Electronic properties of Mn decorated silicene on hexagonal boron nitride
T. P. Kaloni, S. Gangopadhyay, N. Singh, B. Jones, U. Schwingenschlögl
aa r X i v : . [ c ond - m a t . m e s - h a ll ] D ec Electronic properties of Mn decorated silicene on hexagonalboron nitride
T. P. Kaloni , S. Gangopadhyay , N. Singh , B. Jones , and U. Schwingenschl¨ogl , ∗ PSE Division, KAUST, Thuwal 23955-6900, Kingdom of Saudi Arabia and IBM Almaden Research Center, San Jose, California 95120-6099, USA
Abstract
We study silicene on hexagonal boron nitride, using first principles calculations. Since hexagonalboron nitride is semiconducting, the interaction with silicene is weaker than for metallic substrates.It therefore is possible to open a 50 meV band gap in the silicene. We further address the effect ofMn decoration by determining the onsite Hubbard interaction parameter, which turns out to differsignificantly for decoration at the top and hollow sites. The induced magnetism in the system isanalyzed in detail. ∗ Electronic address: [email protected],+966(0)544700080 sp and sp -type bonding results in a buckledstructure, which leads to an electrically tunable band gap [4, 5]. First-principles geometryoptimization and phonon calculations as well as temperature dependent molecular dynamicssimulations predict a stable low-buckled structure [6]. Moreover, stability of silicene underbiaxial tensile strain has been predicted up to 17% strain [7].Silicene and its derivatives experimentally have been grown on Ag and ZrB substrates[8–10], though there is still discussion about the quality of the results [11]. On a ZrB thinfilm an asymmetric buckling due to the interaction with the substrate is found, which opensa band gap. In general, transition metal decorated graphene has been studied extensivelyboth in experiment and theory. It has been predicted that 5 d transition metal atoms showunique properties with topological transport effects. The large spin orbit coupling of 5 d transition metal atoms together with substantial magnetic moments leads to a quantumanomalous Hall effect [12]. A model study also has predicted the quantum anomalous Halleffect for transition metal decorated silicene nanoribbons [13]. Energy arguments indicatethat transition metal atoms bond to silicene much stronger than to graphene. As a result,a layer by layer growth of transition metals could be possible on silicene [14]The deposition of isolated transition metal atoms on layers of hexagonal boron nitride on aRh(111) substrate has been studied in Ref. [15]. The authors have demonstrated a reversibleswitching between two states with controlled pinning and unpinning of the hexagonal boronnitride from the metal substrate. In the first state the interaction of the hexagonal boronnitride is reduced, which leads to a highly symmetric ring in scanning tunneling microscopyimages, while the second state is imaged as a conventional adatom and corresponds tonormal interaction. Motivated by this work, we present in the following a first-principlesstudy of the transition metal decoration of silicene on hexagonal boron nitride. We willfirst address the interaction with the substrate and then will deal with the electronic and2agnetic properties of the Mn decorated system,All calculations have been carried out using density functional theory in the generalizedgradient approximation. We employ the Quantum-ESPRESSO package [16], taking intoaccount the van-der-Waals interaction [17]. The calculations are performed with a planewave cutoff energy of 816 eV, where a Monkhorst-Pack 8 × × − eV and a force convergence of0.001 eV/˚A are reached. To study the interaction of the silicene with the substrate, weemploy a supercell consisting of a 2 × × h -BN, findingonly minor differences (in particular concerning the splitting and position of the Dirac cone)because of the inert nature of the substrate. A thin substrate consequently turns out to befully sufficient in the calculations. Moreover, the 2 × × d transition metal atoms is known to beseveral eV, we explicitly calculate the value in the present study for the different adsorptionsites in order to obtain accurate results for the electronic and magnetic properties of the Mndecorated system.In general, the lattice mismatch of 2.8% between silicene and hexagonal boron nitride canbe expected to be small enough to avoid experimental problems with a controlled growth.Moreover, accurate measurements