Reflectance Difference Spectroscopy Research Articles

Hybridization vs decoupling: influence of an h-BN interlayer on the physical properties of a lander-type molecule on Ni(111).

2D materials such as hexagonal boron nitride (h-BN) are widely used to decouple organic molecules from metal substrates. Nevertheless, there are also indications in the literature for a significant hybridization, which results in a perturbation of the intrinsic molecular properties. In this work we study the electronic and optical properties as well as the lateral structure of tetraphenyldibenzoperiflanthene (DBP) on Ni(111) with and without an atomically thin h-BN interlayer to investigate its possible decoupling effect. To this end, we use in situ differential reflectance spectroscopy as an established method to distinguish between hybridized and decoupled molecules. By inserting an h-BN interlayer we fabricate a buried interface and show that the DBP molecules are well decoupled from the Ni(111) surface. Furthermore, a highly ordered DBP monolayer is obtained on h-BN/Ni(111) by depositing the molecules at a substrate temperature of 170 °C. The structural results are obtained by quantitative low-energy electron diffraction and low-temperature scanning tunneling microscopy. Finally, the investigation of the valence band structure by ultraviolet photoelectron spectroscopy shows that the low work function of h-BN/Ni(111) further decreases after the DBP deposition. For this reason, the h-BN-passivated Ni(111) surface may serve as potential n-type contact for future molecular electronic devices.

Coverage-dependent differential reflectance spectra of MoS2 atomic films synthesized by CVD using a large-diameter quartz tube

Here, we synthesized monolayer MoS2 under atmospheric pressure by chemical vapor deposition (CVD) using a quartz tube with a large-diameter up to 100 mm. The morphology of MoS2 thin films along the airflow direction was initially observed by the optical microscope. Raman spectra and atomic force microscopy (AFM) were utilized to characterize the thickness of samples. Moreover, the optical properties of four regions with continuous different coverage were systematically investigated by differential reflectance spectroscopy (DRS). It is found that MoS2 thin films in the sub-monolayer structure have exhibited the same energy positions of optical characteristic peaks, including A exciton peak at 1.91 eV, B exciton peak at 2.08 eV and C exciton peak at 2.84 eV. And the DRS shows that the intensity of B exciton peak rises at the rate of ~0.46 monolay−1 (ML−1). These results provide an insight for monitoring the growth and evolutions of MoS2 thin films with different coverage.

Manganese ferrite dispersed over graphene sand composite for methylene blue photocatalytic degradation

Abstract Magnetic materials can be considered as promising catalysts for the degradation of various organic pollutants in wastewater treatment as they can be easily separated with visible light response. The purpose of this research was to develop a highly efficient photocatalyst in decomposing methylene blue (MB) was synthesized using MnFe2O4 nanoparticles supported on graphene sand composite (MnFe2O4-GSC), resulting in an easily, magnetically separable and recyclable photocatalyst. MnFe2O4-GSC was prepared by a solvothermal methodology using commercial sand, graphene oxide (GO) and metal ions (Fe3+ and Mn2+) as precursor materials. The photocatalyst was characterized by scanning electron microscopy, X-ray diffraction, differential reflectance spectroscopy, transmission electron microscopy, Fourier-transform infrared spectroscopy and zeta potential analysis. The results indicated that MnFe2O4 had a spherical shape with uniform size of about 200 nm were successfully loaded and dispersed on the graphene nanosheets and on the sand’s surface. The photocatalytic performance of MnFe2O4-GSC for MB degradation under visible light irradiation was investigated with different reaction conditions, H2O2 concentration, catalyst dosage and pH. The use of the MnFe2O4-GSC as a heterogeneous photocatalyst achieved 100 % degradation of 10 mg L−1 of MB solution at natural pH (pH 7.65) in the presence of 5 mL of H2O2 and 180 min of contact time. In cyclic photodegradation experiments, the performance of MnFe2O4-GSC was stable and practically unchanged regarding its efficiency after five runs. Thus, this research provided a promising strategy for designing efficient and magnetically recyclable photocatalysts for decontamination of dye wastewater under visible light irradiation.

