Charge Neutrality Point Research Articles

Controlling two-dimensional twist

Solid-State Physics In heterostructures assembled from two-dimensional materials such as graphene, electron tunneling between layers varies strongly with the rotation angle between the crystal lattices. Usually, the twist angle between layers is fixed after assembly. Ribeiro-Palau et al. encapsulated graphene with boron nitride, but the top boron nitride flake was shaped so that an atomic force microscope tip could push on it to vary the twist angle by as little as 0.2°. They observed variations with twist angle in properties such as the charge neutrality point, which would be difficult to observe in static rotated structures. Science , this issue p. [690][1] [1]: /lookup/doi/10.1126/science.aat6981

Counterpropagating Fractional Hall States in Mirror-Symmetric Dirac Semimetals.

The Landau bands of mirror symmetric 2D Dirac semimetals (e.g., odd layers of ABA graphene) can be identified by their parity with respect to mirror symmetry. This symmetry facilitates a new class of counterpropagating Hall states. We predict the presence of a Laughlin-like correlated liquid state, at the charge neutrality point, with opposite but equal electron and hole filling factors |ν_{±}|=1/m (m odd). This state exhibits fractionally charged quasiparticle-hole pair excitations and counterpropagating edge states with opposite parity. Using a bosonized one-dimensional edge state theory, we show that the fractionally quantized two-terminal longitudinal conductance, σ_{xx}=2e^{2}/(mh), is robust to short-ranged intermode interactions.

Selectivity and Cross-Sensitivity of Transfer-Free Fabricated Nanocrystalline Graphene Field-Effect Gas Sensors

In the course of an increasing awareness for the protection of public health a higher demand for environmental gas sensors, especially for the detection of volatile organic compounds and toxic gases, has been noted. Reasonably-priced devices to cover this demand can be solid-state gas sensors that can be easily fabricated and scaled using planar processing. Furthermore, an appropriate choice of materials may enable the possibility of monolithic integration within silicon CMOS circuitry thus providing the chance of co-integrating read-out and signal processing electronics on the same chip. As a two-dimensional carbon nanomaterial graphene shows a high surface-to-volume ratio and promising electronic properties for the detection of gaseous species. Detection capabilities down to a single molecule have been demonstrated by Schedin et al. [1], using a pristine graphene flake made from mechanical cleavage of highly ordered pyrolytic graphite. In this contribution we port on a novel method for graphene-based sensor fabrication. By means of a CMOS compatible method we have fabricated hundreds of transfer-free nanocrystalline graphene field-effect transistors (ncGFETs) on silicon dioxide [2], which can be used for detection of gaseous species. Under application of a HP4156A semiconductor parameter analyzer the gaseous sensitivity of our ncGFETs is electrically characterized in terms of backgate FET input characteristics as well as in a resistive configuration. For the resistive configuration the backgate bias has been fixed near the charge neutrality point of our nanocrystalline graphene devices at VBG = 0V. All measurements were done using our self-constructed vacuum probing station that can be systematically flooded with calibrated gas mixtures. The gas concentration is then derived from the vacuum pressure using the ideal gas law. Sensitivities of our ncGFETs to different volume concentrations of various toxic gases including ammonia (NH3), nitrogen dioxide (NO2), and carbon monoxide (CO) will be presented. E.g. for devices heated to 425K a sensitivity S of roughly 80% is S = (G-G0)/G0 achieved for 4 parts-per-million-volume (ppmv) of ammonia whereat NO2 already shows a sensitivity of 100% for a concentration of 4 parts-per-trillion-volume (pptv = 10-6 ppmv). The higher sensitivity of our ncGFETs compared to other graphene devices is attributed to modifications of grain boundary potential barriers, which is the dominating mechanism in e.g. SnO2-based Taguchi-type sensors [3]. Furthermore, the influence of water vapor (H2O (g)) on our devices is discussed. Nevertheless, the cross-sensitivity of Taguchi-type sensors, like our ncGFETs, remains a problem. Speciation of gas molecules is feasible by the use of sensor arrays consisting of devices with different sensitivities and electronic characteristics towards different gas species. Therefore, in order to modify the sensitivities towards the different gas species of our ncGFETs towards the different gas species we have characterized the sensitivity and responsivity at various temperatures. Furthermore, during exposure to the different gases the analysis of the backgate ncGFET input characteristics shows shifts of the charge neutrality point, as well as changes in the hysteresis of our devices. A novel method to selectively discriminate between various gaseous species, which is based on the analysis of the hysteresis in the ncGFETs will be presented and discussed. [1]F. Schedin, A. K. Geim, S. V. Morozov, E. W. Hill, P. Blake, M. I. Katsnelson and K. S. Novoselov, Nat Mater, 6, 652 (2007). [2] D. Noll and U. Schwalke, 2017 12th Ieee International Conference on Design & Technology of Integrated Systems in Nanoscale Era (Dtis 2017) (2017). [3] R. K. Srivastava, P. Lal, R. Dwivedi and S. K. Srivastava, Sensor Actuat B-Chem, 21, 213 (1994). Figure 1

