Photothermal Spectroscopy Research Articles

A photonic pH sensor based on photothermal spectroscopy

Although the determination of pH is a standard laboratory measurement, new techniques capable of measuring pH are being developed to facilitate modern technological advances. Bio-industrial processing, tissue engineering, and intracellular environments impose unique measurement requirements on probes of pH. We describe a fiber optic-based platform, which measures the heat released by chromophores upon absorption of light. The optical fibers feature fiber Bragg gratings (FBG) whose Bragg peak redshifts with increasing temperature. Using anthocyanins (pH-sensitive chromophores found in many plants), we are able to correlate visible light absorption by a solution of anthocyanins to heat released and changes in FBG signal over a pH range of 2.5–10. We tested the ability of this platform to act as a sensor coating the fiber within a layer of crosslinked polyethylene glycol diacrylate (PEG-DA). Incorporating the anthocyanins into the PEG, we find that the signal magnitude increases over the observed signal at the same pH in solution. Our results indicate that this platform is viable for assessing pH in biological samples and point at ways to optimize performance.

Defect Absorption in Ge2Sb2Te5 Phase‐Change Films

Defect absorption spectra in amorphous and crystalline Ge2Sb2Te5 phase‐change films are investigated. Photothermal deflection spectroscopy extended to the IR region reveals subgap absorption of the Ge2Sb2Te5 films. Subgap absorption peaks are observed around 0.480, 0.415, and 0.368 eV for as‐deposited films, around 0.475, 0.410, and 0.360 eV for 180 °C‐annealed films, and around 0.410 and 0.350 eV for 300 °C‐annealed films at room temperature. The intensity and energies of the absorption peaks vary with heat treatments in the range of 90–300 °C. Broad absorption of 0.5–0.6 eV is also observed for as‐deposited films. Based on the experimental results, the generation mechanisms of the defect levels in amorphous, metastable, and stable crystalline Ge2Sb2Te5 are discussed.

Intensity Modulated Photothermal Measurements of NO2 with a Compact Fiber-Coupled Fabry-Pérot Interferometer.

Sensors for the reliable measurement of nitrogen dioxide concentrations are of high interest due the adverse health effects of this pollutant. This work employs photothermal spectroscopy to measure nitrogen dioxide concentrations at the parts per billion level. Absorption induced temperature changes are detected by means of a fiber-coupled Fabry–Pérot interferometer. The small size of the interferometer enables small detection volumes, paving the way for miniaturized sensing concepts as well as fast response times, demonstrated down to 3 s. A normalized noise equivalent absorption of cm−1W/ is achieved. Additionally, due to the rigid structure of the interferometer, the sensitivity to mechanical vibrations is shown to be minor.

Photothermal spectroscopy study of fused silica irradiated by a 355 nm wavelength and 68 ns pulse duration laser

Laser-induced damage of fused silica with different parameters was studied by means of photothermal spectroscopy and optical microscope. The development of damage area with laser parameters was discussed, and the photothermal absorption signal and its stability of the damaged pits were studied. It was found that the signal is stronger when the fused silica material was molten or broken. The laser damage area visibility ratio is defined. The change of the laser damage area visibility ratio with laser parameters is studied, and it is suggested that it can be used as a parameter to indicate the degree of damage in engineering.

A Review of Photothermal Detection Techniques for Gas Sensing Applications

Photothermal spectroscopy (PTS) is a technique used for determining the composition of liquids, solids and gases. In PTS, the sample is illuminated with a radiation source, and the thermal response of the analyte (e.g., refractive index) is analyzed to gain information about its content. Recent advances in this unique method of detecting gaseous samples show that photothermal gas spectroscopy can be an interesting alternative to commonly used absorption techniques. Moreover, if designed properly, sensors using PTS detection technique can not only reach sensitivities comparable with other, more complex techniques, but can significantly simplify the design of the sensor. In this review, recent developments in photothermal spectroscopy of gases will be summarized and discussed.

