Infrared Reflectivity Measurements Research Articles

First data set of H<sub>2</sub>O/HDO columns from the Tropospheric Monitoring Instrument (TROPOMI)

Abstract. Global measurements of atmospheric water vapour isotopologues aid to better understand the hydrological cycle and improve global circulation models. This paper presents a new data set of vertical column densities of H2O and HDO retrieved from short-wave infrared (2.3 µm) reflectance measurements by the Tropospheric Monitoring Instrument (TROPOMI) onboard the Sentinel-5 Precursor satellite. TROPOMI features daily global coverage with a spatial resolution of up to 7 km×7 km. The retrieval utilises a profile-scaling approach. The forward model neglects scattering, and strict cloud filtering is therefore necessary. For validation, recent ground-based water vapour isotopologue measurements by the Total Carbon Column Observing Network (TCCON) are employed. A comparison of TCCON δD with ground-based measurements by the Multi-platform remote Sensing of Isotopologues for investigating the Cycle of Atmospheric water (MUSICA) project for data prior to 2014 (where MUSICA data are available) shows a bias in TCCON δD estimates. As TCCON HDO is currently not validated, an overall correction of recent TCCON HDO data is derived based on this finding. The agreement between the corrected TCCON measurements and co-located TROPOMI observations is good with an average bias of (-0.2±3)×1021 molec cm−2 ((1.1±7.2) %) in H2O and (-2±7)×1017 molec cm−2 ((-1.1±7.3) %) in HDO, which corresponds to a mean bias of (-14±17) ‰ in a posteriori δD. The bias is lower at low- and mid-latitude stations and higher at high-latitude stations. The use of the data set is demonstrated with a case study of a blocking anticyclone in northwestern Europe in July 2018 using single-overpass data.

Double Layer Conducting Salts: (CNB-EDT-TTF)4X, X = ClO4−, ReO4−, and SbF6−; Electrical Transport and Infrared Properties

Two new members of the family of bilayer compounds (CNB-EDT-TTF)4X, (CNB-EDT-TTF = 5-cyanobenzene-ethylenedithio-tetrathiafulvalene) with anions X = ReO4− and SbF6− are reported, their electron transport and optical properties investigated, and then compared to the ClO4− salt that was previously described. These compounds share the same structural type, i.e. bilayers of donors, which are packed in a β″-type pattern and then separated by layers of highly disordered anions. The absolute values of the electrical resistivity measured in single crystals within the layers were found in the range of 5 to 18 (Ωcm)−1, with a significantly sample dependence being ascribed to intrinsic disorder effects. The ClO4− and SbF6− salts exhibit metallic behavior with the resistivity decreasing upon cooling almost linearly with temperature until a broad minimum is reached between 15 and 80 K, depending on crystal quality; this is followed by an upturn of resistivity reaching values at T = 1.5 K that were comparable to those attained at room temperature. The electrical resistivity of the ReO4− salt follows a thermally activated behavior already at T = 300 K, although with a small activation energy in the range 16−18 meV. Thermoelectric power measurements yield large positive values (75–80 µV/K) at ambient temperature with a metallic behavior that is identical for all compounds. Temperature and polarization dependent infrared reflection measurements on single crystals of (CNB-EDT-TTF)4X salts, with X = ClO4−, ReO4−, and SbF6−, have been performed to obtain the optical conductivity and analyze the electronic and vibrational properties. For (CNB-EDT-TTF)4ClO4 the molecular vibrations exhibit a significant variation below T = 23 K, which suggests a charge localization phenomena.

Infrared lattice dynamics in negative thermal expansion material in single-crystal ScF3

Simple cubic ‘open’ perovskite ScF3 stands out among trifluoride materials in its large, isotropic negative thermal expansion (NTE), but also its proximity of its zero-temperature state to a structural phase transition. Here we report a temperature- and frequency-dependent lattice dynamical study of Brillouin zone center lattice excitations of single crystals of ScF3 using infrared reflectivity measurements. In addition to quantifying the mode strengths and energies in single crystals of this interesting material, we also find strong evidence for multiphonon absorption processes which excite the zone-edge incipient soft modes associated with NTE and the structural quantum phase transition. In this way, we identify an optically-allowed pathway to excite soft modes provides a means to athermally populate modes associated with NTE in ScF3.