Selectivity and Cross-Sensitivity of Transfer-Free Fabricated Nanocrystalline Graphene Field-Effect Gas Sensors

With respect to its high surface-to-volume ratio graphene shows promising properties for electrochemical sensing. By means of resistive measurements, i.e. having the backgate connected to ground, we demonstrate low-level detection of ammonia (1), nitrogen dioxide (calibration gas 1,000ppm) and carbon monoxide (calibration gas 1,000ppm) under vacuum pressures ranging from 2·10-4 mbar to 4·10-3 mbar using our transfer-free fabricated nanocrystalline graphene field-effect transistors (ncGFETs). A change in conductivity can be observed for all three gases. However, no selectivity between the gases is yet achieved. Therefore, by analyzing the ncGFET input characteristics a mechanism to discriminate between different gaseous species is developed based on the shift of the charge neutrality point (CNP) (2) as well as on the evolution of the hysteresis loop present in our ncGFETs.

Fluctuation-induced tunneling conduction in iodine-doped bilayer graphene

Intrinsic bilayer graphene is a semimetal. Upon applying an electric field, the potential difference between top and bottom layers can open an energy gap and tune bilayer graphene to an insulating state at the charge neutrality point. Here, we demonstrate that the properties of semimetallic bilayer graphene can be controllably tuned to either metallic or insulating by a simple way of iodine molecular doping. The transport properties of iodine-doped bilayer graphene have been systematically investigated. At high iodine doping concentrations, the Fermi level shifts by approximately 0.35 eV to the metallic region because of the symmetric doping on the top and bottom bilayer surfaces. At low iodine doping concentrations, small energy gaps open in local areas due to the asymmetric doping between the top and the bottom graphene layers. In this case, an insulating behavior at low temperatures is observed, which can be well explained by employing the fluctuation-induced tunneling (FIT) model. At medium iodine doping concentrations, both metallic and insulating behaviors can be observed at different temperatures, implying that both FIT and metallic mechanisms take effect. Our work may have potential applications in on/off controllable electronic devices, gas sensors, and transparent flexible electrode in optoelectronics.

(Nano 2nd Place Best Poster Winner) Fabrication of Nanocarbon-Based Field-Effect Transistor Biosensors for Electronic Detection of DNA Sequences

We report on the design and assembly of liquid-gated field-effect transistor devices based on graphene nanoribbons, G-FETs, their functionalization with anchor groups, as well as their response to single-stranded DNA probe tethering and complementary DNA target hybridization in saline buffer solution. The probe DNA was immobilized onto the graphene surface by covalent functionalization with aryldiazonium chemistry. We characterized the electrical response of the devices before and after functionalization. DNA probe tethering and target hybridization were monitored by shifts of the charge neutrality point. Additionally, the electrical response to probe immobilization and probe-target hybridization was recorded in real-time.

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.

