Utilization of plasmonic emission of metal nanoclusters is currently revolutionizing diverse areas of nanotechnology, such as chemical [1] and biological [2] sensing even at single molecule level, plasmon driven enhancement of solar cell efficiency [3], plasmon driven controlled chemical reaction [4,5] and multifold enhancement of catalytic activity [6,7], metamaterial with exotic physical phenomena like negative refractive index and cloaking [8] etc. Though plasmonic emission has been traditionally observed for metal clusters such as gold [9,10], silver [[11], [12], [13]], aluminium [14,15], copper [16], sodium [17,18] etc., recently plasmonic emission of molecules [19,20] also have drawn considerable attention. Molecular plasmonic emission as well as the plasmonic emission of metal nanoclusters [21,22] with size < 2 nm are somewhat analogous as both belong to the area of quantum plasmonics [23]. There is still an ongoing evolution in the ideas of how to analyze and characterize the plasmonic characteristics of molecules and very small nanoclusters [24]. Among metal nanoclusters, gold has now become the standard substrate for utilizing Surface Enhanced Raman Scattering (SERS) phenomena due to its plasmonic emission in the visible spectral range. Silver nanoclusters are also widely employed in various applications [25].

Plasmonic properties of aluminium nanoclusters have drawn special attention [26,27] as the plasmonic band of aluminium falls in the UV region, unlike gold and silver. UV plasmonic emission of aluminium has specialized applications like plasmon nano-thermometry through temperature assessment in single plasmonic hot spot [28] for which other metal nanoclusters are not so effective. Also, aluminium being a cheaper material commercial utilization of aluminium based plasmonic devices has a greater potential. Plasmonic properties of metal nanoclusters depend both on the size and shape of the clusters as well as other environmental factors such as solvent [29]. The aim of the present study is to understand the plasmonic emission characteristics of the recently reported quasi-planar Al13+ nanocluster [30] along with its alkali atom doped counterparts [31].

TDDFT analysis has been employed to investigate the optical spectra of the above mentioned clusters both in gas phase as well as in the presence of different solvents like ethanol, tetrahydrofuran (THF) and water. We have performed an in-depth TDDFT analysis to characterize the excited states of these structures, looked for the possible plasmonic states and the nature of the plasmonic transitions. In the recent literature, various molecular plasmonicity indices like Transition dipole moment (TDM), Natural transition orbital (NTO) and transition inverse participation ratio (TIPR) have been employed to characterize plasmon like transitions of molecular systems as well as small metal nanoclusters. Plasmonic transitions in molecular systems are characterized by the collectiveness of a set of transitions where electrons from different occupied states simultaneously jump to unoccupied states. Through TDM analysis this collectiveness in excitation can be gauged. For a plasmon like transition the overall magnitude of all the contributing TDMs associated with a particular molecular orbital transition has a relatively high value. Besides the magnitude of the TDM, the phase angle of the contributing dipolar transitions is also an important factor. For a plasmon like collective oscillation the individual dipolar transitions corresponding to a particular excited state should have a phase matching also. For an idealistic case, the individual transition dipoles should contribute in the same phase, but in reality, perfect phase matching rarely exists. A high value of TDM along with an overwhelming phase matching of the contributing transitions indicates a collective transition which is a plasmonic one. To characterize plasmonic transitions we have searched for excited states for which at least two TDM components are almost equal in magnitude. Particularly for two component transition, if the components contribute in same phase they give rise to bright plasmon, but when the transition dipoles contribute in opposite phase, it gives rise to dark plasmon. The collectiveness of orbital transitions is also characterized by other parameters like NTO and TIPR. Natural Transition Orbital (NTO) [32] analysis gives reduced transition density matrix which helps one to estimate number of contributing components in a particular excitation. Transition inverse participation ratio (TIPR) [26] can be derived from NTO coefficients which can be utilized to quantify the strength of collectiveness.

TDM, NTO and TIPR analyses reveal that Li and Na doped Al13+ clusters have better plasmonic transition characteristics compared to the pure Al13+ cluster which falls in the ultraviolet region. Dielectric environment of solvent medium has a great impact on the UV–Vis spectra of the alkali doped clusters [33]. It has been found that the presence of solvents like ethanol and tetra hydro furan (THF) can induce plasmonic transitions for the alkali doped clusters. Present study indicates that the plasmonic emission of Al13+ nanocluster can be controlled by doping alkali atoms which can be tuned by solvents.