Respected Researchers !!! I have observed remarkably enhanced (282-fold) PL emission from Bi2Te3 nanosheets. However, its bulk counterpart being a narrow indirect bandgap semiconductor exhibits almost zero photoluminescence. Normally, SPR enhanced PL originates in the vicinity of metallic nanocrystals (The Purcell effect) .
I wonder if this pronounced signal originates solely from indirect to direct bandgap opening/transition or there might be the possibility of surface plasmon resonance (SPR) enhancement in photoluminescence from a single semiconductor ??? It should be noted that 2D nanostructure of Bi2Te3 also support SPR. Your answers would be highly appreciated.
Dear Naveed,
The UV-visible plasmonic properties of bismuth compounds remain not much studied (see for instance Optical Materials Express 7, 7, 2299-2325, 2017), so it is difficult to give a definite answer.
If we first look at the bulk dielectric properties of Bi2Te3, it comes that plasmonic effects (negative epsilon) would be possible in nanostructures at photon energies larger than the bandgap. Therefore what might happen first is an enhanced absorption of the pump beam when its photon energy comes close to the plasmon energy of the nanostructures.
At the photon energy of photoluminescence (i.e. smaller than the bandgap) however, the nanostructures should behave more, on average, as a dielectric (positive epsilon) with high refractive index, and possibly with some absorption losses. Yet some authors (see for instance Adv. Optical Mater. 5(3), 1600768, 2017) have proposed that surface carriers in such materials can make them present a core-shell like response, with a dielectric response in their volume and an UV-Visible plasmonic response at their surface. In the case there would be some plasmonic effect on the photoluminescence, I feel one might have to look at the role of such surface shell.
PL of Bi2Te3 is very weak. The only possible answer of the enhancement is only from the absorption of the pump beam, but this is only if it is close to the resonance of the nanostructures. Actually, we did the study on the Purcell effect caused by the SPR of the Bi2Te3 nanoplates, but in this case, we enhanced the emission of Graphene QDs (which the combination can be interesting for thermoelectric as well). Surprisingly, the loss is relatively small for Bi2Te3 giving large enhancements ( Adv. Optical Mater. 5(17), 1700176, 2017 ). Anyway, other papers about SPR in Bi2Te3 can be found from Kian Ping Loh Group (Nano Letters and Adv. Mater.).
How can you be sure you have pure Bi2Te3 nanosheets? In a normal PL set-up you need to support these flakes somehow and the support-nanosheet interface could possibly give rise to an interface plasmon. Also, every type of surface absorption will change the surface chemistry. So, do you see this effect also when you place the nanosheets onto an inert wide-bandgap semiconductor such as GaN or ZnO, put both into UHV and heat well above 300degC to remove all moisture and organic compounds?
Thank you all. Dear Thomas Walther , structural and chemical composition are examined by Raman, XPS, XRD, TEM/HRTEM analysis. We found no such evidence,other than pure Bi2Te3 nanosheets. since its a layered lattice structure, so it does not need necessary dangling bond formation to stabilize the surfaces. Moreover, Bi2Te3 and other family members exhibit Negligible Surface Reactivity towards Oxygen and Water (ACS Nano, 2013, 7 (6), pp 5181–5191 DOI: 10.1021/nn400908b). Therefore, we were able to rule out the presence of moisture or oxidation. Nevertheless,Si/SiO2 substrate was used in all PL measurements. We never tried to support Bi2Te3 nanosheets on an inert wide-bandgap semiconductor or heat treatment under UHV though. we will try this as well.
Based on above answers and suggestions, i can safely rule out the phenomenon of surface plasmon enhanced PL phenomenon in a single semiconductor system (Bi2Te3). Right ???
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