Charge Neutrality Point Of Graphene Research Articles

Gate‐Tunable Spin Hall Effect in Trilayer Graphene/Group‐IV Monochalcogenide van der Waals Heterostructures

AbstractSpintronic devices require materials that facilitate effective spin transport, generation, and detection. In this regard, graphene emerges as an ideal candidate for long‐distance spin transport owing to its minimal spin‐orbit coupling, which, however, limits its capacity for effective spin manipulation. This problem can be overcome by putting spin‐orbit coupling materials in close contact with graphene leading to spin‐orbit proximity and, consequently, efficient spin‐to‐charge conversion through mechanisms such as the spin Hall effect. Here, the gate‐dependent spin Hall effect in trilayer graphene proximitized with tin sulfide (SnS) is reported and quantified, a group‐IV monochalcogenide that has recently been predicted to be a viable alternative to transition‐metal dichalcogenides for inducing strong spin‐orbit coupling in graphene. The spin Hall angle exhibits a maximum around the charge neutrality point of graphene up to room temperature. The findings expand the library of materials that induce spin‐orbit coupling in graphene to a new class, group‐IV monochalcogenides, thereby highlighting the potential of 2D materials to pave the way for the development of innovative spin‐based devices and future technological applications.

Nanoplasmonic hybrid hotspot in graphene transistor

A field effect transistor based on graphene was introduced to generate a hybrid plasmonic mode. The main reasons to consider graphene in this study are the achievement of high field confinement and the graphene two carrier transport channel near its charge neutrality point. We have observed a nonlinear behavior of the hybrid plasmonic mode near the graphene charge neutrality point because of the existence of the two carrier model. This nonlinear behavior reflects the intrinsic nanoelectronic characteristics of graphene. The proposed nanoelectronic device presents an ultraconfined tunable hybrid hotspot, the mode area of which is ${10}^{5}$ times smaller than the diffraction limited spot area. Moreover, this area is three orders of magnitude smaller than the modal area of the similar metal-based hybrid nanostructure. Such a nanoscale 3D hotspot with plasmonic-photonic character has been reported here. Furthermore, a hybrid structure based on bilayer twisted graphene was numerically investigated which renders ultrahigh confinement and ultralong propagation distance. The proposed nanodevice can be applied to design compact integrated circuits, optical neural networks, and high resolution sensors.

Enhancement of superconductivity upon reduction of carrier density in proximitized graphene

The superconducting transition temperature (Tc) of a single layer graphene coupled to an Indium oxide (InO) film, a low carrier-density superconductor, is found to increase with decreasing carrier density and is largest close to the average charge neutrality point in graphene. Such an effect is very surprising in conventional BCS superconductors. We study this phenomenon both experimentally and theoretically. Our analysis suggests that the InO film induces random electron and hole-doped puddles in the graphene. The Josephson effect across these regions of opposite polarity enhances the Josephson coupling between the superconducting clusters in InO, along with the overall Tc of the bilayer heterostructure. This enhancement is most effective when the chemical potential of the system is tuned between the charge neutrality points of the electron and hole-doped regions.

Graphene field-effect transistors array for detection of liquid conductivities in the physiological range through novel time-multiplexed impedance measurements

In medical applications, graphene field-effect transistors (GFETs) have been employed as sensors for the detection of biological biomarkers and other compounds due to graphene's high sensitivity towards immobilized molecules at its surface. Commonly, the resistivity of a graphene channel is measured under direct drain-source current (DC) and gate voltage sweep. However, like other materials, the electrical response of graphene can also be studied in the form of impedance measurements using alternating drain-source current (AC), which alters the distribution and migration of ionic species across the graphene channel. In this study, an array with 12 non-functionalized GFETs is used for pioneering electrical measurements of prepared liquid solutions in the DC and AC regimes of stimulation. In particular, the transistors were characterized under saline solutions with conductivity levels ranging from 84 μS/cm to 1413 μS/cm. We first report the observation of a larger variation on the graphene's charge neutrality point in gate sweep for the AC regime with varying conductivity levels (ΔVGS = −0.00033∗ΔS) as compared to DC (ΔVGS = −0.00023∗ΔS), which can potentially be explored as a novel way of detecting electrical changes on graphene induced by physiological solutions, chemical analytes, immobilized substances, or cell substrates, when direct current cannot penetrate and influence these structures.