Graphene-based enzyme-modified field-effect transistor biosensor for monitoring drug effects in Alzheimer’s disease treatment

A reduced graphene oxide-based enzyme-modified field-effect transistor (RGO-EnFET) was fabricated to study the enzymatic kinetics between acetylcholinesterase (AChE) and acetylcholine (ACh), which are related to the progression and treatment of Alzheimer’s disease (AD). The RGO-EnFET exhibited typical ambipolar transfer characteristics with a charge neutrality point (the Dirac point, VDirac) in an aqueous environment, and biological events occurring on the RGO surface were evaluated by observing the Dirac point shift (ΔVDirac) according to reactions. First, the pH sensitivity of the RGO-EnFET was measured to be 24.12 mV pH−1 for a pH range of 4–10 with excellent repeatability. Then, ACh quantification in the concentration range of 1 μM–10 mM was evaluated, with a linear slope of 13.9 mV dec−1 being obtained between ΔVDirac and the ACh concentration on a logarithmic scale. The effects of AChE inhibitors (donepezil and rivastigmine) on the AChE enzymatic activity were also investigated. The distinguishable inhibition rates were acquired for donepezil and rivastigmine at saturation levels of 90% and 75%, respectively. These results indicate that the RGO-EnFET biosensor was successfully exploited as an in vitro analytical tool and has potential of future application in the study of enzymatic kinetics and drug screening for therapeutic purposes.

Andreev tunneling and Josephson current in light irradiated graphene

We investigate the Andreev tunneling and Josephson current in graphene irradiated with high-frequency linearly polarized light. The corresponding stroboscopic dynamics can be solved using Floquet mechanism which results in an effective stationary theory to the problem exhibiting an anisotropic Dirac spectrum and modified pseudospin-momentum locking. When applied to an irradiated normal graphene - superconductor (NS) interface, such analysis reveal Andreev reflection (AR) to become an oscillatory function of the optical strength. Specifically we find that, by varying the polarization direction we can both suppress AR considerably or cause the Andreev transport to remain maximum at sub-gap excitation energies even in the presence of Fermi level mismatch. Furthermore, we study the optical effect on the Andreev bound states (ABS) within a short normal-graphene sheet, sandwiched between two s-wave superconductors. It shows redistribution of the low energy regime in the ABS spectrum, which in turn, has major effect in shaping the Josephson super-current. Subjected to efficient tuning, such current can be sufficiently altered even at the charge neutrality point. Our observations provide useful feedback in regulating the quantum transport in Dirac-like systems, achieved via controlled off-resonant optical irradiation on them.

Thermoelectric Properties of Thermally Reduced Graphene Oxide Observed by Tuning the Energy States

Reduced graphene oxide (rGO) possesses a similar electronic structure to graphene but can be synthesized on a larger scale. Hence, rGO is considered as an attractive alternative to graphene. Here we report the carrier transport properties of thermally reduced graphene oxide (TrGO) as a function of reduction temperature. The transfer curve of a field effect transistor fabricated with TrGO exhibited ambipolar properties, and the charge neutrality point of TrGO was shifted from negative to positive as the reduction temperature increased. Furthermore, as revealed in Arrhenius plots of the carrier densities and carrier mobilities, TrGO behaved as a metallic conductor at all reduction temperatures. To investigate the effect of reduction temperature on the thermoelectric properties of TrGO, the Seebeck coefficients of the fabricated TrGOs were calculated from the transfer curve using Mott’s equation for metallic materials. All samples showed ambipolar carrier transport. At Vg = 0 V, the Seebeck coefficient switc...

