Graphene has attracted a great attention since discovered in 2004 [
1]. It is a semi-metal two dimensional (2D) material with extremely high mobility, high transmittance, and excellent electrical conductivity [
2,
3]. The invention of graphene brought a novel family of heterostructure diode based devices, which can function as solar cells [
4,
5], photodetectors [
6–
8] and light emitting diodes (LEDs) [
9]. On the other hand, semiconductor solid-state lighting is replacing traditional incandescent lamps. The commercial blue LEDs are based on GaN materials system [
10]. It is generally believed that ZnO could replace GaN based on: 1) ZnO material can be grown in large scale at low temperature [
11]; 2) The exciton binding energy of ZnO can reach 60 meV [
12], which is twice that of GaN; 3) Zn is abundant in nature and Ga is scarce. However, the lack of stable, low resitivity, single crystalline p-type ZnO hindered the development of ZnO homojunction LED [
13]. Building heterojunctions with ZnO using p-type materials, such as graphene, could solve this problem. In fact, comparing with semiconductor homojunctions, graphene/semiconductor heterostructure has its unique advantage as the junction is located at the surface, which permits a high external quantum efficiency. Up to now, graphene/ZnO heterostructure based LED has been reported [
14–
16]. Other than UV emission, it is common to see green light emission in the reported graphene/ZnO heterostructure LED. However, it remains unclear where the green light emission comes from. The green light emission may come from the intrinsic defects inside the crystal or from the surface of ZnO contacting with graphene.