To achieve quality control, it's also critical to build proper evaluation techniques for swiftly and precisely characterizing 2D materials before and after the transfer process. Alfa Chemistry focuses in developing characterization techniques for evaluating the quality of transferred films and doing post-transfer material quality assessments. The cleanliness, defectivity, and uniformity of the post-transfer film, as well as the electrical and optoelectronic transport properties of nanoscale devices built from the transferred film, can all be used as benchmarks to evaluate the layer transfer method's results.
Our Material Evaluation Methods
Because of its non-contact and sub-micron spatial resolution, Alfa Chemistry prefers microscopy and spectroscopic characterization techniques to assess the quality of transferred 2D material monolayers. At the most basic level, optical microscopy provides a quick and straightforward technique to determine the coverage, cleanliness, homogeneity, and thickness of a transferred 2D material film. We have chosen to employ ellipsometry to gather equivalent information in circumstances where the substrate choice gives insufficient contrast for optical microscopy.
- We used high-resolution scanning electron microscopy (SEM) and transmission electron microscopy (TEM) imaging to analyze the structural integrity and morphology of the transferred films, detecting nanoscale cracks, wrinkles, flaws, and residues.
- Atomic force microscopy (AFM) is also our common choice for mapping film morphology before and after transfer, as well as measuring the influence of transfer on film uniformity and cleanliness.
Although high-energy electron beam techniques can harm 2D materials, most of these techniques can be used to characterize transferred 2D films without causing damage.
Fig 1. (a) Optical microscopy is used for appraisal of film cleanliness and structural integrity. Cracks resulting from transfer can be seen in the top image. (b) SEM and (c) TEM are used to observe μm/nm-scale disorder indicative of damage/contamination. (d) AFM is used to assess changes in step height indicative of cracks/wrinkles and/or contamination. The white lines seen in the top image are indicative of wrinkles in the transferred film. (e) Raman, (f) PL, and (g) XPS are used to assess the number of layers, presence of contamination, doping, and strain in 2D films. (Schranghamer T. F, et al. 2021)
To optically characterize 2D materials and their constituent layers, we also provide Raman, photoluminescence (PL), and X-ray photoelectron spectroscopy (XPS) technologies. These spectroscopic techniques' characteristic peaks' width and intensity are markers of film quality and can be linked to transfer-induced disorder, contamination, doping, and strain. Furthermore, PL can directly measure the TMDC layer-dependent band gap.
To map strain and electrical properties over broad areas, we've recently used more advanced strain-induced second harmonic production and terahertz time-domain spectroscopy. We've also employed contact angle measurements to detect changes in surface chemistry following transfer. Many 2D materials are hydrophobic and have large contact angles with water by nature. As a result of the undesirable chemical changes, a lower contact angle after transfer suggests greater hydrophilicity.
Additional methods, such as evaluating the quality of the transferred films using the electrical properties of the manufacturing device, were also employed to assess the transfer method's appropriateness for device production by us.
Alfa Chemistry has considerable expertise in assessing transfer films and is experienced with a range of 2D material application platforms in order to find the best assessment technique for the customer's specific application and needs. For a professional post-transfer film evaluation, please contact us.
Reference
- Schranghamer T. F, et al. (2021). "Review and Comparison of Layer Transfer Methods for Two-Dimensional Materials for Emerging Applications." Chem. Soc. Rev. 50: 11032-11054.
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