Theory and Design
Milestones
M6.1. Calculated optimal irradiation conditions for induced magnetism in terms of energy projectiles and their energy
The implantations into HOPG were performed in the ranges of 2- 2.5 MeV for H+, 1.0–4.0MeV for Fe+; 0.4–2.4MeV for F+ and 25–190 keV for B+. Hydrogen and helium implantation was performed at 225 keV, and carbon implantation was performed at 70 keV. Figure 5 presents the depth distributions of vacancies in the implanted HOPG samples simulated with the computer code SRIM (J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Range of Ions in Solids, Pergamon, New York (1985)) Irradiation with light particles induces isolated point defects (vacancies and interstitials), but irradiation with heavy particles induces damage cascades. For the energy of 2 MeV, iron produces 4500 vacancies per ion with the Bragg peak at 1.2 um, carbon produces 190 vacancies per ion with the maximum at 0.15 um, and fluorine produces 900 vacancies per ion peaking at 1.64 um. It is not surprising that comparative analysis of H+ with F+ and Fe+ bombardment at similar energies and doses yield drastically different result: vacancy concentrations differ by four orders of magnitude. Iron is a heavy atom, and its interaction with HOPG results in discontinuous sequence of perturbed zones, in which the lattice is destroyed. Fluorine produces less damage, but the dose 11015 ion/cm2 is too large to induce the ferromagnetic effect. The protons with the energy 225 keV produce 10 times fewer vacancies than helium and 20 times less than carbon. Consequently, at low fluencies the He+ and C+ ions produce higher magnetization values than H+. Increase of irradiation dose destroys magnetism.
To judge the effectiveness of helium bombardment for inducing ferromagnetism, we used numerical calculations of the radiation defects with full damage cascades and compared these values with the concentration of localized spins estimated from the temperature dependencies of magnetic moment. Magnetic moment is estimated as 1.12–1.53 uB per vacancy defect and as 0.5 uB for carbon adatom defect. For 1×1015 and 2×1015 ion/cm2 irradiation doses the experimental values proved to be 1.5–2 times higher than the calculated ones. The surplus can be understood by recalling that the TRIM/SRIM code treats the irradiated sample as an amorphous structure and does not take into account intrinsic target defects at which the collision probability increases. At the dose of 5×1015 the effectiveness of spin generation decreases which is apparently due to subsequent kinetics of the defects with their reconstruction.
In the experiments on proton bombardment the dependence of induced magnetic moment versus irradiation dose shows a maximum after which magnetism quickly decays. We are not going to give a comprehensive explanation to the effect but restrict ourselves to the experiments with He+. The amorphization of HOPG occurs at about 51016 He+/cm2 whereas induced magnetism decays at a dose which is an order of magnitude less. The damaging process includes orientational disordering within the basal planes, i.e. fragmentation into small crystallites and rotation of their crystalline axes, change of stacking order and elongation of the interplanar spacing. Some information of these processes may be obtained from Raman spectra. Irradiation changes the in-plane size La of nanographites which is determined using the D/G ratio (relative intensities of the peaks marked D and G in Fig.7). As expected, the crystalline size gradually decreases with the increase of irradiation dose. This parameter reflects the concentration of in-plane defects, i.e. vacancies. We have observed a spatial dependence of the peaks intensity ratio which is in line with the AFM/MFM results. Comparing the spectra of magnetic and nonmagnetic samples, we noticed that the D/G ratio for the magnetic samples reaches 0.5 (La = 9 nm) whereas for nonmagnetic it reaches 0.7 (La = 6.5 nm). While the D/G ratio, i.e. the concentration of in-plane defects, is the same, we observe the difference between the out-of-plane defects through the changes in the second order spectra. The shape of the 2D band around 2700 cm-1 tells us that the “overbombarded” samples show true, strong disorder, i.e. loss of turbostratic stacking: 2700 cm-1 doublet coalesces into a single broad peak. This is an indication of strongly perturbed ABAB stacking with a significant number of c-axis translation faults. On the contrary, the spectrum of the magnetic He+ bombarded sample has a two-peak shape which is typical of the 2D band in three-dimensional graphite samples. This observation confirm the expectations that relative abundance of single and double vacancies plays a great role in graphite magnetism.
M 6.2. Energy and electronic structure of graphite fluorides obtained from DFT calculations
The fluorine pattern in the semifluorinated graphene is unknown and it is impossible to solve this task analysing the data obtained using traditional structural methods such as XRD and Raman spectroscopy. Here we unraveled the structure of C2F from the modelling of experimental near-edge X-ray absorption fine structure (NEXAFS) of graphite fluoride samples. To check how the spin distribution is dependent on the fragment size we increased the size preserving the same symmetry of the model. The spin distribution has a triangular-like shape as it was observed for the C80FH22 model and a ferromagnetic ordering mostly. It is surprising that probability of unpaired electron location is different for different part of the fragment. Probably, attachment of fluorine to graphene sheet will result in formation of magnetic domain with certain size and shape.
