Half-metallic Heusler alloy/MoS2 based magnetic tunnel junction

The paper was published in ACS Applied Materials & Interfaces.

In spite of considerable progress in the last decades, ultra-thin oxide (MgO and Al2O3) spacers serving as tunnel barriers provided no sufficient magnetoresistance (MR) value in vertical spin-valve. Recently, 2D materials displaying extremely large diversity of electronic and structural properties became under consideration as alternative spacer materials. Graphene and h-BN were considered as low-resistance barriers for a vertical spin-valve. Additionally to them, usage of the transition metal dichalcogenides (TMDs) can significantly expand the variety of electronic properties and tune the effectiveness of magnetic junctions. Introducing TMDs into magnetic tunnel junction can bring the advantage of adjustable electrical resistance of magnetic tunnel junction which is technically important from the viewpoint of application. Plenty of TMDs based junctions including MoS2 were studied both experimentally and theoretically and found MR values are in the wide range of 10^0–10^5 %.

Besides a proper choice of 2D spacers, a search of ideal spin-polarized electrons source is of high importance as well. For that purpose, half-metallic materials have been under consideration for decades including Heusler alloys such as Co2FeGe1/2Ga1/2, Co2MnSi, CoFeMnSi, Co2FeAl1/2Si1/2 and others.

In this work, novel magnetic tunnel junction based on Co2FeGe1/2Ga1/2 Heusler alloy electrodes and MoS2 spacer is proposed and theoretically studied as a promising element for spintronics devices. By DFT method electronic and magnetic properties of the MoS2/CFGG interface are explored both for the case of the FeGeGa‑ and Co‑termination of the CFGG surface. Robust ferromagnetism is demonstrated through the whole thickness of the CFGG film. Spin polarization is shown to be suppressed at the outermost few atomic layers of CFGG caused by interfacial interactions together with its quick recovery within four atomic layers (upward of 5 Å). Next, spin-dependent ballistic transport of CFGG/MoS2/CFGG MTJ is studied within the non-equilibrium green function formalism for MoS2 spacers varying from monolayer to four-layer films. In the zero-bias case, the MR values are found to be in the range of 10^4‑10^5 %. The I-V curves are derived as well demonstrating preservation of the large MR values under bias voltage. Together with recent advances in the graphene/CFGG heterostructure synthesis the current work supports further experimental and theoretical studies of half-metallic Heusler alloy based magnetic junctions possessing high effectiveness in spintronics applications.

Transport properties of CFGG/MoS2/CFGG MTJ with FeGeGa-termination. (a) Atomic structure of studied MTJ. (b) Zero-bias spin-resolved conductance for mono-, bi- and three-layer MoS2 spacer. Both parallel (left column) and antiparallel (right column) conductance is presented. (c) Г-centered k||-resolved zero-bias conductance at the Fermi level for parallel scheme with MoS2 monolayer. Majority- and minority-spin conductance are indicated as G↑↑ and G↓↓, respectively

Diamane Oxide. Two-dimensional Film with Mixed Coverage and Variety of Electronic Properties

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

The possibility of mild oxidation of sp2-hybridized carbon gives access to graphene oxide, one of the oldest and the most extensively studied graphene derivative. The relatively inexpensive and largely available GO is an attractive material for a variety of applications in sensing, energy storage, two-dimensional electronics and optoelectronics, photocatalysis and memristors etc.

Graphene oxide is a monolayer material which further development can be devoted with the study of the thicker structure such as bilayer graphene oxide. Hydrogenation or fluorination of bilayer graphene leads to the barrier-free bonding of the layers into a sp3-hybridized structure called diamane. This effect of chemically induced transition has been numerously confirmed in the experiment.

It is important to note that despite the successful synthesis of diamane by hydrogenation and fluorination processes, in the most papers binding of graphene is associated with deposition of oxide groups on its surface. In contrast to hydrogenated and fluorinated diamane, the structure of oxidized diamane has not been studied in detail yet. Only limited number of works exist where relatively simple models are proposed. This is an obstacle to further analysis and interpretation of the experimental data. The main problem is that graphene oxide (as well as diamane oxide) can be considered as two-dimensional solid solution of different functional groups statistically distributed on the graphene surface. This is probably valid for diamane oxide as well, so the description of its structure requires its consideration as a solid solution of different functional groups as we proposed for the graphene oxide.

In the presented paper we try to fill the gap and elucidate details of diamane oxide formation along with its properties. We study the idea that oxygen-containing groups are able to break the π-system and completely cover the outer surface of multilayer graphene changing the hybridization of carbon atoms from sp2 to sp3.

First, we found energetically favorable structures of diamanes with a full surface coverage by H, -OH or peroxide functional groups. Then we revealed thermodynamic stability range depending on the external pressure and chemical environment as denoted by precursor choice. In particularly, we found that commonly used source of oxygen, H2O, requires the pressure applying to form the stable oxidized diamane which is in a full accordance with experimental data (see the figure below).