Green-photodegradation of model pharmaceutical contaminations over biogenic Fe3O4/Au nanocomposite and antimicrobial activity

A simple biogenic approach for synthesis of Fe3O4/Au nanocomposite with 31 nm size using aqueous extract of Carum carvi L. seeds has been reported. Phytochemicals of Carum carvi L. seeds extract play three roles, including reducing, capping, and stabilizing agents during the nanocomposite fabrication process. Resulting nanocomposite was characterized by X-ray diffraction, Fourier-transform infrared spectroscopy, differential reflectance spectroscopy, vibrating-sample magnetometer, energy-dispersive X-ray spectroscopy and X-ray elemental mapping. Differential reflectance spectroscopy investigated optical property and absorption spectra display a sharp absorption agreeing to a bandgap of 1.6 eV. Photocatalytic activity of biogenic Fe3O4/Au nanocomposite has been investigated for degradation of imatinib and imipenem drugs under UV and visible light irradiation, due to pharmaceuticals have shown to be recalcitrant in wastewater and conventional wastewater treatments do not remove them, often. Degradation efficiency of imatinib, and imipenem are about 92% and 96% after 1200 s exposure UV light and about 82% and 84% after 3600s exposure visible light in the concentration of 10 ppm drugs. Also, antimicrobial activity of biogenic Fe3O4/Au nanocomposite was investigated on three human pathogens and best result can see in 25 mg/mL of nanocomposite versuse Bacillus subtilis that inhibition zone is about 27 mm. Design of nanocomposites capable of simultaneously removing pharmaceutical and microbial contaminations is important in environments such as hospitals wastewater treatment.

Application of novel and reusable Fe3O4@CoII(macrocyclic Schiff base ligand) for multicomponent reactions of highly substituted thiopyridine and 4H‐chromene derivatives

In this research study we designed and synthesized CoII(macrocyclic Schiff base ligand containing 1,4‐diazepane) immobilized on Fe3O4 nanoparticles as a novel, recyclable, and heterogeneous catalyst. The nanomaterial was fully characterized using various techniques such as Fourier‐transform infrared spectroscopy, scanning electron microscopy, transmission electron microscopy, X‐ray diffraction, energy‐dispersiveX‐ray spectroscopy, thermogravimetric analysis, vibrating sample magnetometry, differential reflectance spectroscopy, Brunauere–Emmette–Teller method, inductively coupled plasma, and elemental analysis (CHNS). Then, the catalytic performance was successfully investigated in the multicomponent synthesis of 2‐amino‐4‐aryl‐6‐(phenylsulfanyl)pyridine‐3,5‐dicarbonitrile and 2‐amino‐5,10‐dioxo‐4‐aryl‐5,10‐dihydro‐4H‐benzo[g]chromene‐3‐carbonitrile derivatives. Furthermore, the catalyst was isolated using a simple filtration, and recovery of the nanocatalyst was demonstrated five times without any loss of activity.

In situ reorganization of Bi3O4Br nanosheet on the Bi24O31Br10 ribbon structure for superior visible-light photocatalytic capability

In situ synthesized 2D/2D heterojunctions often exhibit a more homogeneous mixture and stronger interface interaction, thereby providing higher interfacial charge transfer and larger interface contact area compared with 0D/2D and 1D/2D heterojunctions. Herein, novel 2D/2D Bi24O31Br10/Bi3O4Br heterojunctions with a “nanosheet-on-ribbon” hierarchical structure were fabricated via in situ reorganization of original elements in a hydrothermal process, and this method was accomplished through the intervention of NH3·H2O. The Bi24O31Br10/Bi3O4Br heterojunctions exhibit outstanding visible light activity than pure Bi3O4Br and Bi24O31Br10, reaching photodegradation efficiency of 85% for sulfamethoxazole in 180 min and removal rate of 32.5% for NO in 30 min. Crystalline phases, optical absorption properties, morphologies, microstructures, and band structures were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, differential reflectance spectroscopy, valence band X-ray photoemission spectroscopy, Mott–Schottky methods, density functional theory, photoluminescence spectroscopy, photocurrent analysis, and electrochemical impedance spectroscopy. The superior photocatalytic activity could be ascribed to the special structure and interface interaction between Bi3O4Br and Bi24O31Br10, thereby enhancing surface-active sites, prolonging light absorption, and promoting charge separation capability during the photocatalytic reaction. The formation mechanism of Bi24O31Br10/Bi3O4Br heterojunctions was proposed, and NH3•H2O was not only a pH adjuster but also a structure-controlling agent. This work provides a reference for in situ synthesis of multifunctional 2D/2D photocatalytic nanocomposites.

Effects of Interlayer Coupling and Band Offset on Second Harmonic Generation in Vertical MoS2/MoS2(1-x)Se2x Structures.

