Nanoscale Raman Characterization of a 2D Semiconductor Lateral Heterostructure Interface

Abstract

The nature of the interface in lateral heterostructures of 2D monolayer semiconductors including its composition, size, and heterogeneity critically impacts the functionalities it engenders on the 2D system for next-generation optoelectronics. Here, we use tipenhanced Raman scattering (TERS) to characterize the interface in a single-layer MoS 2 /WS 2 lateral heterostructure with a spatial resolution of 50 nm. Resonant and nonresonant TERS spectroscopies reveal that the interface is alloyed with a size that varies over an order of magnitudefrom 50 to 600 nmwithin a single crystallite. Nanoscale imaging of the continuous interfacial evolution of the resonant and nonresonant Raman spectra enables the deconvolution of defect activation, resonant enhancement, and material composition for several vibrational modes in single-layer MoS 2 , Mo x W 1−x S 2 , and WS 2 . The results demonstrate the capabilities of nanoscale TERS spectroscopy to elucidate macroscopic structure−property relationships in 2D materials and to characterize lateral interfaces of 2D systems on length scales that are imperative for devices.

Importance of Multiple Excitation Wavelengths for TERS Characterization of TMDCs and Their Vertical Heterostructures

“…Before we come to the conclusions, it is necessary to note that 638 nm excitation is not necessarily the one providing a spectrally deficient TERS response, the completeness of the TERS spectra depends on matching the excitations to the resonant absorption profile of corresponding sample. For example, in recent work on the TERS characterization of the MoS 2 −WS 2 lateral heteromonolayers, Garg et al showed 28 that the TERS spectra of the alloyed Mo x W (1−x) S 2 heterojunction collected with 638 nm excitation featured strong Raman bands in the 210 to 220 cm −1 range, which were completely absent in the TERS spectra of the same alloyed junction area collected with 785 nm excitation, which is significantly below the optical band gap for both MoS 2 and WS 2 .…”

Theoretical Analysis of the Nanoscale Composition, Tip-Enhanced Raman Spectroscopy, and Electronic Properties of Alloys in 2D MoS2–WS2 Heterostructures

“…From the bottom-up, material stoichiometry can be tailored through the introduction of defects, ,, interfaces in lateral heterostructures, − or alloying, , providing additional opportunities to tune electronic structure and optoelectronic phenomena. For localized bottom-up engineering, interfaces play a central role, and the functionality that they engender in a lateral 2D heterostructure is intricately intertwined with that of alloys as the size of the transition between pristine materials can range from atomically sharp to hundreds of nanometers. , …”

“…The potential of tailored properties at alloyed interfaces has been demonstrated by state-of-the-art computational and experimental studies of how the exchange of metal or chalcogen atoms alters the properties of the material, complementing layer- and thickness-dependent properties in stacked heterostructures . Computational and experimental studies of alloyed systems have been performed for Mo x W 1– x S 2 , ,,− Mo x W 1– x Te 2 , MoS 2 x Se 2(1– x ) , Mo 1– x W x Te 2 , and MoTe x Se 2– x , among others . A shared focal point of the past computational studies has been to predict the macroscopic properties of the 1L alloys.…”

“…Experimentally, alloyed systems have been synthesized in many forms from bulk crystals to 1L crystallites, , and multiple prior studies have used optical spectroscopy and electron microscopy to characterize the fundamental physical properties (i.e., atomic structure, vibrational modes, electronic and optical band gaps, etc.) of alloyed systems. ,, These prior studies include investigations of composition-dependent valleytronic phenomena and tunable spin–orbit coupling. ,, However, this sizable canon of prior work has neglected the structure and physical properties of localized nanoscale alloy systems that can form at the interface of a lateral 2D heterostructure. , The structure–property relationships and theoretical predictions established from homogeneous/bulk systems are challenging to apply in interpreting nanoscale characterization measurements that probe such localized systems. Such measurements are more sensitive to local heterogeneity and use specialized approaches, environments, and/or probes to gain nanoscale resolution .…”

“…Here, we present a computational study of the vibrational and optoelectronic properties of 2D alloys of 1L-Mo x W 1– x S 2 with and without sulfur vacancies that is informed by recent nanoscale tip-enhanced Raman scattering (TERS) spectroscopy of heterogeneous alloying at the interface in a 1L-MoS 2 /WS 2 lateral heterostructure . The study employs efficient algorithms and is designed to quantitatively describe these recent measurements.…”