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.