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Atomically thin molybdenum disulfide (MoS2) is a two-dimensional semiconductor with versatile applications. The recent adoption of liquid molybdenum precursors in chemical vapor deposition has contributed significantly to the reproducible wafer-scale synthesis of MoS2 monolayer and few-layer films. In this work, we study the effects of the carrier gas flow rate on the properties of two-dimensional molybdenum disulfide grown by liquid-precursor-intermediate chemical vapor deposition on SiO2/Si substrates. We characterized the samples using Optical Microscopy, Scanning Electron Microscopy, Raman spectroscopy, and Photoluminescence spectroscopy. We analyzed samples grown with different nitrogen carrier flows, ranging from 150 to 300 sccm, and discussed the effect of carrier gas flows on their properties. We found a correlation between MoS2 flake lateral size, shape, and number of layers, and we present a qualitative growth model based on changes in sulfur provision caused by different carrier flows. We show how the use of liquid precursors can allow for the synthesis of homogeneous, single-layer flakes up to 100 µm in lateral size by optimizing the gas flow rate. These results are essential for gaining a deeper understanding of the growth process of MoS2. [ABSTRACT FROM AUTHOR]

Unintentionally doped (001)‐oriented orthorhombic κ‐Ga2O3 epitaxial films on c‐plane sapphire substrates are characterized by the presence of ≈ 10 nm wide columnar rotational domains that can severely inhibit in‐plane electronic conduction. Comparing the in‐ and out‐of‐plane resistance on well‐defined sample geometries, it is experimentally proved that the in‐plane resistivity is at least ten times higher than the out‐of‐plane one. The introduction of silane during metal‐organic vapor phase epitaxial growth not only allows for n‐type Si extrinsic doping, but also results in the increase of more than one order of magnitude in the domain size (up to ≈ 300 nm) and mobility (highest µ ≈ 10 cm2V−1s−1, with corresponding lowest ρ ≈ 0.2 Ωcm). To qualitatively compare the mean domain dimension in κ‐Ga2O3 epitaxial films, non‐destructive experimental procedures are provided based on X‐ray diffraction and Raman spectroscopy. The results of this study pave the way to significantly improved in‐plane conduction in κ‐Ga2O3 and its possible breakthrough in new generation electronics. The set of cross‐linked experimental techniques and corresponding interpretation here proposed can apply to a wide range of material systems that suffer/benefit from domain‐related functional properties. [ABSTRACT FROM AUTHOR]

Raman spectroscopy, a versatile and nondestructive technique, was employed to develop a methodology for gallium oxide (Ga2O3) phase detection and identification. This methodology combines experimental results with a comprehensive literature survey. The established Raman approach offers a powerful tool for nondestructively assessing phase purity and detecting secondary phases in Ga2O3 thin films. X-ray diffraction was used for comparison, highlighting the complementary information that these techniques may provide for Ga2O3 characterization. Few case studies are included to demonstrate the usefulness of the proposed spectroscopic approach, namely the impact of deposition conditions such as metal–organic vapor-phase epitaxy and pulsed electron deposition (PED), and extrinsic elements provided during growth (Sn in the case of PED) on Ga2O3 polymorphism. In conclusion, it is shown that Raman spectroscopy offers a quick, reliable, and nondestructive high-resolution approach for Ga2O3 thin film characterization, especially concerning phase detection and crystalline quality. [ABSTRACT FROM AUTHOR]

