Estimation of Young’s Modulus of Graphene by Raman Spectroscopy

The Young’s modulus of graphene is estimated by measuring the strain applied by a pressure difference across graphene membranes using Raman spectroscopy. The strain induced on pressurized graphene balloons can be estimated directly from the peak shift of the Raman G band. By comparing the measured strain with numerical simulation, we obtained the Young’s modulus of graphene. The estimated Young’s modulus values of single- and bilayer graphene are 2.4 ± 0.4 and 2.0 ± 0.5 TPa, respectively

Davydov Splitting and Excitonic Resonance Effects in Raman Spectra of Few-Layer MoSe₂

Raman spectra of few-layer MoSe₂ were measured with eight excitation energies. New peaks that appear only near resonance with various exciton states are analyzed, and the modes are assigned. The resonance profiles of the Raman peaks reflect the joint density of states for optical transitions, but the symmetry of the exciton wave functions leads to selective enhancement of the A_1g mode at the A exciton energy and the shear mode at the C exciton energy. We also find Davydov splitting of <i>intra</i>layer A_1g, E_1g, and A_2u modes due to <i>inter</i>layer interaction for some excitation energies near resonances. Furthermore, by fitting the spectral positions of <i>inter</i>layer shear and breathing modes and Davydov splitting of <i>intra</i>layer modes to a linear chain model, we extract the strength of the <i>inter</i>layer interaction. We find that the second-nearest-neighbor interlayer interaction amounts to about 30% of the nearest-neighbor interaction for both in-plane and out-of-plane vibrations

Effects of Hydrogen Partial Pressure in the Annealing Process on Graphene Growth

Graphene domains with different sizes and densities were successfully grown on Cu foils with use of a chemical vapor deposition method. We investigated the effects of volume ratios of argon to hydrogen during the annealing process on graphene growth, especially as a function of hydrogen partial pressure. The mean size and density of graphene domains increased with an increase in hydrogen partial pressure during the annealing time. In addition, we found that annealing with use of only hydrogen gas resulted in snowflake-shaped carbon aggregates. Energy-dispersive X-ray spectroscopy (EDX) and high-resolution photoemission spectroscopy (HRPES) revealed that the snowflake-shaped carbon aggregates have stacked sp² carbon configuration. With these observations, we demonstrate the key reaction details for each growth process and a proposed growth mechanism as a function of the partial pressure of H₂ during the annealing process

Polarization-Independent Light Emission Enhancement of ZnO/Ag Nanograting via Surface Plasmon Polariton Excitation and Cavity Resonance

In this study, we observed that the photoluminescence (PL) intensity of ZnO/Ag nanogratings was significantly enhanced compared with that of a planar counterpart under illumination of both transverse magnetic (TM) and transverse electric (TE)-mode light. In the TM mode, angle-resolved reflectance spectra exhibited dispersive dips, indicating cavity resonance as well as grating-coupled surface plasmon polariton (SPP) excitation. In the TE mode, cavity resonance only was allowed, and broad dips appeared in the reflectance spectra. Strong optical field confinement in the ZnO layers, with the help of SPP and cavity modes, facilitated polarization-insensitive PL enhancement. Optical simulation results were in good agreement with the experimental results, supporting the suggested scenario

Growth and Device Characteristics of CZTSSe Thin-Film Solar Cells with 8.03% Efficiency

The improvement of the efficiency of Cu₂ZnSn­(S,Se)₄ (CZTSSe)-based solar cells requires the formation of high-grain-sized pure CZTSSe throughout the film. We have successfully selenized precursor samples of Cu/SnS/ZnS/Mo/Soda lime glass in an almost sealed selenium furnace. Owing to the presence of confined and high-pressure Se vapor in the furnace, Se easily diffused into the precursor samples, and high-quality Se-rich CZTSSe absorbers were obtained. To understand the effect of the growth mechanism in our precursor and annealing system, this study examines the phase evolution and grain formation. Device parameters are discussed from the perspective of a material microstructure in order to improve performance. At a selenization temperature of 570 °C, a CZTSSe film showed fully developed grains with a size of around 2 μm without noticeable pore development near the Mo back contact. Solar cells with up to 8.03% efficiency were obtained with a layer thickness of about 1.2 μm. Detailed electrical analysis of the device indicated that the performance of the device is mainly associated with shunt resistance

Thickness-Dependent Phonon Renormalization and Enhanced Raman Scattering in Ultrathin Silicon Nanomembranes

