Handbook of Modern Coating Technologies

Films of two-dimensional materials

2D materials are attractive because they possess unique and exceptional properties [140]. They have been used widely for smart or flexible electronic [141,142], optoelectronics [143,144] and energy devices [145]. For these devices, 2D materials can be used not only as the host functional coatings, but also as the protective layer or ultrasmooth substrates for other functional materials [146,147]. 2D materials have been one of the hottest research topics. Especially 2D materials coatings are ultrathin and SE which has atomic scale sensitiv­ity is an ideal characterization tool for them.

Graphene, the most widely studied 2D material, has been attracted wide attention because of the unique and excellent physical, chemical and mechanical properties [140]. The first results on the SE results of graphene flakes on dielectric substrates were reported by Kravets et al. [148]. They modeled the graphene layers as a uniaxial anisotropic material and assumed the thickness of graphene layer as 3.35 A, and extracted the optical constants of gra­phene layers successfully. Then the graphene layers obtained by various methods, including CVD grapheme [149—152], exfoliated grapheme [153—156] as well as epitaxial grapheme [157—159] were characterized by using SE in near-IR—visible—ultraviolet region. To deter­mine the optical constants and thickness of graphene flakes accurately and simultaneously by analyzing the SE data, Weber et al. [155] parameterized the optical constants with B- spline function method. The fitted thickness of graphene was 3.4 A and this value was con­sistent with the interlayer spacing in graphite. Isic et al. [160] used SE to extract the optical properties of few-layer graphene flake. Since the size of graphene flake was smaller than that of the ellipsometric light spot (50 pm), the island-film model was introduced to deconvolute ellipsometric data. The obtained extinction coefficients for graphene were in consistent with the previously reported data [148,154,155], while the refractive index data were around 30% lower and without sharp decrease in the UV range. Matkovic et al. [156] employed the IE with small spot size (1 pm) to investigate the surface state of the exfoliated graphene on a Si/ SiO₂ substrate in the experimental environment where some water existed. A Fano model was employed to parameterize the optical constants of graphene. Then the thickness map­ping revealed that there was a water layer on the graphene sample when the sample was exposed to the experimental environment and the spatial distribution of water was obtained. To investigate the effect of the substrates on the optical properties of graphene, Wurstbauer et al. [154] used the IE to determine the shape and layer number of exfoliated graphene deposited on the flat amorphous SiO₂ or crystalline GaAs substrates. Gaskell et al. [159] also employed a high spatial resolution ellipsometer to determine the thickness of exfoliated or epitaxial graphene films. The sensitivity to measure layer number of graphene increased when the layer number and substrate refractive index decreased. Base on IE, recently ellipso- metric contrast micrography with the spatial resolution below 1 pm and atomic layer thick­ness resolution was developed by Hofmann et al. [161]. This technique is a powerful tool for characterizing the layer number, defects, and contamination of the 2D materials.

In situ RTSE experiments were carried out by Losurdo et al. [162] to investigate the depo­sition process of graphene with low-pressure chemical vapor deposition approach on nickel and copper supports. As a nondestructive and nonintrusive optical method, SE was used to monitor all steps of graphene growth, including the cleaning and annealing of metal catalyst, the diffusion of carbon, covering of carbon on the surface, formation of graphene, and so on. These information provided by SE would be useful to control the graphene quality and in situ SE technique was proved to be an effective quality-control tool.

Besides graphene, graphene oxide (GO) was also be studied by using SE. For example, Shen et al. [163] characterized the optical responses of GO and few layer reduced graphene oxide (FRGO) with SE in visible range. To analyze the ellipsometric data of GO and FRGO, a Lorentz oscillator model was employed. It was found that the optical responses of FRGO were very simi­lar with these of monolayer exfoliated graphene within the visible wavelength range. Jung et al. [164] measured the optical properties and thicknesses of single and multiple layers of GO using IE with 2 pm lateral resolution and standard SE. The refractive index (n) of multiple GO layers increased while their thicknesses reduced upon the thermally treatment. To explain the change of the thicknesses, a model was proposed in which some interlamellar water was involved.

The layered transition metal dichalcogenides (TMDs), whose general chemical formula could be written as MX₂ (M = Mo, W, Ti, Zr, Ta, and Nb;X = S, Se, and Te), are other prom­ising 2D materials. SE has been used to extract the optical properties of TMDs [69,70,165] and obtain the 1T/2H phase ratio [166]. IE is also applied to detect the MoS₂ flakes with a lateral resolution about 1 pm [167,168]. Recently some new 2D materials such as hexagonal boron nitride, Ti₂C, Nb₂C, and PtSe₂ were synthesized successfully and their optical and electric properties were determined precisely by using SE [169—171].

• Summary and perspectives

SE is a powerful tool for characterization of coatings with high precision and sensitivity. By using SE, various physical properties such as the thickness, optical, and electric properties can be obtained, which makes the application area of SE quite wide, including characteriza­tion of photovoltaic films, display coatings, protective coatings, biological films as well as 2D materials layers, etc. Moreover, SE measurement is nondestructive and very fast, so that it is easy to in situ real-time monitor the formation and change of coatings on the atomic scale. On the other hand, SE technique is an indirect characterization method. Analyzing ellipso- metric data requires construction of complicated optical models, which can be considered as the greatest disadvantage of SE. To overcome this shortcoming, the complementary techni­ques, for example, XPS, AFM, SEM, etc., should be applied to provide enough information for building reliable models to deconvolute SE data.

In the future, SE would be developed in the following aspects: (1) enlarging spectral range so that the light beam at a given wavelength used in an ellipsometer could provide extra characteristic information about the composition or properties of samples; (2) shortening the acquisition time for in situ RTSE measurements to extract the evolution information of dynamic systems; (3) reducing the diameter of the light beam to improve the spatial resolu­tion of ellipsometers and develop IE;(4) developing more reliable and accurate models to describe the N—X relationships for anisotropic and complicated samples;and (5) proposing new formula system based on the optical principles to deconvolute the ellipsometric data.

Acknowledgment

This work was supported by the National Natural Science Foundation of China (21773019, 21573028, and 21972012) and the Program for New Century Excellent Talents in University (NCET-12-0587 and NCET- 13-0633).