of materials properties can be difficult to achieve on metal-lic substrates, whereas the interaction is reduced on semiconducting substrates. Our calcu-lated binding energy for the interface between silicene and hexagonal boron nitride is only100 meV per Si atom, as compared to typically 500 meV per Si atom for an interface toa metallic substrate. Experimental realizations of graphene based electronic devices usinghexagonal boron nitride as substrate on a Si wafer support are subject to various limita-tions, such as a poor on/off ratio [18]. However, on this substrate graphene exhibits thehighest mobility [19] and a sizable band gap [20–22]. Since silicene resembles the structureof graphene, synthesis on hexagonal boron nitride thus has great potential.The structural arrangement of the system under study is depicted in Fig. 1(b), together3ith the charge redistribution introduced by the interaction with the substrate. We obtainSi − Si bond lengths of 2.24 ˚A to 2.26 ˚A and a buckling of 0.48˚A to 0.54 ˚A, which is slightlyhigher than in free-standing silicene [3, 6]. For the angle between the Si − Si bonds and thenormal of the silicene sheet we observe values of 113 ◦ to 115 ◦ , again close to the findingsfor free-standing silicene (116 ◦ ). The optimized distance between the silicene and hexagonalboron nitride sheets forming the interface turns out to be 3.57 ˚A, which is similar to thedistance at the contact between graphene and hexagonal boron nitride. In addition, theinterlayer distance within the hexagonal boron nitride amounts to be 3.40 ˚A, whereas in abilayer configuration values of 3.30 ˚A to 3.33 ˚A have been reported [23, 24].The interaction between silicene and hexagonal boron nitride recently has been addressedby Liu and coworkers [25], who have reported a perturbation of the Dirac cone with an energygap of 4 meV. This study has taken into account only a single layer of hexagonal boron nitrideas substrate, so that a more realistic description may yield a different result. Indeed, weobserve a perturbed Dirac cone with an energy gap of 50 meV in the band structure shownin Fig. 1(a). The π and π ∗ bands forming the Dirac cone are due to the p z orbitals of the Siatoms, while the bands related to the B and N atoms are located about 0.5 eV above and1 eV below the Fermi energy. We find a small but finite charge redistribution across theinterface to the substrate; see the charge density difference isosurfaces plotted in Fig. 1(b).As a result the Dirac cone is perturbed and the 50 meV energy gap is realized, which can beinteresting for nanoelectronic device applications, in particular because an external electricfield can be used to tune the gap. The isosurface plot also demonstrates that the Si atomclosest to a B atom is subject to the strongest charge transfer, while for all other Si atomscharge transfer effects are subordinate due to longer interatomic distances.The possible decoration sites for a Mn atom on silicene can be classified as top, bridge,and hollow. Decoration at the bridge site is not considered in the following because theMn atom immediately transfers to the top site. A side view of the relaxed structure forMn decoration at the top site is given in Fig. 2(a), together with a spin density map. Weobtain the onsite interaction parameter using a constraint-GGA method [26], and calculatethe values of 3.8 eV for the Mn atom at the top site and 4.5 eV for the Mn atom at thehollow site. For the top site configuration, structural optimization reveals that the Mn atommoves close to an original Si position and thereby strongly displaces this Si atom, resultingin a short Mn − Si bond length of 2.43 ˚A. Moreover, the Mn atom is bound to three Si atoms4
K M Γ -1.5-1-0.500.511.5 E - E F ( e V ) K -0.050.05 (a) (b) FIG. 1: (a) Electronic band structure and (b) charge transfer for silicene on a bilayer of BN (sideview). The isosurfaces correspond to isovalues of ± × − electrons/˚A . The black, blue, andred spheres denote Si, N, and B atoms, respectively. Red and blue isosurfaces refer to positive andnegative charge transfer. Note the significant charge transfer of the Si closest to the BN. with equal bond lengths of 2.45 ˚A. Si − Si bond lengths of 2.24 ˚A to 2.28 ˚A are observed,which corresponds to a slight modification as compared to the pristine configuration. Thebuckling of the silicene, on the other hand, is strongly altered, now amounting to 0.45 ˚Ato 0.67 ˚A. Accordingly, angles of 113 ◦ to 117 ◦ are found between the Si − Si bonds and thenormal of the silicene sheet. The height of the Mn atom above the silicene sheet is 1.30˚A. Finally, we note that the separation between the atomic layers of the hexagonal boronnitride is virtually not modified by the Mn decoration.A side view of the relaxed structure for Mn decoration on the hollow site is shown inFig. 