Chemical Doping Method Research Articles

Chemically-doped graphene with improved surface plasmon characteristics: an optical near-field study.

One of the most fascinating and important merits of graphene plasmonics is their tunability over a wide range. While chemical doping has proven to be a facile and effective way to create graphene plasmons, most of the previous studies focused on the macroscopic behaviors of the plasmons in chemically-doped graphene and little was known about their nanoscale responses and related mechanisms. Here, to the best of our knowledge, we present the first experimental near-field optical study on chemically-doped graphene with improved surface plasmon characteristics. By using a scattering-type scanning near-field optical microscope (s-SNOM), we managed to show that the graphene plasmons can be tuned and improved using a facile chemical doping method. Specifically, the plasmon interference patterns near the edge of the monolayer graphene were substantially enhanced via nitric acid (HNO3) exposure. The plasmon-related characteristics can be deduced by analyzing such plasmonic fringes, which exhibited a longer plasmon wavelength and reduced plasmon damping rate. In addition, the local carrier density and therefore the Fermi energy level (EF) of graphene can be obtained from the plasmonic nano-imaging, which indicated that the enhanced plasmon oscillation originated from the injection of free holes into graphene by HNO3. These findings were further corroborated by theoretical calculations using density functional theory (DFT). We believe that our findings provide a clear nanoscale picture on improving graphene plasmonics by chemical doping, which will be helpful for optimizing graphene plasmonics and for elucidating the mechanisms of two-dimensional light confinement by atomically thick materials.

Radio-frequency characteristics of graphene monolayer via nitric acid doping

Abstract The radio-frequency, i.e. 0.5–40 GHz, characteristics of chemical vapor deposition-grown graphene monolayer via HNO 3 doping is experimentally investigated. According to the obtained results, the sheet resistance of HNO 3 -treated graphene decreases about half compared to bare graphene. In the case of radio-frequency characteristics, the transmission coefficient and effective conductivity of the HNO 3 -treated graphene are more enhanced than those of the bare graphene. Moreover, the intrinsic resistance and inductance of the HNO 3 -treated graphene itself show diminishing tendency with frequency increase. As a result, it is verified that the direct current as well as high frequency characteristics of graphene are improved by using the chemical doping method.

Facile graphene n-doping by wet chemical treatment for electronic applications.

We report a post-synthetic n-doping method for chemical-vapor-deposition (CVD) grown graphene using wet chemical processing. An ammonium fluoride solution was found effective in converting pristine hole doping into electron doping in addition to the mobility improvement of charge carriers. We verified the doping by electrical measurements, Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) analyses and suggest that the mechanism of n-doping is electrostatic doping by ionic physisorption of ammonium ions on the graphene surface. This simple chemical doping method provides a facile and robust route to n-doping of large area graphene for the realization of high performance graphene-based electronic devices.

(Invited) Flexible 3-Dimensional Graphene Foam-Based NO2 Gas Sensors

Graphene has attracted much attention due to its great optical and electronic properties. Since indium-tin-oxide which is common transparent-conductive-layer in GaN-based light-emitting diodes (LEDs) has reached its limitation, many researchers have focused on the applications of graphene as transparent-conductive-layer in optoelectronic devices, for instances LEDs and solar-cells. Especially, the exceptional transmittance in UV (ultra-violet) and IR (infra-red) spectral ranges opens a new possibility for the graphene to be integrated with UV-LEDs. However, the high sheet resistance of the graphene is still an issue in its implementations. The methods to improve the sheet resistance and the stability by a chemical doping method will be discussed.

Graphene based salisbury screen for terahertz absorber

A graphene-based, multiband absorber operating in terahertz (THz) frequency range was demonstrated. Graphene film was transferred onto the top of a flexible polymer substrate backed with a gold reflector. The graphene acts as a resistive film that partially attenuates and reflects THz waves. The destructive interference between THz waves reflected from graphene and backside reflector gives rise to perfect absorbance at multiple frequencies. To enhance the absorbance on/off ratio (AR), the conductivity of graphene was varied using a chemical doping method. The resulting p-doped, graphene-based THz absorber exhibited absorbance at maxima and AR higher than 0.95 and 25 dB, respectively.