Formation of the n = 0 Landau level in hybrid graphene

The minimum of 4-terminal conductance occurring as carrier density is tuned through its charge neutral point has proven to be a robust empirical feature of graphene, persisting with changes to temperature, applied magnetic field, substrate, and layer thickness, though the theoretical mechanisms involved in transport about this point—vanishing density of states, conventional band gap opening, and broken-symmetry quantum Hall mobility gaps—vary widely depending on the regime. In this paper, we report on observations of a regime where the 4-terminal conductance minimum ceases to exist: transport in monolayer graphene connected to bilayer graphene during the onset of the quantum Hall effect. This observed increase in conductance is accompanied by decreases in conductance at the half-filling of the Landau levels adjacent to the charge-symmetric, zero-energy level. As monolayer and bilayer graphene have distinct zero-energy levels that form about the charge neutral point, our observations suggest that competitions between the differing many-body orderings of these states as they emerge may underlie these anomalous conductances.

Anomalously large resistance at the charge neutrality point in a zero-gap InAs/GaSb bilayer

We report here our recent electron transport results in spatially separated two-dimensional electron and hole gases with nominally degenerate energy subbands, realized in an InAs(10 nm)/GaSb(5 nm) coupled quantum well. We observe a narrow and intense maximum (∼500 kΩ) in the four-terminal resistivity in the charge neutrality region, separating the electron-like and hole-like regimes, with a strong activated temperature dependence above T = 7 K and perfect stability against quantizing magnetic fields. We discuss several mechanisms for that unexpectedly large resistance in this zero-gap semi-metal system including the formation of an excitonic insulator state.

Selective Detection of a Protein Biomarker Utilizing a Large Area CVD-Grown Graphene-Based Field Effect Transistor

Lysozyme is an antimicrobial protein, often called the “body’s own antibiotic.” It is a ubiquitous enzyme widely available in diverse organisms, such as bacteria, bacteriophages, fungi, plants and mammals. Lysozyme is also extensively used in food industries for several purposes such as preserving meat and dairy products, as well as fruits and vegetables. Lysozyme has a molecular weight of 14,400 with a primary sequence containing 129 amino acids and an isoelectric point of 11.0 that causes lysozyme to behave as positively charged at neutral pH. Apart from its extensive use in food industry, lysozyme also plays a vital role as a biomarker for diagnosing various diseases such as breast cancer, Alzheimer’s disease, and rheumatoid arthritis. Several analytical methods have been deployed for effective detection of lysozyme molecules. Some of these methods include chromatographic or antibody based techniques, sensitive colorimetric detection, surface plasmon resonance (SPR), and electrochemical impedance spectroscopy (EIS) measurement. In this work, we describe the graphene field effect transistor (GFET)-based selective detection of lysozyme molecules by utilizing a large area CVD-grown graphene. To enhance the selective detection, the graphene is successively functionalized with aptamers in the form of single-stranded probe DNAs (pDNAs) which are specifically designed for lysozyme binding. These aptamers were anchored to the graphene surface through the cross-linking molecule 1-pyrenebutanoic acid succinimidyl ester (PBASE). The GFET devices were configured as electrolyte-gated FETs where the graphene is the conducting channel formed between the source and the drain electrodes on the SiO2/Si substrate as schematically depicted in Figure 1 of the supplemented image. A PBS solution (0.01X) was used as the top gating dielectric. The pyrene group terminated PBASE is coupled to the graphene surface via the π-π stacking forces. The 5’-amine-modified pDNAs were attached to the amine-reactive succinimide group of PBASE by the conjugation reaction between the amine groups. The Id−Vgs characteristics of the GFET devices were measured sequentially after each functionalization step and exposure to the target lysozyme molecules. The binding of the lysozyme molecules with the pDNAs induces changes in the charge carrier density in the graphene channel. This causes a detectable change in the Dirac voltage (VDirac) or the charge neutrality point (VCNP) in the Id−Vgs characteristics of the GFET. Our experimental results demonstrate detectable change in the VCNP with varying concentrations of lysozyme indicating a selective detection capability of lysozyme up to 1 µM. Figure 2 in the attached image shows the transfer process of large area CVD-grown graphene from SiO2/Si substrate onto the pre-fabricated 4 independently addressable gold electrodes. The CVD grown graphene sample was purchased from Graphene Supermarket (NY, USA). The transfer process starts with spin coating of the graphene by a support layer of poly(methyl methacrylate) (PMMA) at 3000 RPM followed by immersion into 6M KOH solution for 30 minutes at 80oC. This results in the etching of the underneath SiO2 layer and separation of the top PMMA/graphene bilayer from the substrate. The PMMA protected graphene layer was then collected on top of the pre-fabricated gold electrodes and dried at room temperature. The electrode was then immersed into acetone for 8 hours to dissolve the top PMMA layer followed by annealing at 250°C for 2 h in argon-filled furnace. Figure 3 shows the Id-Vgs characteristics of the GFET devices at different functionalization steps. A left-shift of VCNP was observed after each step of functionalization with PBASE linker, Tween 20 and the pDNA attachment. This is due to the n-doping effect of graphene by the attached species on the surface. After exposing to 1 nM Lysozyme solution, a right-shift in VCNP was observed due to the positive charge of lysozyme molecules at neutral pH of 7.0. The concentration dependent shifting of the FET measurements is demonstrated in Figure 4. With increasing concentration of lysozyme, the amount of right-shift in VCNP increases and saturates at a lysozyme concentration of 1µM as shown in the calibration curve in Figure 5. The target selectivity of the GFET-based sensor, when investigated against the bovine serum albumin (BSA) is presented in Figure 6. The sensor shows reasonable selectivity toward lysozyme molecules and against the BSA molecules. Finally, the specificity of the aptamer modified sensor is tested against the unmodified GFET which shows negligible response to the lysozyme, verifying the effectiveness of the aptamers as bio-recognition elements for the GFET-based biosensor. Figure 1