Noncentrosymmetric monolayers (MLs) of transition metal dichalcogenides (TMDCs) and their 3R-type vertical stacks provide an ideal platform for studying atomic-scale nonlinear light-matter interaction in terms of second harmonic generation (SHG). Unlike the case of MLs, SHG from artificial stacks can be nontrivially affected by interlayer coupling and band offset between the constituent MLs, where the latter occurs for band-gap-engineered vertical heterostructures (VHs). In order to study these effects, we produced different sets of 3R-type homobilayers (homo-BLs) and heterobilayers (hetero-BLs) composed of MoS2 and its ternary alloy MoS2(1-x)Se2x. We first investigated the impact of interlayer coupling on the SHG response across the A- and B-exciton resonances in the MoS2 homo-BLs. The coupling strength was varied by preparing (i) decoupled BLs (SiO2 intercalated), (ii) weakly coupled BLs (dry transferred), and (iii) strongly coupled BLs (postannealed) and monitored by photoluminescence, Raman, and reflectance difference spectroscopy, and atomic force microscopy. Unlike the decoupled BL, SHG in the coupled BLs cannot be explained by the simple square law in thickness due to coupling-induced band modification. The impact of exciton-resonance offset on SHG was also investigated in the hetero-BLs by controlling the Se concentration in MoS2xSe2(x-1). Although these VHs can significantly broaden the spectral range for efficient SHG by vertically superposing distinct resonances of the constituent MLs, coherent reinforcement of SHG cannot be achieved basically because of the π/2 phase difference between the on-resonance SHG field in one ML and the off-resonance SHG field in the other ML. Upon postannealing, however, the overlapping resonance regime exhibited unexpectedly high SHG enhancement. This may arise from the formation of the strong resonance when the VHs approach ideal 3R-type hetero-BLs. Our approach may be utilized for fully exploiting the TMDC VHs for highly efficient broadband SHG applications.

Green Synthesis of CuFe2O4@Ag Nanocomposite Using the Chlorella vulgaris and Evaluation of its Effect on the Expression of norA Efflux Pump Gene Among Staphylococcus aureus Strains

Increasing drug resistance among Staphylococcus aureus is a global health threat and finding alternative antimicrobial agents against it has been considered. Multidrug resistance efflux pumps, including NorA, are involved with resistance to different drugs, especially fluoroquinolones, in S. aureus. Using metal nanoparticles against pathogenic bacteria is a promising approach; however, physio-chemical synthesis of nanoparticles has limitations. Biosynthesis of metal nanoparticles with antibacterial activity has gained interest, recently. In this study, biosynthesis of CuFe2O4@Ag nanocomposite using aqueous extract from microalgae Chlorella vulgaris was performed, and its antibacterial property and effect on expression of norA efflux pump gene were investigated. Fourier transform infrared spectroscopy (FTIR), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), energy dispersive X-ray mapping analysis (EDX-map), differential reflectance spectroscopy (DRS), and dynamic light scattering (DLS) were used to characterize synthesized nanocomposite. Antibacterial activity of the prepared nanocomposite individually and combined with ciprofloxacin against S. aureus strains was evaluated using the disk assay method, and minimum inhibitory concentration (MIC) of each agent was determined using the broth dilution method. Anti-biofilm activity of this nanocomposite was checked. Finally, the effect of CuFe2O4@Ag nanocomposite alone and in combination with ciprofloxacin on the expression of norA was assessed by real-time PCR. The physical analysis revealed proper synthesis of spherical and well-dispersed CuFe2O4@Ag nanocomposite with an average diameter of 20nm. Synthesized nanocomposite had synergistic antibacterial activity with ciprofloxacin. Moreover, expression of norA gene among clinical and standard strains treated with CuFe2O4@Ag nanocomposite combined with ciprofloxacin reduced by 59% and 65%, respectively. Thus, CuFe2O4@Ag nanocomposite synthesized in this study can be considered as a promising candidate to be used to inhibit staphylococcal efflux pump genes and increasing the antibiotic efficacy.

