Objective  Graphene has been widely and actively used for photodetector due to its unique properties of extremely high mobility and broad optical bandwidth. However, the weak optical absorption of single-layer graphene and short photogenerated carriers' lifetime result in low responsivity of graphene photodetectors. In order to overcome the above problems, doping of heteroatoms is a good option for changing the charge transport properties and manipulating the chemical behavior of the graphene. The nitrogen atom is considered to be an exceptional dopant for carbon material, which has comparable atomic radius to the carbon atom to minimize the disruption of graphene and form strong covalent bond with the carbon atom. However, the traditional methods of doping nitrogen atoms often require complex experimental conditions and long reaction times. Therefore, the development of a convenient, time-saving and efficient strategy for the in-situ preparation of nitrogen-doped graphene has become a critical problem to be solved. Laser induced graphene (LIG) has been demonstrated to be an effective technique for fabricating designed device. On the one hand, the high porosity of LIG overcomes the weak light absorption of single layer graphene. On the other hand, the laser preparation process facilitates selective doping. Therefore, LIG provide a facile method for the fabrication of nitrogen-doped graphene photodetector.

  Methods  A one-step strategy for in-situ fabricating nitrogen-doped porous graphene by laser was demonstrated (Fig.1). The doping contents of melamine in poly (amic acid) (PAA) solution are 0, 1, 3 and 5 wt%, respectively. A continue-wave CO₂ laser with an aid of a control software is applied to mark patterns on melamine/polyimide layer. The length and width of pattern are 10 mm and 700 µm, respectively. The laser scribing time is 6 with the adjacent of two scribing lines of 100 µm. The wavelength and spot size of laser beam are 10.6 and ~180 µm, respectively. The laser power is set to 6 W and the scanning speed is 300 mm·s⁻¹. Laser processing is performed under ambient conditions. Scanning Electron Microscopy (SEM) was used to test the surface morphology and structure characteristics of nitrogen-doped LIG (N-LIG) and LIG. The crystal structure and defect degree of LIG and N-LIG were characterized by Raman spectroscopy. Energy Dispersion Spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS), were used to characterize the elemental composition of LIG and N-LIG and the specific doping ratio of N element in N-LIG. The wavelength of 343, 630 and 1550 nm light sources were used to test the photoelectric response.

  Results and Discussions   Both LIG and N-LIG show porous structures according to the SEM images (Fig.2). Raman spectra show that LIG and N-LIG have the characteristic structure of graphene, and the lattice defect degree of N-LIG is higher (Fig.4). EDS and XPS measurement results show that the main component of LIG and N-LIG are carbon. There is a small amount of oxygen in LIG and N-LIG. There is almost no nitrogen element in LIG, but the proportion of nitrogen element in N-LIG reaches 8.14 wt%, which proves that nitrogen element is successfully introduced into graphene by laser in-situ doping (Fig.5). The time-dependent photocurrents of LIG and N-LIG with light on/off cycles under illumination of 630 nm show good repeatability and stability (Fig.8). The photocurrent of N-LIG with doping content of 5 wt% is 12 µA, which is 5 times higher than that of LIG (1.8 µA). The N-LIG photodetector shows good responsivity in the range of 343 to 1550 nm (Fig.10).

  Conclusions  In summary, a nitrogen-doped graphene photodetector is prepared by one-step laser direct writing. The photoelectric response of N-LIG with different doping concentrations are tested under 630 nm light illumination. The test results show that the photoelectric response increased with the increase of doping concentration. Especially for the N-LIG at the doping concentration of 5 wt%, the responsivity is an order of magnitude more than LIG photodetector. Finally, the photoresponse of N-LIG photodetector at 343 nm and 1550 nm wavelength are tested. The results show that the device can achieve a wide spectral response from ultraviolet to infrared. This work shows that the photoelectric properties of graphene can be regulated by laser induced doping, which provides a feasible scheme for low-cost and efficient fabrication of high-performance broadband photodetectors.