Exciton Dynamics in Functionalized Germanane

Two-dimensional (2D) materials are presently one of the most actively explored platforms for the development of nanoscaled opto- and nano-electronic devices [1-3]. In particular, monoelemental 2D materials (Xenes) represent an effective option due to the tuning of their physical properties by choosing the proper ligand. In this respect, Methyl-substituted Germanane (GeCH₃) is an emerging material that has been proposed for novel applications in optoelectronics, photoelectrocatalysis, and biosensors [4-6]. GeCH₃ is a 2D semiconductor with an intriguing strong above-gap fluorescence associated with water intercalation. More in detail, hydration of the sample switches the PL spectrum reversibly between a bright red peak centered around 1.97 eV – significantly above the 1.62 eV bandgap – for the hydrated material, and a broad band-tail emission for the dry one [7]. Photoluminescence Excitation spectroscopy revealed that the absorption of the state that emits at 1.97eV starts at 2.1 eV and has its maximum at 3.5eV, hence demonstrating that this emission arises from allowed electronic transitions with strong dipole moment from bands above the conduction band minimum and/or below the valence band maximum with no allowed relaxation channel towards the band edges.<br/>Here we exploit time-resolved photoluminescence spectroscopy at different temperatures to investigate the origin of the above bandgap emission in GeCH₃ samples.<br/>From the analysis of the time- and temperature-dependent emission peak energy and integrated quantum yield, we assign the observed fluorescence to the interplay of two distinct exciton populations and discuss their electronic nature. Intriguingly, the temperature dependence of the two emissions arises from coupling to different phonons suggesting a significant coupling to out-of-plane Ge-C vibrations for one population and in-plane Ge-Ge modes for the other. Our results can be described in terms of a periodic modulation of the electronic band structure by low-energy phonons between a direct and an indirect gap. This modulation leads to a recombination rate that has a temperature dependence very similar to an Arrhenius behavior with formal activation energy much higher than the energies of the phonons involved [8].<br/>To summarize, for GeCH₃ we find two distinct populations of emitting excitons localized at RCs within puddles of intercalated water. Since the research on Xenes and Xanes is still in its infancy, our results represent the groundwork for future applications in optoelectronics, light-harvesting, and sensing.<br/>[1] S. Z. Butler et al. ACS Nano <b>7,</b> 2898-2926 (2013);<br/>[2] Y. Yan, B. Y. Xia, Z. C. Xu, X. Wang, ACS Catalysis <b>4</b>, 1693-1705 (2014);<br/>[3] M. Chhowalla, D. Jena, and H. Zhang, Nat Rev Mater <b>1</b>, 16052 (2016);<br/>[4] S. Ng, J. Sturala, J. Vyskocil, P. Lazar, J. Martincova, J. Plutnar, and M. Pumera, ACS Nano <b>15</b>, 7, 11681-11693 (2021);<br/>[5 ]A. Kouloumpis, A. V. Chatzikonstantinou, N. Chalmpes, T. Giousis, G. Potsi, P. Katapodis, H. Stamatis, D. Gournis, and P. Rudolf, ACS Appl. Nano Mater. <b>4</b>, 3, 2333-2338 (2021);<br/>[6] Z. Song, W. L. Ang, J. Sturala, V. Mazanek, P. Marvan, Z. Sofer, A. Ambrosi, C. Ding, X. Luo, and A. Bonanni, ACS Appl. Nano Mater. <b>4</b>, 5, 5164–5175 (2021);<br/>[7] T. J. Asel, W. L. B. Huey, B. Noesges, E. Molotokaite, S.-C. Chien, Y. Wang, A. Barnum, C. McPherson, S. Jiang, S. Shields, C. D’Andrea, W. Windl, E. Cinquanta, L. J. Brillson, and J. E. Goldberger, Chem. Mater. <b>32</b>(4), 1537-1544 (2020);<br/>[8] Z. Wang et al., Nano Lett. <b>21</b>, 2165-2173 (2021);