Novel NiFe/Si/Au magnetic nanocatalyst: Biogenic synthesis, efficient and reusable catalyst with enhanced visible light photocatalytic degradation and antibacterial activity

The aim of this paper is to biosynthesize NiFe2O4/SiO2/Au (NiFe/Si/Au) magnetic nanocatalyst using the sonochemical method. This is the first study to synthesize this compound using this method. To obtain optimum morphology and size of products, the synthesis was performed in the presence of polyvinylpyrrolidone, maltose, glucose, and fructose as capping agents and Crataegus pentagyna leaf extract and NaBH4 as reducing agents. Vibrating‐sample magnetometer, field‐emission scanning electron microscopy, Fourier‐transform infrared, transmission electron microscopy, X‐ray diffraction, energy‐dispersive X‐ray spectroscopy, and differential reflectance spectroscopy techniques were performed to confirm the preparation of magnetic products. The as‐synthesized magnetic samples were used to enhance the photocatalytic degradation and antibacterial activity. The NiFe/Si/Au nanocatalysts exhibited an excellent photocatalytic activity by degradation of rhodamine b, methylene blue, and erythrosine as organic contaminants under ultraviolet and visible light irradiation. The degradation curves illustrate that the degradation of anionic dyes is more than that of cationic dyes. In addition, antibacterial activity of as‐prepared nanocatalyst against seven bacterial species was investigated. Because of their antibacterial effects, as‐synthesized products show a high antibacterial activity against various bacteria. Consequently, the NiFe/Si/Au nanocatalysts with high antibacterial activity and excellent potential photocatalyst activity can be used in environmental and medical sciences for wastewater treatment.

Growth oscillation of MoSe2 monolayers observed by differential reflectance spectroscopy

The precisely controlled growth of transition metal dichalcogenide (TMDC) monolayers requires sensitive and nondestructive techniques to monitor the morphology and coverage in situ and in real time. In the current work, differential reflectance spectroscopy (DRS) was applied to monitor the molecular beam epitaxy (MBE) growth of atomically thin MoSe2 layers on mica. The optical evolution exhibits an oscillation with monolayer periodicity, revealing a two-dimensional (2D) layer-by-layer growth of the MoSe2 thin films. The observed sensitivity of DRS to the step density is associated to the modified electronic structures at the edges of TMDC monolayers. As DRS works in any transparent ambient, we speculate that it could be of great use for realizing precisely controlled growth of TMDC monolayers using not only MBE but also chemical vapor deposition (CVD).

Dielectric Properties of Pulsed Laser Deposited Nanoscale CeNi5 Thin Films

Dielectric properties of pulsed laser deposited, nanoscale CeNi5 alloy layers, on glass or SiO2 substrate are described using the complex dielectric function. The UV–Vis–NIR spectral behavior of this function is studied separately for its real part e1 (the dielectric constant or dielectric permittivity), and for its imaginary part e2 (the dielectric loss function). The layers were obtained from grinded CeNi5 bulk powder using short, modulated laser pulses. The absolute reflectance of the obtained nanoscale alloy layers was measured at the 632.8 nm wavelength of a liquid nitrogen cooled and stabilized He–Ne source. This value was further used to renormalize the relative differential reflectance spectroscopy measurements performed in the UV‒Vis‒NIR domain. The obtained absolute reflectance spectra were processed using the Kramers–Kronig formalism, so that the real and imaginary parts of the complex dielectric function could be computationally determined, also leading to the calculation of the electron loss functions –Im e–1 and –Im(1 + e)–1. The behavior of these functions near the spectral inflexion points was determined using appropriate theoretical considerations. The variation of the dielectric functions was explained, electron density of states and the shape of the energy bands were inferred. This study reveals the layer thickness and deposition substrate dependent optical and electrical properties of the produced nanoscale CeNi5 structures.

Direct Observation of Monolayer MoS2 Prepared by CVD Using In-Situ Differential Reflectance Spectroscopy

The in-situ observation is of great significance to the study of the growth mechanism and controllability of two-dimensional transition metal dichalcogenides (TMDCs). Here, the differential reflectance spectroscopy (DRS) was performed to monitor the growth of molybdenum disulfide (MoS2) on a SiO2/Si substrate prepared by chemical vapor deposition (CVD). A home-built in-situ DRS setup was applied to monitor the growth of MoS2 in-situ. The formation and evolution of monolayer MoS2 are revealed by differential reflectance (DR) spectra. The morphology, vibration mode, absorption characteristics and thickness of monolayer MoS2 have been confirmed by optical microscopy, Raman spectroscopy, ex-situ DR spectra, and atomic force microscopy (AFM) respectively. The results demonstrated that DRS was a powerful tool for in-situ observations and has great potential for growth mechanism and controllability of TMDCs prepared by CVD. To the best of the authors’ knowledge, it was the first report in which the CVD growth of two-dimensional TMDCs has been investigated in-situ by reflectance spectroscopy.