Dual-function graphene-based electronic devices

Transparent electrodes are an essential element in numerous electronic devices, including displays, solar cells, LEDs, and touch panels for smartphones and tablets. Indium tin oxide (ITO) is currently the most dominant material for this purpose due to its good electrical and optical performance, and moisture resistance. However, indium is becoming a scarce and expensive resource and requires a costly vacuum deposition process. In addition, its mechanical properties are weak, which severely restricts its application in flexible electronics. These issues have stimulated numerous developments with the aim of discovering new transparent electrode materials. Graphene has recently attracted considerable attention as an alternative to ITO due to its good electrical conductivity, optical transparency, and high mechanical strength.1, 2 However, for adoption in practical electronic devices, the electrical conductivity of graphene must be made comparable to that of ITO.3, 4 Although a chemical doping method has been developed, the initially high conductivity tends to degrade due to the adsorption of moisture and other chemical molecules under environmental conditions. As an alternative approach to overcoming these limitations, we have developed a nonvolatile doping method using a piezoelectric material as substrate5 that could pave the way toward the application of graphene in high-performance flexible and transparent devices. Our electrostatic doping method employs the ferroelectric polarization of a piezoelectric material, which can effectively enhance the conductivity of graphene while maintaining good mechanical and optical properties, without degradation. After a poling process in which the material is subjected to large voltages (50–150V), the carbonfluorine dipoles of the ferroelectric poly(vinylidene fluorideco-trifluoroethylene)—P(VDF-TrFE)—polymer align along the poling direction. In other words, the net negative charge of the Figure 1. (a) Image of a graphene/poly(vinylidene fluoride-cotrifluoroethylene)/graphene (GPG) device. The device is interconnected with a sound source to measure acoustic actuation, a red LED to measure its performance as a nanogenerator, and an amplifier. Inset shows the structure of GPG multilayer film. Two graphene layers work as top and bottom electrodes for the central poly(vinylidene fluoride-cotrifluoroethylene)—P(VDF-TrFE)—layer. (b) Schematic depiction of the dual functionality of the device as an acoustic actuator and a nanogenerator.

Tunable Photoluminescence of Monolayer MoS2 via Chemical Doping

We demonstrate the tunability of the photoluminescence (PL) properties of monolayer (1L)-MoS2 via chemical doping. The PL intensity of 1L-MoS2 was drastically enhanced by the adsorption of p-type dopants with high electron affinity, but reduced by the adsorption of n-type dopants. This PL modulation results from switching between exciton PL and trion PL depending on carrier density in 1L-MoS2. Achievement of the extraction and injection of carriers in 1L-MoS2 by this solution-based chemical doping method enables convenient control of optical and electrical properties of atomically thin MoS2.

Styrene butadiene copolymer-based transparent conducting thin films

Conducting plastics emerge as a new area of providing a cost effective and unique alternative material for applications ranging from consumer electronics to optoelectronics, solar cells, lighting, memory and a host of new photonic applications. In this work, an attempt has been made to produce conducting elastomeric thin films pertinent to optoelectronic applications. A thermoplastic elastomer, styrene butadiene rubber, is doped with suitable dopants by chemical-doping method. The spectroscopic analysis reveals the formation of conjugated sequences, which is responsible for high intrinsic conductivity in polymeric materials. The polystyrene segment of the elastomer seems to affect the doping rate and the length of the conjugated sequences formed. In this study, the electrical conductivity is found to increase from 10−13 S cm−1 to 10−1 S cm−1 upon doping, and the conduction mechanisms involved are also proposed.

