Session: Government Agency Student Posters

Paper Number: 173187

Probing Topography and Electronic States of (Sb2te3)m(sb2)n Misfit Materials 

Misfit layered compounds are a developing class of 2D materials with applications in nanoscale device structures for classical and quantum computing attributed to their novel quantum states. These materials consist of sequences of 2D materials with distinct chemistries held together by weak van der Waals interactions. For example, (Sb2Te3)m(Sb2)n is a superlattice consisting of m quintuple layers of Sb2Te3 and n bilayers layers of Sb2. Although Sb2Te3 is a semiconductor, as additional Sb2 layers are added to the repeat unit, narrowing of the bandgap is predicted, with a transition to a semimetal for (Sb2Te3)1(Sb2)2. For these studies, (Sb2Te3)m(Sb2)n ingots with m=1 and n varied from 0 to 4 (not including n=1) were prepared using solid-state synthesis followed by 60 days of annealing in a sealed quartz tube. Scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) were performed on samples of (Sb2Te3)m(Sb2)n at both room temperature and 77K. Prior to scanning, samples were exfoliated in ultrahigh vacuum to prevent surface contamination. For STS, the voltage dependence of the tunneling current and the differential conductance (dI/dV) are measured simultaneously using a lock-in amplifier. To analyze STS data, box car averaging and spectral normalization are performed prior to estimation of effective band gaps using extrapolation of the linear onsets of the band edges. For Sb2Te3 (i.e. m=1, n=0), STS reveals an effective bandgap of 0.25 eV, which is consistent with earlier measurements and predictions. However, as n is increased up to 4, STS spectra appear to reveal bandgaps that consistently increase up to 0.62 eV in (Sb2Te3)1(Sb2)4 at 77K. Furthermore, STS spectra reveal features in the local density of states at -0.5 eV and 0.2 eV which increase in magnitude as n is increased. In addition, mid-gap features in the STS spectra are also apparent in the structures with n > 1, suggesting the possibility of semi-metallic behavior. For (Sb2Te3)1(Sb2)2 superlattices, quantitative STM topographic images in conjunction with STS enabled differentiation of the Sb2Te3 and Sb2 surface terminations and identification of effective bandgaps of 0.3 eV (Sb2Te3) and < 0.1 eV (Sb2) within individual surface terminations. In this case, the presence of an apparent band gap is consistent with computational spectra performed for (Sb2Te3)1(Sb1)2 to integrate the density of states over all of k-space. Current efforts are ongoing to quantify the spectral features in (Sb2Te3)m(Sb2)n superlattices using second harmonic analysis of the lock in amplifier and high resolution quasiparticle interference (QPI) to map the local density of states in STM.

Presenting Author: Jakob Hammond-Renfro University of Michigan - Ann Arbor

Presenting Author Biography: Jakob Hammond-Renfro is an undergraduate student of Physics and research assistant in the Goldman group at University of Michigan - Ann Arbor. Their research focuses on scanning tunneling microscopy and scanning tunneling spectroscopy of misfit layered compounds to understand their electronic structure.

Authors:

Jakob Hammond-Renfro University of Michigan - Ann Arbor
Yi-Hsin Shen University of Michigan - Ann Arbor
Katharine Moncrieffe Cooper Union
Ming Wen University of Michigan - Ann Arbor
Pierre Poudeu University of Michigan - Ann Arbor
Dominika Zgid University of Michigan - Ann Arbor
Rachel Goldman University of Michigan - Ann Arbor