Nanocarbon-Based Field-Effect Transistor Biosensors (bioFETs) for Real-Time Detection of DNA Sequences

Field-effect transistor devices based on functionalized nanocarbon materials (2-D graphene thin layers and 1-D carbon nanotubes) are a promising technology for biomolecular sensing applications. In particular, this type of devices allows for label-free detection of DNA hybridization with the advantage of an easy and direct electrical readout in comparison with most routine techniques. In this work, we report on the design and fabrication of nanocarbon-based bioFETs for DNA hybridization detection. Employing a combination of microfluidics and real-time electrical measurements, we investigated the response of bioFETs to single-stranded DNA (ssDNA) probe tethering and hybridization with complementary target ssDNA in saline buffer solution. The bioFETs were assembled from individual carbon nanotubes or graphene nanoribbons connected by pairs of metallic electrodes using photolithography techniques. Transfer curves of bioFETs in saline buffer were measured using an immersed pseudo-reference electrode. We obtained transfer curves showing current modulation with high OFF-current at the charge-neutrality point. Probe and target ssDNA were chosen as 22-nucleotide-long complementary sequences. Probe tethering on nanocarbon was obtained using diazonium chemistry followed by EDC/NHS coupling of an amine moiety placed at the extremity of the DNA probe [1]. Transfer characteristics were measured before/after probe and target injection. Moreover, to further characterize the bioFETs response, probe immobilization and probe-target hybridization were recorded in real-time using electrical readout. The development of highly selective and ultra-sensitive sensors for DNA will open novel technological avenues to improve the detection of genetic biomarkers for various diseases. [1] Bouilly, D. et al. Nano Letters 16, 4679–4685 (2016)

Ammonia Sensing Using Transfer-Free in Situ CCVD Grown Nanocrystalline Graphene Field Effect Transistors

