Graphene-based Spintronic Devices Research Articles

Spin polarization properties of benzene/graphene with transition metals as dopants: First principles calculations

First principles calculations are used to study the spin polarization properties of benzene molecule adsorbed on the graphene surface which doped with transition metals including Mn, Cr, Fe, Co, and Ni. The densities of states (DOS) of the benzene molecule can be induced to be spin split at the Fermi level only when it is adsorbed on Mn-, and Cr-doped graphene. The p-orbital of the benzene molecule will interact with the d orbital of the doped atoms, which will generate new spin coupling states and lead to obvious spin polarization of the benzene molecule. The spin-polarized density distributions as well as the differential charge density distributions of the systems also suggest that Mn-doped graphene will induce bigger spin polarization than that of Cr-doped graphene. Benzene molecule could be spin-polarized when it is adsorbed on the graphene surface with transition metal dopants, which could be a new method for researching graphene-based organic spintronic devices.

Voltage-dependent spin flip in magnetically substituted graphene nanoribbons: Towards the realization of graphene-based spintronic devices

We examine the possibility of using graphene nanoribbons (GNRs) with directly substituted chromium atoms as spintronic device. Using density functional theory, we simulate a voltage bias across a constructed GNR in a device setup, where a magnetic dimer has been substituted into the lattice. Through this first principles approach, we calculate the electronic and magnetic properties as a function of Hubbard U, voltage, and magnetic configuration. By calculating of the total energy of each magnetic configuration, we determine that initial antiferromagnetic ground state flips to a ferromagnetic state with applied bias. Mapping this transition point to the calculated conductance for the system reveals that there is a distinct change in conductance through the GNR, which indicates the possibility of a spin valve. We also show that this corresponds to a distinct change in the induced magnetization within the graphene.

Theoretical perspective on structural, electronic and magnetic properties of 3d metal tetraoxide clusters embedded into single and di-vacancy graphene

Structural, electronic and magnetic properties of 3d transition metal tetraoxide TMO4 superhalogen clusters doped single vacancy (SV) and divacancy (DV) monolayer graphene have been studied using first-principles calculations. We found that in both cases of TMO4 cluster substitution, all the impurity atoms are tightly bonded with graphene, having significant formation energy and large charge transfer occurs from graphene to TMO4 clusters. CrO4 and MnO4 substituted SV graphene structures exhibit dilute magnetic semiconductor behavior in their spin down channel with 2.15μB and 3.51μB magnetic moment, respectively. However, CoO4, FeO4, TiO4 and NiO4 substitution into SV graphene, leads to Fermi level shifting to conduction band, thereby causing the Dirac cone to move into valence band and a band gap appears at high symmetric K-point. Interestingly, CoO4, CrO4, FeO4 and MnO4 substituted DV graphene structures exhibit dilute magnetic semiconductor behavior in their spin up channel with 1.74μB, 3.27μB, 3.09μB and 1.99μB magnetic moment, respectively. Detailed analysis of density of states (DOS) plots show that d orbitals of 3d TM atoms should be responsible for inducing magnetic moments in graphene. We believe that our results are appropriate for experimental exploration and graphene-based spintronic and magnetic storage devices.

Spin polarization properties of thiophene molecule adsorbed to the edge of zigzag graphene nanoribbon

Spin polarization properties of thiophene molecule adsorbed to the edge of zigzag graphene nanoribbon (GNR) are studied based on density functional theory calculations. It is shown that there is charge transfer between the thiophene molecule and the zigzag GNR, which changes the bond lengths of the thiophene molecule. Another consequence of the charge transfer is the spontaneous spin polarization of the thiophene molecule. The mechanism of the spontaneous spin polarization is also discussed based on the analysis of the density of states. It is found that the p orbitals of the carbon atoms and the sulfur atom in thiophene molecule become spin split near the Fermi energy, so preferential fillings of the spin-splitted p orbitals make the thiophene molecule spin polarized. This spontaneous spin polarization of thiophene molecules adsorbed to the zigzag GNR could be a new route to graphene-based organic spintronic devices.

