Tuning the electronic and magnetic properties of germanene by surface adsorption of small nitrogen-based molecules

The structural, energetic and electronic properties of germanene adsorbed with small nitrogen-based molecules, including N2, NH3, NO2 and NO, have been investigated by using first-principles calculations. The results show that all nitrogen-based molecules considered bind much stronger to germanene than to graphene due to the hybridized sp2-sp3 bonding of Ge atoms. The N2, NO and NO2 molecules all act as an acceptor, while the NH3 molecule donates electrons to germanene. We also found sizable band gaps (2-158 meV) are opened at the Dirac point of germanene through N2, NH3, and NO2 adsorptions, but with only slightly destroying its Dirac cone shape. The NO2 molecule also shows a heavy p-type doping character and makes germanene to be metallic. Moreover, when adsorbed by NO molecule, the germanene can change to be a ferromagnetic half-metal with 100% spin-polarization at the Fermi level. Overall, the different adsorption behaviors of small nitrogen-based gas molecules on germanene provide a feasible way to exploit chemically modified germanene for a wide range of practical applications, such as field-effect transistors, gas sensors and spintronic devices.