Doped Graphene Layers Research Articles

High-performance electrochemical detection of glucose in human blood serum using a hierarchical NiO2 nanostructure supported on phosphorus doped graphene

This report describes the development of high-performing non-enzymatic glucose sensors based on a combination of a phosphorous doped graphene (PG) layer and a composite metal oxide. Using a reflux technique and high-temperature annealing, a combined sheet-like NiO2@PG nanocomposite was created. This structure was examined using a variety of physicochemical methods, including thermogravimetric analysis, X-ray photoelectron spectroscopy, and scanning electron microscopy. The NiO2@PG nanocomposite offers a large surface area and an easily penetrable structure, allowing for greater chemical sensitivity to glucose oxidation. The dynamic range of the developed glucose sensor is relatively broad, ranging 10–170 μM. It exhibits a sensitivity of 70.28 μA μM−1 cm−2 , a small limit of detection of 7.83 μM, with a rapid response time of 1.07 s. The sensor also shows outstanding selectivity, long-term stability, reproducibility, and favorable repeatability, and can detect glucose in human serum samples. Additionally, the NiO2@PG composites sensing ability can be enhanced through ligand-to-metal charge-transfer, as validated by density functional theory. Studies on the mechanism of sensing and chemical structure of the composite after stabilization suggest it should prove useful in real-world applications.

Stopping and image forces acting on a charged particle moving near a graphene- Al2O3 -graphene heterostructure

We study the impact of plasmon-phonon hybridization on the stopping and image forces acting on a charged particle moving parallel to a graphene-${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$-graphene sandwichlike composite system by considering a broad domain in the parameter space. The effective dielectric function of the system is obtained using two descriptions of the electronic response of doped graphene: an ab initio method based on the time-dependent density functional theory calculations and an analytical expression based on the massless Dirac fermion (MDF) approximation for graphene $\ensuremath{\pi}$ bands. It is found that the main discrepancies between the two methods come from the high-energy interband electron transitions, which are included in the ab initio method but not in the MDF method. Special attention is paid to the regime of low-particle speeds, where the MDF method compares well with the ab initio method, but the modeling is sensitive to the effects of finite temperature and the treatment of phenomenological damping. It is observed at low particle speeds that both forces exhibit an interesting interplay between the hybrid modes of the phononic type and the continuum of the intraband electron-hole excitations in graphene. Furthermore, the effects of the lowest-energy antisymmetric modes, which exhibit acoustic dispersions at long wavelengths in a system with equally doped graphene layers, are exposed in calculations of the stopping force on a co-moving pair of incident particles with opposite charges that are symmetrically positioned around the target system.

Nanohybrids that consisit of p-type, nitrogen-doped ZnO and graphene nanostructures: synthesis, photophysical properties, and biosensing application

ZnO, a promising material for optoelectronic applications, has attracted considerable attention due to its wide and direct band gap and large exciton binding energy. To understand the applications of this material, fabrication of high quality p-type ZnO is a key step. However, a reliable p-type doping of this material remains a major challenge. In this study, we report p-type nitrogen-doped ZnO nanoparticle, grown in a nitrogen doped graphene layer matrix by a plasma heating process using a natural protein and zinc nitrate as the precursors. The structural characterizations are developed by several microscopic techniques including the field emission electron microscopy, high resolution transmission electron microscopy, x-ray photoelectron spectroscopy, and micro-Raman analysis. In addition, the ultraviolet (UV)–visible absorption characteristics and photoluminescence properties of the samples are studied. Its p-type conduction behaviour is confirmed by the Hall effect measurement, which was ascribed to the high nitrogen dopant concentration in the Zn-poor ZnO, and the related mechanism for the p-type behaviour is also discussed. Moreover, the results of the glucose detection based on the strong green luminescence of glucose indicate that the nitrogen-doped ZnO nanodots/nitrogen-doped graphene layer nanohybrid is also a competitive candidate in the biosensing field.

CuZn@N‑doped graphene layer for upgrading of furfural to furfuryl alcohol

The upgrading of furfural to furfuryl alcohol is still a challenge due to the multiple conversion pathways of furfural. Herein, an efficient Zn doped Cu@NG catalyst (Cu, Zn anchored in N‑doped graphene layer) was built through one-pot ammonia evaporation strategy. The characterization and experimental results confirmed that the introduction of Zn promotes the dispersion of Cu nanoparticles, thus improving the activity of the catalyst; On the other hand, the side reaction of decarbonylation is inhibited, thereby improving the selectivity of the furfuryl alcohol. The above two aspects endows the catalyst excellent catalytic performance. Under the optimal reaction conditions, the yield of furfuryl alcohol is close to 99.0%, and the catalyst showed robust stability in the process of five recycles, which makes it has a great industrial application prospect.