Electrical and optical characteristics of boron doped nanocrystalline diamond films

Boron doped diamond is a prospective material which can be used as a conductive and optically transparent thin-film electrode in a variety of optoelectronic applications. In this work, we present the results of the temperature resolved electrical conductivity, optical reflection, transmission and absorption of thin boron doped nanocrystalline diamond films grown by a microwave plasma enhanced chemical vapor deposition. Optical transmittance, reflectance and absorptance properties of layers were studied by a photo-thermal deflection spectroscopy analysis. Raman spectroscopy with various excitation wavelengths was employed for the analysis of nanocrystalline diamond layers. The measured position, shift and broadening of the characteristic boron doped diamond Raman lines were used for the determination of the boron concentration. Correlation between the results of the atomic boron concentration estimated via the Raman analysis and measured electrical conductivity values is presented.

Key signal contributions in photothermal deflection spectroscopy

We report on key signal contributions in photothermal deflection spectroscopy (PDS) of semiconductors at photon energies below the bandgap energy and show how to extract the actual absorption properties from the measurement data. To this end, we establish a rigorous computation scheme for the deflection signal including semianalytic ray tracing to analyze the underlying physical effects. The computation takes into account linear and nonlinear absorption processes affecting the refractive index, thus leading to a deflection of the probe beam. We find that besides the linear mirage effect, nonlinear absorption mechanisms make a substantial contribution to the signal for strongly focused pump beams and sample materials with high two-photon absorption coefficients. For example, the measured quadratic absorption contribution exceeds 5% at a pump beam intensity of about 1.3×105W/cm2 in Si and at 5×104W/cm2 in GaAs. In addition, our method also includes thermal expansion effects as well as spatial gradients of the attenuation properties. We demonstrate that these effects result in an additional deflection contribution that substantially depends on the distance of the photodetector from the readout point. This distance dependent contribution enhances the surface related PDS signal up to two orders of magnitude and may be misinterpreted as surface absorption if not corrected in the analysis of the measurement data. We verify these findings by PDS measurements on crystalline silicon at a wavelength of 1550 nm and provide guidelines on how to extract the actual attenuation coefficient from the PDS signal.

Uptake and Retention of Nanoplastics in Quagga Mussels.

Here, a set of experiments to assess the feasibility of using an invasive and widespread freshwater mussel (Dreissena rostrformis bugensis) as a sentinel species for nanoplastic detection is reported. Under laboratory experimental conditions, mussels ingest and retain fluorescent polystyrene (PS) beads with carboxylic acid (—COOH) termination over a size range of 200–2000 nm. The number of beads the mussels ingested is quantified using fluorescence spectroscopy and the location of the beads in the mussels is imaged using fluorescence microscopy. PS beads of similar size (1000–2000 nm) to mussels' preferred food are trafficked in the ciliated food grooves of the gills. Beads of all sizes are observed in the mussels' digestive tracts, indicating that the mussels do not efficiently reject the beads as unwanted foreign material, regardless of size. Fluorescence microscopy shows all sizes of beads are concentrated in the siphons and are retained there for longer than one month postexposure. Combined atomic force microscopy–infrared spectroscopy and photothermal infrared spectroscopy are used to locate, image, and chemically identify the beads in the mussel siphons. In sum, these experiments demonstrate the potential for using mussels, specifically their siphons, to monitor environmental accumulation of aquatic nanoplastics.

Autonomous Supramolecular Interface Self-Healing Monitored by Restoration of UV/Vis Absorption Spectra of Self-Assembled Thiazole Layers.

Longevity of complex organic devices critically depends on the supramolecular integrity of the constituting layers and interfaces. Because the latter are soft matter, they can structurally respond to perturbation of their supramolecular structure by relaxing back to a thermodynamically favorable state. To use this response for self-healing of optoelectronically active layers and particularly interfaces, the degraded dyes in these layers need to be exchanged with non-degraded ones. Here, we present a dye layer interfaced between a solid surface and a dye reservoir that autonomously self-heals after photo-degradation of single molecules to restore its optical function. Surface sensitive in situ photothermal deflection spectroscopy reveals that this supramolecular self-healing approach critically depends on the thermodynamic stability of the layer, the chemical change of the dye upon degradation, and the medium dissolving the degraded dye and providing the reservoir dyes. Hence, the interplay of these parameters is key to successfully using this supramolecular self-healing approach to thin layers and interfaces in organic device for increased sustainability of organic optoelectronics and related fields.