Proximity-Induced Spin Polarization of Graphene in Contact with Half-Metallic Manganite.

The role of proximity contact with magnetic oxides is of particular interest from the expectations of the induced spin polarization and weak interactions at the graphene/magnetic oxide interfaces, which would allow us to achieve efficient spin-polarized injection in graphene-based spintronic devices. A combined approach of topmost-surface-sensitive spectroscopy utilizing spin-polarized metastable He atoms and ab initio calculations provides us direct evidence for the magnetic proximity effect in the junctions of single-layer graphene and half-metallic manganite La0.7Sr0.3MnO3 (LSMO). It is successfully demonstrated that in the graphene/LSMO junctions a sizable spin polarization is induced at the Fermi level of graphene in parallel to the spin polarization direction of LSMO without giving rise to a significant modification in the π band structure.

Adsorption sensitivity of metal atom decorated bilayer graphene toward toxic gas molecules (CO, NO, SO2 and HCN)

Based on the first-principles calculations, the sensing performances of Fe embedded graphene sheets (including monolayer Fe-MG and bilayer Fe-BG) toward toxic gases (NO, CO, HCN and SO2) are comparably investigated. Compared with the Fe-MG, the stable configuration of Fe-BG sheet exhibits the stronger affinity toward the gas molecules. The adsorbed NO has the largest energy difference between Fe-MG and Fe-BG substrate as compared with the other gases, as well as inducing the change in electronic structure and magnetic property of Fe-graphene systems. In addition, the supported Pt(111) substrate can effectively regulate the strength of interaction between gas molecule and Fe-graphene substrates. As a result, the increased layer of graphene substrate can be utilizing as good sensor for toxic gas molecules, yet the metal Pt supported substrate can enhance the magnetic property of adsorbed gas on the Fe-graphene systems. These results could provide important information for controlling the adsorption sensoring of gas molecules, which opens up a new avenue for the design and fabrication of the graphene-based gas sensors or spintronic devices.

Optical signatures of electric-field-driven magnetic phase transitions in graphene quantum dots

Experimental challenges in identifying various types of magnetic ordering in graphene quantum dots (QDs) pose a major hurdle in the application of these nanostructures for spintronic devices. Based upon phase diagrams obtained by employing the $\ensuremath{\pi}$-electron Pariser-Parr-Pople (PPP) model Hamiltonian, we demonstrate that the magnetic states undergo phase transition under the influence of an external electric field. Our calculations of the electroabsorption spectra of these QDs indicate that the spectrum in question carries strong signatures of their magnetic state (FM vs AFM), thus suggesting the possibility of an all-optical characterization of their magnetic nature. Further, the gaps for the up and the down spins are the same in the absence of an external electric field, both for the antiferromagnetic (AFM) and the ferromagnetic (FM) states of QDs. But, once the QDs are exposed to a suitably directed external electric field, gaps for different spins split and exhibit distinct variations with respect to the strength of the field. The nature of variation exhibited by the energy gaps corresponding to the up and down spins is different for the AFM and FM configurations of QDs. This selective manipulation of the spin-polarized gap splitting by an electric field in finite graphene nanostructures can open up new frontiers in the design of graphene-based spintronic devices.

Tunable magnetic moment and potential half-metal behavior of Fe-nanostructure-embedded graphene perforation

Doping with transition metal elements is an effective method to introduce magnetism in graphene, which could enable future graphene-based spintronic devices. Motivated by the recent experimental observation of a stable single layer iron membrane embedded in graphene perforation, we investigate the electronic and magnetic properties of the Fe-nanostructure-embedded graphene system based on first principles calculations. The results demonstrate that strain could lead to dramatic changes in the magnetic configurations for both small Fe clusters bonded to the edge carbon atoms of graphene perforation and the single layer Fe membrane fully embedded in the graphene layer. For optimal doping, a delicate balance can be achieved, which leads to a half-metallic electronic structure. This work suggests an easy and effective method to introduce and tune the magnetic properties of graphene, which offers a new direction for the development of future graphene-based spintronic devices.

