form spontaneously due to layer shifting during indentation.

For the ABC stacking pattern, a cubic diamond embryo with a threefold rotational axis surrounded by regions of hexagonal diamond (lonsdaleite) emerged first. As indentation proceeded, both cubic diamond and lonsdaleite regions grew in size, see Fig. (a).

a,b) atomic structure of multilayer graphene with ABC (a) and AA' (b) stacking with the formed diamond region caused by indentation. Left: a slice of the structure, side view. Right: a top view of the film, where red lines show the slice area. The atomic environment of the structure is analyzed, and atoms belonging to the cubic diamond structure are highlighted in blue, and hexagonal diamond atoms are highlighted in orange; c) phase transition pressure dependence of the number of layers in graphene for the case of an indenter of infinite size (plane).

The AA' stacking pattern exhibited different behavior. Initially, a layer of hexagonal diamond with mirror symmetry appeared, followed by the formation of cubic diamond regions around it, see Fig. (b). The thickness of the lonsdaleite layer remained constant as indentation pressure increased, while the fraction of cubic diamond increased significantly. At small indenter sizes, hexagonal diamond acted as a twinning defect precursor within the cubic diamond regions, ultimately transforming into the cubic diamond phase.

Another crucial finding was the substantially lower pressure required for the phase transition from AA' stacked multilayer graphene to lonsdaleite compared to ABC stacked multilayer graphene transforming into cubic diamond for thin films indented by a flat surface (representing the limit of an infinitely large indenter), see Fig. (c).

Using ab initio analysis, we demonstrated that this difference arises from the redistribution of electrons on the lonsdaleite surface, leading to a lower surface energy for hexagonal diamond compared to cubic diamond.

Currently, scientists possess techniques for measuring Raman spectra during the indentation process using an optically transparent indenter objective. The spectra of cubic and hexagonal diamond can be distinguished. Samples containing mixed lonsdaleite and diamond phases exhibit a shift in the broad main peak from 1332 cm-1 to lower frequencies (down to 1320 cm-1).

Therefore, the possibility of experimentally verifying our predictions exists, which we hope to achieve in the near future.

Diamondization of graphene under irradiation with a swift heavy ions

The paper was published in Carbon.

The formation of single crystal 2D nanodiamonds (diamane) within the graphene structure during irradiation is an intriguing question. This is particularly important considering the challenges associated with the destabilizing effect of surface effects on the 2D structure. Irradiating graphene with swift heavy ions at MeV energies shows promise in forming diamond due to the sharp increase in temperature (~3000 K) and the emergence of shock waves. This approach could enable the formation of two-dimensional diamond films with surfaces unaffected by graphitization, such as films with the (100) surface.

This approach opens new perspectives for obtaining ultrathin diamond films with unique electronic properties. Nanodiamonds produced by explosions and laser irradiation are typically a few nanometers in size and are known as detonation nanodiamonds. However, the stability of such nanodiamonds can be disrupted due to surface impact, which requires rigorous conditions during the synthesis process.

Interestingly, the use of multilayer graphene as an irradiation target opens up the possibility of creating ultrathin diamond films with unique electronic properties. For example, electron irradiation can locally bind graphene layers, creating a barrier for charge carriers in the irradiated region. This allows the electronic properties of the films to be effectively tuned.

In this study, we have explored the possibility of forming nanodiamonds in multilayer graphene through irradiation with swift heavy ions for the first time. The high surface fraction allows graphene to be locally transformed into diamond over the entire film thickness, which opens up the possibility of efficiently tuning its electronic properties.

In our study, we focused on examining the behavior of few-layer graphene films that were transferred onto specialized TEM grids as substrates under ion irradiation. The sketch of the sample preparation is shown in Figure (a). The procedure involved suspending the few-layer graphene locally and then subjecting it to irradiation for TEM observation. Upon ion irradiation with Xe ions of energies 26 and 167 MeV, the few-layer graphene films exhibited the formation of embedded nanostructures. High-resolution TEM images in Figure (b) clearly show that these nanostructures possess a regular diamond structure with lateral sizes ranging from a few to tens of nanometers. XRD, Raman and atomistic simulation evidence the presence the nanodiamonds in graphene too, see Figure (c).

a) A sketch of samples irradiation with high-energy Xe ions. b) High-resolution transmission electron microscopy (HRTEM) images of zoo of nanosized diamonds embedded in few-layer graphene films irradiated with high-energy Xe ions. The scale bar is 10 nm. с) Corresponding atomic models (top and perspective views). The sp2 and sp3 bonds are represented by black and blue sticks, respectively

Our simulations showed that in graphene films with less than six layers it is possible to form exceptionally diamond clusters with surface area (110), while four-layer (and thinner) films cannot stabilize the diamond structure (see figure below). The prediction of the formation of thinner diamond films awaits experimental confirmation, and we hypothesize that this contributes to achieving a more homogeneous distribution of two-dimensional nanodiamonds in future studies.

