Huajun Liu

Spin-dependent transport properties of Fe3O4/MoS2/Fe3O4 junctions.

Magnetite is a half-metal with a high Curie temperature of 858 K, making it a promising candidate for magnetic tunnel junctions (MTJs). Yet, initial efforts to exploit its half metallic nature in Fe3O4/MgO/Fe3O4 MTJ structures have been far from promising. Finding suitable barrier layer materials, which keep the half metallic nature of Fe3O4 at the interface between Fe3O4 layers and barrier layer, is one of main challenges in this field. Two-dimensional (2D) materials may be good candidates for this purpose. Molybdenum disulfide (MoS2) is a transition metal dichalcogenide (TMD) semiconductor with distinctive electronic, optical, and catalytic properties. Here, we show based on the first principle calculations that Fe3O4 keeps a nearly fully spin polarized electron band at the interface between MoS2 and Fe3O4. We also present the first attempt to fabricate the Fe3O4/MoS2/Fe3O4 MTJs. A clear tunneling magnetoresistance (TMR) signal was observed below 200 K. Thus, our experimental and theoretical studies indicate that MoS2 can be a good barrier material for Fe3O4 based MTJs. Our calculations also indicate that junctions incorporating monolayer or bilayer MoS2 are metallic.

Enhanced Shubnikov–De Haas Oscillation in Nitrogen-Doped Graphene

N-doped graphene displays many interesting properties compared with pristine graphene, which makes it a potential candidate in many applications. Here, we report that the Shubnikov-de Haas (SdH) oscillation effect in graphene can be enhanced by N-doping. We show that the amplitude of the SdH oscillation increases with N-doping and reaches around 5k Ω under a field of 14 T at 10 K for highly N-doped graphene, which is over 1 order of magnitude larger than the value found for pristine graphene devices with the same geometry. Moreover, in contrast to the well-established standard Lifshitz-Kosevich theory, the amplitude of the SdH oscillation decreases linearly with increasing temperature and persists up to a temperature of 150 K. Our results also show that the magnetoresistance (MR) in N-doped graphene increases with increasing temperature. Our results may be useful for the application of N-doped graphene in magnetic devices.

Transport Gap Opening and High On–Off Current Ratio in Trilayer Graphene with Self-Aligned Nanodomain Boundaries

Trilayer graphene exhibits exceptional electronic properties that are of interest both for fundamental science and for technological applications. The ability to achieve a high on-off current ratio is the central question in this field. Here, we propose a simple method to achieve a current on-off ratio of 10(4) by opening a transport gap in Bernal-stacked trilayer graphene. We synthesized Bernal-stacked trilayer graphene with self-aligned periodic nanodomain boundaries (NBs) on the technologically relevant vicinal cubic-SiC(001) substrate and performed electrical measurements. Our low-temperature transport measurements clearly demonstrate that the self-aligned periodic NBs can induce a charge transport gap greater than 1.3 eV. More remarkably, the transport gap of ∼0.4 eV persists even at 100 K. Our results show the feasibility of creating new electronic nanostructures with high on-off current ratios using graphene on cubic-SiC.

Large positive in-plane magnetoresistance induced by localized states at nanodomain boundaries in graphene

Graphene supports long spin lifetimes and long diffusion lengths at room temperature, making it highly promising for spintronics. However, making graphene magnetic remains a principal challenge despite the many proposed solutions. Among these, graphene with zig-zag edges and ripples are the most promising candidates, as zig-zag edges are predicted to host spin-polarized electronic states, and spin–orbit coupling can be induced by ripples. Here we investigate the magnetoresistance of graphene grown on technologically relevant SiC/Si(001) wafers, where inherent nanodomain boundaries sandwich zig-zag structures between adjacent ripples of large curvature. Localized states at the nanodomain boundaries result in an unprecedented positive in-plane magnetoresistance with a strong temperature dependence. Our work may offer a tantalizing way to add the spin degree of freedom to graphene.