Direct fabrication and characterization of vertically stacked Graphene/h-BN/Graphene tunnel junctions

We have used a lithography free technique for the direct fabrication of vertically stacked two-dimensional (2D) material-based tunnel junctions and characterized by Raman, AFM, XPS. We fabricated Graphene/h-BN/Graphene devices by direct deposition of graphene (bottom layer), h-BN (insulating barrier) and graphene (top layer) sequentially using a plasma enhanced chemical vapor deposition on Si/SiO2 substrates. The thickness of the h-BN insulating layer was varied by tuning the plasma power and the deposition time. Samples were characterized by Raman, AFM, and XPS. The I-V data follows the barrier thickness dependent quantum tunneling behavior for equally doped graphene layers. The resonant tunneling behavior was observed at room temperature for oppositely doped graphene layers where hydrazine and ammonia were used for n-doping of one of the graphene layers. The resonance with negative differential conductance occurs when the band structures of the two electrodes are aligned. The doping effect of the resonant peak is observed for varying doping levels. The results are explained according to the Bardeen tunneling model.

(Invited) Machine Learned Deep Neural Networks to Simulate Raman Spectrum of Defective Graphene Systems

Raman spectrum is a common spectroscopic tool used in materials synthesis and characterization. The width and shift of Raman peaks are commonly used in characterization of graphene samples. In this presentation, I will highlight our efforts in using machine learned models to develop an efficient and accurate methodology for simulating accurate spectroscopic signatures in graphene. We are using deep neural networks trained with density functional theory calculations to take advantage of the accuracy of this approach while being able to simulate large atomic systems corresponding to doped graphene layers. We are then able to evaluate the empirical models presently used to assess the defect levels in graphene and compare them to state-of-the-art calculation on such systems.

Manipulation of electronic property of epitaxial graphene on SiC substrate by Pb intercalation

Manipulating the electronic properties of graphene has been a subject of great interest since it can aid material design to extend the applications of graphene to many different areas. In this paper, we systematically investigate the effect of lead (Pb) intercalation on the structural and electronic properties of epitaxial graphene on the SiC(0001) substrate. We show that the band structure of Pb-intercalated few-layer graphene can be effectively tuned through changing intercalation conditions, such as coverage, location of Pb, and the initial number of graphene layers. Lead intercalation at the interface between the buffer layer (BL) and the SiC substrate decouples the BL from the substrate and transforms the BL into a $p$-doped graphene layer. We also show that Pb atoms tend to donate electrons to neighboring layers, leading to an $n$-doping graphene layer and a small gap in the Dirac cone under a sufficiently high Pb coverage. This paper provides useful guidance for manipulating the electronic properties of graphene layers on the SiC substrate.

Finite Element Modelling of Bandgap Engineered Graphene FET with the Application in Sensing Methanethiol Biomarker.

In this work, we have designed and simulated a graphene field effect transistor (GFET) with the purpose of developing a sensitive biosensor for methanethiol, a biomarker for bacterial infections. The surface of a graphene layer is functionalized by manipulation of its surface structure and is used as the channel of the GFET. Two methods, doping the crystal structure of graphene and decorating the surface by transition metals (TMs), are utilized to change the electrical properties of the graphene layers to make them suitable as a channel of the GFET. The techniques also change the surface chemistry of the graphene, enhancing its adsorption characteristics and making binding between graphene and biomarker possible. All the physical parameters are calculated for various variants of graphene in the absence and presence of the biomarker using counterpoise energy-corrected density functional theory (DFT). The device was modelled using COMSOL Multiphysics. Our studies show that the sensitivity of the device is affected by structural parameters of the device, the electrical properties of the graphene, and with adsorption of the biomarker. It was found that the devices made of graphene layers decorated with TM show higher sensitivities toward detecting the biomarker compared with those made by doped graphene layers.