Electroactive organic anion‐doped polypyrrole as a low cost and renewable cathode for sodium‐ion batteries

AbstractWe demonstrate here a remarkable electrochemical activation of polypyrrole chains by doping with redox‐active diphenylamine sulfonate anions. The organic redox dopant can not only serve as anionic counterions to enhance electrochemical activity of the polymer chains, but also contributes their redox capacity to the material. This organic‐polymer composite exhibits a quite high reversible capacity of 115 mA h g−1, excellent rate capability and cycling stability, capable of serving as a low cost, and renewable cathode for Na‐ion batteries. Since the chemical doping method is simple and easily extendable for a large variety of organic anions and polymer networks, it is possible to adopt this new strategy for creating low cost and electrochemically active polymer materials for widespread electric storage applications. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2013

Semiconducting and conducting transition of covalent-organic polymers induced by defects

The chemical doping method is often adopted to obtain metal-free conducting materials. To date, it is still a great challenge to controllably prepare metal-free semiconducting and conducting materials by tuning the inherent structure of a material. In this work, a class of novel one-dimensional (1D) covalent-organic polymer (COP) has been designed, whose cross-sections are triangular, tetragonal, pentagonal and hexagonal, and their electronic properties are explored. The tetragonal 1D COP exhibits unique phenomena in electronic properties, i.e. the tetragonal COPs with mono- or trilayer defects (odd defects) show semiconducting properties, while they become conductors for the two cases of non- or bilayer defects (even defects). This observation indicates that they comply with the characteristics of semiconducting and conducting switches induced by the odd–even defects. Therefore, we infer that for the tetragonal configuration, the odd–even defects could potentially manipulate the electrical behavior of the COP material. The discovery provides a new direction for the targeted synthesis of semiconducting and conducting materials by tuning the inherent structure of materials, which is entirely different from the chemical doping method yielding metal-free conducting materials.

Chemical vapor doping of transparent and conductive films of carbon nanotubes

An alternative chemical vapor doping (CVDo) method for doping transparent and conductive films of carbon nanotubes (CNTs) is presented. The different effects between the traditional solution doping process and the proposed vapor doping method are compared in detail by means of optical microscopy, SEM, sheet resistance measurements, and Raman and UV–Vis–NIR spectroscopy. It was shown that the level of reduced sheet resistance was comparable for the two treatments but the vapor-treated film displayed better stability in air as well as other significant advantages that include its appropriateness for different types of flexible substrates. A mechanism for chemical vapor doping is proposed.

Facile Preparation of Nitrogen-Doped Few-Layer Graphene via Supercritical Reaction

To achieve the applications of graphene, the modulation of its electrical properties is of great significance. The element doping might give a promising approach to produce fascinating properties of graphene. Herein we report a facile chemical doping method to obtain nitrogen-doped (N-doped) few-layer graphene sheets through supercritical (SC) reaction in acetonitrile at temperature as low as 310 °C, using expanded graphite as starting material. X-ray photoelectron spectroscopy analysis revealed that the level of nitrogen-doping (N-doping) increased from 1.57 to 4.56 at % when the reaction time was tuned from 2 to 24 h. Raman spectrum confirmed that the resulting N-doped few-layer graphene by SC reaction maintain high quality without any significant structural defects. Electrical measurements indicated that N-doped few-layer graphene sheets exhibit a typical n-type field-dependent behavior, suggesting the N-doping into the lattice of graphene. This work provides a convenient chemical route to the scalable production of N-doped graphene for potential applications in nanoelectronic devices.

Low temperature preparation and electrical properties of sodium–potassium bismuth titanate lead-free piezoelectric thick films by screen printing

Sodium–potassium bismuth titanate (NKBT) thick films with thickness of 40 μm were prepared by screen printing. To improve the homogeneity, the sintering aids were added into the pastes as a chemical liquid-phase doping method. The results show that the addition of Bi–Li sintering aids was beneficial for both the reduction of the sintering temperature and the improvement of the electrical performance of the thick films. The thick films containing 5 wt.% Bi–Li sintering aids demonstrated optimal dielectric properties with the maximum dielectric constant of 725 and minimum dielectric loss of 2.5%. Moreover, the NKBT thick films containing 3 wt.% Bi–Li sintering aids sintered at 950 °C exhibited the remanent polarization of 19.6 μC/cm 2, room-temperature pyroelectric coefficient of 1.56 × 10 −4 C/(m 2 °C), figure of merit for specific detectivity of 0.48 × 10 −5 Pa −0.5, and effective longitudinal piezoelectric coefficient of 88 pm/V, which are comparable to that of the high-temperature sintered thick films without sintering aids.