2(b). In this case, the Mn atom does not displace a specific Si atom but stays close tothe center of the Si hexagon. It is bound equally to the neighboring Si atoms with bondlengths of 2.40 ˚A to the upper three and 2.77 ˚A to the lower three Si atoms. A bucklingof 0.46 ˚A to 0.62 ˚A, Si − Si bond lengths of 2.23 ˚A to 2.28 ˚A, and angles to the normal of112 ◦ -117 ◦ are obtained. The Mn atom is located 1.01 ˚A above the silicene sheet and theseparation between the atomic layers in the hexagonal boron nitride is slightly increasedto 3.44 ˚A. In contrast, the distance between silicene and substrate here amounts to 3.55 ˚Aand thus is significantly larger than in the case of decoration at the top site, because in thelatter case one Si atom is displaced from the silicene sheet, which modifies the distance tothe substrate. The calculated total energies indicate that decoration at the hollow site is by33 meV favorable as compared to decoration at the top site.We find total magnetic moments of 4.56 µ B and 3.50 µ B per supercell for Mn decorationat the top and hollow sites, a reduction of spin from the free Mn value of 5.0 unpaired5 a)(b) FIG. 2: The spin density map for silicene decorated by Mn at the (a) top and (b) hollow site ofthe h -BN substrate. The hollow site is energetically favorable. electrons. The magnetization reduction is notable on the hollow site, which can be seenfrom Fig. 2 and 3 to involve greater immersion in and hybridization with the Si than thetop site. By far the largest contribution to the magnetic moment comes from the Mn atomand only small moments are induced on the Si atoms. This can be clearly seen in the spindensity maps presented in Figs. 3(a) and (b). For Mn decoration at the top site we obtain aMn moment of 4.40 µ B and a total of 0.16 µ B from all the Si atoms, whereas for decorationat the hollow site the Mn moment amounts to 4.16 µ B and the Si atoms contribute a total − . µ B . These results indicate that the Mn and Si moments are ordered ferromagneticallyand antiferromagnetically for decoration at the top and hollow sites, respectively.In Fig. 3 we address the density of states (DOS) for decoration at the (a) top and (b)hollow sites. The left panel of the figure shows the total DOS and the right panel the partialDOSs of the Mn 3 d and 4 s orbitals. In contrast to pristine silicene (band gap of 1.55 meV[3]), the DOSs show a region without states around 0.5 eV below the Fermi energy. Thisobservation corresponds to an n -doping due to the mentioned charge transfer from Mn tosilicene. Closer inspection of the partial DOSs for decoration at the top site shows thatthe spin majority s and d r − z as well as the spin minority d r − z , d x − y , and d xy statescontribute in the vicinity of the Fermi energy, while there are essentially no contributionsfrom the d zx and d zy states. A sharp Mn peak is obsvered about 0.8 eV, which is due to thespin minority d r − z states. For decoration at the hollow site almost exclusively the spinmajority s and spin minority d x − y and d xy states contribute around the Fermi energy. Twoless pronounced DOS peaks appear 0.75 eV below the Fermi energy, contributed by the spin6 DO S ( / e V ) -2-101 sd -z d zx d zy d x -y d xy -2 -1 0 1 2E-E F (eV)-10-50510 DO S ( / e V ) updn-2 -1 0 1 2E-E F (eV)-2-101 (a)(b) FIG. 3: Total (left) and Mn partial (right) densities of states of silicene decorated by Mn at the(a) top and (b) hollow site of the h -BN substrate. minority d xy and d x − y states.In conclusion, we have employed density functional theory to discuss the structure andchemical bonding of silicene on hexagonal boron nitride. The interaction results in a bandgap of 50 meV. Furthermore, we have calculated the onsite Hubbard interaction parameterfor Mn decoration at the top and hollow sites of the silicene, finding values of 3.8 eV and4.5 eV, respectively. The electronic and magnetic properties of Mn decorated silicene havebeen studied in detail. 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