Photo-induced selective gas detection based on reduced graphene oxide/Si Schottky diode
Introduction
Graphene is a carbon allotrope consisted of single-layered honeycomb sp2 hybridized carbon atoms [1], [2], [3], and it has been proved to form Schottky barrier with both p-Si and n-Si material [4], [5], [6], [7]. Consequently, graphene-based materials have been successfully used in solar cells [8], [9], [10], [11], [12], [13], photodiodes [14], [15], [16], [17] and chemical sensors [18], [19], [20], [21] thus far. Particularly, it was found that the photoelectric conversion efficiency of graphene/Si systems can reach up to 15% [22], and such devices often exhibit excellent responsivity, detectivity, and response time [16].
Aside from the important characteristics mentioned above, graphene also possesses exceptionally large specific surface area [23], [24], thus they can provide enormous contact area for various gas molecules. As a matter of fact, molecular doping is one of the most efficient methods to change the work function of graphene [25], [26]. When different gas molecules are adsorbed on graphene surface through van der Waals interaction or chemical bonding, its work function will be increased or reduced depending on whether the gas molecules are electron acceptors or donators [18], [21], [27]. Consequently, the Schottky barrier height (SBH) and current response of the graphene/Si diode will be significantly changed, which potentially can extend the application of graphene/Si diodes to gas detection. As previously reported, plain graphene can be employed as gas sensor to a number of molecules, because it often exhibits pronounced conductivity change upon molecular doping [28], [29], [30], [31], [32], [33]. However, the responses and sensitivity of such devices are not ideal, plus, they are hard to control. Singh, et al. assembled a graphene/Si diode gas sensor on the same chip as employed in the conventional graphene-only ones, and then compared their detecting performance. Surprisingly, they found that the sensitivity of the former is 13 times higher than the latter for NO2 detection [18]. In most work published, the detection mechanism has been attributed to the variation of work function originated from the chemical doping of graphene. Notably, electrical acceptor molecules like NO2 can lower the SBH of graphene/Si diodes, whereas electrical donator molecules like NH3 would increase the SBH [18], [21].
In our earlier work, we demonstrated that the reduced graphene oxide (RGO) can be used as the functional layers to form Schottky diode with silicon material [14]. Specifically, we discovered that RGO devices with an “optimal” reduction level exhibited photodetective performance superior to chemical vapor deposition-graphene-based ones. Due to the low production cost and ease of synthesis, RGO systems have been considered as one of the most promising graphene-based devices [34], [35]. However, even after intense reducing treatments, significant amounts of oxygen-containing functional groups (OFGs) can be still remained on the graphene basal plane, which might impose undesirable influence on the gas detection performance of RGO devices.
In this work, a RGO/n-Si diode was prepared to serve as a gas detector for typical combustible/toxic gases, of which the responses under various conditions were examined. Plus, the influences of OFGs on the performance of RGO were thoroughly discussed.
Section snippets
Experimental
Graphene oxide (GO) dispersion (0.25 mg/mL) was purchased from Nanjing XFNano Co. Ltd. First, GO dispersion was drop-casted onto an n-Si substrate with SiO2 layers (3–5 nm) on the top margins. Subsequently, the resulting GO coated silicon wafer was annealed at 400 °C under the protection of Ar and H2 (20:1 in flow rate). At this stage, GO was reduced to RGO, and the wafer side coated with RGO was treated with HF (40 w.t.%) vapor for 15–20 s to remove SO2 produced during the process of annealing. The
Schottky barrier and current changes when exposed in different gases
As previously reported, most graphene-based heterojunction gas sensors are based on the mechanism that charge density changes as gas adsorption takes place. Specifically, whenever gas molecules bind to graphene, either by van der Waals interaction or covalent bonding, the local charge density near the bonding areas on the graphene basal plane will certainly change. The adsorption ability of graphene to some gas molecules is comparatively stronger than others, which enables it to achieve
Conclusion
We have prepared a RGO/n-Si diode by a simple drop-casting/annealing process that was explored for the possibility of selective gas detection. Differing from conventional graphene/Si diodes, OFGs on the surface of RGO/n-Si diodes play important roles on their detection behavior. Specifically, the RGO/n-Si-based device showed remarkable selectivity to different gas molecules, which could allow it to be used for gas identification in complicated environments. Two types of unprecedented effects
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