Differential Reflectance Spectra Research Articles

Evidence for Charging and Discharging of MoS2 and WS2 on Mica by Intercalating Molecularly Thin Liquid Layers

Transition metal dichalcogenides (TMDCs) are often mechanically exfoliated on mica and examined under ambient conditions. It is known that above a certain relative humidity, a molecularly thin layer of water intercalates between the mica and the TMDC. Herein, the effect of molecularly thin liquid layers on the optical spectra of MoS2 and WS2 exfoliated on dry mica and exposed to the vapors of water, ethanol, and tetrahydrofuran (THF) is investigated. Photoluminescence and differential reflectance (ΔR/R) spectra on the TMDCs on dry mica show dominant trion emission due to n‐doping. Intercalation of water removes charge doping and results in purely neutral exciton emission, while an ethanol layer, which can be reversibly exchanged with water, does not completely suppress charge. Similarly, THF intercalates between TMDC and mica, as shown by atomic force microscopy, but it does not suppress the charging of mica. In MoS2 bi‐ and trilayers, an intercalated water layer leads to a near doubling of the intensity of the indirect band transition. The described charging/discharging of TMDCs by molecular thin liquid layers can provide important clues to better control the optical properties of TMDCs under environmental conditions.

Analysis of Photo-Generated Carrier Escape in Multiple Quantum Wells

Recent experiments have shown that more than 85% of photo-generated carriers can escape from multiple quantum wells (MQWs) sandwiched between p-type and n-type layers (PIN). In this work, we quantitatively analyze the relationship between the energy of carriers and the height of potential barriers to be crossed, based on the GaAs/InGaAs quantum well structure system, combined with the Heisenberg uncertainty principle. It was found that that the energy obtained by electrons from photons is just enough for them to escape, and it was found that the energy obtained by the hole is just enough for it to escape due to the extra energy calculated, based on the uncertainty principle. This extra energy is considered to come from photo-generated thermal energy. The differential reflection spectrum of the structure is then measured by pump–probe technology to verify the assumption. The experiment shows that the photo-generated carrier has a longer lifetime in its short circuit (SC) state, and thus it possesses a lower structure temperature than that in open circuit (OC). This can only explain a thermal energy reduction caused by the continuous carrier escape in SC state, indicating an extra thermal energy transferred to the escaping carriers. This study is of great significance to the design of new optoelectronic devices and can improve the theory of photo-generated carrier transports.

Exciton insulator in a moiré lattice

Interlayer electron and hole can be paired up through coulomb interaction to form an exciton insulator when their kinetic energy is substantially smaller than the interaction energy. The traditional platform to realize such an interlayer interaction is the double quantum well with dielectric material between electron and hole, for which an external magnetic field is required to generate Landau level flat bands that can reduce the kinetic energy of charged carriers. When both quantum wells are at the half filling of the lowest landau level, the electron-electron repulsive interaction, by the particle-hole transformation in one well, will be equivalent to electron-hole attractive interaction, from which interlayer exciton and its condensation can emerge. In a two-dimensional twisted homostructure or an angle aligned heterostructure, there exists a moiré superlattice, in which bands are folded into the mini-Brillouin zone by the large moiré period. Gap opening at the boundary of mini-Brillouin zone can form the well-known moiré flat band. This review will discuss how to use the moiré flat bands to generate exciton insulator in the absence of external magnetic field in transitional metal dichalcogenide (TMD) moiré heterostructure. Unlike the double quantum well where symmetric well geometry is used, the moiré related sample can have multiple different geometries, including monolayer TMD-hexagonal boron nitride-moiré structure, moiré-moiré structure, and monolayer TMD-bilayer TMD structure. The carriers in those structures can be well tuned to locate equally in different layers, and particle-hole transformation in the moiré first Hubbard band can transform the interlayer repulsive coulomb interaction into attractive interaction, which is the same as that in quantum well under magnetic field. We will show that by using differential contrast reflection spectrum, interlayer photoluminescence, 2s exciton sensing, quantum capacitance and microwave impedance microscopy, the signature of exciton fluid can be identified. The excitonic coherence features in those structures will promise by using the coulomb drag technique and counter flow technique in future. In general, exciton in moiré lattice is a promising candidate for studying the Bose-Hubbard model in solids and can well realize exciton superfluidity, excitonic mott insulator as well as the crossover between them.