Carbon Tetragons as Definitive Spin Switches in Narrow Zigzag Graphene Nanoribbons.

Precise spatial control of the spin propagation channels is of fundamental and practical importance in future graphene-based spintronic devices. Here we use first-principles calculations to show that when narrow zigzag graphene nanoribbons are connected to form junctions or superlattices, properly placed square-shaped carbon tetragons not only serve as effective bundles of the two incoming spin edge channels, but also act as definitive topological spin switches for the two outgoing channels. The nanoribbon segments are largely drawn from different acene molecules. We further show that such spin switches can lift the degeneracy between the two spin propagation channels, which enables tunability of different magnetic states upon charge doping. Preliminary experimental supports for the realization of such tetragons connecting nanoribbon segments are also presented.

B21-P-07Elucidating structure and thermal reaction of low-dimeansional Bi growth on monolayer epitaxial graphene

The doping graphene with heavy metals can modify the number of carriers and spin-orbit coupling (SOC) can be exploited for novel electronic and spintronic applications. 1-2 This study elucidates the epitaxial growth structure and electronic properties of Bi on monolayer epitaxial graphene (MEG) surface by using scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). Bi adatoms undergo a structural transition from linear structures at lower Bi coverage to two-dimensional (2D) triangular islands at thicker Bi coverage, which is affected by the corrugated substrate. Upon Bi deposition, a little charge transfer occurs and a characteristic peak can be observed in the tunneling spectrum, reflecting the distinctive electronic structure of the Bi adatoms. After annealing to ∼500K, 2D triangular Bi islands aggregate into Bi nanoclusters (NCs) of uniform size. A well-controlled fabrication method is thus demonstrated. The approaches adopted herein provide perspectives for fabricating and characterizing periodic networks on MEG and related systems, which are useful in realizing graphene-based electronic, energy, sensor and spintronic devices.

Transport properties of zigzag graphene nanoribbons adsorbed with single iron atom**Project supported by the National Natural Science Foundation of China (Grant Nos. 11374162 and 51032002) and the Key Project of the National High Technology Research and Development Program of China (Grant No. 2011AA050526).

We have performed density-functional calculations of the transport properties of the zigzag graphene nanoribbon (ZGNR) adsorbed with a single iron atom. Two adsorption configurations are considered, i.e., iron adsorbed on the edge and on the interior of the nanoribbon. The results show that the transport features of the two configurations are similar. However, the transport properties are modified due to the scattering effects induced by coupling of the ZGNR band states to the localized 3d-orbital state of the iron atom. More importantly, one can find that several dips appear in the transmission curve, which is closely related to the above mentioned coupling. We expect that our results will have potential applications in graphene-based spintronic devices.

Role of edge dehydrogenation in magnetization and spin transport of zigzag graphene nanoribbons with line defects

We investigate the effects of edge dehydrogenation on magnetism and spin transport of zigzag graphene nanoribbons (ZGNRs) with line defects (558defect and 57defect) by the first-principles calculations. Results show that magnetization can be induced or strengthened obviously in 558defect-ZGNRs unterminated by hydrogen, but not for 57defect-ZGNRs. This is because a spin-polarized σ edge state appears near the Fermi level and strengthens spin-splitting of energy bands at bare edges of the 558defect-ZGNRs. Moreover, compared with pristine ZGNRs, the 558defect-ZGNRs with bare edges can realize a transition from antiferromagnetic coupling to ferromagnetic coupling between both edges. In addition, the spin-filter efficiency can be effectively improved in our proposed devices by edge dehydrogenation. Our results demonstrate that the presence of σ edge state near the Fermi level plays an important role in controlling spin transport of the graphene-based spintronic devices.

