P-doped Graphene Research Articles

Fabrication of a Graphene/ZnO based p-n junction device and its ultraviolet photoresponse properties

Graphene with a zero-bandgap energy is easily doped using a chemical dopant, and a shift upwards or downwards in the Fermi level is generated. Moreover, the integration of inorganic material into the doped graphene changes the physical and chemical properties of the material. For this purpose, we successfully fabricated a p-n junction device by depositing an n-typed ZnO layer on p-doped graphene and studied the ultraviolet (UV) photoresponse properties under a photocurrent (UV light on) and a dark current (UV light off). Two devices, lateral and vertical, were developed by alternating the thickness of the ZnO layer, and the photoresponse mechanisms were described on the basis of the contact potential difference.

Substrate and atmosphere influence on oxygen p-doped graphene

The mechanisms responsible for p-type doping of substrate supported monolayer graphene (Gr) by thermal treatments in oxygen ambient have been investigated by micro-Raman spectroscopy, atomic force microscopy (AFM) and X-ray Photoelectron Spectroscopy (XPS), considering commonly employed dielectric substrates, such as SiO2 and Al2O3 thin films grown on Si. While a high p-type doping (∼1013 cm−2) is observed for Gr on SiO2, no significant doping is found for Gr samples on the Al2O3 substrate, suggesting a key role of the Gr/SiO2 interface states in the trapping of oxygen responsible for the Gr p-type doping. Furthermore, we investigated the doping stability of Gr on SiO2 during subsequent thermal treatments in nitrogen (N2), carbon dioxide (CO2), water (H2O) or in vacuum controlled atmospheres. These processes induce only minor effects on the doping of Gr but for H2O and principally affect its defectiveness, suggesting that the literature reported air influence on the doping depends on water present in the atmosphere.

Three Dimensional P-doped Graphene Synthesized by Eco-Friendly Chemical Vapor Deposition for Oxygen Reduction Reactions.

Heteroatom doping provides possibilities for changing the electronic properties of graphene. Three Dimensional P-doped graphene (3DPG) was fabricated via chemical vapor deposition (CVD) using nickel foam as template and triphenylphosphine (TPP) as C and P sources simultaneously without using toxic organic solvent as carrier liquid. The invasion of P atoms into graphene networks make them non-electroneutral and consequently favor the adsorption of oxygen and O-O bond cleavage due to the charge polarization increase of the P-C bond. Thus, the as-prepared 3DPG served as an efficient electrocatalyst for oxygen reduction reaction (ORR). Additionally, the 3D porous structure is favorable for the mass transfer of electrolytes ions, hence 3DPG exhibit better electrocatalytic activity, long-term stability, and tolerance to crossover effect of methanol than pristine 3D graphene and Pt/C for ORR.

Interpenetrated Graphene/WSe2 Lateral Heterostructures for Barrierless Ohmic-Contacted p-FETs

High contact resistance between TMDs and metals has been identified as an outstanding issue. TMDs tend to form a substantial Schottky barrier (SB) with commonly used metals. Air-stable graphene electrodes stacked on the WSe2 by transfer technique have been reported. However, unipolar conduction has not been achieved in FETs based on TMD materials and graphene electrodes. Here we report a synthetic process to directly grow WSe2 monolayers along the edge of patterned few-layered graphene, forming the lateral heterojunction between the graphene and monolayer WSe2. The few-layered WSe2 bridges the few-layer graphene and monolayer WSe2, and an interpenetrating layered structure is formed between graphene and WSe2. As a result, the WSe2 monolayer channel and the junction to the graphene can be formed at the same time by the synthesis method, where graphene serves as source and drain. Graphene is in-situ heavily p-doped by H2 with covent bond during WSe2 growing process. Such a unique junction structure leads to a barrier less ohmic contact between doped graphene and WSe2. Unipolar WSe2 PFET with laterally contacted p-doped graphene source/drain (p-G/WSe2 PFET) can be achieved for the first time. With Pd contacted source/drain metal, the unipolar PFET perform Ion/Ioff > 105 and field effect mobility ~20 cm2/Vˑs. Moreover we can still have unipolar PFET with Ti source/drain metal exceeds 103 Ion/Ioff ratio even Ti work function is low (4.3 eV) and close to the conduction band edge of WSe2 (electron affinity of WSe2~ 4.03 eV).