Assessment of firmness and soluble solids content of peaches by spatially resolved spectroscopy with a spectral difference technique

Spatially resolved spectroscopy (SRS) provides a new approach to the measurement of optical scattering and absorption properties and quality assessment of horticultural products. In this research, spatially resolved (SR) spectra over 550 – 1,650 nm were acquired for 600 peaches using a recently developed SRS system covering the light source-detector (S-D) distances from 1.5 to 36 mm with 30 fibers of three sizes (i.e., 50 µm, 105 µm and 200 µm). A new method of calculating spectral differences, between the first S-D distance and the remaining 14 S-D distances was proposed to enhance firmness and soluble solids content (SSC) predictions. Partial least squares (PLS) regression models based on SR and difference reflectance (DR) spectra were developed and compared for firmness and SSC prediction. Results showed that when using SR spectra, prediction results for firmness and SSC varied greatly with the S-D distance; SSC prediction results became worse with the increasing S-D distance, while larger S-D distances of 12–32 mm resulted in better firmness predictions. DR spectra gave consistently better results for both firmness and SSC predictions than SR spectra, with the best correlation coefficients of 0.853 and 0.839 for firmness and SSC prediction, respectively. Overall, SRS coupled with the spectral difference technique can enhance the prediction of firmness and SSC for peach fruit.

Magneto‐Optical Properties of Oxidized Co Nanowires on Pt Substrate

This work presents the results of the first‐principals study of magneto‐optical properties of oxidized Co nanowires on vicinal Pt surfaces. Two types of vicinal platinum surface (Pt(332) and Pt(322)) are studied to analyze the effect of surface structure on oxidation process and on the magneto‐optical properties of oxidized Co nanowires. The study reveals the change of magnetic coupling in Co nanowire under oxidation, which is sensitive to step edge geometry of vicinal surface. The dielectric tensor and reflectivity are calculated for oxide structures in order to find their main features in optical characteristic. Besides, the reflectance difference spectra are constructed for pure and oxidized nanowires in order to reveal the effect of interplay between magnetic state and oxidation on the optical properties of magnetic nanowires, which can be useful for tailoring their magnetic properties in spintronic devices.

Selectively Strong Coupling of MoS2 Excitons to a Metamaterial at Room Temperature

Light emitters in the vicinity of a hyperbolic metamaterial (HMM) show a range of quantum optical phenomena from spontaneous decay-rate enhancement to strong coupling. In this study, we integrate a monolayer molybdenum disulfide (${\mathrm{Mo}\mathrm{S}}_{2}$) emitter in the near-field region of the HMM. The ${\mathrm{Mo}\mathrm{S}}_{2}$ monolayer has $A$ and $B$ excitons, which emit in the red region of the visible spectrum. We find that the $B$ excitons couple to the HMM differently compared to $A$ excitons. The fabricated HMM transforms to a hyperbolic dispersive medium at 2.14 eV, from an elliptical dispersive medium. The selective coupling of $B$ excitons to the HMM modes is attributed to the inbuilt field gradient of the transition. The $B$ exciton energy lies close to the transition point of the HMM, relative to the $A$ exciton. So, the HMM modes couple more to the $B$ excitons and the metamaterial functions as a selective coupler. The coupling strength calculations show that coupling is 2.5 times stronger for $B$ excitons relative to $A$ excitons. High near field of HMM, large magnitude, and the in-plane transition dipole moment of ${\mathrm{Mo}\mathrm{S}}_{2}$ excitons, result in strong coupling of $B$ excitons and formation of hybrid light-matter states. The measured differential reflection and photoluminescence spectra indicate the presence of hybrid light-matter states, i.e., exciton polaritons. Rabi splitting of $143.5\phantom{\rule{0.2em}{0ex}}\mathrm{meV}\ifmmode\pm\else\textpm\fi{}14.4$ meV at room temperature is observed. The low-temperature photoluminescence measurement shows mode anticrossing, which is a characteristic feature of hybrid states. Our results show that the HMM works as an energy-selective coupler for multiexcitonic systems as ${\mathrm{Mo}\mathrm{S}}_{2}$.

