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It remains a challenge to characterize inhomogeneities in two-dimensional (2D) layered materials associated with strain and defects. Spectroscopic ellipsometry and reflectance/transmittance measurement can be used to characterize complex dielectric functions. However, these techniques are not well suited for submicron spectroscopic imaging of inhomogeneities across a macroscale sample. Here, we report hyperspectral phase microscopy (HPM) to image spatially-resolved complex dielectric functions of refractive index and extinction coefficient of transition metal dichalcogenides. This was implemented by measuring wavelength-dependent phase shifts in optical interference, which were then analyzed by a simple Kramers-Kronig-consistent model. We successfully image not only the phase difference but also patterns of modulated carrier density and strain in mono-/bi-layer heterostructured transition metal dichalcogenides. [Display omitted] • Challenges to characterize inhomogeneties in two-dimensional materials remain. • Inhomogeneties caused by strain and doping effects influence on optical constants. • Conventional spectroscopic methods are not suitable for imaging optical constants. • Spatially-resolved optical constants were imaged through hyperspectral microscopy. [ABSTRACT FROM AUTHOR]

Abstract: We report on the systematic studies of the parameters governing the integrity of graphene film during general “wet” transfer from a thermodynamic point of view. We chose polystyrene (PS) as a test carrier material and attempted to use eight different solvents to find optimal conditions for the graphene transfer from a catalyst film to a desired substrate without defects. When parameterizing the conventional chemical properties of solvents, the boiling points and surface tension were found to be critical in determining the quality of the transferred graphene. During the formation step of a conformal PS film on a graphene surface before catalyst etching, a solvent with a boiling point over ∼140°C was essential. During the following PS film removal step, a solvent with surface tension higher than approximately 29dyn/cm led to the formation of a continuous graphene film without cracks and holes. In addition, a high spin-coating velocity and lower concentration of PS in the solvent enhanced the quality of a transferred graphene film. The UV treatment of Si/SiO2 (100nm) was also found to improve the adhesion of the graphene on substrates. By electrical characterization, morphological differences were found to affect the electrical properties markedly. [Copyright &y& Elsevier]

Abstract: Raman scattering is used to study the temperature-driven structural transformations of bundled single-walled carbon nanotubes (SWCNTs) observed in HiPCO and ARC synthesis by electron microscopy, i.e., tube–tube coalescence ∼1300–1400°C, coalesced tubes to multi-walled tubes (MWCNT) at ∼1600–1800°C and finally (only ARC tubes) MWCNT to graphitic nanoribbons (GNRs) at ∼1800°C. All these transformations occurred in vacuum. Here, we present the details of these transformations as seen through the “eyes” of Raman scattering via changes in the radial (R) SWCNT band, the G-band (and its substructure) and the relative intensity of the disorder-induced D- and D′-band scattering. The Raman spectrum of GNRs is also discussed in detail. For 514.5nm laser excitation, five relatively broad GNR Raman bands are observed: 1350, 1580, 1620, 2702 and 3250cm−1. A Knight plot is used to estimate the GNR width and we find w ∼9nm, which is in reasonable agreement with the estimate of 7.6nm based on TEM and the model that a GNR is a collapsed MWCNT. [Copyright &y& Elsevier]

We report a method to evaluate the electrical properties of chemical-vapor-deposited (CVD) graphene samples from their Raman characteristics, regardless of the synthesis method or catalyst type. We synthesized five types of graphene on Cu and CuNi alloy catalysts by thermal CVD and inductively coupled plasma (ICP) CVD, respectively, and measured the Raman properties and carrier mobilities of hundreds of field effect transistors (FETs) fabricated with each CVD graphene. The Raman characteristics of the five CVD graphene types were plotted as a function of carrier mobility. The full width at half maximum (FWHM) of the 2D peak of CVD graphene, Γ 2 D , was found to decrease in proportion to carrier mobility, but the intensity ratio of the Raman D and G bands ( I D / I G ) was not related. In particular, the mobility due to long range scattering, μ l , obtained by fitting the transport curves of the FET devices, showed a linear relationship with the logarithm of the carrier mobility and could be expressed by log μ l = α + β ⋅ Γ 2 D , where α and β are 12.157 and −0.116, respectively, regardless of the CVD graphene type. We proposed a general method to evaluate the electrical properties of all CVD graphenes without directly measuring the carrier mobility by analyzing the 2D FWHM of the CVD graphene. [ABSTRACT FROM AUTHOR]

While Ga 2 O 3 crystals are considered as the next generation optoelectronic materials, the exploration of HVPE α-Ga 2 O 3 crystals and their use as solar-blind UV photodetector are still insufficient. In this paper, we investigated the structural characterization of HVPE α-Ga 2 O 3 materials and their properties as solar-blind photodetector. HVPE α-Ga 2 O 3 exhibited excellent structural properties with relatively good crystallinity, and smooth surface morphology, without any complicated process. Raman investigations confirmed the existence of slight compressive strain in the as-grown HVPE α-Ga 2 O 3. UV–visible spectrophotometry proved that HVPE α-Ga 2 O 3 had an optical bandgap of 5.15 eV, evidencing the suitability of this material for application as solar-blind photodetector. Furthermore, the photodetector synthesized using HVPE α-Ga 2 O 3 showed excellent photo-response characteristics. We believe that this study will support further development of functional oxide semiconductor materials and opto-electronic devices. • The exploration of the HVPE α-Ga 2 O 3 crystals and photodetector application was implemented. • α-Ga 2 O 3 exhibited excellent structural properties with good crystallinity, surface morphology without any complicated process. • Raman investigations confirmed the existence of compressive strain in the as-grown α-Ga 2 O 3. • The photodetector realized on HVPE α-Ga 2 O 3 showed excellent solar-blind and photo-response characteristics. [ABSTRACT FROM AUTHOR]

While the importance of α-Ga 2 O 3 crystals is increasingly gaining much interest, the structural behavior of halide vapor phase epitaxy (HVPE) α-Ga 2 O 3 crystals in wet chemical etching has not been explored yet. In this study, we investigate the structural characterization of HVPE α-Ga 2 O 3 materials via alkali KOH solution etching. Further, the activation energy of the etch rate for the materials, etch pit, and surface roughness after wet chemical treatment have been examined. The cross-sectional TEM analysis demonstrates that the triangular-shaped etch pits with (11 2 ‾ 6) plane are caused by the propagation of threading dislocations in the HVPE α-Ga 2 O 3 crystals. [ABSTRACT FROM AUTHOR]