Interaction between Yb3+ doped glasses substrates and graphene layers by raman spectroscopy

We report the interaction between 5.0 wt.% and 20 wt.% Yb3+ doped phosphate glasses substrates with graphene single layer (1L) and bilayer (2L) by Raman spectroscopy. This interaction is studied in terms of shifting of the G, 2D and 2D’ bands of graphene, which is higher in 1L than in 2L graphene on the 20% Yb3+ substrate, in respect to the 5.0 wt.% substrate. Furthermore, changes in the full width at half maximum of the G, 2D and 2D’ bands, together with the I2D/IG ratio, for doped and undoped substrates, reveal this interaction. Additionally, when a single layer graphene is found on the surface of both substrates, some Raman peaks on the less doped sample are absent, whereas they are present on the more doped sample, indicating that the presence of Yb3+ ions increases the interaction between the substrate and the single layer graphene. The reported results give insights about graphene interaction with doped rare earth substrates.

Microwave second-harmonic generation due to mobile carriers in doped graphene lacking center-of-inversion symmetry

We present a Boltzmann equation analysis of microwave second-harmonic generation (SHG) due to mobile carriers in the doped graphene layer grown epitaxially on a SiC substrate (or, alternatively, deposited on a h-BN substrate) and lost its space inversion symmetry owing to the graphene-substrate interaction. The latter also induces a finite gap between the conduction and the valence band at the corners of the graphene’s Brillouin zone. Within such a gapped band model, which includes, as a key necessary ingredient, the trigonal warping of the graphene conduction band involved, we derive an explicit analytical expression for the SHG response function of the graphene’s mobile carriers at room temperature, when the effect of electron-momentum relaxation is primarily due to acoustic phonon scattering. The results obtained show that the possibility to tune the doping level of graphene by an external gate voltage allows one to maximize the output power of the microwave SHG, which may be of practical interest for the designs of graphene-based nonlinear devices operating in the millimeter-wave range.

Electronic and optical properties of vacancy and B, N, O and F doped graphene: DFT study

Structural and optical properties of graphene with a vacancy and B, N, O and F doped graphene have been investigated computationally using density functional theory (DFT). Absorption spectrum of all considered structures for both cases of in plane E ⊥ c and out of plane E ‖ c polarization of light are illustrated in figure a and b respectively. Optical properties of O and F doped graphene were calculated for the first time and other result compared with experimental and theoretical reports. • In all cases, the graphene with a vacancy and doped graphenes remain planar so that new structures like pure graphene have sp 2 hybridization. • It was found that 3.125% boron doped graphene is a p-type and oxygen and nitrogen doped graphene are a n-type semiconductor. (a) For requesting structures, band gap opened and for F doped graphene band gap was different for spin up and down that is caused this structure could be a good candidate for spintronic device design. the magnetic moment of this structure is 0.71 μB/cell. • The graphene with a vacancy undergoes a John-Teller distortion which leads to the saturation of two of the three dangling bonds toward the missing atom. Graphene with a vacancy has magnetic moment of 0.89 μB/cell and two of the carbon atoms close to the vacancy site move towards each other forming a C–C distance of 2.226Å. • All absorption peaks for different structures have shifted toward larger energies (blue shift) and peak intensities decrease with increasing number of electrons entering the structure. Structural and optical properties of graphene with a vacancy and B, N, O and F doped graphene have been investigated computationally using density functional theory (DFT). We find that B is a p-type while N, O and F doped graphene layers, as well as graphene with a vacancy are n-type semiconductors. Optical properties for both cases of in plane E ⊥ c and out of plane E ‖ c polarization of light are investigated. It is observed that with the increase in the number of electrons entering the supercell, the amount of absorption of the system decreases and the absorption peaks are transferred to higher energies (blue shift).

Hierarchical heterostructure based on molybdenum dichalcogenide nanosheets assembled nitrogen doped graphene layers for efficient hydrogen evolution reaction

A novel hierarchical heterostructure of molybdenum dichalcogenide nanosheets homogeneously assembled on nitrogen doped graphene layer (MoS2 NSs/N-doped GR) was successfully developed by an easy and cost-effective strategy. The MoS2 NSs/N-doped GR was then utilized for accelerating hydrogen evolution reaction (HER) in alkaline medium. It was found that the nanohybrid exhibited highly efficient catalytic activity towards HER with higher current density and smaller overpotential as compared to those of other synthesized materials. In this context, the nanohybrid required an overpotential of only 208 mV to achieve a current density of 10 mA cm−2, along with a small Tafel slope of 79 mV dec-1, which is close to those of a commercial Pt/C. An outstanding stability of the nanohybrid for long-term operation was also demonstrated. The obtained results suggest that the MoS2 NSs/N-doped GR nanohybrid can be a potential candidate for HER in overall water splitting application.

