P-doping Of Graphene Research Articles

Enhanced electrochemical performance of supercritical fluid aided P-doped graphene nanoflakes by I3−/I− redox couple

Heteroatom doped graphene framework is considered as a promising candidate for electrochemical hybrid supercapacitor applications. Herein, we report the synthesis of P-doped graphene using sodium phosphate as a phosphorus source by supercritical fluid assisted processing method. The chemical nature and the quantity of P-doping in graphene are analyzed using XPS and EDAX techniques. Three different P-doped graphene samples are prepared by varying the weight ratio of graphene oxide and sodium phosphate in which the P-doped graphene with a 1:1 weight ratio (graphene oxide: sodium phosphate) exhibits a high specific capacitance of 518 F/g and capacitance retention of 98% over 5000 cycles with 1 M H2SO4 electrolyte. Interestingly, there could be 4-fold increase in the specific capacitance is observed after the incorporation of KI with H2SO4 (1772 F/g) when compared with H2SO4 (396 F/g) at 4 A/g. In addition, specific energy of symmetric supercapacitor with KI/H2SO4 (29 Wh/kg) electrolyte is 3.4-fold higher than that of H2SO4 (8 Wh/kg).

Effect of silicon doping on graphene/silicon Schottky photodiodes

We realize two graphene/Si devices with the same structure but on different Si type and characterize their current-voltage properties. We observe that the gr/n-Si junction has a higher Schottky barrier and rectification factor than the gr/p-Si junction. The low Schottky barrier of the gr/p-Si junction is explained by the p-doping of graphene, induced by polymethylmethacrylate residues and chemicals used for the transfer process, which align the graphene Fermi level to the Si valence band. Under illumination, both devices show a reverse current greater than the forward one. This phenomenon is attributed to the metal-oxidesemiconductor capacitor connected in parallel with the gr/Si Schottky junction. Although both junctions possess a high responsivity, the gr/p-Si junction shows a high dark current that hampers its use as photo detector.

Doping and band gap control at poly(vinylidene fluoride)/graphene interface

Using the density-functional first-principles calculations, we investigate the electronic structures of poly(vinylidene fluoride) PVDF/graphene composite systems. The n- and p-doping of graphene can be flexibly switched by reversing the ferroelectric polarization of PVDF, without scarifying the intrinsic π-electron band dispersions of graphene that are usually undermined by chemical doping. The doping degree is also dependent on the thickness of PVDF layers, which will get saturated when PVDF is thick enough. In PVDF/bilayer graphene (BLG) heterostructure, the doping degree directly determines the local energy gap of the charged BLG. The sandwich structure of PVDF/BLG/PVDF can further enhance the local energy gap as well as keep the electric neutrality of BLG, which will be of great application potentials in graphene-based nanoelectronics.

P-Doping of graphene in hybrid materials with 3,10-diazapicenium dications.

N,N'-Didodecyl-substituted 3,10-diazapicenium salts featuring bromide and hexafluorophosphate counterions have been designed as novel dopants to realize individualized graphene sheets in a series of cutting edge experiments and to intrinsically stabilize them via p-doping. Importantly, electrochemical studies revealed two consecutive irreversible one-electron reductions of the N,N'-didodecyl-substituted 3,10-diazapicenium salts to yield the corresponding radical cation and neutral quinoidal species. Formation of both species was accompanied by characteristic changes in the absorption spectra. The 3,10-diazapicenium bromide was found to be a potent dopant to produce hybrid materials with exfoliated graphene. Microscopy based on AFM and TEM imaging and spectroscopy based on Raman probing corroborated that, upon drying, the hybrid material consists of few layer (5-8 layers) turbostratic graphene sheets that are p-doped. Our findings identify the newly synthesized N,N'-dialkylated 3,10-diazapicenium salts as highly promising candidates for the fabrication of functional graphene materials with tailored properties.

Graphene oxide as a p-dopant and an anti-reflection coating layer, in graphene/silicon solar cells.

It is shown that coating graphene-silicon (Gr/Si) Schottky junction based solar cells with graphene oxide (GO) improves the power conversion efficiency (PCE) of the cells, while demonstrating unprecedented device stability. The PCE has been shown to be increased to 10.6% (at incident radiation of 100 mW cm(-2)) for the Gr/Si solar cell with an optimal GO coating thickness compared to 3.6% for a bare/uncoated Gr/Si solar cell. The p-doping of graphene by the GO, which also serves as an antireflection coating (ARC) has been shown to be a main contributing factor to the enhanced PCE. A simple spin coating process has been used to apply GO with thickness commensurate with an anti-refection coating (ARC) and indicates the suitability of the developed methodology for large-scale solar cell assembly.

Tuning the Electronic Structure of Graphene by Molecular Dopants: Impact of the Substrate.

