Search

Proper derivation of CH3NH3PbX3 (MAPbX3; where X = Cl−, Br−, I−) optical constants is a critical step toward the development of high‐performance perovskite devices. To date, the optical dispersions at all wavelengths have been inconsistently characterized by under‐approximating or omitting anomalous spectral features. Herein, a rigorous optical dispersion data analysis of single‐crystal MAPbBr3 involving variable‐angle spectroscopic ellipsometry data appended with transmission intensity data is presented. This approach yields a more robust derivation of the refractive index and extinction coefficient for both anomalous (absorptance) and normal (no absorptance) optical dispersion regimes. Using the derived optical constants, illustrative modeled perovskite solar cell device designs are presented in relation to nonrealistic designs prepared using representative optical constants reported in the literature. In comparison, the derived optical constants enables the modeling of layer thicknesses to maximize absorption by the active layer (MAPbBr3) and minimize parasitic optical absorptance by the nonactive layers at broad angles of incidence (≈0°–70°). This robust derivation of MAPbBr3 optical constants is expected to impact the optical dispersion data analysis of all perovskite analogs and expedite targeted development of, for example, solar cell, light‐emitting diode, photo‐ and X‐ray/γ‐ray detector, and laser system technologies. [ABSTRACT FROM AUTHOR]

Metal halide perovskites have garnered considerable interest for their potential uses in high-efficiency photonics, particularly in the construction of on-chip lasers. Despite extensive efforts to understand the mechanisms underlying perovskite-based lasing, no clear consensus has emerged. Moreover, the fabrication of practical lasing emitters requires the challenging integration of a low-defect active material into a device architecture with minimized complexity. In this study, we demonstrate a simple, multimode lasing emitter composed of a millimeter-scale single-crystalline thin film of CsPbBr3. Dislocations, created during vapor-based film deposition, function as lasing cavity walls and form close-packed sets of resonators with random sizes at two orthogonal orientations within the thin film. Collecting ensemble temperature and power-dependent lasing characteristics of multiple, independent lasing modes in a single sample enables a statistical analysis of the underlying lasing mechanism. Our results reveal that the power-dependent red-shift in the stimulated emission envelope is caused by coupling between the radiatively recombining excitons and the collective oscillations of a photoexcited electron–hole plasma within the perovskite. [ABSTRACT FROM AUTHOR]

The development of high-precision large-area optical coatings and devices comprising low-dimensional materials hinges on scalable solution-based manufacturability with control over exfoliation procedure-dependent effects. As such, it is critical to understand the influence of technique-induced transition metal dichalcogenide (TMDC) optical properties that impact the design, performance, and integration of advanced optical coatings and devices. Here, we examine the optical properties of semiconducting MoS2 films from the exfoliation formulations of four prominent approaches: solvent-mediated exfoliation, chemical exfoliation with phase reconversion, redox exfoliation, and native redox exfoliation. The resulting MoS2 films exhibit distinct refractive indices (n), extinction coefficients (k), dielectric functions (ε1 and ε2), and absorption coefficients (α). For example, a large index contrast of Δn ≈ 2.3 is observed. These exfoliation procedures and related chemistries produce different exfoliated flake dimensions, chemical impurities, carrier doping, and lattice strain that influence the resulting optical properties. First-principles calculations further confirm the impact of lattice defects and doping characteristics on MoS2 optical properties. Overall, incomplete phase reconfiguration (from 1T to mixed crystalline 2H and amorphous phases), lattice vacancies, intraflake strain, and Mo oxidation largely contribute to the observed differences in the reported MoS2 optical properties. These findings highlight the need for controlled technique-induced effects as well as the opportunity for continued development of, and improvement to, liquid phase exfoliation methodologies. Such chemical and processing-induced effects present compelling routes to engineer exfoliated TMDC optical properties toward the development of next-generation high-performance mirrors, narrow bandpass filters, and wavelength-tailored absorbers. [ABSTRACT FROM AUTHOR]

High throughput characterization and processing techniques are becoming increasingly necessary to navigate multivariable, data-driven design challenges for sensors and electronic devices. For two-dimensional materials, device performance is highly dependent upon a vast array of material properties including the number of layers, lattice strain, carrier concentration, defect density, and grain structure. In this work, laser crystallization was used to locally pattern and transform hundreds of regions of amorphous MoS2 thin films into 2D 2H-MoS2. A high throughput Raman spectroscopy approach was subsequently used to assess the process-dependent structural and compositional variations for each illuminated region, yielding over 6000 distinct nonresonant, resonant, and polarized Raman spectra. The rapid generation of a comprehensive library of structural and compositional data elucidated important trends between structure–property processing relationships involving laser-crystallized MoS2, including the relationships between grain size, grain orientation, and intrinsic strain. Moreover, extensive analysis of structure/property relationships allowed for intelligent design and evaluation of major contributions to device performance in MoS2 chemical sensors. In particular, it is found that NO2 sensor performance is strongly dependent on the orientation of the MoS2 grains relative to the crystal plane. [ABSTRACT FROM AUTHOR]