The dependence of ΔG of diamane oxide [C16H4(OH)4]m film on pressure

Next, we studied the tuning of electronic properties in energetically favorable diamane films. We showed that depending on surface OH-groups concentration the band gap of diamane oxide can vary from 4.6 eV to 6.5 eV while the effective mass ranges from 1.1 m0 to 0.6 m0.

For two most representative films, namely H-diamane and OH-diamane, we studied how their electronic states are changed by the film thickness. We demonstrated that bilayer diamane behaves as uniform semiconductor while thicker films with more than 5 layers include surface and bulk-like regions with different conduction properties (see the figure below).

a) Band gap resolved from partial DOS in H-diamane (circles) and OH-diamane (rhombuses) depending on the inversed number of carbon layers. Band gap is calculated for surface carbon atoms (solid markers) and for inner carbon atoms in the middle of diamane (empty markers). Inset shows 5-layer H-diamane with inner carbon atoms indicated; Wave functions distribution for 2- and 9-layer H- (b) and OH- (c) diamanes are plotted for bottom conduction band with green color and top valence band and the with pink color

We believe that the present work will motivate experimental groups to discover novel approaches of ultrathin diamond films fabrication and their application.

The role of structural defects in the growth of two-dimensional diamond from graphene

The paper was published in Nanomaterials.

The synthesis of a 2D diamond is the most challenging field since unlike graphene and many other two-dimensional materials, diamane cannot be cleaved from the bulk. Moreover, a thermodynamic analysis shows that a few-layered diamond film without coverage layer is simply unstable and decomposes into multilayered graphene because the diamond surface energy is higher than the one of graphite. This conclusion is well supported by experiment where the direct pressure in diamond anvil cells was used to induce conversion of the whole graphene-flake while the diamondization pressure was much higher than in the bulk case, and instability of the formed diamondized film was apparent after pressure release.

The most promising way to obtain a two-dimensional diamond seems to be the use of graphene as a precursor, by deposition of reference atoms (e.g. hydrogen) on its surface. In this case the thermodynamic stability of the material is reversed, the previously unstable diamond film becomes energy favorable and graphene layers tend to bond to each other. Despite a number of encouraging experimental results confirming such predictions, the question of diamane synthesis is far from being resolved. Indeed, the nucleation of the diamane in graphene is hindered by the high stability of the graphene π-system resisting attachment of reference atoms. As a result, only two layers of graphene can be connected relatively easily, and only in the case of using hydrogen plasma as a hydrogen source. In the case of using H2, we can expect the appearance of a significant nucleation barrier which can be overcome only by high pressure and temperature.

However, the real structure of graphene contains structural defects that can be used as nucleation centers which may allow the synthesis of diamane under less severe conditions. Here we investigated such an effect in detail. For this purpose, we studied ones of the most common structural defects in graphene and reveal their impact on diamond nucleation. We found that type and concentration of structural defects can sufficiently impact the initial stages of diamond nucleation. At the same time, it does not influence further diamond formation. Defects impact on C-H bonding strength is being disappeared at the second coordination sphere already. We show that vacancies agglomeration (that can be produced by low energy ion irradiation) can sufficiently expand the reactive region which vanishes the nucleation barrier for the first stages of nucleation (see the figure below).

Average binding energy as a function of numbers of atoms in H cluster for AB bilayer graphene contained 1 (blue line), 2 (red line), 3 (yellow line) and 4 (purple line) cross layer vacancies. In the inset top view of graphene with 4 vacancies agglomerate is presented. H bonding energies in the H2 molecule (εH2) and in infinite diamane (εb(∞)) are marked by dashed and solid horizontal lines, respectively. The εb(n) dependence for hydrogen on the surface of defectless AB bilayer graphene is shown by black

Stone-Wales defects impact is lower but still promotes the hydrogenation and bonding of the graphene layers. We show that 1D defect (dislocation) not only facilitates the diamondization but also may lead to the appearance of 2D diamond consisting of chemically connected grains of different crystallographic orientations. Therefore, polycrystalline graphene usually observed in the experiment can produce specific 2D diamond polycrystals containing different surfaces. Even hexagonal and cubic 2D diamonds can coexist together in the same film with grain boundary energy comparable with the same values for other two-dimensional carbon structures (see the figure below).

(left) the atomic structure of polycrystalline bilayer graphene containing symmetrically inclined grains by θ = 11.5° with 5|7 defects highlighted by blue and red colors corresponding to first and second layer, respectively; (right) atomic structure of polycrystalline diamane produced by hydrogenation of graphene presented in left consisting of grains with cubic diamond and lonsdaleite structures.