Optical Tomography of Polymeric Microsphere Layers Using Confocal Raman Microscopy.

In confocal Raman microscopy, depth profiling is a key application that enables analysis of the structural and chemical composition and size of three-dimensional (3D) transparent objects. However, the precise interpretation of a probed sample's Raman depth profile measurement can be significantly affected by both its size and surrounding objects. This study provides a more comprehensive understanding of the observed optical effects at the interface between polymer spheres and different substrates. Ray- and wave-optical simulations support our results. We derive a correction factor that, depending on the instrumental configuration, allows us to determine the nominal dimensions of the scanned objects more accurately from Raman depth profiles. Our studies support the need for careful consideration when employing depth profiling in confocal Raman microscopy for nondestructive, quantitative tomography of 3D objects.

Impact of Polymer-Assisted Epitaxial Graphene Growth on Various Types of SiC Substrates.

The growth parameters for epitaxial growth of graphene on silicon carbide (SiC) have been the focus of research over the past few years. However, besides the standard growth parameters, the influence of the substrate pretreatment and properties of the underlying SiC wafer are critical parameters for optimizing the quality of monolayer graphene on SiC. In this systematic study, we show how the surface properties and the pretreatment determine the quality of monolayer graphene using polymer-assisted sublimation growth (PASG) on SiC. Using the spin-on deposition technique of PASG, several polymer concentrations have been investigated to understand the influence of the polymer content on the final monolayer coverage using wafers of different miscut angles and different polytypes. Confocal laser scanning microscopy (CLSM), atomic force microscopy (AFM), Raman spectroscopy, and scanning electron microscopy (SEM) were used to characterize these films. The results show that, even for SiC substrates with high miscut angles, high-quality graphene is obtained when an appropriate polymer concentration is applied. This is in excellent agreement with the model understanding that an insufficient carbon supply from SiC step edge decomposition can be compensated by additionally providing carbon from a polymer source. The described methods make the PASG spin-on deposition technique more convenient for commercial use.

Confocal Raman Microscopy for the Analysis of the Three-Dimensional Shape of Polymeric Microsphere Layers.

The reconstruction of the three-dimensional (3D) morphology of polymeric microsphere layers based on confocal Raman microscopy was studied. Refraction of the Raman laser beam at the curved surface of the spheres broadens the focus volume inside the sphere. Compared to planar layers, the focus gets trapped inside the spheres such that the measured depth profiles are shifted and broadened. Additionally, the Raman signal of the underlying substrate is already observed for nominal focus positions above the microsphere layer. The results are successfully modeled with ray-optical simulations that allow for a clear understanding of the relevant mechanisms that lead to the generation of the Raman signals in the complex three-dimensional structures.

Traceable Quantitative Raman Microscopy and X-ray Fluorescence Analysis as Nondestructive Methods for the Characterization of Cu(In,Ga)Se2 Absorber Films.

The traceability of measured quantities is an essential condition when linking process control parameters to guaranteed physical properties of a product. Using Raman spectroscopy as an analytical tool for monitoring the production of Cu(In1-xGax)Se2 thin-film solar cells, proper calibration with regard to chemical composition and lateral dimensions is a key prerequisite. This study shows how the multiple requirements of calibration in Raman microscopy might be addressed. The surface elemental composition as well as the integral elemental composition of the samples is traced back by reference-free X-ray fluorescence analysis. Reference Raman spectra are then generated for the relevant Cu(In1-xGax)Se2 related compounds. The lateral dimensions are calibrated with the help of a novel dimensional standard whose regular structures have been traced back to the International System of Units by metrological scanning force microscopy. On this basis, an approach for the quantitative determination of surface coverage values from lateral Raman mappings is developed together with a complete uncertainty budget. Raman and X-ray spectrometry have here been proven as complementary nondestructive methods combining surface sensitivity and in-depth information on elemental and species distribution for the reliable quality control of Cu(In1-xGax)Se2 absorbers and Cu(In1-xGax)3Se5 surface layer formation.

Direct Growth of Patterned Graphene.

The direct growth of single-layer graphene patterns via electron irradiation of aromatic self-assembled monolayers and subsequent annealing is demonstrated. In this way, a reduction in the number of necessary manufacturing steps is achieved. The formed micro- and nanostructures can be arbitrarily shaped and eventually implemented in a manifold of applications.

Epitaxial graphene on SiC: modification of structural and electron transport properties by substrate pretreatment.

The electrical transport properties of epitaxial graphene layers are correlated with the SiC surface morphology. In this study we show by atomic force microscopy and Raman measurements that the surface morphology and the structure of the epitaxial graphene layers change significantly when different pretreatment procedures are applied to nearly on-axis 6H-SiC(0 0 0 1) substrates. It turns out that the often used hydrogen etching of the substrate is responsible for undesirable high macro-steps evolving during graphene growth. A more advantageous type of sub-nanometer stepped graphene layers is obtained with a new method: a high-temperature conditioning of the SiC surface in argon atmosphere. The results can be explained by the observed graphene buffer layer domains after the conditioning process which suppress giant step bunching and graphene step flow growth. The superior electronic quality is demonstrated by a less extrinsic resistance anisotropy obtained in nano-probe transport experiments and by the excellent quantization of the Hall resistance in low-temperature magneto-transport measurements. The quantum Hall resistance agrees with the nominal value (half of the von Klitzing constant) within a standard deviation of 4.5 × 10(-9) which qualifies this method for the fabrication of electrical quantum standards.

All-carbon vertical van der Waals heterostructures: non-destructive functionalization of graphene for electronic applications.

Non-destructive chemical functionalization of graphene for applications in electronic devices (e.g., sensors or transducers) is achieved via assembly of carbon nanomembrane (CNM)/single-layer graphene (SLG) van der Waals heterostructures. The CNMs are 1 nm-thick, dielectric molecular sheets terminated with functional amino groups. The structure and performance of heterostructured field-effect transistors (FETs) are characterized by photoelectron/Raman spectroscopy and by electric transport measurements in vacuum, ambient conditions and water.

Functional single-layer graphene sheets from aromatic monolayers.

Self-assembled monolayers of aromatic molecules on copper substrates can be converted into high-quality single-layer graphene using low-energy electron irradiation and subsequent annealing. This two-dimensional solid state transformation is characterized on the atomic scale and the physical and chemical properties of the formed graphene sheets are studied by complementary microscopic and spectroscopic techniques and by electrical transport measurements. As substrates, Cu(111) single crystals and the technologically relevant polycrystalline copper foils are successfully used.

Versatile sputtering technology for Al2O3 gate insulators on graphene.

We report a novel, sputtering-based fabrication method of Al2O3 gate insulators on graphene. Electrical performance of dual-gated mono- and bilayer exfoliated graphene devices is presented. Sputtered Al2O3 layers possess comparable quality to oxides obtained by atomic layer deposition with respect to a high relative dielectric constant of about 8, as well as low-hysteresis performance and high breakdown voltage. We observe a moderate carrier mobility of about 1000 cm2 V-1 s-1 in monolayer graphene and 350 cm2 V-1 s-1 in bilayer graphene, respectively. The mobility decrease can be attributed to the resonant scattering on atomic-scale defects, likely originating from the Al precursor layer evaporated prior to sputtering.