Ag/Co/B tri-doped TiO2/SiO2 film for enhancing photocatalytic degradation

Ag/Co/B tri-doped TiO2/SiO2 film was prepared by the sol–gel method, and the structure and properties of this film were characterized by the X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET) test method, field emission scanning electron microscopy (FE-SEM), differential thermal analysis-thermogravimetry (DTA-TG), photoluminescence (PL), and UV-visible diffuse reflectance spectrum (UV-vis DRS). XRD results indicated that the film structure of this film conforms to the anatase single crystal form. The presence of SiO2 can inhibit the formation of brookite and prevent the conversion of anatase into rutile. BET and FE-SEM results revealed that the film had a higher specific surface area and smaller grain size compared with pure TiO2 film. DTA-TG results exposed that the film had excellent thermal stability at 450[Formula: see text]C. Due to the spectacular adsorption capacity of SiO2 and the Schottky barrier formed between Ag and TiO2, this film improved the h[Formula: see text]/e[Formula: see text] separation efficiency and optical absorption performance showed by PL results. UV-vis DRS results displayed that the band gap energy (2.47 eV) of the film was clearly lower than that of pure TiO2 film, due to the intermediate energy levels generated by Co and B ions doping. The photocatalytic activity of the film was verified by ultraviolet and visible light degradation experiments of two different organic pollutants. The experimental results exhibited that the film had excellent photocatalytic degradation ability and stability. Ultimately, a possible synergistic mechanism of photocatalysis was proposed in this paper.

Extracting quantitative dielectric properties from pump-probe spectroscopy

Optical pump-probe spectroscopy is a powerful tool for the study of non-equilibrium electronic dynamics and finds wide applications across a range of fields, from physics and chemistry to material science and biology. However, a shortcoming of conventional pump-probe spectroscopy is that photoinduced changes in transmission, reflection and scattering can simultaneously contribute to the measured differential spectra, leading to ambiguities in assigning the origin of spectral signatures and ruling out quantitative interpretation of the spectra. Ideally, these methods would measure the underlying dielectric function (or the complex refractive index) which would then directly provide quantitative information on the transient excited state dynamics free of these ambiguities. Here we present and test a model independent route to transform differential transmission or reflection spectra, measured via conventional optical pump-probe spectroscopy, to changes in the quantitative transient dielectric function. We benchmark this method against changes in the real refractive index measured using time-resolved Frequency Domain Interferometry in prototypical inorganic and organic semiconductor films. Our methodology can be applied to existing and future pump-probe data sets, allowing for an unambiguous and quantitative characterisation of the transient photoexcited spectra of materials. This in turn will accelerate the adoption of pump-probe spectroscopy as a facile and robust materials characterisation and screening tool.

Inspection of Line Defects in Transition Metal Dichalcogenides Using a Microscopic Hyperspectral Imaging Technique

The line defects of two-dimensional (2D) transition metal dichalcogenides (TMDs) play a vital role in determining their device performance. In this work, a microscopic hyperspectral imaging technique based on differential reflectance was introduced for the online inspection of line defects in TMDs. Upon comparison of the measurement results of imaging and spectra, the relationship between optical contrast and differential reflectance spectra was established. A light selection method was proposed to optimize the optical contrast of line defects. Via application of an image processing algorithm, an automatic detection of the line defects with a classification accuracy of 95% was achieved for WS2, MoS2, and MoSe2. This work not only provides a microscopic hyperspectral imaging technique for detecting 2D material defects but also introduces a versatile design strategy for developing an advanced machine vision spectroscopic system.

Biaxial versus uniaxial strain tuning of single-layer MoS2

Strain engineering has arisen as a powerful technique to tune the electronic and optical properties of two-dimensional semiconductors like molybdenum disulfide (MoS2). Although several theoretical works predicted that biaxial strain would be more effective than uniaxial strain to tune the band structure of MoS2, a direct experimental verification is still missing in the literature. Here we implemented a simple experimental setup that allows to apply biaxial strain through the bending of a cruciform polymer substrate. We used the setup to study the effect of biaxial strain on the differential reflectance spectra of 12 single-layer MoS2 flakes finding a redshift of the excitonic features at a rate between -40 meV/% and -110 meV/% of biaxial tension. We also directly compare the effect of biaxial and uniaxial strain on the same single-layer MoS2 finding that the biaxial strain gauge factor is 2.3 times larger than the uniaxial strain one.

Gold Coated VO2 Nanogratings Based Plasmonic Switches

This paper presents 2 dimensional (2D) and 1 dimensional (1D) gold (Au) coated VO2 (Vanadium Dioxide) nanogratings based tunable plasmonic switch. VO2 is a phase changing material and hence exhibits phase transition from semiconductor to metallic phase approximately at 67 ºC or 340 K (critical temperature) which can be achieved by exposure to IR radiation, application of voltage, heating, etc. and there is a huge contrast between optical properties of its metallic and insulating phases and hence that can be utilized to implement VO2 based optical switches. These VO2 based gratings couple the incident optical radiation to plasmonic waveguide modes which in turn leads to high electromagnetic field enhancement in the gaps between the nanogratings. The proposed Au coated VO2 nanogratings can be fabricated by using current state of art fabrication techniques and provides switchability of the order of femtoseconds. Hence the optical switching explained in our paper can be used fast switching applications. For an optimum switch our aim is to maximize its differential reflectance spectra between the 2 states of VO2, i.e., metallic and semiconductor phases. Rigorous Coupled Wave Analysis (RCWA) reveals that wavelengths for maximum differential reflectance can be optimized over a large spectral regime by varying various parameters of nanogratings for example groove height (h), width (w), gap (g) between the gratings, and thickness (t) of Au coating over VO2 by simulation using RCWA for maximum differential reflectance between VO2 metal and semiconductor phase, i.e., the switching wavelengths can be tuned by varying grating parameters and thus we can have optimum optical switch.