[Display omitted] • Structure, chemical composition, morphology and photoluminescence of mechanically exfoliated multilayer PbI 2. • The direct free exciton peak at 2.44 eV and defect-related bound exciton peaks at 2.41, 2.35, 2.19 and 2.04 eV are found. • DFT and experimental results allow to clarify peaks: 2.41 eV to O I , 2.35 eV to V Pb and 2.19/2.04 eV to I Pb. • The known absorption transition at ∼ 1.55 eV is related to V I , which behaves as a nonradiative recombination center. • The thermal dissociation energy of free exciton in PbI 2 is 0.22 eV. A complete study of structure, chemical composition, morphology and photoluminescence (PL) is performed for multilayer PbI 2 flakes. The reduction of thickness results in uneven spatial mapping of PL and Raman intensities, while the positions and shapes of the spectra do not undergo notable changes. The most intense peaks in the PL spectra are the direct free exciton (FX) peak at 2.44 eV (508 nm) and the bound exciton (BX) at 2.41 eV (516 nm). The variation of above/below-bandgap excitation has allowed to find three more defect-related peaks at 2.35, 2.19 and 2.04 eV. So far, these three BXs have not been studied. Our DFT calculations combined with the experimental results allow to clarify the origins of BX peaks as the recombination from the optical conduction band edge to the defect levels located above the valence band: 2.41 eV to O substituting I (O I), 2.35 eV to Pb vacancy (V Pb), and 2.19/2.04 eV to I Pb with split level. We have defined that the known absorption at 1.55 eV (800 nm) is related to V I nonradiative recombination center. The temperature behaviors allow to determine the thermal dissociation energy of FX of 0.22 eV. The dependences also show a co-interaction between defect levels. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) MoS 2 is emerging as a promising candidate for novel optoelectronics and flexible devices, but the photoluminescence (PL) efficiency is still limited for actual photonic applications. Plasmonic enhancement is the best way to increase the PL emission of 2D materials. In this study, the PL enhancement of 2D MoS 2 decorated with Au nanorods (NRs) and nanospheres (NSs) at room temperature is investigated, focusing in particular on the effect of the nonspherical shape of NRs. To have a proper comparison, both nanoparticles have very similar diameters of 13–14 nm, while the NR average length is 81 nm. After decorating with Au NRs and NSs, the average PL enhancement of few-layer MoS 2 is about 4.3 and 2 times, respectively. The Raman spectra are similarly enhanced, 3.8 and 2.4 times (A 1g mode), respectively. The maximal enhancement is observed for the samples with Au NRs: this feature can be related to highly localized plasmonic hot spots in vicinity of the NR tips. A strong electric field within the hot spots causes the generation of additional free electron-hole pairs and, in turn, the enhancement of exciton excitation and the increase of the PL intensity. Simultaneously, the injection of hot electrons from the metal increases the probability of trions generation, thus contributing to the PL enhancement. [Display omitted] • Photoluminescence (PL) enhancement of 2D MoS 2 by Au nanorods (NRs) and nanospheres (NSs). • The focus on the effect of the nonspherical shape of NRs on the PL enhancement. • Au NRs and NSs cause the average PL enhancement of about 4.3 and 2 times, respectively. • The stronger enhancement can be related to high localized plasmonic hot spots in vicinity of NR tips. • The formation of trion due to the hot electron transfer from metal after decorating with Au NRs. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) MoS 2 possesses unique optical and electrical properties and has many practical applications in nano-optoelectronics. Raman spectroscopy allows readily identify monolayer of MoS 2 , but the unambiguous determination of the number of layers in a few-layer structure remains a challenging task. In this work, the modification of frequencies of the most intense bands arising from in-plane and out-of-plane vibrations in the spectra of mono- and few-layer MoS 2 crystals is studied experimentally, using the Raman scattering technique, and systematically analyzed theoretically, considering the influence of the interlayer interaction as well as the anharmonic interactions within the layer. A good agreement between experimental spectra and theoretical simulations is demonstrated, which allows to estimate the thickness of few-layer MoS 2 and also determine parameters of the interlayer and anharmonic interactions. Possible effects of Fermi resonance in 2D MoS 2 and the corresponding frequency shift of fundamental modes are considered. The proposed theoretical approach can be used to analyze Raman spectra, estimate the interlayer interaction, anharmonism, and the number of layers for other 2D layered materials. [Display omitted] • Raman spectra in mono- and few-layer MoS 2 are experimentally and theoretically studied. • The influence of anharmonic and weak van der Waals interactions on the Raman profiles is shown. • The theory allows to determine parameters of the interlayer interaction and anharmonicity. • Theoretical approach allowing estimation of the thickness of few-layer MoS 2 is proposed. • Possible effects of Fermi resonance in 2D MoS 2 is considered. [ABSTRACT FROM AUTHOR]

• Variation of exciton and trion PL and structural properties on the number of MoS 2 layers. • PL of few-layer MoS 2 redshifts with temperature, while the intensity decreases. • The exciton and trion bands are deconvoluted and their behaviors are studied in-detail. • The temperature behavior of low-energy tail of trion PL is studied for few-layer MoS 2. • Evaluation of the spin-orbit splitting, quenching activation energy and trion effective size. Molybdenum disulfide (MoS 2) is one of the most interesting 2D materials. However, while monolayer MoS 2 is well-explored, the few-layer flakes, also promising for flexible device and nano-optoelectronics application, remain less investigated. We study the variation of exciton and trion photoluminescence (PL) and structural properties of few-layer MoS 2 grown on SiO 2 /Si by chemical vapor deposition. Scanning electron and atomic force microscopies show that most of the flakes have a triangular shape. The µ-PL spectra of the flakes show two broad bands associated to the A exciton overlapped by trion and B exciton. The light emission at room temperature redshifts when the number of layers increases, controlled by µ-Raman spectroscopy. In the temperature-dependent measurements from 80 to 290 K, the PL redshifts according to bandgap narrowing, while the intensity decreases due to the thermally activated nonradiative recombination related to the increase of the electron-phonon interaction. Exciton and trion bands are deconvoluted and their behaviors with temperature are studied by fitting the PL data with modelling equations. This allows to evaluate the spin-orbit splitting, quenching activation energy, trion effective size and other important parameters. These new findings are useful for the potential applications of few-layer MoS 2 in nano-optoelectronics involving external modulation of optical properties. [ABSTRACT FROM AUTHOR]