Multiple-peak resonance of optical second harmonic generation arising from band nesting in monolayer transition metal dichalcogenides TX2 on SiO2/Si(001) substrates (T=Mo,W;X=S,Se)

The nonlinear optical response of monolayer transition-metal dichalcogenides $T{X}_{2}\phantom{\rule{0.28em}{0ex}}(T=\mathrm{Mo},\mathrm{W};\phantom{\rule{0.28em}{0ex}}X=\mathrm{S},\phantom{\rule{0.28em}{0ex}}\mathrm{Se})$ on ${\mathrm{SiO}}_{2}/\mathrm{Si}(001)$ substrates was examined using second-harmonic generation (SHG) spectroscopy in combination with differential reflectance (DR) spectroscopy. In the SHG spectroscopy, the second-harmonic intensity corrected by a crystalline quartz reference was obtained for the wavelength of the incident femtosecond laser varied from 750 to 1040 nm. The SHG spectra of all the four ${TX}_{2}$ were dominated by two-photon resonance around the C peaks in the DR spectra that was previously assigned to interband transitions at critical points formed as a result of band nesting. Multiple peaks were observed in each resonance spectrum, which was then decomposed into a few components. The overall feature of the main components---denoted as $\overline{\mathrm{C}1}$ and $\overline{\mathrm{C}2}$---was common among the four ${TX}_{2}$ materials. The optical transitions contributing to these components were concluded to occur at different $k$ points in a ring-shaped area around the \cyrchar\CYRG{} point with the aid of the published results of optical spectrum calculations and the SHG spectrum of bilayer ${\mathrm{MoS}}_{2}$. In addition, one-photon resonance at the $A$ exciton and two-photon resonance at the ${A}^{\ensuremath{'}}$ exciton were observed in ${\mathrm{MoSe}}_{2}$ and ${\mathrm{WSe}}_{2}$, respectively, and their intensities were proven to be significantly lower than those of the resonances around the C peaks.

Revealing Suppressed Intermolecular Coupling Effects in Aggregated Organic Semiconductors by Diluting the Crystal: Model System Perfluoropentacene:Picene.

In order to investigate the effects of intermolecular interactions on the optical properties of organic semiconductors, we employ mixing of the organic semiconductor perfluoropentacene (PFP; C22F14) with the wide band-gap organic semiconductor picene (PIC; C22H14). The binary mixed thin films are prepared by simultaneous coevaporation of PIC and PFP in vacuum. We determine the optical properties of the blends by differential reflectance spectroscopy (absorption) and photoluminescence (emission). PFP:PIC thin films are a rare case of mixed thin films with a known molecular packing. The formation of equimolar mixed domains with a crystal structure clearly different from that of the pure compounds is, in the case of nonequimolar blends, accompanied by pure domains of the excess compound. Due to the wide band gap of PIC, the effect of reduced intermolecular interactions between PFP molecules can be studied in detail without any direct contributions of PIC to the spectra. We find a strongly enhanced emission from PFP in the mixed thin films, which can be explained by decoupling. Real-time investigations of the absorption spectra during growth provide further insight into intermolecular coupling effects on optical properties.

The Opposite Anisotropic Piezoresistive Effect of ReS2.

Mechanical strain induced changes in the electronic properties of two-dimensional (2D) materials is of great interest for both fundamental studies and practical applications. The anisotropic 2D materials may further exhibit different electronic changes when the strain is applied along different crystalline axes. The resulting anisotropic piezoresistive phenomenon not only reveals distinct lattice-electron interaction along different principle axes in low-dimensional materials but also can accurately sense/recognize multidimensional strain signals for the development of strain sensors, electronic skin, human-machine interfaces, etc. In this work, we systematically studied the piezoresistive effect of an anisotropic 2D material of rhenium disulfide (ReS2), which has large anisotropic ratio. The measurement of ReS2 piezoresistance was experimentally performed on the devices fabricated on a flexible substrate with electrical channels made along the two principle axes, which were identified noninvasively by the reflectance difference microscopy developed in our lab. The result indicated that ReS2 had completely opposite (positive and negative) piezoresistance along two principle axes, which differed from any previously reported anisotropic piezoresistive effect in other 2D materials. We attributed the opposite anisotropic piezoresistive effect of ReS2 to the strain-induced broadening and narrowing of the bandgap along two principle axes, respectively, which was demonstrated by both reflectance difference spectroscopy and theoretical calculations.