Study of vibrational modes in (Ga2S3)x−(Ga2Se3)1-x mixed crystals by Raman and infrared reflection measurements

Raman and infrared (IR) reflection characteristics were investigated in the frequency region of 100–450 cm−1 for (Ga2S3)x − (Ga2Se3)1-x mixed crystals for compositions of x increasing from 0.0 to 1.0 by intervals of 0.25 obtained by Bridgman crystal growth technique. In the Raman spectra of these crystals four dominant peak features were observed while two bands were detected in the IR spectra of interest samples. Kramers-Kronig dispersion relations applied to IR spectra presented the frequencies of transverse optical modes. The compositional dependencies of revealed Raman- and IR-active mode frequencies on (Ga2S3)x − (Ga2Se3)1-x crystals were established. One-mode behavior was displayed from indicated dependencies.

Influence of annealing on the optical properties of monoclinic vanadium oxide VO2 prepared in nanoscale by hydrothermal technique

Terahertz and infrared reflectivity measurements were carried out on monoclinic vanadium oxide VO2 prepared directly in nanoscale by hydrothermal technique. The product was annealed in argon atmosphere at pressure 100 Pa for 6 h at temperatures 350 °C, 450 °C and 550 °C. The as-grown and annealed samples were investigated by XRD, HRTEM, VSM, DSC, THz, and FTIR techniques to explore the effect of annealing on the optical and electronic properties. The obtained results reveal the existence of monoclinic M-B-VO2 phase for the as-grown sample, while the annealed samples include two other oxidation phases V6O13 and V2O5 together with the monoclinic VO2 phase. This observation is supported by the data obtained from XRD, VSM, and DSC measurements. Both THz transmittance and infrared reflectivity measurements performed at room temperature show insulating behavior for the as-grown monoclinic VO2 sample whereas subtle modifications of monoclinic VO2 phase by annealing induces a metallic behavior via enhancement of surface area to volume ratio and increasing the grain boundaries as a result of embedded and distributed of two other phases V6O13 and V2O5 within the monoclinic VO2 matrix. Fitting of the optical conductivity deduced from both THz and infrared measurements clearly shows Drude-like behavior for the annealed sample while this behavior was not observed in case of the as-grown sample.

Hierarchical laser patterning of indium tin oxide thin films

Multiscale laser patterning of indium tin oxide thin films using the combination of direct laser interference patterning (DLIP) and laser induced periodic surface structuring (LIPSS) was studied. By balancing DLIP and LIPSS, structures in an ordered hierarchical pattern consisting of 75 nm LIPSS, 650 nm DLIP, and 50 µm laser spot size were demonstrated. The effects of laser fluence on the DLIP, LIPSS, and grain growth are discussed. The anisotropic conductance due to LIPSS was shown to exceed 50,000:1. Infrared reflectance measurements indicated that the anisotropy was uniform across the laser patterned samples.

New Proximal Sensing Approach to Measure Soil Aggregate Stability

New Proximal Sensing Approach to Measure Soil Aggregate Stability

Infrared spectroscopic measurements of structural transition and charge dynamics in 1T−TiTe2 under pressure

Layered semimetallic $1\mathit{T}\ensuremath{-}{\mathrm{TiTe}}_{2}$ has been reported to exhibit pressure-induced superconductivity, and some nontrivial topological states have also been suggested to appear under pressure. Here we investigate the optical properties of this compound by infrared reflectance measurements over a broad energy range under quasihydrostatic pressures up to 22 GPa. The observed conspicuous changes of infrared phonon spectra, in combination with x-ray diffraction data, provide clear evidence of an irreversible structural transition into a noncentrosymmetric space group ($P\overline{3}m1\phantom{\rule{0.16em}{0ex}}\ensuremath{\rightarrow}\phantom{\rule{0.16em}{0ex}}P3m1$) at a low pressure ($\ensuremath{\sim}2.3$ GPa), before undergoing a subsequent transition to a monoclinic phase ($C2/m$) above 12 GPa. The pressure-induced irreversible gain of the low-frequency spectral weight of optical conductivity further supports the above crystal structure modification. The irreversible structural modification and associated change in electronic structure have been argued to be responsible for the emergence of superconductivity with enhanced ${T}_{c}$ and its persistence upon decompression. The low-pressure structural instability in $1\mathit{T}\text{\ensuremath{-}}{\mathrm{TiTe}}_{2}$ makes it necessary to revisit the theoretical investigation of its topological nature.

