## 1 Introduction
Thanks to their specific properties, these 1D-nanostructures present, in comparison with their 2D-film counterparts, important characteristics to fundamentally improve the gas-sensing efficiency in terms of sensitivity, response and spatial resolution [19; 20; 21; 22; 23; 24; 25; 26; 27; 28]: (i) their large surface to volume ratio greatly enhances the detection limit, thus leading to a higher sensitivity [29] and performances [30], and induces enhanced and tunable surface reactivity implying possible room temperature operation [31; 32], faster response and recovery time [30]; (ii) their nanometer scale dimensions are compatible with the size of species being sensed, opening up the way for nano-detection; and finally (iii) due to these morphological properties combined with their quasi-lattice perfection, the NWs offer superior mechanical properties leading to large elastic deformation without plastic deformations or fractures [33], higher flexibility, higher robustness and higher resistance to fatigue, then extending the operational lifetime of nano-systems [34]. As for 2D based sensors, the main materials used for developing gas-sensors, from room temperature to 400 \({}^{\circ}\)C, are chemiresistor based on metal oxide NWs such as SnO\({}_{2}\), ZnO, In\({}_{2}\)O\({}_{3}\) and Ga\({}_{2}\)O\({}_{3}\). However, in spite of the considerable efforts devoted to developing highly efficient gas sensors based on 1D-nanostructures working on a large temperature range, further improvements of the sensor performances are needed. To enhance the sensing performances of NW-based sensors, several approaches have been considered to modify the surface properties, such as doping, surface functionalization or hybridization [23] and core-shell heterostructures [23; 35]. Concerning this last approach, carbon nanotubes-core/vanadium oxide-shell [36], Ga\({}_{2}\)O\({}_{3}\)-core/SnO\({}_{2}\)-shell NWs [37], ZnO-core/Co\({}_{3}\)O\({}_{4}\)-shell NWs [38], Pb nanocubes-core/In\({}_{2}\)O\({}_{3}\) oxide-shell [39] and Ga\({}_{2}\)O\({}_{3}\)-core/GaN-shell NWs [40] have demonstrated enhanced sensing performances or sensing at a lower operating temperature. These systems are based on the NW resistance modulation process. For example, Ga\({}_{2}\)O\({}_{3}\)/GaN core/shell-based sensor has demonstrated detection of 10-200 ppm CO at 150 \({}^{\circ}\)C with responses 1.6-3.1 times stronger than a pure Ga\({}_{2}\)O\({}_{3}\) nanowire sensor [40]. In spite of these encouraging results, the system presents a strong response to other gases, which indicates its poor selectivity to CO.  
To develop highly sensitive and selective CO-gas sensor devices operating in a large range of temperature, from ambient temperature (smart object) to high temperatures (up to 600 \({}^{\circ}\)C for industrial combustion processes), we propose to consider GaN/Ga\({}_{2}\)O\({}_{3}\) core-shell NW heterostructures combining the advantages of the GaN-core and Ga\({}_{2}\)O\({}_{3}\)-shell materials. GaN and its alloys present favorable properties to develop gas-sensors and especially in playing the NW-core role: (i) the III-Nitrides present high thermal and chemical stability as well as radiation hardness, thus allowing their use in harsh environments; (ii) they exhibit less intrinsic leakages and are capable of operating at high temperatures due to their larger bandgap and stronger bond energy [19]; (iii) the GaN, as same as AlN, has a high Pauling electronegativity difference (1.23 eV), which confers a high sensitivity of the electronic properties to its surface states (K Parameter) [19]; and (iv) the nitrides are characterized by conductivity in the range of 10\({}^{-8}\) to 10\({}^{3}\) S/cm, allowing us to keep the influence of the surface charge effects on the charge carrier concentration and on the modulation of the Fermi level [19].