Preparation and characterization of (V 1− xAl x) 2O 3 ultrafine powders by chemical doping method

(V 1− x Al x ) 2O 3 ultrafine powders have been synthesized successfully by pyrolysizing poly-crystalline Al-doped precursor, Al-doped (NH 4) 5[(VO) 6(CO 3) 4(OH) 9]·10H 2O in H 2 atmosphere at 1373 K for the first time. The relation between the ratio of Al/V in the VOCl 2 solution and that in polycrystalline Al-doped precursor was studied. The result shows that this chemical doping method is successful, and the Al (III) content in precursor is controllable. The lattice parameters of (V 1− x Al x ) 2O 3 determined by XRD at room temperature and high-temperature reveal that the Al (III) enter the V 2O 3 lattice. The SEM micrograph shows that the size of the powder particles is about 100–200 nm. The DSC and magnetic susceptibility results of the (V 1− x Al x ) 2O 3 demonstrate that the powders exhibit the intrinsic phase transition properties.

Preparation and characterization of (V 1− xCr x) 2O 3 nano-powders by chemical doping method

(V 1- x Cr x ) 2O 3 nano-powders have been synthesized successfully for the first time in powder form by pyrolising polycrystalline Cr-doped precursors, Cr-doped (NH 4) 5[(VO) 6(CO 3) 4(OH) 9]·10H 2O in H 2 atmosphere at 1373 K. Development of single-crystal of the Cr-doped precursor showed that Cr (III) possibly entered the precursor lattice or a solid solution formed in the precursor. Comparing the ratio of Cr/V in the raw materials (VOCl 2 solution) to the Cr/V in the polycrystalline Cr-doped precursor indicated that the chemical doping method was feasible and Cr content in the precursor was controlled easily. The lattice parameters of Cr-doped V 2O 3 powders determined by X-ray diffraction (XRD) were consistent with those of single-crystalline Cr-doped V 2O 3, revealing that Cr (III) entered the V 2O 3 lattice. Both the results of differential scanning calorimetry (DSC) and magnetic susceptibility of the (V 1− x Cr x ) 2O 3 demonstrated that the Cr-doped V 2O 3 powders exhibit intrinsic phase transition properties. Homogeneously distributed spherical (V 1− x Cr x ) 2O 3 powders with a particle size of about 40 nm have been obtained.

Screen-printed piezoelectric ceramic thick films with sintering additives introduced through a liquid-phase approach

Lead zirconate titanate (PZT) ceramic films, 30 μm thick, were deposited onto Pt-coated silicon wafers by a screen-printing process. A chemical liquid-phase doping method was used to introduce sintering aids with improved homogeneity in the PZT thick films. Porous thick films with good adhesion were formed on the substrates after heating at a temperature of 925 °C. The morphology, dielectric, ferroelectric and piezoelectric properties of the thick films were characterized. Stable out-of-plane piezoelectric vibration of the thick films was observed with a laser scanning vibrometer (LSV), and the piezoelectric dilatation magnitude was determined accordingly. The effective longitudinal piezoelectric coefficient d 33 was evaluated according to numerical simulations with the elastic clamping of the substrate taken into account. High piezoelectric voltage constants were obtained due to the very low dielectric constant of the porous thick films. These films have application potential as integrated piezoelectric sensors.

Synthesis and characterization of liquid-phase prepared RbC 60

Development of the rapid synthesis of the polymer phase of RbC 60 using the chemical doping method in liquid phase is reported first. X-ray powder diffraction characterization has proved that the sample prepared by the present method has a polymer phase with the lattice parameters a=9.14, b=10.1 and c=14.2 Å identical to those of the sample prepared by solid-phase reaction. The electron spin resonance (ESR) observation of the sample has shown a similar magnetic behavior as that of the usual solid-phase prepared sample.