| Feb 10, 2021 |
Light interference can help in quicker identification of defective graphene surface
(Nanowerk News) Transferring graphene grown on metal catalysts to desired functional surfaces is essential for fabricating electronic devices. However, this often damages the graphene surface. Fast characterization of surface defects, generally unachievable with usual techniques, is highly desired.
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To this end, scientists from Korea and the USA resorted to phase-shift interferometry, a well-established surface profiling technique, demonstrating it to be equally robust in characterizing graphene, paving the way for its implementation in the 2D materials industry.
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Since its discovery, graphene has been a ubiquitously popular material for fundamental research and industrial applications alike, thanks to its intriguing physical properties unmatched by any conventional material.
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Before these properties can be studied or utilized, however, a graphene sample needs to be transferred from the metal catalyst substrate (on which it is grown) to a non-conducting substrate, which often leaves tears, wrinkles, and residues, which degrade the sample quality and affect performance. Consequently, a defect-free transfer protocol is highly sought after, along with a tool for reliable monitoring of the quality of the transferred graphene.
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Traditionally, techniques such as Raman spectroscopy and atomic force microscopy (AFM) have been employed for testing the quality of transferred graphene. Although reliable, they are expensive and time-consuming, and they only provide “information” from small areas. This makes them unsuitable for use in industrial-scale quality control.
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Fortunately, phase shift interferometry (PSI), a technique that utilizes the optical interference principle, can overcome this problem because it can scan a large area in seconds, with a vertical resolution of less than 0.1 nm.
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In a recent study published in Advanced Materials ("Facile Morphological Qualification of Transferred Graphene by Phase-Shifting Interferometry"), scientists from Korea and the USA have now put this ability to the test by using PSI to investigate the quality of graphene transferred via four different protocols.
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“During the transfer process, a polymer supporting thin film such as poly methyl-methacrylate or PMMA, or cellulose is usually coated on the graphene surface and then removed after transferring the graphene to another substrate. In our study, we characterized graphene prepared via four different methods, wet transfer using PMMA, rapid thermal annealing after PMMA assisted wet transfer, wet transfer by cellulose acetate, and dry transfer, to find out which of these enables the cleanest and most flawless transfer,” explains Prof. Hyungbin Son from Chung-Ang University, who was part of this study.
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The scientists used a “single shot” PSI technique, which allowed multiple phase-shifted interferograms to be captured, and then utilized a five-step algorithm to extract phase information from the interferograms, which, in turn, provided height information of the sample. The team successfully inspected a relatively large area (~1 mm2) of the sample in just 4 seconds. In addition, they used Raman spectroscopy and AFM to characterize the same area for comparative evaluation.
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The scientists found that unlike PSI, Raman spectroscopy could not provide information on polymer residues on the transferred graphene. AFM yielded noisy images, which also took a long time to acquire (10 minutes).
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Furthermore, while both PSI and AFM reported the sample thickness to be higher than original, the thickness “uncertainty” was higher with AFM. In case of PSI, scientists attributed the measured thickness to the refractive index difference between graphene and air, whereas for AFM, they attributed it to the presence of residual contaminants.
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Lastly, they found that, among the four samples, the one with cellulose transfer was the smoothest and cleanest, with a surface roughness of ~0.8 nm, while the sample with dry transfer was the roughest, with a roughness of ~7 nm.
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While the PSI technique itself has been long-known and widely used, the study is the first to demonstrate that it is possible to reliably analyze defects in transferred graphene using such a method. Consequently, new possibilities have emerged on the horizon.
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“PSI can be extended to monitoring the transfer of other two-dimensional materials such as transition metal dichalcogenides, or TMDS, which are being actively studied for applications to various electronic devices. The electronic structure of these materials holds promise for new physical phenomena that can be discovered and explored using PSI,” comments Prof. Son.
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