Tuning of Graphene Work Function by Alkyl Chain Length in Amine-Based Compounds

In this study, the effect of alkyl chain length in amine-based compounds on the work function of graphene was investigated. The graphene was synthesized by the chemical vapor deposition method. The graphene layers were functionalized by amine-based groups using a simple spin-coating method. The amine-based compounds were composed of phenyl amine and methyl-, ethyl-, propyl-, n/t-butyl-, and octyl-phenyl amine groups. Materials were confirmed by X-ray photoelectron spectroscopy to show the C and N bonding. The work function of the doped graphene layers decreased because of the effect of the doping agents. Among the doped graphene samples, t-butyl-phenyl amine functionalized graphene achieved the lowest work function of 3.89 eV (compared with 4.43 eV for pristine graphene). Further, the sheet resistance of n-doped graphene increased, confirming the high concentration of n-doping agents on the graphene layers. These results suggest the best alkyl chain is the t-butyl group to reduce the work function of graphene, and promise the use of these materials as cathodes for opto-electronic applications.

Quantum plasmons with optical-range frequencies in doped few-layer graphene

Although plasmon modes exist in doped graphene, the limited range of doping achieved by gating restricts the plasmon frequencies to a range that does not include the visible and infrared. Here we show, through the use of first-principles calculations, that the high levels of doping achieved by lithium intercalation in bilayer and trilayer graphene shift the plasmon frequencies into the visible range. To obtain physically meaningful results, we introduce a correction of the effect of plasmon interaction across the vacuum separating periodic images of the doped graphene layers, consisting of transparent boundary conditions in the direction perpendicular to the layers; this represents a significant improvement over the exact Coulomb cutoff technique employed in earlier works. The resulting plasmon modes are due to local field effects and the nonlocal response of the material to external electromagnetic fields, requiring a fully quantum mechanical treatment. We describe the features of these quantum plasmons, including the dispersion relation, losses, and field localization. Our findings point to a strategy for fine-tuning the plasmon frequencies in graphene and other two-dimensional materials.

Excitation of hybridized Dirac plasmon polaritons and transition radiation in multi-layer graphene traversed by a fast charged particle

We analyze the energy loss channels for a fast charged particle traversing a multi-layer graphene (MLG) structure with N layers under normal incidence. Focusing on a terahertz (THz) range of frequencies, and assuming equally doped graphene layers with a large enough separation d between them to neglect interlayer electron hopping, we use the Drude model for two-dimensional conductivity of each layer to describe hybridization of graphene’s Dirac plasmon polaritons (DPPs). Performing a layer decomposition of ohmic energy losses, which include excitation of hybridized DPPs (HDPPs), we have found for N = 3 that the middle HDPP eigenfrequency is not excited in the middle layer due to symmetry constraint, whereas the excitation of the lowest HDPP eigenfrequency produces a Fano resonance in the graphene layer that is first traversed by the charged particle. While the angular distribution of transition radiation emitted in the far field region also shows asymmetry with respect to the traversal order by the incident charged particle at supra-THz frequencies, the integrated radiative energy loss is surprisingly independent of both d and N for N ≤ 5, which is explained by a dominant role of the outer graphene layers in transition radiation. We have further found that the integrated ohmic energy loss in optically thin MLG scales as ∝1/N at sub-THz frequencies, which is explained by exposing the role of dissipative processes in graphene at low frequencies. Finally, prominent peaks are observed at supra-THz frequencies in the integrated ohmic energy loss for MLG structures that are not optically thin. The magnitude of those peaks is found to scale with N for N ≥ 2, while their shape and position replicate the peak in a double-layer graphene (N = 2), which is explained by arguing that plasmon hybridization in such MLG structures is dominated by electromagnetic interaction between the nearest-neighbor graphene layers.

Relativistic effects in the energy loss of a fast charged particle moving parallel to a two-dimensional electron gas

Abstract We present a fully relativistic formulation for the energy loss rate of a charged particle moving parallel to a sheet containing two-dimensional electron gas, allowing that its in-plane polarization may be described by different longitudinal and transverse conductivities. We apply our formulation to the case of a doped graphene layer in the terahertz range of frequencies, where excitation of the Dirac plasmon polariton (DPP) in graphene plays a major role. By using the Drude model with zero damping we evaluate the energy loss rate due to excitation of the DPP, and show that the retardation effects are important when the incident particle speed and its distance from graphene both increase. Interestingly, the retarded energy loss rate obtained in this manner may be both larger and smaller than its non-retarded counterpart for different combinations of the particle speed and distance.