Strong dependence of flattening and decoupling of graphene on metals on the local distribution of intercalated oxygen atoms

We show that the flattening of a highly corrugated graphene layer grown on Rh(111) is linked to its decoupling from the metal substrate taking place during oxygen intercalation in the lowest moiré sites. We have been able to track this process at the atomic scale by combining scanning tunneling microscopy experiments and first-principles calculations. Initially, intercalated and non intercalated areas can coexist through a careful experimental control of the oxygen dosage and temperature. This allows a direct comparison of the corrugation profiles of both areas. Although the corrugation increases for very low coverages, higher oxygen concentrations provoke the occupation of the lowest moiré sites leading to a flattening of the graphene sheet. This mechanism is confirmed by calculations of the equilibrium geometries for different oxygen concentrations. Electronic structure calculations and scanning tunneling spectroscopy measurements reveal that the flattening of the graphene layer is linked with the local uncoupling from the metal induced by the increasing oxygen concentration. This process, completed only when the lowest moiré sites are filled with oxygen, finally converts a strongly coupled system into a free-standing like p-doped graphene layer.

Density functional study on the adsorption characteristics of O, O2, OH, and OOH of B-, P-doped, and B, P codoped graphenes

Over past years, the excessive use of fossil fuel has posed serious problems such as greenhouse effect and environmental pollution, which threaten human life. Regarded as an ideal substitution for traditional internal combustion engine, low temperature proton exchange membrane fuel cell (PEMFC) converts chemical energy through electrode reaction directly into electrical energy with high efficiency and low pollution. However, the main problem behind the industrialization of PEMFC, is that oxygen reduction reaction (ORR) occurring on the cathode needs precious metal platinum (Pt) as catalyst, which has a limited reserve and is costly. Owing to high activity and stability, the graphenes doped with non-metal B and P, have proven to be excellent alternatives to Pt experimentally. However, the relevant theoretical work is scarce.Adsorptions of the ORR intermediates, i.e., O, O2, OH, and OOH, of doped graphenes are essential for the cathode reaction, which also bring some difficulties to the next step reaction. Therefore, in this paper, based on density functional theory, the adsorption characteristics of O, O2, OH, and OOH of B-doped, P-doped and B, P-codoped graphenes are studied using first-principles calculation code VASP first. By analyzing the adsorption energies, bond lengths, densities of states and charge transfers, the influences of the different dopants on the intermediates are evaluated. Then, the ORR steps are discussed, and the free energy change of each step is further given. The results show that for B-doped and P-doped graphenes, the adsorption energies of various intermediates exhibit similar linear relationships. The adsorption energy of OOH of P-doped graphene (3.26 eV) is much larger than that in B-doped grapheme (0.73 eV). The large adsorption energy of P-doped graphene is beneficial to the fracture reaction of OO bond in OOH, while the small adsorption energy of B-doped graphene can promote the reaction of OH converting into water. Owing to the synergistic effect, the graphene codoped with B and P possesses better catalyzing ability than single B-and P-doped ones. The results are helpful for understanding the excellent performances of codoped graphenes.

Nitrogen and phosphorus dual-doped graphene as a metal-free high-efficiency electrocatalyst for triiodide reduction.

Alternative high-performance electrocatalysts for triiodide (I3-) reduction of low-cost dye-sensitized solar cells (DSSCs) are urgently sought after. To address the concerned issues, we report a facile strategy for engineering the nitrogen and phosphorus dual-doped graphene (NPG) via an efficient ball-milling process, followed by a simple thermal annealing approach utilizing melamine (C3H6N6) and triphenylphosphine ((C6H5)3P) as the N and P source, respectively. When employed as the counter electrode (CE) in DSSCs, such a metal-free material exhibits excellent electrocatalytic activity towards the I3-/I- redox reaction. Dual-doping of N and P heteroatoms can markedly enhance the photovoltaic performance of DSSCs by a synergistic effect and a high conversion efficiency of 8.57% is achieved, which is superior to Pt CE, and much higher than that of the single-component N- or P-doped graphene electrodes. In addition, the NPG CE also shows an outstanding electrochemical stability. The present results demonstrate that the NPG as a low-cost and high-efficiency electrocatalyst for reduction of I3- will be one of the promising CE materials in DSSCs.