We report on the thickness-dependent Raman spectroscopy of ultrathin silicon (Si) nanomembranes (NMs), whose thicknesses range from 2 to 18 nm, using several excitation energies. We observe that the Raman intensity depends on the thickness and the excitation energy due to the combined effects of interference and resonance from the band-structure modulation. Furthermore, confined acoustic phonon modes in the ultrathin Si NMs were observed in ultralow-frequency Raman spectra, and strong thickness dependence was observed near the quantum limit, which was explained by calculations based on a photoelastic model. Our results provide a reliable method with which to accurately determine the thickness of Si NMs with thicknesses of less than a few nanometers

Ising-Type Magnetic Ordering in Atomically Thin FePS₃

Magnetism in two-dimensional materials is not only of fundamental scientific interest but also a promising candidate for numerous applications. However, studies so far, especially the experimental ones, have been mostly limited to the magnetism arising from defects, vacancies, edges, or chemical dopants which are all extrinsic effects. Here, we report on the observation of <i>intrinsic</i> antiferromagnetic ordering in the two-dimensional limit. By monitoring the Raman peaks that arise from zone folding due to antiferromagnetic ordering at the transition temperature, we demonstrate that FePS₃ exhibits an Ising-type antiferromagnetic ordering down to the monolayer limit, in good agreement with the Onsager solution for two-dimensional order–disorder transition. The transition temperature remains almost independent of the thickness from bulk to the monolayer limit with <i>T</i>_N ∼ 118 K, indicating that the weak interlayer interaction has little effect on the antiferromagnetic ordering

In Situ Imaging of an Anisotropic Layer-by-Layer Phase Transition in Few-Layer MoTe₂

Understanding the phase transition mechanisms in two-dimensional (2D) materials is a key to precisely tailor their properties at the nanoscale. Molybdenum ditelluride (MoTe2) exhibits multiple phases at room temperature, making it a promising candidate for phase-change applications. Here, we fabricate lateral 2H–Td interfaces with laser irradiation and probe their phase transitions from micro- to atomic scales with in situ heating in the transmission electron microscope (TEM). By encapsulating the MoTe2 with graphene protection layers, we create an in situ reaction cell compatible with atomic resolution imaging. We find that the Td-to-2H phase transition initiates at phase boundaries at low temperatures (200–225 °C) and propagates anisotropically along the b-axis in a layer-by-layer fashion. We also demonstrate a fully reversible 2H-Td-2H phase transition cycle, which generates a coherent 2H lattice containing inversion domain boundaries. Our results provide insights on fabricating 2D heterophase devices with atomically sharp and coherent interfaces

In Situ Imaging of an Anisotropic Layer-by-Layer Phase Transition in Few-Layer MoTe₂

Understanding the phase transition mechanisms in two-dimensional (2D) materials is a key to precisely tailor their properties at the nanoscale. Molybdenum ditelluride (MoTe2) exhibits multiple phases at room temperature, making it a promising candidate for phase-change applications. Here, we fabricate lateral 2H–Td interfaces with laser irradiation and probe their phase transitions from micro- to atomic scales with in situ heating in the transmission electron microscope (TEM). By encapsulating the MoTe2 with graphene protection layers, we create an in situ reaction cell compatible with atomic resolution imaging. We find that the Td-to-2H phase transition initiates at phase boundaries at low temperatures (200–225 °C) and propagates anisotropically along the b-axis in a layer-by-layer fashion. We also demonstrate a fully reversible 2H-Td-2H phase transition cycle, which generates a coherent 2H lattice containing inversion domain boundaries. Our results provide insights on fabricating 2D heterophase devices with atomically sharp and coherent interfaces

Band Tail Engineering in Kesterite Cu₂ZnSn(S,Se)₄ Thin-Film Solar Cells with 11.8% Efficiency

Herein, we report a facile process, i.e., controlling the initial chamber pressure during the postdeposition annealing, to effectively lower the band tail states in the synthesized CZTSSe thin films. Through detailed analysis of the external quantum efficiency derivative (<i>d</i>EQE/<i>d</i>λ) and low-temperature photoluminescence (LTPL) data, we find that the band tail states are significantly influenced by the initial annealing pressure. After carefully optimizing the deposition processes and device design, we are able to synthesize kesterite CZTSSe thin films with energy differences between inflection of d­(EQE)/dλ and LTPL as small as 10 meV. These kesterite CZTSSe thin films enable the fabrication of solar cells with a champion efficiency of 11.8% with a low <i>V</i>_oc deficit of 582 mV. The results suggest that controlling the annealing process is an effective approach to reduce the band tail in kesterite CZTSSe thin films