Work Function Of Graphene Oxide Research Articles

Engineering the electrical and optical properties of graphene oxide via simultaneous alkali metal doping and thermal annealing

In order to extend the application of graphene oxide (GO) in the area of electronic industries, enhancing the electrical properties of GO as a cost-effective alternative for graphene seems mandatory. Engineering the electrical properties of GO can be achieved in two different approaches: the oxygen functional group reduction and doping GO with chemical dopants. Here, both approaches were utilized to tune the electrical properties of GO toward its application as cathode; first, GO was doped with alkali metal dopants, and later, the doped samples were thermally reduced. Energy-dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy were utilized to study the chemical composition of the doped samples. The even distribution of the dopants on the GO surface presented via the EDX elemental map, with no sign of particle development. After doping GO with alkali metals followed by thermal reduction, the sheet resistance of the doped samples was decreased from 311.0 kΩ/sq to as low as 32.1 kΩ/sq. Moreover, the optical properties of GO were effectively engineered via the different doping agents. The ultra-violet photoelectron spectroscopy showed that the shift of the work function of GO was as high as 1.74 eV, after doping followed by thermal reduction.

Optimization of Reducing Agents for Selective Bandgap Manipulation in Visible Region of Graphene Oxide and Its Work Function Estimation

Abstract In the present study, we report an effective way for reduction of graphene oxide (GO) as well as tuning of bandgap using various reducing agents. It is found that sodium borohydride (NaBH4) can be a promising reducing agent, as it can reduce the bandgap to 2.30 eV and has a higher capacity to restore the carbon–carbon bonds compared with other reducing agents. Further bandgap tuning has been obtained by hydrogenation of GO as well as NaBH4-reduced GO (NGO). It was observed that after hydrogenation, the bandgap of GO was reduced from 2.48 to 1.95 eV, whereas it reduced from 2.45 to 1.87 eV in NGO. Hydrogenation of GO results in rearranging of C=O functionalities and removal of oxygen-containing groups. Furthermore, transmission electron microscopy observations corroborate the observation that after reduction with NaBH4, GO shows a more uniform nature than GO. The intensity ratio (I{1,100}/I{1,200}) of the {1,200} and {1,100} planes is found to be 1. After reduction with NaBH4, the ID/IG ratio decreased from 1.05 to 0.92, suggesting a decrease in defect density and better reduction. The intrinsic work function of GO and hydrogenated NGO is determined using electrostatic force microscopy and found to be 5.19 ± 0.1 eV and 4.39 ± 0.1 eV, respectively.

Engineering work function of graphene oxide from p to n type using a low power atmospheric pressure plasma jet.

In this work, we demonstrate doping graphene oxide (GO) films using a low power atmospheric pressure plasma jet (APPJ) with subsequent tuning of the work function. The surface potential of the plasma functionalized GO films could be tuned by 120 ± 10 mV by varying plasma parameters. X-ray spectroscopy used to probe these changes in electronic structure of systematically functionalized GO films by plasma. Detailed investigation using X-ray photoelectron spectroscopy and near edge X-ray absorption fine structure spectroscopy revealed the reactive nitrogen species in the plasma induce finite changes in the surface chemistry of the GO films, introducing additional density of states near the top of the valence band edge. Nitrogen introduced by the atmospheric pressure plasma is predominantly in a graphitic configuration with a varying concentration of pyridinic nitrogen. Additionally, evidence of gradual de-epoxidation of these GO films with increasing plasma exposure was also observed. We attribute this variation in work function values to the configuration of nitrogen in the graphitic structure as revealed by X-ray spectroscopy. With pyridinic nitrogen the electronic states of GO became electron deficient, inducing a p-type doping whereas an increase in graphitic nitrogen increased the electron density of GO leading to an n-type doping effect. Nitrogen doping was also found to decrease the resistivity from 138 MΩ sq-1 to 4 MΩ sq-1. These findings are extremely useful in fabricating heterojunction devices like sensors and optoelectronic devices where band structure alignment is key to device performance when GO is used as a charge transport layer. This technique can be extended to other known 2D systems.