Density-Functional Calculation of Methane Adsorption on Graphenes

The adsorption behaviors of methane adsorbed on different graphenes (pristine, and B-, N-, P-, and Al-doped monolayer and multilayer) are analyzed using density-functional theory. The results demonstrate that the sensing performance of graphene as a methane sensor strongly depends on the selection of dopants and the number of layers. The adsorption energy on monolayer P-doped or Al-doped graphene shows about one order of magnitude higher than that with other dopants. In addition, graphenes doped with different impurities show different responses to the charge transfer. A further analysis indicates that the multilayer structure has a positive effect on the adsorption energy on the pristine, B-doped, and N-doped graphene, while the P-doped or Al-doped graphene shows a significant decrease with the increase in the number of layers. Moreover, the multilayer structure has a minor effect on the charge transfer. Based on the combined effects on the adsorption energy and the charge transfer, Al-doped monolayer graphene is the optimal candidate for methane sensing.

Impact of doping on the carrier dynamics in graphene.

We present a microscopic study on the impact of doping on the carrier dynamics in graphene, in particular focusing on its influence on the technologically relevant carrier multiplication in realistic, doped graphene samples. Treating the time- and momentum-resolved carrier-light, carrier-carrier, and carrier-phonon interactions on the same microscopic footing, the appearance of Auger-induced carrier multiplication up to a Fermi level of 300 meV is revealed. Furthermore, we show that doping favors the so-called hot carrier multiplication occurring within one band. Our results are directly compared to recent time-resolved ARPES measurements and exhibit an excellent agreement on the temporal evolution of the hot carrier multiplication for n- and p-doped graphene. The gained insights shed light on the ultrafast carrier dynamics in realistic, doped graphene samples.

Graphene-Based D-Shaped Polymer FBG for Highly Sensitive Erythrocyte Detection

Graphene-based silica fiber-optic sensors, with high sensitivity, fast response, and low cost, have shown great promise for gas sensing applications. In this letter, by covering a monolayer of p-doped graphene on a D-shaped microstructured polymer fiber Bragg grating (FBG), we propose and demonstrate a novel biochemical probe sensor, the graphene-based D-shaped polymer FBG (GDPFBG). Due to the graphene-based surface evanescent field enhancement, this sensor shows high sensitivity to detect surrounding biochemical parameters. By monitoring the Bragg peak locations of the GDPFBG online, human erythrocyte (red blood cell) solutions with different cellular concentrations ranging from 0 to $10^{\mathrm {\mathbf {4}}}$ ppm were detected precisely, with the maximum resolution of sub-ppm. Such a sensor is structurally compact, is clinically acceptable, and provides good recoverability, offering a state-of-the-art polymer-fiber-based sensing platform for highly sensitive in situ and in vivo cell detection applications.

Terahertz conductivity of graphene on boron nitride

The conductivity of graphene on a boron nitride substrate exhibits features in the terahertz (THz) and infrared (IR) frequency regimes that are associated with the periodic moir\'e pattern formed by the weakly coupled two-dimensional materials. The THz and IR features are strongest when the two honeycomb lattices are orientationally aligned, and in this case are Pauli blocked unless the Fermi level is close to $\pm 150$ meV relative to the graphene sheet Dirac point. Because the transition energies between moir\'e bands formed above the Dirac point are small, ac conductivity features in n-doped graphene tend to be overwhelmed by the Drude peak. The substrate-induced band splitting is larger at energies below the Dirac point, however, and can however lead to sharp features at THz and IR frequencies in p-doped graphene. In this Letter we focus on the strongest few THz and IR features, explaining how they arise from critical points in the moir\'e-band joint density-of-states, and commenting on the interval of Fermi energy over which they are active.

Solution-processed transparent blue organic light-emitting diodes with graphene as the top cathode.

Graphene thin films have great potential to function as transparent electrodes in organic electronic devices, due to their excellent conductivity and high transparency. Recently, organic light-emitting diodes (OLEDs)have been successfully demonstrated to possess high luminous efficiencies with p-doped graphene anodes. However, reliable methods to fabricate n-doped graphene cathodes have been lacking, which would limit the application of graphene in flexible electronics. In this paper, we demonstrate fully solution-processed OLEDs with n-type doped multilayer graphene as the top electrode. The work function and sheet resistance of graphene are modified by an aqueous process which can also transfer graphene on organic devices as the top electrodes. With n-doped graphene layers used as the top cathode, all-solution processed transparent OLEDs can be fabricated without any vacuum process.