Emissivity of solar cell cover glass calculated from infrared reflectance measurements

The thermal emissivity of solar cell cover glasses with differences in glass composition or manufacture and surface texture are evaluated using specular and specular+diffuse infrared reflectance at different angles of incidences. Non-textured and textured glasses all exhibit similar emissivity at all angles of incidence regardless of composition and patterning. Both diffuse and specular reflectance must be included for textured glass at any angle of incidence and may be needed for planar glass at a high angle of incidences to properly determine emissivity.

Ternary Pb1−xCdxSe films grown by molecular beam epitaxy on GaAs/ZnTe hybrid substrates

Thin epitaxial Pb1−xCdxSe films with cadmium content x ≤ 0.36 have been grown by molecular beam epitaxy (MBE) on highly resistive, lattice matched (0 0 1) ZnTe/GaAs hybrid substrates. The High Resolution X-ray Diffraction (HRXRD) indicates that the films crystallize in the rock salt structure. Up to x = 0.12 the Pb1−xCdxSe films are (0 0 1) oriented and monocrystalline with negligible amount of other phases. The lattice constant of the films decreases proportionally to the Cd content. As evidenced by low-temperature photoluminescence (PL) the energy gap of Pb1−xCdxSe films increases with the increasing Cd content. The parameters of optical phonons and plasmons in the material are established by infrared reflectance measurements. The most interesting property of the Pb1−xCdxSe films is revealed by classical transport measurements. The alloying of PbSe with CdSe reduces the hole concentration in the material from a typical value of 3 × 1018 cm−3 in the pure PbSe film down to 3 × 1016 cm−3 in the film containing x = 0.052 of Cd. The reduction of the hole concentration by two orders of magnitude makes the Pb1−xCdxSe films very attractive for infrared detector applications. Simple photoresistors made of Pb1−xCdxSe films exhibit sensitivity to the infrared radiation up to the room temperature.

A Novel Shortwave Infrared Proximal Sensing Approach to Quantify the Water Stability of Soil Aggregates

A novel proximal sensing approach to determine soil aggregate stability is demonstrated. Change in pre‐ vs. post‐wetted aggregate reflectance yields aggregate stability (AS) from surface structure alteration. Results were well‐correlated with those of the high energy moisture characteristic (HEMC) method, which involves rapid and slow wetting of aggregates. The new method only employs rapid wetting of aggregates, saving time for sample processing and evaluation. Soil structure and aggregate stability (AS) are critical soil properties affecting water infiltration, root growth, and resistance to soil and wind erosion. Changes in AS may be early indicators of soil degradation, pointing to low organic matter (OM) content, reduced biological activity, or poor nutrient cycling. Hence, efficient and reliable AS measurement techniques are essential for detection, management, and remediation of degraded soil resources. Here we quantify soil AS by developing a novel proximal sensing technique based on shortwave infrared (SWIR) reflectance measurements. The novel approach is similar to the well‐documented high energy moisture characteristic (HEMC) method, which yields a stability ratio (SR) derived from comparison of hydraulic and structural characteristics of slowly‐ and rapidly‐wetted soil samples near‐saturation. We rapidly wetted aggregated soil samples (i.e., high energy input) and hypothesized that an AS index can be derived from SWIR surface reflectance spectra due to differences in post‐wetting surface roughness that is intimately linked to AS. To test this hypothesis, surface reflectance spectra from a wide range of structured soil textures under both slowly‐ and rapidly‐wetted samples, were measured with a SWIR spectroradiometer (350–2500 nm). The ratio between pre‐ and post‐wetting spectra was determined and compared with the HEMC method's volume of drainable pore ratio (VDPR). We found a strong correlation (R 2 = 0.87) between the VDPR and the SWIR‐derived reflectance index (RI) and also between the SR (R 2 = 0.90) and the RI for all soils. These results point to the feasibility and appeal of quantifying AS using the newly proposed and more time‐efficient proximal sensing method.