MoS2-nanosheet/graphene-oxide composite hole injection layer in organic light-emitting diodes

In this work, composite layers comprising two-dimensional MoS2 and graphene oxide (GO) were employed as hole injection layers (HILs) in organic light-emitting diodes (OLEDs). MoS2 was fabricated by the butyllithium (BuLi) intercalation method, while GO was synthesized by a modified Hummers method. The X-ray diffraction patterns showed that the intensity of the MoS2 (002) peak at 14.15° decreased with increase in GO content; the GO (001) peak was observed at 10.07°. In the C 1s synchrotron radiation photoemission spectra, the contributions of the C-O, C=O, and O-C=O components increased with increase in GO content. These results indicated that GO was well mixed with MoS2. The lateral size of MoS2 spanned from a few hundreds of nanometers to 1 μm, while the size of GO was between 400 nm and a few micrometers. Thus, the coverage of the MoS2-GO composite on the ITO surface improved as the GO content increased, owing to the large particle size of GO. Notably, GO with large size could fully cover the indium tin oxide film surface, thus, lowering the roughness. The highest maximum power efficiency (PEmax) was exhibited by the OLED with MoS2-GO 6:4 composite HIL, indicating that similar contents of MoS2 and GO in MoS2-GO composites provide the best results. The OLED with GO HIL showed very high PEmax (4.94 lm W−1) because of very high surface coverage and high work function of GO. These results indicate that the MoS2-GO composites can be used to fabricate HILs in OLEDs.

Copper(II) chloride doped graphene oxides as efficient hole transport layer for high-performance polymer solar cells

Effective hole transport layers (HTLs) are still critical and desirable for the fabrication of high-performance, high-stability and cost-effective polymer solar cells (PSCs). In this work, we report that the graphene oxide (GO) simply modified with CuCl2-doping (GO:CuCl2) can be used as an effective HTL to produce high-performance PSC based on poly[(ethylhexyl-thiophenyl)-benzodithiophene-(ethylhexyl)-thienothiophene]:[6,6]-phenyl C71 butyric acid methyl ester (PTB7-Th: PC71BM). As a result, the average power conversion efficiency (PCE) reaches 7.68%, remarkably higher than that of the device using pristine GO as HTL (7.10%) and comparable to that of the device with conventional HTL of poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) (7.69%). UPS measurements suggest that the work function of GO:CuCl2 is slightly increased relative to that of undoped GO. AFM characterizations also reveal a more continuous film morphology for the deposited GO:CuCl2 layer on indium tin oxide (ITO). Such improvements render an ohmic contact as well as a better interfacial contact between the electrode and active layer, yielding improved open-current voltage (Voc), fill factor (FF) and thus enhanced PCE.

Significantly improved photovoltaic performance in polymer bulk heterojunction solar cells with graphene oxide /PEDOT:PSS double decked hole transport layer

This work demonstrates the high performance graphene oxide (GO)/PEDOT:PSS doubled decked hole transport layer (HTL) in the PCDTBT:PC71BM based bulk heterojunction organic photovoltaic device. The devices were tested on merits of their power conversion efficiency (PCE), reproducibility, stability and further compared with the devices with individual GO or PEDOT:PSS HTLs. Solar cells employing GO/PEDOT:PSS HTL yielded a PCE of 4.28% as compared to either of individual GO or PEDOT:PSS HTLs where they demonstrated PCEs of 2.77 and 3.57%, respectively. In case of single GO HTL, an inhomogeneous coating of ITO caused the poor performance whereas PEDOT:PSS is known to be hygroscopic and acidic which upon direct contact with ITO reduced the device performance. The improvement in the photovoltaic performance is mainly ascribed to the increased charge carriers mobility, short circuit current, open circuit voltage, fill factor, and decreased series resistance. The well matched work function of GO and PEDOT:PSS is likely to facilitate the charge transportation and an overall reduction in the series resistance. Moreover, GO could effectively block the electrons due to its large band-gap of ~3.6 eV, leading to an increased shunt resistance. In addition, we also observed the improvement in the reproducibility and stability.