Coulomb Drag between a Carbon Nanotube and Monolayer Graphene.

We have measured Coulomb drag between an individual single-walled carbon nanotube (SWNT) as a one-dimensional (1D) conductor and the two-dimensional (2D) conductor monolayer graphene, separated by a few-atom-thick boron nitride layer. The graphene carrier density is tuned across the charge neutrality point (CNP) by a gate, while the SWNT remains degenerate. At high temperatures, the drag resistance changes sign across the CNP, as expected for momentum transfer from drive to drag layer, and exhibits layer exchange Onsager reciprocity. We find that layer reciprocity is broken near the graphene CNP at low temperatures due to nonlinear drag response associated with temperature dependent drag and thermoelectric effects. The drag resistance shows power-law dependences on temperature and carrier density characteristic of 1D Fermi liquid-2D Dirac fluid drag. The 2D drag signal at high temperatures decays with distance from the 1D source slower than expected for a diffusive current distribution, suggesting additional interaction effects in the graphene in the hydrodynamic transport regime.

Effect of an optimal oxide layer on the efficiency of graphene‐silicon Schottky junction solar cell

The harvesting of solar energy through silicon/graphene Schottky junction photovoltaic cell has been widely investigated in the last decade but the surface recombination at interface limits the high conversion efficiency. We have demonstrated the utilization of the optimum thickness (1.5 nm) of Al2O3 between the interfaces of graphene and n-type Si as an interlayer to reduce the surface recombination along with chemical doping of perfluorinated polymeric sulfonic acid (PFSA) is used as a p-type dopant to modulate the work function of graphene. The Kelvin probe force microscopy (KPFM) analysis revealed that the p-doping enhances the graphene work function from 4.65 to 4.8 eV. The transport measurements are performed to study the shift in charge neutrality point of graphene. Furthermore, the effect of PFSA doping on graphene is also confirmed through Raman spectroscopy. The maximum value of power conversion efficiency (PCE) was found to be 13.52% (100 mW cm−2, AM 1.5) by introducing an optimal oxide layer and PFSA doping. The device exhibits the photoresponsivity of 0.10 AW−1. We believe that our findings will provide a route toward the development of new photovoltaic (PV) cells.

Trigonal quasicrystalline states in 30^circ rotated double moir\xe9 superlattices

We study the lattice configuration and electronic structure of a double moiré superlattice, which is composed of a graphene layer encapsulated by two other layers in a way such that the two hexagonal moiré patterns are arranged in a dodecagonal quasicrystalline configuration. We show that there are between 0 and 4 such configurations depending on the lattice mismatch between graphene and the encapsulating layer. We then reveal the resonant interaction, which is distinct from the conventional 2-, 3-, 4-wave mixing of moiré superlattices, that brings together and hybridizes twelve degenerate Bloch states of monolayer graphene. These states do not fully satisfy the dodecagonal quasicrystalline rotational symmetry due to the symmetry of the wave vectors involved. Instead, their wave functions exhibit trigonal quasicrystalline order, which lacks inversion symmetry, at the energies much closer to the charge neutrality point of graphene.

Persistent and reversible electrostatic control of doping in graphene/hexagonal boron nitride heterostructures

Since its first application as a substrate for graphene field effect transistors (FETs), hexagonal boron nitride (hBN) has become a prominent component in two-dimensional (2D) material devices. In addition, hBN has been shown to host defects that can be manipulated to change the electronic properties of adjacent 2D materials. Despite the wide use of such defect manipulations, no focused efforts have been made to further the understanding of defect excitations and their influence in graphene/hBN FETs. In this study, we explore the effect of high electric fields (∼10V/nm) on graphene/hBN FETs and find that persistent and reversible shifts in graphene's charge neutrality point (CNP) occur. By increasing the applied electric field and temperature of our device, we find that this CNP shift is enhanced. With this insight, we propose a mechanism that explains these observations based on Poole–Frenkel emissions from defects in hBN. Finally, we show that such an effect may be suppressed by using graphite as a backgate, thus preventing unintended changes in the electrical properties of graphene/hBN FETs.