Optical properties of (Bi1-xInx)2Se3 thin films

Bi2Se3 is a topological insulator with unique optical properties, including linearly-dispersing surface states. Many of the proposed device applications for Bi2Se3 require a lattice-matched trivially-insulating component. It is known that (Bi1-xInx)2Se3 is a trivial band insulator for moderate indium concentrations. In this paper, we grow and characterize the optical properties of (Bi1-xInx)2Se3 films with varying indium concentrations. We find that the lattice constant and optical bandgap for (Bi1-xInx)2Se3 varies linearly with concentration. We perform infrared reflection measurements as a function of polarization and angle, enabling us to model the permittivity for these materials. Again, we find that most parameters vary linearly with concentration. Our results for the pure end members are consistent with the literature values. This is the first report of optical values for the intermediate compounds, which are likely to be integral components of future topological insulator optical devices.

Characterization of carrier concentration and mobility of GaN bulk substrates by Raman scattering and infrared reflectance spectroscopies

n-Type GaN bulk substrates with carrier concentrations (n) of 1016–1020 cm−3 were characterized by Raman scattering (350–5000 cm−1) and infrared reflectance (400–5000 cm−1) spectroscopies. Experimental spectra were fitted with the curves calculated from the dielectric function and carrier concentration and mobility of the GaN bulk. The obtained values agree well with the values from Hall-effect measurements for n of 1016–1019 cm−3 in Raman scattering measurements and for n of 1018–1020 cm−3 in infrared reflectance measurements.

Synthesis and optical properties of La1−XCeXMnO3 studied by infrared reflectivity measurements

The bulk polycrystalline La1−xCexMnO3 (0.0 ≤ x ≤ 0.8) manganites were synthesized through citrate sol-gel auto-combustion method for a wide range of composition 0.0 ≤ x ≤ 0.8. The x-ray diffraction (XRD) analysis suggests the single phase rhombohedral (R-3c) symmetry for parent LaMnO3 compound, however a secondary cubic (Fm-3 m) phase of cerium oxide (CeO2) is appeared when Ce is incorporated into the LaMnO3. A detailed XRD study of these materials were discussed, and specifically the infrared reflectivity measurements on La1−xCexMnO3 materials are discussed in this study for the first time. The phonon softening and hardening was observed for different modes but softening was found to be more pronounced which implied strong electron–phonon coupling. The most affected phonon mode was appeared for the doping range 0.2 ≤ x ≤ 0.4, in which the largest softening was observed. The Lyddane–Sachs–Teller relation is verified by experimental FTIR data.

Thermochromic VO2 thin films on ITO-coated glass substrates for broadband high absorption at infra-red frequencies

Thin films of vanadium dioxide (VO2) are deposited on indium tin oxide (ITO), stainless steel (SS), and glass substrates using chemical vapour deposition. X-ray diffraction and Raman spectroscopy measurements confirmed the single phase nature of the VO2, which showed a phase transition from a low conducting state at low temperature(<68 °C) to a high conducting state at high temperature (>68 °C). This was confirmed by electrical conductance and infra-red reflectance measurements. X-ray photoelectron spectroscopy was used to measure the charge states of vanadium species. The optical constants of VO2 were determined using visible and near-infra red(NIR) reflectivity and show that the VO2 film on ITO has a lowered plasma frequency compared with VO2 on glass substrates. The thin films of VO2 enable a broadband of ultra-high absorption at mid-wave infra-red frequencies due to a Fabry-Pérot (F-P) like resonance due to the dielectric properties of ITO, SS, or glass. The tunability of this absorption band via VO2 thickness and the switchability by temperature makes the system attractive for absorptive coatings with controllable emissivity.

Study of vibrational modes in CuxAg1−xIn5S8 mixed crystals by infrared reflection measurements

Infrared reflection spectra of CuxAg1−xIn5S8 mixed crystals, grown by Bridgman method, were studied in the wide frequency range of 50–2000 cm−1. All four infrared-active modes were detected, which are in full agreement with the prediction of group-theoretical analysis. Real and imaginary parts of the dielectric function, refractive index and the energy losses function were evaluated from reflectivity measurements. The frequencies of TO and LO modes and oscillator strengths were also determined. The bands detected in IR spectra of studied crystals were assigned to various vibration types (valence and valence-deformation) on the basis of the symmetrized displacements of atoms obtained employing the Melvin projection operators. The linear dependencies of optical mode frequencies on the composition of CuxAg1−xIn5S8 mixed crystals were obtained. These dependencies display one-mode behavior.