Observing electron extraction by monolayer graphene using time-resolved surface photoresponse measurements.

Graphene is considered a next-generation electrode for indium tin oxide (ITO)-free organic photovoltaic devices (OPVs). However, to date, limited numbers of OPVs containing surface-modified graphene electrodes perform as well as ITO-based counterparts, and no devices containing a bare graphene electrode have been reported to yield satisfactory rectification characteristics. In this report, we provide experimental data to learn why. Time-resolved surface photoresponse measurements on templated pentacene-on-graphene films directly reveal that p-doped monolayer graphene efficiently extracts electrons, not holes, from photoexcited pentacene. Accordingly, a graphene/pentacene/MoO3 heterojunction displays a large surface photoresponse and, by inference, efficient dissociation of photogenerated excitons, with graphene serving as an electron extraction layer and MoO3 as a hole extraction layer. In contrast, a graphene/pentacene/C60 heterojunction yields a comparatively insignificant surface photoresponse because both graphene and C60 act as competing electron extraction layers. The data presented herein provide experimental insight for future endeavors involving bare graphene as an electrode for organic photovoltaic devices and strongly suggest that p-doped graphene is best considered a cathode for OPVs.

High sensitivity surface enhanced Raman spectroscopy of R6G on in situ fabricated Au nanoparticle/graphene plasmonic substrates

Plasmonic gold nanoparticles (AuNP) with controllable dimensions have been fabricated in situ on graphene at moderately elevated temperature for high sensitivity surface enhanced Raman spectroscopy (SERS) of Rhodamine 6G (R6G) dye molecules. Significantly enhanced Raman signature of R6G dyes were observed on AuNP/graphene substrates as compared to the case without graphene with an improvement factor of 400%, which is remarkably greater than previous results obtained in ex situ fabricated SERS substrate. Simulation of localized electromagnetic field around AuNPs with and without the underneath graphene layer reveals an enhanced local electromagnetic field due to the plasmonic effect of AuNPs, while additional Ohmic loss occurs when graphene is present. The enhanced local electromagnetic field by plasmonic AuNPs is unlikely the dominant factor contributing to the observed high SERS sensitivity on R6G/AuNP/graphene substrate. Instead, the p-doped graphene, which is supported by the large positive Dirac point shift away from “zero” observed in AuNP/graphene field effect transistors, promotes SERS signals through enhanced molecule adsorption and non-resonance molecular–substrate chemical interaction.

High-performance deformable photoswitches with p-doped graphene as the top window electrode

Deformable polymer photoswitches with p-doped single layer graphene as the top window electrode exhibit an on/off ratio as high as 8.5 × 105.

Four-wave-mixing generated by femto-second laser pumping based on graphene coated microfiber structure

Nonlinear optics researches of graphene-based four waves mixing (FWM) effect are important for a new generation of photonic devices. Compared with the ordinary graphene materials, the P-doped graphene based hybrid waveguide structure is more conducive to the simulating of the third-order nonlinear effect in low power due to its smaller transmission loss. In this work, we propose a P-doped graphene coated microfiber hybrid waveguide structure for femto-second laser pumping excited FWM. By the simulations, we analyze the HE11 mode distribution and the effective refractive index of the silica microfiber and P-doped graphene coated microfiber hybrid waveguide with different fiber diameters at a wavelength of ～1550 nm. We also implement the fabrication processing and characterize this P-doped graphene coated microfiber hybrid waveguide. In the experiments, we utilize a femto-second laser as the pump laser with a peak power up to kW. As the graphene material and the microfiber contribute to the nonlinearity, the cascade FWM could be obtained. Experimental results demonstrate that when the peak power of the injection pump is fixed at 1.03 kW, by adjusting the detuning in wavelength to the length less than 10.0nm, there are four sets of frequency components that can be observed. In the present paper, we provide the relationship among the detuning in wavelength, the pump power and the the power of stokes peak. These results indicate that under the condition of a few nanometer detuning wavelength, when the pump power is fixed at 14.1 dBm and the detuning wavelength is 6.7 nm, there are second order stokes light and the second order anti-stokes light, which can be observed, here the obtained conversion efficiency is up to-60 dB, which can be improved by optimizing the waveguide structure and increasing the pump power. Meanwhile, this FWM processing is also fast due to the fast pumping laser.#br#The simulation and experimental results demonstrate that this P-doped graphene coated microfiber hybrid structure has the advantages of highly nonlinearity, compact size and withstanding high power ultrafast laser, showing the important research value and potential applications in fields based on ultrafast optics, such as multi-wavelength laser, phase-sensitive amplification, comb filters and all-optical regeneration.