Supervised dimension reduction for optical vapor sensing.

Detecting and identifying vapors at low concentrations is important for air quality assessment, food quality assurance, and homeland security. Optical vapor sensing using photonic crystals has shown promise for rapid vapor detection and identification. Despite the recent advances of optical sensing using photonic crystals, the data analysis method commonly used in this field has been limited to an unsupervised method called principal component analysis (PCA). In this study, we applied four different supervised dimension reduction methods on differential reflectance spectra data from optical vapor sensing experiments. We found that two of the supervised methods, linear discriminant analysis and least-squares regression PCA, yielded better interclass separation, vapor identification and improved classification accuracy compared to PCA.

Optical properties of monolayer, multilayer, and bulk BiI3 studied using time-dependent density functional theory

We investigate the optical property of monolayer and layered BiI$_3$ and reveal the presence of exciton only in the monolayer crystal. We evaluate the energy spectrum of a dielectric function by using time-dependent density functional theory. Bulk crystal of BiI$_3$ is an atomic layered semiconductor with the band gap corresponding to the frequency of visible light. The numerical result for the bulk crystal is confirmed to be consistent with the previous experimental results and it does not depend on the number of layers except the monolayer. We reveal the excitons appearing below the resonant peak associated with the inter-band excitation in the monolayer crystal. The unique optical property can be directly observed in the optical absorption or differential reflectance spectrum and distinguish the monolayer crystal from the stacked BiI$_3$.

Broadband femtosecond spectroscopic ellipsometry.

We present a setup for time-resolved spectroscopic ellipsometry in a pump-probe scheme using femtosecond laser pulses. As a probe, the system deploys supercontinuum white light pulses that are delayed with respect to single-wavelength pump pulses. A polarizer-sample-compensator-analyzer configuration allows ellipsometric measurements by scanning the compensator azimuthal angle. The transient ellipsometric parameters are obtained from a series of reflectance-difference spectra that are measured for various pump-probe delays and polarization (compensator) settings. The setup is capable of performing time-resolved spectroscopic ellipsometry from the near-infrared through the visible to the near-ultraviolet spectral range at 1.3 eV-3.6 eV. The temporal resolution is on the order of 100 fs within a delay range of more than 5 ns. We analyze and discuss critical aspects such as fluctuations of the probe pulses and imperfections of the polarization optics and present strategies deployed for circumventing related issues.

Degradation of MB and RhB by modified ZrO2 nanoparticles via sunlight

In this study, CuO–ZrO2 nanoparticles (ZrO2 modified by copper(II)oxide) were prepared by using the impregnation method. The physicochemical properties of produced nanoparticles were characterized using differential reflectance spectra, X-ray diffraction, field-emission scanning electron microscopy, transmission electron microscopy, energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy. The occurrence of copper(II) was confirmed by XPS technique. The calculated average crystallite size was determined by TEM spectroscopy. The DRS studies exhibited a reduction in the band gap of ZrO2 from 4.7 to 2.9 eV after modification with copper(II) oxide. The photocatalytic method with sunlight was used to examine the ability of copper(II) oxide-modified zirconium(IV) oxide nanoparticles in removing toxic chemicals such as rhodamine B (RhB) and methylene blue (MB). Based on the analysis of experiments, copper(II)oxide-modified ZrO2 showed a good photocatalytic activity, such that 1 mg of the photocatalyst degraded 10 ppm of MB and RhB within 30 and 60 min, respectively. The degradation of MB and RhB dyes was tested four and three times, respectively, to ensure reusability of photocatalyst. Copper(II)oxide-modified zirconium(IV) oxide nanoparticles were examined in regard to their ability in removing toxic chemicals such as rhodamine B (RhB) and methylene blue (MB) through the use of the photocatalytic method. Based on the analysis of experiments, copper(II)oxide-modified ZrO2 showed a good photocatalytic activity, such that 1 mg of the photocatalyst degraded 10 ppm dye within 30 and 60 min for MB and RhB, respectively.