Systematic study of Nd3+ on structural properties of ZnO nanocomposite for biomedical applications; in-vitro biocompatibility, bioactivity, photoluminescence and antioxidant properties

Owing to the inconformity in ionic radius between Nd3+ and Zn2+, the successful incorporation of Nd3+ ion into the ZnO nanocrystals still remains a great challenge. In the present study various doping ratios containing 1 wt%, 5 wt%, 7 wt% and 10 wt% of Nd3+ doped ZnO nanoparticles (Nd/ZnO NPs) were synthesized in which a bio-layer caped the NPs. SEM/EDX analysis was performed on the ZnO and Nd/ZnO NPs. In addition, the as-synthesized NPs were characterized using X-ray diffraction (XRD), dynamic light scattering (DLS), differential reflectance spectroscopy (DRS) and photoluminescence (PL) spectroscopy. The average size of Nd (5 wt%)/ZnO NPs was in the range of 6.22 and 15–18 nm based on XRD and SEM techniques, respectively. The measured band gap values for pure ZnO and Nd/ZnO NCs with doping ratios of 1 wt%, 5 wt%, 7 wt% and 9 wt% were equal to 3.46, 3.26, 3.05, 3.25 and 3.29, respectively. After inhalation, nanoparticles first interact with lung surfactant system and accordingly their toxic effects will appear on lungs cells such as A549 cell line. The effect of Nd/ZnO NPs to interact by human A549 cell line was evaluated by means of cell viability test. According to cell viability test the concentrations of 0.3 and 0.5 mg/mL of Nd/ZnO NPs induce a low toxicity. The present study shows that these toxic effects of Nd/ZnO NPs can be rectified by capping its surface via the addition of a bio-layer around particles in order to prevent them from interacting A549 cell line.

Plasma emission correction in reflectivity spectroscopy during sputtering deposition

Surface differential reflectivity spectroscopy is a fast non-destructive in situ and real-time measurement technique which allows following the first stages of thin film deposition. However, when applied to sputtering technique, spectra can strongly be distorted by residual light coming from plasma in a way, as shown herein, that depends on sample reflectivity. Thanks to suitable measurements, before and after growth with and without plasma or illumination lights, a protocol of signal correction is proposed to get rid of the spurious plasma contribution. The interest of the method is illustrated in the case of silver deposition on a silicon substrate.

Silicide phase formation by Mg deposition on amorphous Si. Ab initio calculations, growth process and thermal stability

Formation of magnesium silicides on amorphous silicon by deposition of Mg at room temperature is studied by electron energy loss spectroscopy, differential reflectance spectroscopy and high resolution transmission electron microscopy. Optimal crystal structures of Mg silicides under high pressure are found by ab initio DFT calculations. These structures are related to the particular minima of enthalpy. Dielectric functions are calculated for these structures. The transitions from the cubic phase c-Mg2Si to orthorhombic o-Mg2Si at 5.6 GPa and then from o-Mg2Si to hexagonal h-Mg2Si at 22.3 GPa are predicted using the USPEX code. The experimental spectra and the data obtained from the calculated dielectric functions are mutually consistent. Optical reflectance is suitable for monitoring the growth and transformations of the phases during experiments. During Mg deposition onto amorphous Si, the o-Mg2Si phase forms first, then the c-Mg2Si phase grows upon it. The observed sequence of phase formation is related with the compression stress arising in the depth of the Mg-Si mixture.

Investigation of InGaN Layer Grown Under In-Rich Condition by Reflectance Difference Spectroscopy Microscope

Investigation of InGaN Layer Grown Under In-Rich Condition by Reflectance Difference Spectroscopy Microscope

Thickness-Dependent Differential Reflectance Spectra of Monolayer and Few-Layer MoS₂, MoSe₂, WS₂ and WSe₂.

The research field of two dimensional (2D) materials strongly relies on optical microscopy characterization tools to identify atomically thin materials and to determine their number of layers. Moreover, optical microscopy-based techniques opened the door to study the optical properties of these nanomaterials. We presented a comprehensive study of the differential reflectance spectra of 2D semiconducting transition metal dichalcogenides (TMDCs), MoS2, MoSe2, WS2, and WSe2, with thickness ranging from one layer up to six layers. We analyzed the thickness-dependent energy of the different excitonic features, indicating the change in the band structure of the different TMDC materials with the number of layers. Our work provided a route to employ differential reflectance spectroscopy for determining the number of layers of MoS2, MoSe2, WS2, and WSe2.