In-situ investigation of graphene oxide under UV irradiation: Evolution of work function

Using in-situ Kelvin probe force microscopy (KPFM) to measure surface potential, we investigated the time-dependent work function evolution of solution-processed graphene oxide (GO) under ultraviolet (UV) irradiation. We found that the work function of GO exposed in UV shows a notable decrease with increasing irradiation time, which is proposed to be attributed to the gradual disappearance of oxygen-containing functional groups in GO during the UV-induced reduction reaction process. Fourier transform infrared spectrum and Raman spectrum were used to confirm the reduction of GO under UV irradiation. Our study would give an insight into understanding the transformation of GO’s electronic structures during the reduction process.

Graphene oxide as a promising hole injection layer for MoS₂-based electronic devices.

The excellent physical and semiconducting properties of transition metal dichalcogenide (TMDC) monolayers make them promising materials for many applications. The TMDC monolayer MoS2 has gained significant attention as a channel material for next-generation transistors. However, while n-type single-layer MoS2 devices can be made with relative ease, fabrication of p-type transistors remains a challenge as the Fermi-level of elemental metals used as contacts are pinned close to the conduction band leading to large p-type Schottky barrier heights (SBH). Here, we propose the utilization of graphene oxide (GO) as an efficient hole injection layer for single-layer MoS2-based electronic and optoelectronic devices. Using first-principles computations, we demonstrate that GO forms a p-type contact with monolayer MoS2, and that the p-type SBH can be made smaller by increasing the oxygen concentration and the fraction of epoxy functional groups in GO. Our analysis shows that this is possible due to the high work function of GO and the relatively weak Fermi-level pinning at the MoS2/GO interfaces compared to traditional MoS2/metal systems (common metals are Ag, Al, Au, Ir, Pd, Pt). The combination of easy-to-fabricate and inexpensive GO with MoS2 could be promising for the development of hybrid all-2D p-type electronic and optoelectronic devices on flexible substrates.

Photoreduction of graphene oxide with polyoxometalate clusters and its enhanced saturable absorption

We investigated the photocatalytic abilities of three Keggin-type polyoxometalate (POM) clusters, H3PW12O40 (PW), H4SiW12O40 (SiW), and H3PMo12O40 (PMo) to reduce graphene oxide (GO) under UV-irradiation in water. UV–vis absorption and X-ray photoelectron spectroscopy were performed and show that PW and SiW can photoreduce GO effectively, in contrast to PMo. We conclude that the LUMO levels of POMs should be located energetically above the work function of GO to enable electron transfer from POM to GO. We also investigated the saturable absorption of GO and reduced GO by means of z-scan experiments at 532nm. The POM-assisted photoreduction of GO can greatly enhance the saturable absorber properties of GO, which appears useful for modelocking in ultrafast laser systems.

Comparison of Graphene Oxide with Reduced Graphene Oxide as Hole Extraction Layer in Organic Photovoltaic Cells

A comparison was performed between the use of graphene oxide (GO) and reduced graphene oxide (rGO) as a hole extraction layer (HEL) in organic photovoltaic (OPV) cells with poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester. Hydrazine hydrate (HYD) and the thermal method (Thermal) were adopted to change the GO to rGO. The GO HEL was deposited on an indium tin oxide electrode by spin coating, followed by the reduction process to form the rGO HELs. The success of the reduction processes was confirmed by X-ray diffraction, Raman spectroscopy, X-ray photoemission spectroscopy, transmittance, and 2-point probe method. The OPV cell with the GO (-3 nm) HEL exhibits an increased power conversion efficiency (PCE) as high as 2.5% under 100 mW/cm2 illumination under air mass conditions, which is higher than that of the OPV cell without HEL, viz. 1.78%. However, the PCE of the OPV cell with rGO HEL is not high as the values of 1.8% for the HYD-rGO and 1.9% for the Thermal-rGO. The ultraviolet photoemission spectroscopy results showed that the work function of GO was 4.7 eV, but those of HYD-rGO and Thermal-rGO were 4.2 eV and 4.5 eV, respectively. Therefore, it is considered that GO is adequate to extract the holes from the active layer, but HYD-rGO and Thermal-rGO are not appropriate to use as HELs in OPV cells from the viewpoint of the energy alignment.