Surface enhanced Raman scattering of monolayer MX 2 with metallic nano particles

Monolayer transition metal dichalcogenides MX2 (M = Mo, W; X = S) exhibit remarkable electronic and optical properties, making them candidates for application within flexible nano-optoelectronics. The ability to achieve a high optical signal, while quantitatively monitoring strain in real-time is the key requirement for applications in flexible sensing and photonics devices. Surface-enhanced Raman scattering (SERS) allows us to achieve both simultaneously. However, the SERS depends crucially on the size and shape of the metallic nanoparticles (NPs), which have a large impact on its detection sensitivity. Here, we investigated the SERS of monolayer MX2, with particular attention paid to the effect of the distribution of the metallic NPs. We show that the SERS depends crucially on the distribution of the metallic NPs and also the phonon mode of the MX2. Moreover, strong coupling between MX2 and metallic NPs, through surface plasmon excitation, results in splitting of the and modes and an additional peak becomes apparent. For a WS2-Ag system the intensity of the additional peak increases exponentially with local strain, which opens another interesting window to quantitatively measure the local strain using SERS. Our experimental study may be useful for the application of monolayer MX2 in flexible nano-optoelectronics.

Probing thermal expansion coefficients of monolayers using surface enhanced Raman scattering

Monolayer transition metal dichalcogenides exhibit remarkable electronic and optical properties, making them candidates for application within flexible nano-optoelectronics, however direct experimental determination of their thermal expansion coefficients (TECs) is difficult. Here, we propose a non-destructive method to probe the TECs of monolayer materials using surface-enhanced Raman spectroscopy (SERS). A strongly coupled Ag nanoparticle over-layer is used to controllably introduce temperature dependent strain in monolayers. Changes in the first-order temperature coefficient of the Raman shift, produced by TEC mismatch, can be used to estimate relative expansion coefficient of the monolayer. As a demonstration, the linear TEC of monolayer WS2 is probed and is found to be 10.3 × 10−6 K−1, which would appear support theoretical predictions of a small TEC. This method opens a route to probe and control the TECs of monolayer materials.

Competition Between Anti-Phase Boundaries and Charge-Orbital Ordering in Epitaxial Stepped Fe3O4(100) Thin Films

Magnetite is a highly utilized transition metal oxide with many interesting magnetic and transport properties. The presence of anti-phase boundaries (APBs) and charge-orbital ordering (COO) are two of the most exciting properties of epitaxial magnetite thin films. Here, epitaxial stepped Fe3O4 films were prepared to investigate the competition between APBs and COO via measurements of in-plane anisotropy. The anisotropy was probed for two orthogonal configurations, with magnetic field applied or electrical-contacts aligned either along or perpendicular to the steps. We reveal that the APBs dominate the magnetic and transport properties of the films above the Verwey transition temperature (TV). However, below TV film thickness becomes a decisive factor in determining the magnetic nature of stepped magnetite films, due to its correlation with domain size. When the film is thinner than a critical thickness, the anisotropy is dominated by the APBs, and a higher anisotropy constant and MR ratio are observed when the magnetic field or contacts are oriented along the steps. Conversely, for sufficiently thick films, below TV, the magnetic and electrical transport properties are dominated by COO. Thus a higher anisotropy constant and MR ratio are observed when the magnetic field or contacts are oriented perpendicular to the steps.

Threshold magnetoresistance in anistropic magnetic 2D transition metal dichalcogenides

Recently many novel magnetoresistance (MR) phenomena have been reported from studies of two dimensional (2D) materials. Here, we report on the exotic transport behavior of VS2. A large negative and quadratic MR of −10% is observed for an in-plane magnetic field B up to 14 T. Remarkably, when the applied field deviates from the in-plane orientation there is a threshold field, Bc, and the MR shows a plateau of near zero MR. When B < Bc, only a single state exists and the transition between quantum spin states is forbidden. Our work sheds new light on the MR of magnetic 2D materials with localized states and may spur further investigations.