Semitransparent Flexible Organic Solar Cells Employing Doped-Graphene Layers as Anode and Cathode Electrodes.

Semitransparent flexible photovoltaic cells are advantageous for effective use of solar energy in many areas such as building-integrated solar-power generation and portable photovoltaic chargers. We report semitransparent and flexible organic solar cells (FOSCs) with high aperture, composed of doped graphene layers, ZnO, P3HT:PCBM, and PEDOT:PSS as anode/cathode transparent conductive electrodes (TCEs), electron transport layer, photoactive layer, and hole transport layer, respectively, fabricated based on simple solution processing. The FOSCs do not only harvest solar energy from ultraviolet-visible region but are also less sensitive to near-infrared photons, indicating semitransparency. For the anode/cathode TCEs, graphene is doped with bis(trifluoromethanesulfonyl)-amide or triethylene tetramine, respectively. Power conversion efficiency (PCE) of 3.12% is obtained from the fundamental FOSC structure, and the PCE is further enhanced to 4.23% by adding an Al reflective mirror on the top or bottom side of the FOSCs. The FOSCs also exhibit remarkable mechanical flexibilities through bending tests for various curvature radii.

Tunable graphene/dielectric laminate for adaptive low-gigahertz shielding and absorbing screens

Shielding and absorbing screens made of tunable graphene/ dielectric laminate (GL) doped by an electrostatic field bias are designed applying simple modelling procedures in the low-gigahertz frequency range. The adaptive response of both types of screens is achieved through the control of the effective sheet resistance of the GL, consisting of a proper number of doped graphene layers separated by thin films of polyethylene terephthalate (PET). The performances of the shielding screen are predicted in the frequency range up to 1 THz, using simple approximate expressions and rigorous simulation models. The multilayer absorber is a twoperiod dielectric Salisbury screen, in which the outer lossy sheet is made of a single-layer graphene and the inner one is a GL with two electrically doped graphene layers. The reflection coefficient is minimized at each frequency through the proper setting of the electrostatic field bias. The optimal absorbing performances of the screen are predicted in the frequency range between 2 GHz and 25 GHz.

Ab initio study of the electron energy loss function in a graphene-sapphire-graphene composite system

The propagator of a dynamically screened Coulomb interaction W in a sandwichlike structure consisting of two graphene layers separated by a slab of Al2O3 (or vacuum) is derived from single-layer graphene response functions and by using a local dielectric function for the bulk Al2O3. The response function of graphene is obtained using two approaches within the random phase approximation (RPA): an ab initio method that includes all electronic bands in graphene and a computationally less demanding method based on the massless Dirac fermion (MDF) approximation for the low-energy excitations of electrons in the π bands. The propagator W is used to derive an expression for the effective dielectric function of our sandwich structure, which is relevant for the reflection electron energy loss spectroscopy of its surface. Focusing on the range of frequencies from THz to mid-infrared, special attention is paid to finding an accurate optical limit in the ab initio method, where the response function is expressed in terms of a frequency-dependent conductivity of graphene. It was shown that the optical limit suffices for describing hybridization between the Dirac plasmons in graphene layers and the Fuchs-Kliewer phonons in both surfaces of the Al2O3 slab, and that the spectra obtained from both the ab initio method and the MDF approximation in the optical limit agree perfectly well for wave numbers up to about 0.1 nm−1. Going beyond the optical limit, the agreement between the full ab initio method and the MDF approximation was found to extend to wave numbers up to about 0.3 nm−1 for doped graphene layers with the Fermi energy of 0.2 eV.

Work function of graphene multilayers on SiC(0001)

The work function and electronic structure of epitaxial graphene as well as of quasi-freestanding graphene multilayer samples were studied by Kelvin probe and angle resolved photoelectron spectroscopy. The work function converges towards the value of graphite as the number of layers is increased. Thereby, n-type doped epitaxial graphene layers have a work function lower than graphite and p-type doped quasi-freestanding graphene layers exhibit a work function higher than graphite. We explain the behaviour by the filling of the -bands due to substrate interactions.