Chemical Identification of Interlayer Contaminants within van der Waals Heterostructures

Abstract

Van der Waals heterostructures (vdWHs) leverage the characteristics of two-dimensional (2D) material building blocks to create a myriad of structures with unique and desirable properties. Several commonly employed fabrication strategies rely on polymeric stamps to assemble layers of 2D materials into vertical stacks. However, the properties of such heterostructures frequently are degraded by contaminants, typically of unknown composition, trapped between the constituent layers. Such contaminants therefore impede studies of the intrinsic properties of heterostructures and hinder their application. Here, we use the photothermal induced resonance (PTIR) technique to obtain infrared spectra and maps of the contaminants down to a few attomoles and with nanoscale resolution. Heterostructures comprised of WSe 2 , WS 2 , and hBN layers were found to contain significant amounts of polydimethylsiloxane (PDMS) and polycarbonate, corresponding to the stamp materials used in their construction. Additionally, we verify that an atomic force microscope-based "nano-squeegee" technique is an effective method for locally removing contaminants by comparing spectra within as-fabricated and cleaned regions. Having identified the source of the contaminants, we demonstrate that cleaning PDMS stamps with isopropanol or toluene prior to vdWH fabrication reduces PDMS contamination within the structures. The general applicability of the PTIR technique for identifying the sources corrupting vdWHs provides *

High-Q dark hyperbolic phonon-polaritons in hexagonal boron nitride nanostructures

“…For this we analyze sample-1 ( Ar = 4.65) in both contact-mode ( Figure 2A ) and tapping-mode ( Figure 2B ) using probe-A. In contrast with previous s-SNOM images [ 15 ] that showed strong intensities only on the sidewall of the frusta (see reproduction in Figure S3b ), the PTIR contact-mode images show strong intensities both on the frustum sidewall and top plateau ( Figure 2A ), reproducing our previous measurements on similar frusta well [ 16 ], but with 8.3-fold higher throughput thanks to the resonant detection scheme [ 24 , 49 ] adopted here. The PTIR images in Figure 2A show polariton near-field patterns evolving as a function of the incident frequency, as expected for a hyperbolic medium where the polariton propagation angle is dictated by the frequency-dependent dielectric function [ 13 ].…”

“…The dynamics of this process can be captured directly in PTIR using fast nanophotonic probes [ 43 ]; however, the expansion is too rapid for the conventional AFM probes (tens of μs response time) [ 44 ], which, instead, are shocked by the expansion and kicked into oscillation like to a struck tuning fork. The main interest in this approach stems from the proportionality of the cantilever oscillation amplitude (measured by the AFM detector) to the absorbed energy in the sample [ 44 , 46 , 47 ], which enables mapping of chemical composition [ 26 , 48 , 49 ], molecular conformation [ 50 ] and electronic bandgap [ 51 ] at the nanoscale. As reviewed recently [ 19 , 52 ], PTIR applications in material science [ 48 , 53 , 54 ], biology [ 50 , 55 , 56 ], geology [ 57 ] and other fields are growing rapidly.…”

Substrate-mediated hyperbolic phonon polaritons in MoO3

“…Figure 4 shows PTIR absorption maps at 985 and 910 cm −1 of the same MoO 3 flake seen in Figure 3 (different location), revealing relatively strong absorption in regions localized over trenches in the substrate. Absorption spectra measured on these features exhibit a distinctive peak at ≈1264 cm −1 that we attribute to the Si-CH 3 asymmetric deformation of polydimethylsiloxane (PDMS) residue [40]. Since no significant topographic features were observed on the exposed surface, we conclude that these features represent PDMS contaminants trapped primarily in the trenches beneath the crystal, derived from the polymer stamp used to prepare the sample (see Section 4).…”

“…With PTIR, light absorption in a sample is measured by transducing the photo-induced thermal expansion of the sample directly beneath the probe tip of an atomic force microscope [37]. Notably, PTIR can sense sample composition far below the exposed surface [38][39][40], at depths exceeding 1 µm [33,41], with its signal proportional to the local sample absorption coefficient [42], enabling easy comparison of PTIR spectra with far-field IR databases [40,42,43]. These characteristics enable broad applications in materials science [44][45][46] and biology [47][48][49][50][51].…”

Growth andin situcharacterization of 2D materials by chemical vapour deposition on liquid metal catalysts: a review

“…This method suffers from very low yield and is incredibly prone to interfacial contamination. 154,155 For that reason, CVD is a widely used technique to achieve larger scale heterostructures. The development of a direct one-step synthesis for 2D heterostructures is a significant milestone with many technological prospects.…”