Unexpected Hole Doping of Graphene by Osmium Adatoms

AbstractThe electronic transport of monolayer graphene devices is studied before and after in situ deposition of a sub‐monolayer coating of osmium adatoms. Unexpectedly, and unlike all other metallic adatoms studied to date, osmium adatoms shift the charge neutrality point to more positive gate voltages. This indicates that osmium adatoms act as electron acceptors and thus leave the graphene hole‐doped. Analysis of transport data suggest that Os adatoms behave as charged impurity scatterers, albeit with a surprisingly low charge‐doping efficiency. The charge neutrality point of graphene is found to vary non‐monotonically with gate voltage as the sample is warmed to room temperature, suggesting that osmium diffuses on the surface but is not completely removed.

Photosensing System Using Photosystem I and Gold Nanoparticle on Graphene Field-Effect Transistor.

In this study, a light sensor is fabricated based on photosystem I (PSI) and a graphene field-effect transistor (FET) that detects light at a high quantum yield under ambient conditions. We immobilized PSI on a micrometer-sized graphene FET using Au nanoparticles (AuNPs) and measured the I-V characteristics of the modified graphene FET before and after light irradiation. The source-drain current (Isd) increased upon illumination, exhibiting a photoresponsivity of 4.8 × 102 A W-1, and the charge neutrality point of graphene shifted by -12 mV. This system represents the first successful photosensing system based on proteins and a solution-gated graphene FET. The probable mechanism of this negative shift can be explained by the increase in negative charge carriers in graphene induced by a hole trap in the AuNP resulting from electron transfer from the AuNP to PSI. Photoresponses were only observed in the presence of two surface-active agents, n-hexyltrimethylammonium bromide and sodium dodecylbenzenesulfonate, because they caused the formation of a hydrophobic environment on the surface of the graphene. The lipid layer of these agents caused dissociation of ascorbate ions from the graphene sheet, thereby expanding the Debye screening length of the electrolyte solution. The hydrophobic environment above graphene also enhanced hole storage in the AuNP through electron transfer from the AuNP to PSI.

Reconfigurable edge-state engineering in graphene using LaAlO3/SrTiO3 nanostructures

The properties of graphene depend sensitively on doping with respect to the charge-neutrality point (CNP). Tuning the CNP usually requires electrical gating or chemical doping. Here, we describe a technique to reversibly control the CNP in graphene with nanoscale precision, utilizing LaAlO3/SrTiO3 (LAO/STO) heterostructures and conductive atomic force microscope (c-AFM) lithography. The local electron density and resulting conductivity of the LAO/STO interface can be patterned with a conductive AFM tip [Cen et al., Nat. Mater. 7, 298 (2008)] and placed within two nanometers of an active graphene device [Huang et al., APL Mater. 3, 062502 (2015)]. The proximal LAO/STO nanostructures shift the position of graphene CNP by ∼1012 cm−2 and are also gateable. Here, we use this effect to create reconfigurable edge states in graphene, which are probed using the quantum Hall effect. Quantized resistance plateaus at h/e2 and h/3e2 are observed in a split Hall device, demonstrating edge transport along the c-AFM written edge that depends on the polarity of both the magnetic field and direction of currents. This technique can be readily extended to other device geometries.

Semiconductor channel-mediated photodoping in h-BN encapsulated monolayer MoSe2 phototransistors

In optically excited 2D phototransistors, charge transport is often affected by photodoping effects. Recently, it was shown that such effects are especially strong and persistent for graphene/h-BN heterostructures, and that they can be used to controllably tune the charge neutrality point of graphene. In this work we investigate how this technique can be extended to h-BN encapsulated monolayer MoSe2 phototransistors at room temperature. By exposing the sample to 785 nm laser excitation we can controllably increase the charge carrier density of the MoSe2 channel by Δn ≈ 4.45 × 1012 cm−2, equivalent to applying a back gate voltage of ~60 V. We also evaluate the efficiency of photodoping at different illumination wavelengths, finding that it is strongly correlated with the light absorption by the MoSe2 layer, and maximizes for excitation on-resonance with the A exciton absorption. This indicates that the photodoping process involves optical absorption by the MoSe2 channel, in contrast with the mechanism earlier described for graphene/h-BN heterostroctures.

