# Study on Nitrogen-Doped Graphene Ink and Its Effects on the Heat Dissipation for the LED Lamps
## 2 Historical Review
### Graphene Literature  
In 2004, [PERSON] and [PERSON] [1] of the University of Manchester, England used mechanical exfoliation to separate graphene from layered graphite. However, this method cannot accurately control the area of graphene and the number of layers was not uniform for mass production. Nowadays, there are many ways in which to make graphene. Later, many methods were developed for producing graphene [2; 3; 4; 5]. Defects and ruptures were often found in the structure of graphene, so related doping methods were developed to repair the structure of graphene. In 2008, [PERSON], [PERSON], [PERSON] et al. doped graphene with metal contacts [3]. [PERSON], [PERSON], [PERSON] et al. used chemical vapor deposition (CVD) to synthesize nitrogen doping graphene in 2009 [4]. In 2017, [PERSON] exploited the method of pulsed laser deposition to produce nitrogen doping graphene [5]. The development of graphene was very rapid. In 2010, [PERSON], [PERSON], and [PERSON] published a paper related to a historical review on graphene [6].  
In 2010, [PERSON] et al. [7] used chemical vapor deposition (CVD) to synthesize graphene films. The deposition of Au particles on the film was observed through an atomic force microscope and a scanning electron microscope. The size of the Au particles was 10-100 nm. At the same time, the film had a transmittance of 87%, and the surface resistance value was 150 \(\Omega\)/sq. Generally, CVD needs to be performed at high temperature. In 2017, [PERSON] et al. [8] used a dilute methane vapor source and a molten gallium catalyst to produce high-quality graphene at a controlled ambient temperature of about 50 \({}^{\circ}\)C and 100 \({}^{\circ}\)C on sapphire and polycarbonate substrates. Based on this development, significant progress has been made in low-temperature graphene synthesis methods.  
Liquid phase exfoliation mainly uses graphite blocks as a carbon source dispersed in a solvent. The solvent is usually N-methyl pyrrolidone (NMP) or dimethylformamide (DMF). Ultra-sonication is applied when molecules are adsorbed on the surface of graphite. Molecules oscillate in the liquid phase, allowing graphene to overcome van der Waals forces between the layers of graphite. There is no chemical oxidation effect or defect during the graphene stripping process, and the crystal quality of graphene is high. The CVD method requires ultrasonic vibration for tens of hours, which is very time-consuming. The peeled graphene is shattered to a size less than 1 um, and the graphene dispersion solution cannot produce a uniform single-layer graphene. [PERSON] et al. [9] indicated that the surface tension of the solvent for stripping graphene was about 40-50 m\({}^{2}\)/m, which is the best range for graphene production. The thermal conductivity of NDG was studied by [PERSON] in 2015 [10]. In the study, the effects of temperature and doping density of nitrogen on thermal conductivity were discussed. The results showed that the thermal conductivity of NDG will decrease as the temperature or the doping density of nitrogen increases. At room temperature (300 K), it showed that the thermal conductivity of NDG was in the range of 750-1300 Wm\({}^{-1}\)K\({}^{-1}\) for the doping density range of 0.5-7%. In 2010, [PERSON] et al. [11] used ammonia plasma exposure to investigate the features of NDG and showedthat the electrical conductance of NDG decreased due to the creation of excess defects, which could be observed by the increase in the D peak intensity of the Raman spectrum.  
Regarding the application of graphene to coating paints, [PERSON] [12] and others used a sol-gel method to make a conductive heat-dissipating composite coating, which contained graphene powder and polydimethylsiloxane. The graphene coating had a thermal conductivity four times better than that of the original polydimethylsiloxane coating and the graphene composite coating had a thermal conductivity of 1.02 W/mK. This graphene coating had hydrophobicity, excellent adhesion and hardness, and good performance for the actual coating.