Elsevier

Thin Solid Films

Volume 521, 30 October 2012, Pages 163-167
Thin Solid Films

Photoconductivity of reduced graphene oxide and graphene oxide composite films

https://doi.org/10.1016/j.tsf.2011.12.086Get rights and content

Abstract

A photoconductive device was fabricated by patterning magnetron sputtered Pt/Ti electrode and Reduced Graphene Oxide (RGO)/Graphene Oxide (GO) composite films with a sensitive area of 10 × 20 mm2. The surface morphology of as-deposited GO films was observed by scanning electronic microscopy, optical microscopy and atomic force microscopy, respectively. The absorption properties and chemical structure of RGO/GO composite films were obtained using a spectrophotometer and an X-ray photoelectron spectroscopy. The photoconductive properties of the system were characterized under white light irradiation with varied output power and biased voltage. The results show that the resistance decreased from 210 kΩ to 11.5 kΩ as the irradiation power increased from 0.0008 mW to 625 mW. The calculated responsiveness of white light reached 0.53 × 10 3 A/W. Furthermore, the device presents a high photo-conductivity response and displays a photovoltaic response with an open circuit voltage from 0.017 V to 0.014 V with irradiation power. The sources of charge are attributed to efficient excitation dissociation at the interface of the RGO/GO composite film, coupled with cross-surface charge percolation.

Introduction

Graphene, as the thinnest material known in the universe, is a single-atom-thick sheet of sp2-bonded carbon atoms in a hexagonal two-dimensional lattice. These thin graphene sheets have attracted considerable attention for next-generation electronics as a substitute for silicon [1]. Recent research indicates that graphene sheets offer extraordinary electronic, thermal, and mechanical properties that are expected to provide a variety of applications in various technological fields, such as field effect transistors [1], transparent conductors [2], [3], photovoltaics [4], [5], ultrafast photonics applications [6] and detectors [7].

Due to their high electron mobility (up to 170,000 cm2/Vs) [8], much effort has been made to develop graphene-based electronic devices as a substitute for traditional semiconductor silicon materials. Besides applying graphene to the development of electric devices, there is a strong interest in opening a band gap in graphene for optoelectronic applications. Single-layer graphene should transmit ~ 97% of the incident light and absorb 2.3%, independent of the wavelength of the light [9]. Indeed, a number of experimental studies have verified the 2.3% inter-band absorption in a graphene monolayer over a wide wavelength scale, spanning the visible and infrared ranges [9]. The wide range and constant absorption of graphene make it a promising material for photo detectors.

Graphene has metallic-like properties and good electrical conductivity, which limits its applications in conventional photoconductors. Hwang et al. verified that there is no clear photoconductive behavior in a monolayer of graphene under irradiation with visible light [10]. Reduced Graphene Oxide (RGO) is a known semiconductor. Progress has been made to modify Graphene Oxide (GO) or RGO samples by compositing other materials that are intended to act as sensitive layer photoconductors. Obvious photocurrents have been detected under bias voltages and light irradiation [11], [12], [13], [14]. However, Lv Xin observed high photocurrent generation efficiency for graphene-based films [15].

There is some controversy regarding the charge generation mechanism of RGO and GO/RGO composite films. Venkatran et al. reported a conjugated polymer-graphene oxide with better photoconductivity than that of GO. They determined that the GO is the source of photo-electron generation, while the polymer merely collected and mobilized these electrons [11]. Yao et al. presented RGO and poly-diallyldimethylammonium\titania hybrid films that exhibit high photocurrent generation. The photo-electron generation was ascribed to the photo-electro conversion of TiO2 nano-sheets, where the RGO was only an electronic collector and transporter [12]. However, the efficient photocurrent conversion in RGO and titania multilayered films, reported by Lon et al., was attributed to the efficient excitation dissociation at the interface coupled with cross-surface charge percolation [13]. Moreover, the charge generation for RGO and copper phthalocyanine composite, reported by Zhai et al., was also attributed to the presence of donor/acceptor materials and large donor/acceptor interfaces [14].

So, a large number of experiments under different conditions, using different films (Graphene, GO, GO/RGO, others Graphene/GO/RGO based composite films), are needed in order to clarify the fundamental understanding of these systems, as well as to enhance the photoconductivity properties before the commercialization of Graphene/GO/RGO-based photoconductor devices is possible.

Semiconductor light detectors can be divided into two major categories: junction and bulk effect devices. Junction devices, including Schottky photodiodes, Metal-Semiconductor-Metal (MSM) photodiodes, p-i-n/p-n and avalanche photodiodes, and phototransistors, utilize the reverse characteristic of a junction when operated in the photoconductive mode. Under reverse bias, the PN junction acts as a light controlled current source. The output is proportional to the incident illumination and is relatively independent of applied voltage. In contrast, bulk effect photoconductors have no junction. The bulk resistivity decreases with increasing illumination, allowing more photocurrent to flow. This resistive characteristic gives bulk effect photoconductors a unique quality: the signal current from the detector can be varied over a wide range by adjusting the applied voltage.

In our work, GO films were patterned on Pt/Ti electrodes with 20 mm gaps on the SiO2/Si substrate to form typical bulk effect light detector devices. After annealing at a temperature of 1173 K, with a pressure of 0.1 Pa, RGO and GO composite films were formed because parts of the GO were reduced to graphene. We observed high photocurrents under visible light. Our results show that only carbon nano-sheet (RGO and GO) composite films could be applied to the development of sensitive photoconductor or energy conversion materials. The sources of charge were attributed to efficient excitation dissociation at the interface of the RGO/GO composition, coupled with cross-surface charge percolation.

Section snippets

Device preparation

The GO was prepared by oxidation of graphite using the method of Hummer and Offeman [15]. A GO water solution with a density of 8 mg/10 ml was spin-coated on a quartz slide for absorption properties and on a SiO2/Si substrate for fabricating device, scanning electronic microscopy (SEM), optical microscopy, X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) measurements. The device electrodes were prepared by argon sputtering at sequential Ti and Pt targets with a 150 W power

Results and discussion

The formation of single-layered GO sheets and measurements of their thickness were confirmed by AFM measurements, as shown in Fig. 2(a). The thickness was found to be in the range of 1–1.3 nm, which is typical for the thickness of a single, functionalized GO sheet. Large numbers of flakes, dozens of micrometers in size, were observed, as shown in Fig. 2(c), depicting the surface structure and morphology of the GO/RGO composite film coated on a SiO2/Si substrate for photocurrent measurements

Conclusions

A photoconductive device was fabricated by patterning the magnetron sputtered Pt/Ti electrode and multi-layer graphene/graphene oxide films with a sensitive area of 10 × 20 mm2. Absorption curves, XPS spectrum and AFM images indicate that the as-deposited films are a GO/RGO composite. In this system, the resistance decreased from 210 kΩ to 11.5 kΩ as the irradiation power increased from 0.0008 mW to 625 mW. The calculated responsiveness of white light reached 0.53 × 10 3 A/W. Furthermore, high

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 61007015 and 60978040).

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