Long-distance spin transport through a graphene quantum Hall antiferromagnet

Antiferromagnetic insulators (AFMI) are robust against stray fields, and their intrinsic dynamics could enable ultrafast magneto-optics and ultrascaled magnetic information processing. Low dissipation, long distance spin transport and electrical manipulation of antiferromagnetic order are much sought-after goals of spintronics research. Here, we report the first experimental evidence of robust long-distance spin transport through an AFMI, in our case the gate-controlled, canted antiferromagnetic (CAF) state that appears at the charge neutrality point of graphene in the presence of an external magnetic field. Utilizing gate-controlled quantum Hall (QH) edge states as spin-dependent injectors and detectors, we observe large, non-local electrical signals across a 5 micron-long, insulating channel only when it is biased into the nu=0 CAF state. Among possible transport mechanisms, spin superfluidity in an antiferromagnetic state gives the most consistent interpretation of the non-local signal's dependence on magnetic field, temperature and filling factors. This work also demonstrates that graphene in the QH regime is a powerful model system for fundamental studies of antiferromagnetic, and in the case of a large in-plane field, ferromagnetic spintronics.

Large spin relaxation anisotropy and valley-Zeeman spin-orbit coupling in WSe2 /graphene/ h -BN heterostructures

Large spin-orbital proximity effects have been predicted in graphene interfaced with a transition metal dichalcogenide layer. Whereas clear evidence for an enhanced spin-orbit coupling has been found at large carrier densities, the type of spin-orbit coupling and its relaxation mechanism remained unknown. We show for the first time an increased spin-orbit coupling close to the charge neutrality point in graphene, where topological states are expected to appear. Single layer graphene encapsulated between the transition metal dichalcogenide WSe$_2$ and hBN is found to exhibit exceptional quality with mobilities as high as 100000 cm^2/V/s. At the same time clear weak anti-localization indicates strong spin-orbit coupling and a large spin relaxation anisotropy due to the presence of a dominating symmetric spin-orbit coupling is found. Doping dependent measurements show that the spin relaxation of the in-plane spins is largely dominated by a valley-Zeeman spin-orbit coupling and that the intrinsic spin-orbit coupling plays a minor role in spin relaxation. The strong spin-valley coupling opens new possibilities in exploring spin and valley degree of freedom in graphene with the realization of new concepts in spin manipulation.

Investigation of the two-gap superconductivity in a few-layer NbSe2 -graphene heterojunction

We investigated the superconductivity in a few-layer ${\mathrm{NbSe}}_{2}$-graphene heterojunction by differential conductance spectroscopy. Because of the gate-tunable Fermi level of the few-layer graphene, used here as a tunneling electrode in a nano-point-contact spectroscopy setup, the differential conductance of the heterojunction showed highly sensitive dependence on the gate voltage, which allowed us to probe the nature of the superconducting gap functions with unprecedented detail by continuously tuning the transparency of the junction between the spectroscopic tunneling and the Andreev reflection limits. Characteristic features associated with a two-gap superconductivity in ${\mathrm{NbSe}}_{2}$ were reproducibly observed in both limits and between, e.g., in the form of a central conductance dip with two sets of coherence peaks when the Fermi level was close to the charge neutrality point of graphene. From fits with the Blonder-Tinkham-Klapwijk model, two gaps with their temperature dependence were extracted. The two gaps associated with the two-band superconductivity in ${\mathrm{NbSe}}_{2}$ followed the expected temperature behavior in the limit of weak interband scattering, with a gap to ${T}_{c}$ ratio suggesting a weak to moderately strong coupling in few-layer systems.

Voltage Scaling of Graphene Device on SrTiO3 Epitaxial Thin Film.

Electrical transport in monolayer graphene on SrTiO3 (STO) thin film is examined in order to promote gate-voltage scaling using a high-k dielectric material. The atomically flat surface of thin STO layer epitaxially grown on Nb-doped STO single-crystal substrate offers good adhesion between the high-k film and graphene, resulting in nonhysteretic conductance as a function of gate voltage at all temperatures down to 2 K. The two-terminal conductance quantization under magnetic fields corresponding to quantum Hall states survives up to 200 K at a magnetic field of 14 T. In addition, the substantial shift of charge neutrality point in graphene seems to correlate with the temperature-dependent dielectric constant of the STO thin film, and its effective dielectric properties could be deduced from the universality of quantum phenomena in graphene. Our experimental data prove that the operating voltage reduction can be successfully realized due to the underlying high-k STO thin film, without any noticeable degradation of graphene device performance.

