MXenes are two-dimensional (2D) materials with versatile applications in optoelectronics, batteries, and catalysis. To unlock their full potential, it is crucial to characterize MXene interfaces and intercalated species in more detail than is currently possible with conventional optical spectroscopies. Here, we combine ultra-broadband ellipsometry and transmission spectroscopy from the mid-infrared (IR) to the deep-ultraviolet (UV) to probe quantitatively the composition, structure, transport, and optical properties of spray-coated Ti₃C₂T_x MXene thin films with varying material properties. We find film thickness heterogeneity and surface roughness in the low-nanometer range as well as depth-dependent conductivity properties, which we quantify with a graded Drude model. The optically determined sheet resistance is confirmed by four-point probe measurements. Furthermore, we employ density-functional-theory calculations to assign the observed absorption bands in the MXene dielectric function to various interband transitions from mixed MXene surface terminations. The prominent 1.48 eV (833 nm) spectral feature is found to be related to oxygen termination. Additional plasmonic effects are also suggested. Finally, we leverage the chemical sensitivity of state-of-the-art IR ellipsometry to separate the fingerprints of intercalated species within the MXene from the dominant Drude contributions, presenting for the first time a set of infrared optical constants of intercalated water. This work lays the foundation for optical metrology for interface engineering of MXene and other 2D materials.

中文翻译：

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Ti3C2Tx MXene 薄膜和插层物质，以 IR-UV 宽带椭圆偏振法为特征


MXenes 是二维 （2D） 材料，在光电子、电池和催化领域具有广泛的应用。为了释放它们的全部潜力，比目前传统光谱学更详细地表征 MXene 界面和插层物质至关重要。在这里，我们结合了超宽带椭圆偏振法和从中红外 （IR） 到深紫外 （UV） 的透射光谱，以定量探测具有不同材料特性的喷涂 Ti₃C₂T_x MXene 薄膜的成分、结构、传输和光学特性。我们发现薄膜厚度异质性和表面粗糙度在低纳米范围内，以及深度依赖性的电导率特性，我们用分级的 Drude 模型对其进行量化。光学确定的薄层电阻通过四点探针测量来确认。此外，我们采用密度泛函理论计算，将 MXene 介电函数中观察到的吸收带分配给来自混合 MXene 表面终端的各种带间跃迁。发现突出的 1.48 eV （833 nm） 光谱特征与氧终止有关。还提出了其他等离子体效应。最后，我们利用最先进的红外椭圆偏振法的化学敏感性，将 MXene 内插层物种的指纹与主要的 Drude 贡献区分开来，首次提出了一组插层水的红外光学常数。这项工作为 MXene 和其他 2D 材料的界面工程的光学计量奠定了基础。