Effective chemical sensor devices must facilitate both the detection of analytes at ultralow concentrations and the ability to distinguish one analyte from another. Sensors built using two-dimensional nanomaterials have demonstrated record-level sensitivity toward certain chemical vapor species, but the specificity of chemical analyte detection remains lacking. To address this deficiency, this work pioneers the use of a broadband fiber-optic sensor coated with thin-film MoS2 where selectivity is achieved through observing changes in the visible spectrum transmission during exposure to different aliphatic and aromatic vapors. A significant loss in transmission across the fiber was observed near peaks in the refractive index associated with the C, B, and A excitons as well as at peaks associated with defect states. Several mechanisms for achieving selectivity are investigated, including deciphering donor/acceptor molecules, aromatic compounds, analytes with high refractive index, and intercalants such as aniline-based compounds. Moreover, the sensor device is entirely reusable and demonstrates reversible, empirical, and selective detection of aniline down to 6 ppm. [ABSTRACT FROM AUTHOR]

The high refractive index, broadband transparency, and low spectral absorption of HfO2 (hafnia) make it a suitable choice for thermally stable nanoscale optical filters, UV mirrors, and antireflection coatings. However, achieving dense thin films of HfO2 with bulk-like optical properties has been a challenge due to differences in film stoichiometric and spatial uniformity at nanoscale thicknesses. Here, we assess HfO2 thin films (i.e., <200 nm thicknesses) prepared using pulsed laser deposition (PLD) at different substrate temperatures (20–675 °C) and the associated chemical, structural, and optical properties are reported. X-ray diffraction analysis reveals that nanoscale-thick thin films of HfO2 deposited at an ambient substrate temperature (20 °C) using PLD are amorphous with embedded nanocrystallites, whereas films deposited on heated substrates are polycrystalline monoclinic HfO2. For the films deposited on heated substrates, further analysis shows that the nanocrystalline phase does not change with increasing substrate temperatures; however, the texture of the crystalline orientation changes to favor (111) at 300–675 °C from the initial (002) orientation at ambient substrate temperature. Such differences in nanoscale-thick thin-film HfO2 PLD process-dependent amorphousness, crystallinity, and surface textures discussed here exhibit minimal influence on the resulting broadband optical properties (250 nm–30 μm). Furthermore, we show that the complex refractive index of high-density nanoscale-thick thin-film HfO2 prepared at an ambient substrate temperature using PLD resembles that of bulk HfO2 responses. The achievement of fully stoichiometric, high-density, thin-film HfO2 from PLD is expected to further enable thermally robust nanoscale photonic device integration involving low-dimensional high-performance optical and optoelectronic applications. [ABSTRACT FROM AUTHOR]

Although there has been progress in studying the electronic and optical properties of monolayer and near-monolayer (two-dimensional, 2D) M o S 2 upon adatom adsorption and intercalation, understanding the underlying atomic-level behavior is lacking, particularly as related to the optical response. Alkali atom intercalation in 2D transition metal dichalcogenides (TMDs) is relevant to chemical exfoliation methods that are expected to enable large scale production. In this work, focusing on prototypical 2D M o S 2 , the adsorption and intercalation of Li, Na, K, and Ca adatoms were investigated for the 2H, 1T, and 1T ' phases of the TMD by the first principles density functional theory in comparison to experimental characterization of 2H and 1T 2D M o S 2 films. Our electronic structure calculations demonstrate significant charge transfer, influencing work function reductions of 1-1.5 eV. Furthermore, electrical conductivity calculations confirm the semiconducting versus metallic behavior. Calculations of the optical spectra, including excitonic effects using a many-body theoretical approach, indicate enhancement of the optical transmission upon phase change. Encouragingly, this is corroborated, in part, by the experimental measurements for the 2H and 1T phases having semiconducting and metallic behavior, respectively, thus motivating further experimental exploration. Overall, our calculations emphasize the potential impact of synthesis-relevant adatom incorporation in 2D M o S 2 on the electronic and optical responses that comprise important considerations toward the development of devices such as photodetectors or the miniaturization of electroabsorption modulator components.