Fluorination of graphite at room temperature allows producing graphite fluoride compounds with a controlled content of fluorine. We combine solid-state NMR spectroscopy and DFT calculations to study the structure and reveal the fluorine patterning in graphite fluorides C2Fx intercalated with acetonitrile. Two major chemical states of carbon, namely the atoms covalently bound to fluorine and the bare atoms, are detected by C-13 MAS NMR irrespective of the degree of fluorination. The data indicate that although all graphene sheets were subjected to fluorination, the near-planar configuration is preserved. The interaction between host C2Fx matrix and acetonitrile molecules is of van der Waals character. Decomposition of the F-19 MAS NMR spectra reveals occurrence of at least six fluorine environments in each sample. By DFT calculations distinct F-19 chemical shifts are attributed to isolated, end chain, “linked” (which include midchain, cyclic, and branched) CF groups and infinite CF arrays. The assignment is confirmed by F-19 RFDR, which is sensitive to dipolar coupling. Analyzing the data for C2Fx samples with different degrees of fluorination x, an evolution of the fluorine pattern is proposed The reported calculated F-19 NMR shielding parameters provide classification criteria for assignment of F-19 NMR chemical shifts in fluorinated carbon materials.
To unravel the chemical bonding in semifluorinated graphite, we apply angle-resolved near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and quantum-chemical modeling. The strong angular dependence of the CK and FK edge NEWS spectra on the Incident radiation indicates that room-temperature-produced graphite fluoride is a highly anisotropic material, where half of the carbon atoms are covalently bonded with fluorTne, while the rest of the carbon atoms preserve pi electrons. Comparison of the experimental CK edge spectrum with theoretical spectra plotted for C2F models reveals that fluorine atoms are more likely to form chains. This conclusion agrees with the atomic force microscopy observation of a chain-like pattern on the surface of graphite fluoride layers.
Analysis of the angle-resolved NEXAFS data obviously indicates the covalent bonding between carbon and fluorine. In contrast to the high-temperature-produced semifluorinated graphite, in our material, the fluorine atoms are attached on both sides of the graphene sheet and the non fluorinated carbon atoms preserve their pi-electrons intact. On the basis of the X-ray spectroscopy data, three models of the fluorinated graphene were considered. The chains (armchair or zigzag) constituted from bare carbon atoms alternated with chains from fluorinated carbon atoms and the double C-C bonds alternated with CF-CF bonds. The theoretical NEXAFS spectra for the models were calculated at the B3LYP/6-31G level within the Zþ1 approach accounting for the hole electron interactions. Comparison between the theory and experiment shows that fluorine forms the most likely zigzag chains with an admixture of armchair chains. This finding is supported by the high-resolution AFM image of the fluorinated layer surface. Direction of the chains is determined by the hexagonal graphene lattice, while the chain length is kinetically controlled. The fluorine pattern and hence the size and shape of the pi-electron regions could be varied by changing the synthesis conditions.
M6.3. Estimates of the stability of carbon magnetism and suitability for spintronic applications
One of the main aspects in the development of magnetic carbon structures is resistance to ageing. Defect-based magnetism is innately unstable due to the ability of carbon to heal defects. Here we show that the second stage samples are resistant to ageing. Moreover, the mixed-stage samples improved their structure during 1 year storage in an inert atmosphere. Simultaneously, their magnetic properties changed drastically: whereas all mixed-stage samples are diamagnetic, the second stage samples demonstrate a non-Curie paramagnetic contribution to magnetic susceptibility, with a pronounced broad peak in the 150-250 K range.
Recent experimental reports have demonstrated unusual properties of fluorinated graphene that make it attractive for various applications and have included observations of tremendous differences between hydrogenated and fluorinated graphene. In contrast to the case of hydrogenation, where single adatoms, small clusters, or total hydrogenation has been obtained for free-standing graphene, fluorination provides formation of a variety of CFx structures for x ranging from 0.10 to 0.98. X-ray measurements evidence the formation of linearly patterned fluorinated areas along the graphene axis in semi-fluorinated graphene, but with no preferred axis. Further experiments have demonstrated that in semi-fluorinated graphene, about 20% of fluorine adatoms do not belong to any uniform fluorination pattern but instead form small clusters, pairs, and, for a few percent, single adatoms.
Theoretical calculations show that the chain-like arrangement is less preferential than the ribbon-like. This seem to contradict our experimental results which clearly demonstrate that in our samples the formation of chains proceeds. When the theory contradicts the experiments, there is no question in that regard. In our case thesequestions can be answered quite simply: we work with bilayer graphenes.