Electrically tunable resonant scattering in fluorinated bilayer graphene

Adatom-decorated graphene offers a promising new path towards spintronics in the ultrathin limit. We combine experiment and theory to investigate the electronic properties of dilutely fluorinated bilayer graphene, where the fluorine adatoms covalently bond to the top graphene layer. We show that fluorine adatoms give rise to resonant impurity states near the charge neutrality point of the bilayer, leading to strong scattering of charge carriers and hopping conduction inside a field-induced band gap. Remarkably, the application of an electric field across the layers is shown to tune the resonant scattering amplitude from fluorine adatoms by nearly twofold. The experimental observations are well explained by a theoretical analysis combining Boltzmann transport equations and fully quantum-mechanical methods. This paradigm can be generalized to many bilayer graphene-adatom materials, and we envision that the realization of electrically tunable resonance may be a key advantage in graphene-based spintronic devices.

A first principles approach to magnetic and optical properties in single-layer graphene sandwiched between boron nitride monolayers

The dependence of the stability of single-layer graphene (SLG) sandwiched between hexagonal boron nitride bilayers (h-BN) has been described and investigated for different types of stacking in order to provide the fingerprint of the stacking order which affects the optical properties of such trilayer systems. Considering the four stacking models AAA-, AAB-, ABA-, and ABC-type stacking, the static dielectric functions (in case of parallel polarizations) for AAB-type stacking possesses maximum values, and minimum values are noticed for AAA. However, AAA-type stacking structures contribute the maximum magnetic moment while vanishing magnetic moments are observed for ABA and ABC stacking. The observed optical anisotropy and magnetic properties of these trilayer heterostructures (h-BN/SLG/h-BN) can be understood from the crystallographic stacking order and inherent crystal lattice symmetry. These optical and magnetic results suggest that the h-BN/SLG/h-BN could provide a viable route to graphene-based opto-electronic and spintronic devices.

Tailoring low-dimensional structures of bismuth on monolayer epitaxial graphene.

To improve graphene-based multifunctional devices at nanoscale, a stepwise and controllable fabrication procedure must be elucidated. Here, a series of structural transition of bismuth (Bi) adatoms, adsorbed on monolayer epitaxial graphene (MEG), is explored at room temperature. Bi adatoms undergo a structural transition from one-dimensional (1D) linear structures to two-dimensional (2D) triangular islands and such 2D growth mode is affected by the corrugated substrate. Upon Bi deposition, a little charge transfer occurs and a characteristic peak can be observed in the tunneling spectrum, reflecting the distinctive electronic structure of the Bi adatoms. When annealed to ~500 K, 2D triangular Bi islands aggregate into Bi nanoclusters (NCs) of uniform size. A well-controlled fabrication method is thus demonstrated. The approaches adopted herein provide perspectives for fabricating and characterizing periodic networks on MEG and related systems, which are useful in realizing graphene-based electronic, energy, sensor and spintronic devices.

Contracted interlayer distance in graphene/sapphire heterostructure

Direct growth of graphene on insulators is expected to yield significant improvements in performance of graphene-based electronic and spintronic devices. In this study, we successfully reveal the atomic arrangement and electronic properties of a coherent heterostructure of single-layer graphene and α-Al2O3(0001). The analysis of the atomic arrangement of single-layer graphene on α-Al2O3(0001) revealed an apparentcontradiction. The in-plane analysis shows that single-layer graphene grows not in a single-crystalline epitaxial manner, but rather in polycrystalline form, with two strongly pronounced preferred orientations. This suggests relatively weak interfacial interactions are operative. However, we demonstrate that unusually strong physical interactions between graphene and α-Al2O3(0001) exist, as evidenced by the small separation between the graphene and the α-Al2O3(0001) surface. The interfacial interaction is shown to be dominated by the electrostatic forces involved in the graphene π-system and the unsaturated electrons of the topmost O layer of α-Al2O3(0001), rather than the van der Waals interactions. Such features causes graphene hole doping and enable the graphene to slide on the α-Al2O3(0001) surface with only a small energy barrier despite the strong interfacial interactions.