Graphene oxide as a chemically tunable 2-D material for visible-light photocatalyst applications

To elucidate the usage of graphene oxide (GO) as a photocatalysis material, we have studied the effect of epoxy and hydroxyl functionalization on the electronic structure, work function, CBM/VBM position, and optical absorption spectra of GO using density functional theory calculations. By varying the coverage and relative ratio of the surface epoxy (O) and hydroxyl (OH) groups, both band gap and work function of the GO materials can be tuned to meet the requirement of photocatalyst. Interestingly, the electronic structures of GO materials with 40–50% (33–67%) coverage and OH:O ratio of 2:1 (1:1) are suitable for both reduction and oxidation reactions for water splitting. Among of these systems, the GO composition with 50% coverage and OH:O (1:1) ratio can be very promising materials for visible-light-driven photocatalyst. Our results not only explain the recent experimental observations about 2-D graphene oxide as promising visible-light-driven photocatalyst materials but can also be very helpful in designing the optimal composition for higher performance.

Optimization of thermally reduced graphene oxide for an efficient hole transport layer in polymer solar cells

In this work, graphene oxide (GO) reduced by thermal annealing was employed as a hole transport layer (HTL) in bulk heterojunction (BHJ) solar cells. Considering the insulating property of nonreduced GO, the annealing temperature plays an important role in recovering the conjugated structure of the graphene sheet, and thereby the conductivity of GO. BHJ solar cells with high-temperature (e.g., 230°C) reduced GO as the HTL showed much larger fill factor (FF) than devices with low-temperature (e.g., 130°C) reduced GO as the HTL, indicating the better conductivity of GO annealed at an elevated temperature due to the removal of oxygen functional groups from the graphene sheet to a much-higher level. On the other hand, the work function of GO may be lowered toward that of graphene (4.5eV) with increasing the reduction temperature, which results in a decreased open-circuit voltage (Voc) for the high-temperature reduced GO devices. By further optimizing the concentration and spin-coating speed of GO dispersion, we achieved a power conversion efficiency (PCE) that is 26% higher than devices without any HTL. This is mainly attributed to the increase in FF as a result of the decreased series resistance (Rs). In addition, the PCE of the optimized GO device was ∼85% of the PCE of the conventional device with poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) HTL. We anticipate that further optimization of the reduction conditions (e.g., using chemical reductants) will lead to the better performance of GO solar cells.

Graphene oxide/PEDOT:PSS and reduced graphene oxide/PEDOT:PSS hole extraction layers in organic photovoltaic cells

AbstractA comparison was made between the use of graphene oxide (GO)/poly(ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) and reduced graphene oxide (rGO)/PEDOT:PSS as a hole extraction layer (HEL) in organic photovoltaic (OPV) cells. Hydrazine hydrate (HYD) and the thermal method were adopted to change the GO to rGO. The OPV cell with the GO (∼2 nm)/PEDOT:PSS HEL exhibits a power conversion efficiency (PCE) as high as 3.53% under 100 mW/cm2 illumination and air mass conditions, which is higher than that of the OPV cell without the HEL, viz. 1.78%. The device with the PEDOT:PSS/GO HEL shows a similar PCE of 3.48%. However, the PCE of the OPV cell with the rGO/PEDOT:PSS HEL is not high as those of the cells with the HYD‐rGO/PEDOT:PSS and T‐rGO/PEDOT:PSS, viz. 3.3 and 3.37%, respectively. The work function of GO was 4.7 eV, but those of HYD‐rGO and T‐rGO were 4.2 and 4.5 eV, respectively, suggesting that the decrease of the barrier height between GO and active materials is higher than that in rGO case.