Hydrogen intercalation: An approach to eliminate silicon dioxide substrate doping to graphene

We present an effective approach to eliminate SiO2 substrate doping to graphene by hydrogen intercalation during thermal annealing treatments. Owing to the high temperature applied, the conventional vacuum annealing process creates oxygen deficiency defects and dangling bonds in the substrate, which change the p-doped graphene to the heavily n-doped type. In contrast, an annealing process in hydrogen ambient leads to hydrogen intercalation at the graphene–SiO2 interface, which could effectively terminate the dangling bonds on the substrate formed during the high-temperature process. Hence, the supported graphene can stay “uncontaminated” by the substrate.

Synthesis of phosphorus-doped graphene and its wide potential window in aqueous supercapacitors.

Phosphorus-doped (P-doped) graphene with the P doping level of 1.30 at % was synthesized by annealing the mixture of graphene and phosphoric acid. The presence of P was confirmed by elemental mapping and X-ray photoelectron spectroscopy, while the morphology of P-doped graphene was revealed by using scanning electron microscopy and transmission electron microscopy. To investigate the effect of P doping, the electrochemical properties of P-doped graphene were tested as a supercapacitor electrode in an aqueous electrolyte of 1 M H2 SO4. The results showed that doping of P in graphene exhibited significant improvement in terms of specific capacitance and cycling stability, compared with undoped graphene electrode. More interestingly, the P-doped graphene electrode can survive at a wide voltage window of 1.7 V with only 3 % performance degradation after 5000 cycles at a current density of 5 A g(-1), providing a high energy density of 11.64 Wh kg(-1) and a high power density of 831 W kg(-1).

Enhanced field emission properties of doped graphene nanosheets with layered SnS2

We report here our experimental investigations on p-doped graphene using tin sulfide (SnS2), which shows enhanced field emission properties. The turn on field required to draw an emission current density of 1 μA/cm2 is significantly low (almost half the value) for the SnS2/reduced graphene oxide (RGO) nanocomposite (2.65 V/μm) compared to pristine SnS2 (4.8 V/μm) nanosheets. The field enhancement factor β (∼3200 for the SnS2 and ∼3700 for SnS2/RGO composite) was calculated from Fowler-Nordheim (F-N) plots, which indicates that the emission is from the nanometric geometry of the emitter. The field emission current versus time plot shows overall good emission stability for the SnS2/RGO emitter. The magnitude of work function of SnS2 and a SnS2/graphene composite has been calculated from first principles density functional theory (DFT) and is found to be 6.89 eV and 5.42 eV, respectively. The DFT calculations clearly reveal that the enhanced field emission properties of SnS2/RGO are due to a substantial lowering of the work function of SnS2 when supported by graphene, which is in response to p-type doping of graphene.

Tunable transport characteristics of p-type graphene field-effect transistors by poly(ethylene imine) overlayer

Tunable electrical transport properties of graphene field-effect transistors (GFETs) are achieved by spin-coating a poly(ethylene imine) (PEI) layer on graphene surface. The initially p-doped graphene recovers the ambipolar characteristics with the PEI overlayer. When increasing the PEI concentration in a methanol solvent, a systematic evolution of the transport properties of GFETs is observed. The carrier mobility of graphene is greatly improved by several tens of times and the hole/electron conductivity saturation is shown. The voltage of the neutrality point VDirac gets closer to 0V and the plateau width around the neutrality point ΔVDirac becomes much smaller. It is proposed that the long-range Coulomb scattering in graphene is suppressed due to the screening effect of PEI and the performances of GFETs are consequently improved. The hysteretic behaviors of the transfer characteristic curves of GFETs are also influenced by the PEI coating. The gradual reversion of the hysteresis direction is observed when increasing the concentration of PEI, which is probably due to the two competing mechanisms between the charge trapping at the graphene/SiO2 interface and the capacitive coupling of graphene and the PEI overlayer.