Band Alignment in WSe2-Graphene Heterostructures.

Using different types of WSe2 and graphene-based heterostructures, we experimentally determine the offset between the graphene neutrality point and the WSe2 conduction and valence band edges, as well as the WSe2 dielectric constant along the c-axis. In a first heterostructure, consisting of WSe2-on-graphene, we use the WSe2 layer as the top dielectric in dual-gated graphene field-effect transistors to determine the WSe2 capacitance as a function of thickness, and the WSe2 dielectric constant along the c-axis. In a second heterostructure consisting of graphene-on-WSe2, the lateral electron transport shows ambipolar behavior characteristic of graphene combined with a conductivity saturation at sufficiently high positive (negative) gate bias, associated with carrier population of the conduction (valence) band in WSe2. By combining the experimental results from both heterostructures, we determine the band offset between the graphene charge neutrality point, and the WSe2 conduction and valence band edges.

Large area graphene ion sensitive field effect transistors with tantalum pentoxide sensing layers for pH measurement at the Nernstian limit

We have fabricated and characterized large area graphene ion sensitive field effect transistors (ISFETs) with tantalum pentoxide sensing layers and demonstrated pH sensitivities approaching the Nernstian limit. Low temperature atomic layer deposition was used to deposit tantalum pentoxide atop large area graphene ISFETs. The charge neutrality point of graphene, inferred from quantum capacitance or channel conductance, was used to monitor surface potential in the presence of an electrolyte with varying pH. Bare graphene ISFETs exhibit negligible response, while graphene ISFETs with tantalum pentoxide sensing layers show increased sensitivity reaching up to 55 mV/pH over pH 3 through pH 8. Applying the Bergveld model, which accounts for site binding and a Guoy-Chapman-Stern picture of the surface-electrolyte interface, the increased pH sensitivity can be attributed to an increased buffer capacity reaching up to 1014 sites/cm2. ISFET response was found to be stable to better than 0.05 pH units over the course of two weeks.

Systematic Doping Control of CVD Graphene Transistors with Functionalized Aromatic Self‐Assembled Monolayers

Recent reports have shown that self‐assembled monolayers (SAMs) can induce doping effects in graphene transistors. However, a lack of understanding persists surrounding the quantitative relationship between SAM molecular design and its effects on graphene. In order to facilitate the fabrication of next‐generation graphene‐based devices it is important to reliably and predictably control the properties of graphene without negatively impacting its intrinsic high performance. In this study, SAMs with varying dipole magnitudes/directions are utilized and these values are directly correlated to changes in performance seen in graphene transistors. It is found that, by knowing the z‐component of the SAM dipole, one can reliably predict the shift in graphene charge neutrality point after taking into account the influence of the metal electrodes (which also play a role in doping graphene). This relationship is verified through density functional theory and comprehensive device studies utilizing atomic force microscopy, X‐ray photoelectron spectroscopy, Raman spectroscopy, and electrical characterization of graphene transistors. It is shown that properties of graphene transistors can be predictably controlled with SAMs when considering the total doping environment. Additionally, it is found that methylthio‐terminated SAMs strongly interact with graphene allowing for a cleaner graphene transfer and enhanced charge mobility.

Quantized coexisting electrons and holes in graphene measured using temperature-dependent magnetotransport

We present temperature-dependent magneto-transport experiments around the charge neutrality point in graphene and determine the amplitude of potential fluctuations $s$ responsible for the formation of electron-hole puddles. The experimental value $s \approx 20$ meV is considerably larger than in conventional semiconductors which implies a strong localization of charge carriers observable up to room temperature. Surprisingly, in the quantized regime, the Hall resistivity overshoots the highest plateau values at high temperatures. We demonstrate by model calculations that such a peculiar behavior is expected in any system with coexisting electrons and holes when the energy spectrum is quantized and the carriers are partially localized.