Highly Sensitive and Selective VOC Vapor Optical Fiber Sensor Using Hierarchical Nanostructures of Butterfly Wing Scales

Volatile organic compound (VOC) is one of the most widely spread and hazardous environmental pollutants. In this work, a VOC vapor optical fiber sensor is proposed and demonstrated based on the multilayer interference and diffraction of Morpho Didius butterfly wing scales. The sensing wing is fabricated and sheathed on the end of the optical fiber probe with a sleeve, while the sensing element could be easily renewed by replacing the sleeve part. It is found that the reflectance spectrum of the butterfly wings changes with varied VOC vapors. By tracing the spectral changes, the categories and concentrations of VOC vapors are investigated experimentally, while the differential reflectance spectrum ΔR is introduced to magnify the subtle differences of the original spectrum. Here, the dichloromethane, acetone, and ethanol vapors are tested, and the maximum sensitivity of the sensor is measured as 3.303E-5%/ppm, 7.762E-5%/ppm, and 2.881E-4%/ppm, respectively. The principal components analysis method is employed to further analyze the ΔR spectrum, and the concentration of varied VOC vapors could be discriminated and identified according to the distribution of the projected points. Considering its advantages including low-cost, easy fabrication, good repeatability, high sensitivity, and selectivity, this wing-based vapor optical fiber sensor has great application potential in the field of air monitoring.

Imaging layer thickness of large-area graphene using reference-aided optical differential reflection technique.

Transparent layers are critical for enhancing optical contrast of graphene on a substrate. However, once the substrate is fully covered by large-area graphene, there are no accurate transparent layer and reference for optical contrast calculations. The thickness uncertainty of the transparent layer reduces the analytical accuracy of graphene. Thus, in this Letter, we propose a reference-aided differential reflection (DR) method with a dual-light path. The accurate thickness of the transparent layer is obtained by improving the DR spectrum sensitivity using a designable reference. Hence, the analytical accuracy of graphene thickness is guaranteed. To demonstrate this concept, a centimeter-scale chemical-vapor-deposition-synthesized graphene was measured on a SiO2/Si substrate. The thickness of underlying SiO2 was first identified with the 1 nm resolution by the DR spectrum. Then, the thickness distribution of graphene was directly deduced from a DR map with submonolayer resolution at a preferred wavelength. The results were also confirmed by ellipsometry and atomic force microscopy. As a result, this new method provides an extra degree of freedom for the DR method to accurately measure the thickness of large-area two-dimensional materials.

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.

Оптичні властивості CdTe, легованого Ca

Досліджено оптичне поглинання, відбивання і люмінесценцію CdTe:Ca. Встановлено, що отримані леговані Ca поверхневі шари характеризуються інтенсивною фотолюмінесценцією з η = 8-10% у крайовій області. Випромінювання формується внаслідок міжзонної рекомбінації вільних носіїв заряду і анігіляцією зв’язаних на ізовалентних домішках Ca екситонів. Зазначені складові спостерігаються у диференційних спектрах оптичного відбивання Ŕω у приповерхневому шарі отриманому при легування ізовалентною домішкою Ca підкладинок CdTe. Встановлено, що легування обумовлює утворення p-типу провідності.

Facile and Reliable Thickness Identification of Atomically Thin Dichalcogenide Semiconductors Using Hyperspectral Microscopy.

Although large-scale synthesis of layered two-dimensional (2D) transition metal dichalcogenides (TMDCs) has been made possible, mechanical exfoliation of layered van der Waals crystal is still indispensable as every new material research starts with exfoliated flakes. However, it is often a tedious task to find the flakes with desired thickness and sizes. We propose a method to determine the thickness of few-layer flakes and facilitate the fast searching of flakes with a specific thickness. By using hyperspectral wild field microscopy to acquire differential reflectance and transmittance spectra, we demonstrate unambiguous recognition of typical TMDCs and their thicknesses based on their excitonic resonance features in a single step. Distinct from Raman spectroscopy or atomic force microscopy, our method is non-destructive to the sample. By knowing the contrast between different layers, we developed an algorithm to automatically search for flakes of desired thickness in situ. We extended this method to measure tin dichalcogenides, such as SnS2 and SnSe2, which are indirect bandgap semiconductors regardless of the thickness. We observed distinct spectroscopic behaviors as compared with typical TMDCs. Layer-dependent excitonic features were manifested. Our method is ideal for automatic non-destructive optical inspection in mass production in the semiconductor industry.