Room-temperature magnetic order on zigzag edges of narrow graphene nanoribbons.

The possibility that non-magnetic materials such as carbon could exhibit a novel type of s-p electron magnetism has attracted much attention over the years, not least because such magnetic order is predicted to be stable at high temperatures. It has been demonstrated that atomic-scale structural defects of graphene can host unpaired spins, but it remains unclear under what conditions long-range magnetic order can emerge from such defect-bound magnetic moments. Here we propose that, in contrast to random defect distributions, atomic-scale engineering of graphene edges with specific crystallographic orientation--comprising edge atoms from only one sub-lattice of the bipartite graphene lattice--can give rise to a robust magnetic order. We use a nanofabrication technique based on scanning tunnelling microscopy to define graphene nanoribbons with nanometre precision and well-defined crystallographic edge orientations. Although so-called 'armchair' ribbons display quantum confinement gaps, ribbons with the 'zigzag' edge structure that are narrower than 7 nanometres exhibit an electronic bandgap of about 0.2-0.3 electronvolts, which can be identified as a signature of interaction-induced spin ordering along their edges. Moreover, upon increasing the ribbon width, a semiconductor-to-metal transition is revealed, indicating the switching of the magnetic coupling between opposite ribbon edges from the antiferromagnetic to the ferromagnetic configuration. We found that the magnetic order on graphene edges of controlled zigzag orientation can be stable even at room temperature, raising hopes of graphene-based spintronic devices operating under ambient conditions.

Graphene spintronics.

The isolation of graphene has triggered an avalanche of studies into the spin-dependent physical properties of this material and of graphene-based spintronic devices. Here, we review the experimental and theoretical state-of-art concerning spin injection and transport, defect-induced magnetic moments, spin-orbit coupling and spin relaxation in graphene. Future research in graphene spintronics will need to address the development of applications such as spin transistors and spin logic devices, as well as exotic physical properties including topological states and proximity-induced phenomena in graphene and other two-dimensional materials.

Conductance recovery and spin polarization in boron and nitrogen co-doped graphene nanoribbons

We present an ab initio study of the structural, electronic, and quantum transport properties of B–N-complex edge-doped graphene nanoribbons (GNRs). We find that the B–N edge codoping is energetically a very favorable process and furthermore can achieve novel doping effects that are absent for the single B or N doping. The compensation effect between B and N is predicted to generally recover the excellent electronic transport properties of pristine GNRs. For the zigzag GNRs, however, the spatially localized B–N defect states selectively destroy the doped-side spin-polarized GNR edge currents at the valence and conduction band edges. We show that the energetically and spatially spin-polarized currents survive even in the fully ferromagnetic metallic state and heterojunction configurations. This suggests a simple yet efficient scheme to achieve effectively smooth GNR edges and graphene-based spintronic devices.

Electrical Spin Switch in Hydrogenated Multilayer Graphene

Electric field control of magnetism has become increasingly important for the next generation of graphene-based spintronic devices. We predict that the magnetic moment induced by chemisorbed H atoms on the top layer of a few-layer graphene system is tunable by an external electric field. Through accurate first-principles electronic structure calculations, we show that this magnetoelectric effect is negligible in one-layer graphene, but becomes pronounced in bilayer and trilayer graphene, saturating in magnitude in quadrilayer graphene. The effect is due to shifting of the Dirac cone of the pure graphene layers relative to the bands of the hydrogenated layer, induced by the external field. The calculated magnetoelectric coefficient (α) has values comparable to those found for ferromagnetic films or perovskite interfaces. The value of α was also used to identify a half-metallic state at low gate bias, which suggests a new class of spin-polarized materials based on hydrogenated multilayer graphene. Our results point to an experimentally feasible way to create a magnetoelectric coupling in graphene using the interplay between covalent functionalization and electric fields.