A combination of ultraviolet and X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, and first principle calculations was used to study the electronic structure at the interface between the strong molecular acceptor 1,3,4,5,7,8-hexafluorotetracyano-naphthoquinodimethane (F6TCNNQ) and a graphene layer supported on either a quartz or a copper substrate. We find evidence for fundamentally different charge redistribution mechanisms in the two ternary systems, as a consequence of the insulating versus metallic character of the substrates. While electron transfer occurs exclusively from graphene to F6TCNNQ on the quartz support (p-doping of graphene), the Cu substrate electron reservoir induces an additional electron density flow to graphene decorated with the acceptor monolayer. Remarkably, graphene on Cu is n-doped and remains n-doped upon F6TCNNQ deposition. On both substrates, the work function of graphene increases substantially with a F6TCNNQ monolayer atop, the effect being more pronounced (∼1.3 eV) on Cu compared to quartz (∼1.0 eV) because of the larger electrostatic potential drop associated with the long-distance graphene-mediated Cu-F6TCNNQ electron transfer. We thus provide a means to realize high work function surfaces for both p- and n-type doped graphene.

(Invited) Electronic Properties of Self-Assembled Trimesic Acid Monolayer on Graphene Layers

The adsorption of trimesic acid (TMA) on graphene surface has been studied with density functional theory (DFT). By considering the adsorption of a single TMA molecule on different sites on graphene, we have been able to perform a detailed analysis of the equilibrium geometry, charge transfer, electronic properties in terms of density of states and band structure, and finally scanning tunnelling microscopy (STM) simulations on those simple systems. The results for isolated adsorption were then compared to the behaviour of TMA unit within two different self-assembled monolayers. Our results indicate that structural deformations of TMA may significantly contribute to the magnitude of p-doping and band gap opening in graphene. The formation of an hydrogen bonding network within the assembly improves the stability of the adlayer but its adhesion on graphene is significantly lowered. The magnitude of p-doping in graphene per TMA unit remains nearly constant from the isolated to the assembled systems, but the magnitude of the band gap opening appears strongly correlated with the breaking of symmetry of $\\pi$-states of graphene by the TMA patterning on the surface. Our results suggest that polymorphism in self-assembled adlayers could be used to tune and control the electronic properties of graphene.\\\\ \\emph{Keywords}: Tunable band gap; Electrical field; Ab-initio calculation; Electronic properties;

One-step etching, doping, and adhesion-control process for graphene electrodes

Enhancing the electrical conductivity and reliability of graphene electrodes is critical for the practical realization of graphene-based electronics, since these factors influence the electrical performance of devices. To achieve such improvements in graphene electrodes requires additional processes such as doping and surface treatments, which inevitably complicate device fabrication. Here, we introduce a novel, straightforward one-step etching method, in which a catalytic copper substrate is etched in imidazole-containing ammonium persulfate solution, of simultaneously enhancing the electrical and adhesion properties of graphene grown on copper foil by chemical vapor deposition. Applying one-step etching method, the sheet resistance of monolayer graphene with 270Ω/sq is obtained, while the adhesion of graphene is improved by 20%. Moreover, the electrical conductivity of graphene remained improved after storage for 30days in ambient conditions without any passivation layers, and the graphene was almost transparent with transmittance of 97.7% at 550nm. The enhancement of the electrical and adhesion properties of graphene originated from the synergistic adsorption of imidazole and etched Cu ions, which results in p-doping of graphene.

Electronic properties of self-assembled trimesic acid monolayer on graphene.

The adsorption of trimesic acid (TMA) on a graphene surface has been studied with density functional theory. By considering the adsorption of a single TMA molecule on different sites on graphene, we have been able to perform a detailed analysis of the equilibrium geometry, charge transfer, electronic properties in terms of density of states and band structure, and finally scanning tunneling microscopy simulations on those simple systems. The results for isolated adsorption were then compared to the behavior of the TMA unit within two different self-assembled monolayers. Our results indicate that structural deformations of TMA may significantly contribute to the magnitude of p-doping and band gap opening in graphene. The formation of a hydrogen bonding network within the assembly improves the stability of the adlayer, but its adhesion on graphene is significantly reduced. The magnitude of p-doping in graphene per TMA unit remains nearly constant from the isolated to the assembled systems, but the magnitude of the band gap opening appears to be strongly correlated with the breaking of symmetry of π-states of graphene by the TMA patterning on the surface. Our results suggest that polymorphism in self-assembled adlayers could be used to tune and control the electronic properties of graphene.

Dirac voltage tunability by Hf1−xLaxO gate dielectric composition modulation for graphene field effect devices

We report that the Dirac voltage of graphene field effect transistors (FETs) can be tuned by controlling the composition of hafnium lanthanum oxide (HfLaO) gate dielectrics. As the lanthanum percentage is increased in the HfLaO film, the charge neutrality point of the graphene FET is gradually shifted in the negative direction. The origin of this tuning is attributed to the hygroscopic nature of the lanthanum oxide, as it is found that lanthanum oxide in the HfLaO film absorbs water molecules below the graphene channel, resulting in the suppression of the p-doping in graphene.

Electronic properties of Au-graphene contacts

The effects of Au grains on graphene conduction and doping are investigated. To obtain a clean Au-graphene contact, Au grains are deposited over graphene before any chemical processing. The bulk and the effective contact resistance versus gate voltage demonstrate that Au grains cause p-doping in graphene. The Fermi level shift is in disagreement with published first-principles calculations, with larger than predicted separation between the graphene and the topmost Au layer. The differential resistance versus bias voltage display anomalies at zero bias and at higher voltages, the latter likely caused by inelastic tunneling across the Au-graphene interface.