Proof-of-concept robot platform for exploring automated harvesting of sugar snap peas

Currently, sugar snap peas are harvested manually. In high-cost countries like Norway, such a labour-intensive practise implies particularly large costs for the farmer. Hence, automated alternatives are highly sought after. This project explored a concept for robotic autonomous identification and tracking of sugar snap pea pods. The approach was based on a combination of visible–near infrared reflection measurements and image analysis, along with visual servoing. A proof-of-concept harvesting platform was implemented by mounting a robotic arm with hand-mounted sensors on a mobile unit. The platform was tested under plastic greenhouse conditions on potted plants of the sugar snap pea variety Cascadia using LED-lights and a partial shade. The results showed that it was feasible to differentiate the pods from the surrounding foliage using the light reflection at the spectral range around 970 nm combined with elementary image segmentation and shape modelling methods. The proof-of-concept harvesting platform was tested on 48 representative agricultural environments comprising dense canopy, varying pod sizes, partial occlusions and different working distances. A set of 104 images were analysed during the teleoperation experiment. The true positive detection rate was 93 and 87% for images acquired at long distances and at close distances, respectively. The robot arm achieved a success rate of 54% for autonomous visual servoing to a pre-grasp pose around targeted pods on 22 untouched scenarios. This study shows the potential of developing a prototype robot for semi-automated sugar snap pea harvesting.

Observation of ultrahigh mobility surface states in a topological crystalline insulator by infrared spectroscopy

Topological crystalline insulators possess metallic surface states protected by crystalline symmetry, which are a versatile platform for exploring topological phenomena and potential applications. However, progress in this field has been hindered by the challenge to probe optical and transport properties of the surface states owing to the presence of bulk carriers. Here, we report infrared reflectance measurements of a topological crystalline insulator, (001)-oriented Pb1−xSnxSe in zero and high magnetic fields. We demonstrate that the far-infrared conductivity is unexpectedly dominated by the surface states as a result of their unique band structure and the consequent small infrared penetration depth. Moreover, our experiments yield a surface mobility of 40,000 cm2 V−1 s−1, which is one of the highest reported values in topological materials, suggesting the viability of surface-dominated conduction in thin topological crystalline insulator crystals. These findings pave the way for exploring many exotic transport and optical phenomena and applications predicted for topological crystalline insulators.

Chemical pressure effect in Sm and La substituted ferroelectric BiFeO3 thin films: Insights from infrared spectroscopy

We investigate the effects of Sm and La substitution in ferroelectric BiFeO3 thin films on the lattice dynamics by infrared reflection measurements at room temperature. The frequencies of the infrared-active phonon modes are studied as a function of Sm and La content in Bi1–x(Sm,La)xFeO3 composition spread films in the range from x = 0 up to x = 0.25, grown on SrTiO3 substrates by pulsed laser deposition. Substitution of the Bi3+ ions with small Sm3+ ions leads to the appearance of a new phase above x ≈ 0.09 coexisting with the ferroelectric BiFeO3 phase up to x ≈ 0.19. In contrast, for the substitution of Bi3+ ions with La3+ ions of similar size a continuous transition from the original BiFeO3 phase to a new phase takes place. In both cases, we assign the new phase to the paraelectric, orthorhombic phase. These findings are discussed in terms of the morphotropic phase boundary in Sm-doped BiFeO3 around x = 0.14 with a phase coexistence, which was suggested as the origin for enhanced piezoelectric properties.

Pressure-induced phase transitions and insulator-metal transitions in VO2 nanoparticles

Abstract We present pressure-induced phase transitions and metallization in VO2 nanoparticles characterized by synchrotron x-ray diffraction, Raman spectroscopy, and infrared reflectivity measurements. The M1-M1′-Mx phase transitions were found in VO2 nanoparticles upon compression. The results of IR reflectivity shown that pressure-induced metallization occurs in the M1′ phase with increasing pressure and the sample becomes fully metallic at the transition of M1′ to Mx. The metallic Mx phase transforms to metastable mixed phases displaying insulating properties upon decompression. We attribute the pressure-induced metallization of the M1′ phase to the strong electron correlations, while the metal-insulator transition (MIT) from the Mx to the mixed phases is found to be associated with the structural phase transitions. Both the electron-correlation-driven Mott transition and the structure-driven MIT can be achieved in VO2 by applying pressure. Compared with bulk samples, the VO2 nanoparticles exhibit larger bulk moduli and increased transition pressure due to their nanosize effects. High pressure provides an effective method to study the MIT in strongly correlated materials and paves the way for modifying electronic properties of VO2.