Solid state gas sensors for monitoring toxins in the environment, chemical exhaust or biological samples have received increasing attention over the recent years. A promising material for this application is graphene, having demonstrated single molecule detection capability and sensitivity towards a variety of gases [1]. In this contribution we present a transfer-free production method of nanocrystalline graphene field effect transistors (nGFETs), which can be used as very sensitive gas sensors. By means of our PMMA-enhanced in situ catalytic chemical vapor deposition (CCVD) process [2] we fabricate few-layered nGFETs. By this method, hundreds of nGFETs are simultaneously fabricated on a single 2 inch oxidized, highly p-doped silicon wafer. After fabrication the individual metal catalyst sites are used as electrical contacts to the nanocrystalline graphene that bridges the gap along the insulating silicon dioxide surface (see figure 1a & b). Hereby, post-growth graphene-transfer and etching as well as cleaning steps are obsolete. Material characterization has been done using a Horiba Labram HR800 Raman microscope with a 632nm laser, yielding spectra showing strong G (1590 cm-1) and D (1350 cm-1) signatures but only a weak 2D (2700 cm-1) signal. Hence, a near edge X-ray absorption fine structure (NEXAFS) analysis at the carbon K-edge of the CCVD graphene was done and analyzed in respect to its C-C sp2 bonding structure. For that purpose, reference spectra of graphene and graphene oxide, both supplied by Graphenea, have been recorded. The NEXAFS experiments were performed at the plane grating monochromator (PGM) [3] beamline of PTB at the BESSY II synchrotron employing radiometrically calibrated instrumentation. By means of a linear combination of reference NEXAFS spectra for graphene and graphene oxide, the composition of the CCVD grown graphene layers is derived. By electrical characterization using an HP4156A precision semiconductor parameter analyzer we investigate the intrinsic electrical properties and gas sensing capabilities of our nGFETs using our self made vacuum probing station. The intrinsic properties have been tested under a vacuum pressure of 2*10-5mbar revealing a negative shift of the charge neutrality point in the input characteristics of our nGFETs, thereby verifying atmospheric hole-doping to our as-fabricated devices. Afterwards, the nGFETs have been thermally annealed under vacuum to desorb remaining adsorbents in order to restore the intrinsic properties of the devices. Subsequently, the devices have been exposed down to a concentration of 200ppbv of ammonia. As a consequence a positive shift of the charge neutrality point is recorded, indicating an increase in electron doping. During this exposure the hysteresis effect of the nGFETs is enhanced and its origin will be discussed. We will also report on the dynamical behavior (e.g. response and recovery times) of our devices in comparison to a simple commercial MQ-5 gas sensor. [1] F. Schedin, A. K. Geim, S. V. Morozov, E. W. Hill, P. Blake, M. I. Katsnelson and K. S. Novoselov; “Detection of individual gas molecules adsorbed on graphene, Nat Mater, 6, 652 (2007). [2] D. Noll, U. Schwalke; ”PMMA-enhancement of the lateral growth of transfer-free in situ CCVD grown graphene”, in 2016 13th International Multi-Conference on Systems, Signals & Devices (SSD), p. 458 (2016). [3] F. Senf, U. Flechsig, F. Eggenstein, W. Gudat, R. Klein, H. Rabus, G. Ulm, J. Synchrotron Rad. (1998) 5, 780-782. Acknowledgement The authors would like to thank PD Dr. Emanuel Ionescu and Benjamin Juretzka from Technische Universität Darmstadt for the opportunity to record Raman spectra at the group of dispersive solids. Figure 1

(Invited) Two-Dimensional Wide Bandgap Materials for Electronic Applications

The emerging two-dimensional (2D) materials, such as graphene, MoS2, and hexagonal boron nitride (hBN), are promising candidates for next-generation electronics. Gapless graphene has intriguing properties of ultrahigh mobility, transparency, tunable charge neutral point, etc. Differently from conventional semiconductors, 2D semiconductors, representatively transition metal dichalcogenides (TMDs), have shown unprecedented characteristics, such as indirect-to-direct band transition, trions, and valleys, which can be utilized to realize new-function devices. Despite rigorous researches on these 2D materials, wide bandgap 2D materials have not been studied widely and actively due to limited material sources. However, it should be noted that these wide bandgap 2D materials are essential for practical applications of 2D-material-based electronics because these insulating blocks are required to form multi-stacked 2D devices. Here I present electrical/optical characteristics of wide bandgap 2D materials, such as hBN and MoO3, and electronic devices consisting of van der Waals heterostructures of the 2D materials. 2D insulators with large bandgap were used as atomically thin insulator or tunnel barrier for ultrathin devices. It was also confirmed that 2D oxide layers can be epitaxially grown on 2D substrates of graphene and hBN. Growth of highly crystalline 2D oxide layers on other 2D layers shows great potential for large scale production of 2D oxides, which are highly useful for future electronics. Our approaches would allow us to come closer to practical applications of 2D materials in industry.