Side view of the relaxed atomic structure of a diamond cluster with surfaces (100) and (110) of different thicknesses. The sp2 and sp3 bonds are represented by black and blue sticks, respectively. The lateral size of the clusters is about 10 nm

Obtaining two-dimensional diamond is an intriguing result in itself, but the properties of the graphene-diamond hybrid structure are also quite interesting. The resulting 2D material, with mixed hybridization states, is anticipated to combine the advantages of each individual hybrid state and exhibit versatile physical properties. Traditional carbon-based composites, such as carbon-fiber-reinforced pyrolytic carbon, consist of sp2-hybridized carbon materials with diverse microstructures, ranging from disordered, poorly graphitic fragments to oriented, highly graphitized crystallites. These composites have found extensive applications in space aircraft, the automobile industry, and biomedical devices due to their high tensile strength. However, further enhancement of their mechanical performance becomes nearly impossible due to the weak van der Waals bonding within and between components. By introducing ultrastrong components that enable strong covalent bonding at the interfaces, the overall mechanical properties can be significantly improved.

The exceptional mechanical properties of the 2D composite are further demonstrated through uniaxial stress calculations. We conducted simulations on structures of experimental size, consisting of a diamane domain with a diameter of 5 nm surrounded by a 9-layer graphene matrix. Our estimates of the mechanical stiffness of the obtained composite (graphene/diamond) indicate that it is more brittle but at least as stiff as the original graphene.

However, such an elastic response is hardly obtainable in experiment. Usually the mechanical properties of 2D membranes are investigated by indentation which is actually local measurements of stiffness. The result of such a measurement depends on where and how it is performed and near the structural defects the local stiffness can significantly overwhelm the pristine value. Therefore, it is crucial to investigate the elastic response of different components within the composites during indentation. Our findings revealed that the local stiffness of the composite significantly exceeded the stiffness of graphene. Specifically, the (110) surface exhibited a Y value of 4.3 TPa, while the (100) surface demonstrated a higher Y value of 8.9 TPa. These values surpass the elastic modulus of graphene and, to the best of our knowledge, represent the highest recorded values for 2D films.

This study opens new perspectives for the creation of ultrathin diamond films using graphene and allows us to better understand the influence of structure and surface orientation on the properties of these materials. The ability to create diamond structures in graphene opens up a range of possibilities for generating the properties of ultrathin diamond films. These films have great potential in a multitude of fields ranging from electronics and optics to the field of biomedicine. The intrinsic stability and extraordinary properties of 2D nanodiamonds position them as promising candidates for future technological solutions. An ultra-stiff, ultra-strong, flexible and conductive 2D carbon composite consisting of graphene and diamond can be considered as a promising material for applications in space aviation, automotive and biomedical industries. Further research in this area could pave the way for the development of new materials and devices with improved performance and functionality.

Fluorination of Single-Wall Carbon Nanotubes: Toward “Diamond Nanoribbons”

The paper was published in J. Phys. Chem. C.

In a presented paper, we have identified a new type of one-dimensional nanomaterial called bilayer diamond-like nanoribbons. These nanoribbons can be synthesized by fluorinating single-wall carbon nanotubes, a process that involves attaching fluorine atoms to the nanotube structure. When zigzag or armchair nanotubes are functionalized with fluorine atoms, the carbon atoms change from sp2 to sp3 hybridization, causing the nanotube to collapse and form interlayer C-C bonding in a specific pattern. This process ultimately results in the formation of bilayer diamond nanoribbons, a new class of diamond-like ribbon and nanofiber structures.

Left: The process of collapsing a carbon nanotube into a bilayer diamond ribbon when exposed to fluorine. Right: unusual dependence of the band gap of a nanoribbon depending on its width and structure

The band gap of these nanoribbons is almost constant regardless of their width and depends on their morphology. However, nanoribbons containing an 8-membered ring exhibit special behavior, with electrons from the ring creating an additional band at the edge of the conduction band, which significantly reduces the band gap of the structure. This unique electronic property could make these nanoribbons useful in the development of advanced electronic devices and energy storage systems.

The mechanical stiffness of these nanoribbons is also high and comparable to that of graphene and carbon nanotubes. This robustness makes them ideal candidates for use in composites, where they could serve as a stiff and strong filler material. The high stiffness and unique electronic properties of bilayer diamond-like nanoribbons could also make them suitable for applications in nanomechanics and energy storage, such as in NEMS (Nanoelectromechanical Systems).

These findings could inspire further experimental and theoretical research into the formation of diamond-like nanoribbons from chiral carbon nanotubes. The discovery of bilayer diamond-like nanoribbons opens up a new world of possibilities for the development of advanced materials with unique electronic, mechanical, and optical properties. As researchers continue to explore and understand these novel nanomaterials, we can expect to see new and exciting applications in various fields, from electronics and energy storage to composites and nanomechanics.