Signature of lattice dynamics in twisted 2D homo/hetero-bilayers
Yang Pan, S. Li, Mahfujur Rahaman, Ilya Milekhin, Dietrich R. T. Zahn
Abstract
Abstract Twisted 2D bilayer materials are created by artificial stacking of two monolayer crystal networks of 2D materials with a desired twisting angle θ . The material forms a moiré superlattice due to the periodicity of both top and bottom layer crystal structure. The optical properties are modified by lattice reconstruction and phonon renormalization, which makes optical spectroscopy an ideal characterization tool to study novel physics phenomena. Here, we report a Raman investigation on a full period of the twisted bilayer (tB) WSe 2 moiré superlattice (i.e. 0 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msup> <mml:mrow/> <mml:mo>°</mml:mo> </mml:msup> <mml:mo>⩽</mml:mo> <mml:mtext> </mml:mtext> <mml:mi>θ</mml:mi> <mml:mtext> </mml:mtext> <mml:mo>⩽</mml:mo> </mml:mrow> </mml:math> 60 ∘ ). We observe that the intensity ratio of two Raman peaks, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mi>B</mml:mi> <mml:mrow> <mml:mn>2</mml:mn> <mml:mi>g</mml:mi> </mml:mrow> </mml:msub> </mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mi>E</mml:mi> <mml:mrow> <mml:mn>2</mml:mn> <mml:mi>g</mml:mi> </mml:mrow> </mml:msub> <mml:mrow> <mml:mo>/</mml:mo> </mml:mrow> <mml:msub> <mml:mi>A</mml:mi> <mml:mrow> <mml:mn>1</mml:mn> <mml:mi>g</mml:mi> </mml:mrow> </mml:msub> </mml:math> correlates with the evolution of the moiré period. The Raman intensity ratio as a function of twisting angle follows an exponential profile matching the moiré period with two local maxima at 0 ∘ and 60 ∘ and a minimum at 30 ∘ . Using a series of temperature-dependent Raman and photoluminescence measurements as well as ab initio calculations, the intensity ratio <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mi>I</mml:mi> <mml:mrow> <mml:msub> <mml:mi>B</mml:mi> <mml:mrow> <mml:mn>2</mml:mn> <mml:mi>g</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:msub> <mml:mrow> <mml:mo>/</mml:mo> </mml:mrow> <mml:msub> <mml:mi>I</mml:mi> <mml:mrow> <mml:mrow> <mml:msub> <mml:mi>E</mml:mi> <mml:mrow> <mml:mn>2</mml:mn> <mml:mi>g</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> <mml:mrow> <mml:mo>/</mml:mo> </mml:mrow> <mml:mrow> <mml:msub> <mml:mi>A</mml:mi> <mml:mrow> <mml:mn>1</mml:mn> <mml:mi>g</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:mrow> </mml:msub> </mml:math> is explained as a signature of lattice dynamics in tB WSe 2 moiré superlattices. By further exploring different material combinations of twisted hetero-bilayers, the results are extended for all kinds of Mo- and W-based transition metal dichalcogenides.