Selective Detection of Lysozyme Biomarker Utilizing Large Area Chemical Vapor Deposition-Grown Graphene-Based Field-Effect Transistor

Selective and rapid detection of biomarkers is of utmost importance in modern day health care for early stage diagnosis to prevent fatal diseases and infections. Among several protein biomarkers, the role of lysozyme has been found to be especially important in human immune system to prevent several bacterial infections and other chronic disease such as bronchopulmonary dysplasia. Thus, real-time monitoring of lysozyme concentration in a human body can pave a facile route for early warning for potential bacterial infections. Here, we present for the first time a label-free lysozyme protein sensor that is rapid and selective based on a graphene field-effect transistor (GFET) functionalized with selectively designed single-stranded probe DNA (pDNA) with high binding affinity toward lysozyme molecules. When the target lysozyme molecules bind to the surface-immobilized pDNAs, the resulting shift of the charge neutrality points of the GFET device, also known as the Dirac voltage, varied systematically with the concentration of target lysozyme molecules. The experimental results show that the GFET-based biosensor is capable of detecting lysozyme molecules in the concentration range from 10 nM to 1 µM.

Two-dimensional Dirac fermions in thin films of Cd3As2

Two-dimensional states in confined thin films of the three-dimensional Dirac semimetal Cd3As2 are probed by transport and capacitance measurements under applied magnetic and electric fields. The results establish the two-dimensional Dirac electronic spectrum of these states. We observe signatures of p-type conduction in the two-dimensional states as the Fermi level is tuned across their charge neutrality point and the presence of a zero energy Landau level, all of which indicate topologically non-trivial states. The resistance at the charge neutrality point is approximately h/e2 and increases rapidly under the application of a magnetic field. The results open many possibilities for gate-tunable topological devices and for the exploration of novel physics in the zero energy Landau level.

Andreev reflection in a proximity junction of graphene: Influence of a naturally formed pn junction

We theoretically study the electron transport in a proximity junction made by partially covering a graphene sheet with a bulk superconductor. Such a system serves as a normal conductor–superconductor (NS) junction and its electron transport properties are mainly governed by Andreev reflection (AR). In this NS junction, the charge neutrality point (CNP) of graphene must spatially vary as a result of the penetration of excess carriers induced from the superconductor. By taking this into account in the electron-doped case, we calculate the differential conductance GNS of the graphene NS junction. Owing to the variation of the CNP in the uncovered region, a smooth pn junction is naturally formed at the point where the Fermi energy crosses the CNP. The resulting pn junction causes unusual behavior of GNS. For example, a resonant peak structure appears in GNS reflecting the fact that quasi-bound states are created by the pn junction.

Magnetotransport of Monolayer Graphene with Inert Gas Adsorption in the Quantum Hall Regime

The surface of graphene is easily accessible from outside, and thus it is a suitable material to study the effects of molecular adsorption on the electric transport properties. We investigate the magnetotransport of inert-gas-adsorbed monolayer graphene at a temperature of 4.4 K under a magnetic field ranging from 0 to 7 T. We introduce 4He or Ar gas at low temperature to graphene kept inside a sample cell. The magnetoresistance change ΔRxx and Hall resistance change ΔRxy from the pristine graphene are measured as a function of gate voltage and magnetic field for one layer of adsorbates. ΔRxx and ΔRxy show oscillating patterns related to the constant filling factor lines in a Landau-fan diagram. Magnitudes of these quantities are relatively higher around a charge neutral point and may be mass-sensitive. These conditions could be optimized for development of a highly sensitive gas sensor.