Published: October 05, 2011 r2011 American Chemical Society 4682 dx.doi.org/10.1021/nl202266h | Nano Lett. 2011, 11, 4682–4687 LETTER pubs.acs.org/NanoLett Negative and Positive Persistent Photoconductance in Graphene Chandan Biswas, † Fethullah G€ unes -, † Duong Dinh Loc , † Seong Chu Lim, † Mun Seok Jeong, ‡ Didier Pribat, † and Young Hee Lee* ,† † SKKU Advanced Institute of Nanotechnology, WCU Department of Energy Science, Department of Physics, Graphene Center, Sungkyunkwan University, Suwon 440-746, Republic of Korea ‡ Advanced Photonics Research Institute, Gwangju Institute of Science and Technology, Gwangju 500-712, Republic of Korea b S Supporting Information G raphene, a single layer of two-dimensional honeycomb carbon lattices, exhibits numerous exotic behaviors such as the anomalous quantum Hall effect, 1 the Klein paradox, 2 and coherent transport. 3 In contrast, optoelectronic, excitonic, and/or impurity related electron dynamics studies are limited despite its scientific and technical relevance. Persistent photoconductance, 4À6 a prolonged light-induced conducting behavior over several hun- dred seconds, has often been observed in three-dimensional semiconductors with high impurity or in functionalized nano- particles. 6 Illuminating light generates mobile charge carriers in the valence and/or conduction bands of the traditional photo- conductors, 4À11 resulting in increased conductivity. 7 Such posi- tive photoconductance (PPC) was observed in both bulk and nanostructured photoconductors. 6,8 In contrast, conductivity could decrease by light excitation (negative photoconductivity, NPC) in doped IIIÀV semiconductors, 9,10 diamond thin films, 11 quantum well heterostructures, 4 and metal nanoparticles. 6 Im- purity-related photoabsorptions allowing electronic transitions between localized or extended states have been employed to explain the above phenomena. The existence of NPC in metals and simultaneous observation of persistent NPC and PPC have not been observed. Enhancement of such effects could be advantageous in numerous optoelectronic devices and is ex- pected to be observed in low-dimensional systems. Here we report persistent NPC in suspended graphene and a successive persistent NPC, followed by a prominent substrate induced persistent PPC in graphene on SiO 2 substrate. The yield and transient response time (over several hours) of photoconduc- tance change upon light illumination distinct from conventional photodetectors 4À11 and were unusually large in monolayer graphene, although this effect was reduced significantly as the number of graphene layers increased. Two-dimensional semi- metallic graphene shows these behaviors due to its planar structure, in which the constituent atoms are exclusively exposed to the ambient, resulting in strong interactions with the environ- ment and/or substrate. These phenomena could not be observed in conventional three-dimensional metal or semiconductor struc- tures due to their bulk natures which differ significantly from an ideal two-dimensional graphene structure. Monolayer graphene was transferred onto SiO 2 /Si substrate and two Au electrodes were formed for source and drain (inset of Figure 1a). A constant dark current (I dark ) was observed at a fixed sourceÀdrain voltage, V SD = 10 mV under ambient conditions (Figure 1a,b). Monochromatic light illumination (200 nm wavelength) onto the device gradually decreases the current level over a prolonged time of 700 s. This phenomenon is called persistent NPC. Subsequently, a rapid increase in current (at point a) reached to a maximum value (point c) in about 300 s of time scale, called a persistent PPC. Four distinct regions were observed during transient current measurement under light illumination: a, transition time from NPC to PPC region; b, an intermediate bump; c, maximum current peak; d, saturated current region (PPC sat ). The formation of an intermediate bump, an overshoot maximum current, and subsequent current satura- tion were consistently observed independent of the incident light wavelengths. Thus, the single-layer graphene films transferred on SiO 2 /Si substrate exhibited persistent NPC and PPC behaviors simultaneously, which have not been observed previously. Received: July 5, 2011 Revised: September 19, 2011 ABSTRACT: Persistent photoconductance, a prolonged light-induced conduct- ing behavior that lasts several hundred seconds, has been observed in semicon- ductors. Here we report persistent negative photoconductance and consecutive prominent persistent positive photoconductance in graphene. Unusually large yields of negative PC (34%) and positive PC (1652%) and remarkably long negative transient response time (several hours) were observed. Such high yields were reduced in multilayer graphene and were quenched under vacuum condi- tions. Two-dimensional metallic graphene strongly interacts with environment and/or substrate, causing this phenomenon, which is markedly different from that in three-dimensional semiconductors and nanoparticles. KEYWORDS: Graphene, positive photoconductance, negative photoconduc- tance, persistent photoconductance, optoelectronics