Anomalous vibrational behavior of two dimensional tellurium: Layer thickness and temperature dependent Raman spectroscopic study

A few layer/multilayer two-dimensional (2D) Tellurium (Te) nanoflakes were mechanically exfoliated and their structural and vibrational properties were probed. Layer thickness reduction was confirmed using optical microscopy, atomic force microscopy and kelvin probe force microscopy. Raman spectroscopy measurements revealed anomalous shift in the frequencies of their intrachain vibrational modes, where, after an initial increasing trend (blue shift) with thickness reduction from bulk, they changed to decreasing trend (red shift) on further layer thinning. Such unusual vibrational behaviour of 2D Te, exhibiting reversal from blue shift to red shift, is attributed mainly to the deformations occurring in the Te chains on layer thinning, followed by modifications in the local structures and bondlengths of the corresponding Te atoms. The deformation under relaxed conditions are also confirmed from first principle calculations. Red shift can also be correlated to lesser hole concentration for thinner flakes, as revealed by work function changes measured on the nanoflakes surfaces. Interestingly, in spite of such anomalous behavior, the difference in frequencies of both modes showed initial increase with thickness and saturation beyond a certain thickness, as generally observed in other 2D materials. Temperature dependent Raman spectroscopic measurements further confirmed anomalous behaviour for thinner flakes.