Impact of surface dipole in NiOx on the crystallization and photovoltaic performance of organometal halide perovskite solar cells

Despite the success of solution processed nickel oxide (s-NiOx) as the hole transporting layer (HTL) in organic solar cells, applying s-NiOx in perovskite solar cells (PVSCs) is not that straight forward. The reported power conversion efficiencies (PCEs) of the s-NiOx based PVSCs span a wide range from 8% to over 20% even with a similar recipe. Here, we report that one of the causes for the performance discrepancy might be the large surface dipole on the s-NiOx surface. We find that the perovskite deposited on the as-prepared sol-gel derived s-NiOx has large number of defects at the s-NiOx/perovskite interface. Based on the in-depth mechanism study with various spectroscopy techniques, we propose that the strong surface dipole of the s-NiOx composite film induces adhesion of perovskite precursor ions on the surface of s-NiOx during the perovskite film formation and creates defects in the perovskite crystals at the interface. Such interfacial dipole-ion attachment has been demonstrated can be dissociated by ultraviolet (UV) light soaking experiment. The high energy of the UV light helps to dissociate the physical dipole-ion attachment and mobilize the ions to accommodate the perovskite defect sites. The defect density of the perovskite film on s-NiOx has been significantly reduced by an amount of 4.1 × 1017 cm−3 after the UV light soaking as evidenced by photothermal deflection spectroscopy (PDS) measurement. By treating the s-NiOx surface with a dipolar molecule n-Butylamine, the surface dipole of the s-NiOx film is efficiently reduced and it significantly reduces the defect density in the perovskite film. As a result, the PVSCs based on the n-Butylamine treated s-NiOx layer have achieved a dramatical enhancement in PCE to 18.9% with decent stability at the maximum power point tracking. It is believed that this work provides insight and strategy to develop highly reproducible PVSCs with solution derived metal oxide as interlayers.

Balanced-detection interferometric cavity-assisted photothermal spectroscopy.

An optical cavity can be utilized as an excellent transducer for highly sensitive gas detection with the application of photothermal spectroscopy, featuring the beneficial property of an ultra-low absorption volume within a rugged sensing element. We report the novel implementation of balanced detection in Fabry-Perot photothermal interferometry via two identical 1 mm-spaced cavities. That way, excess noise limiting the sensitivity of previous cavity-based photothermal sensors was effectively rejected close to the fundamental limit of shot noise. A quantum cascade laser served as mid-infrared excitation source to induce refractive index changes in the sample, and a near-infrared fiber laser served as probe source to monitor the photo-induced variations. The metrological qualities of the sensor were investigated by SO2 detection. For the targeted absorption band centered at 1380.93 cm-1, a 5 ppbv minimum detection limit was achieved with a 1 s integration time, corresponding to a normalized noise equivalent absorption of 7.5 × 10-9 cm-1 W Hz-1/2. Additionally, the sensor showed excellent long-term stability, enabling integration times of a few thousand seconds.

Sensitive determination of DNA based on phosphate-dye interaction using photothermal lens technique.

Photothermal lens spectrometry is a powerful optical detection technique that can be used to investigate biomolecules. In this work, for the first time to our knowledge, photothermal lens spectrometry was used for determination of nanomolar concentrations of three distinct deoxyribonucleic acid (DNA) strands using methylene blue as a labeling dye. Methylene blue interacts with phosphate groups of the DNA in lower DNA concentrations. It was observed that phosphate-methylene blue interaction had no obvious effect on methylene blue absorption and fluorescence spectra, but the photothermal lens spectrometry signal of methylene blue increased with DNA concentration. For this purpose, to evaluate the performance of the presented method, herring sperm DNA, Escherichia coli bacteria DNA, and partial 16S rRNA genes were examined. Under optimum conditions, photothermal lens spectrometry intensity of methylene blue increased linearly with DNA concentration when herring sperm DNA, Escherichia coli DNA, and 16S rRNA gene concentrations increased in the ranges of 0.1-250, 1-700, and 1-800 nmol L-1, respectively. The corresponding detection limits were found to be 0.07, 0.71, and 0.56 nmol L-1, respectively, and relative standard deviations for 50 nmol L-1 of the tested samples were 2.59%, 4.95%, and 4.57%, respectively.

Superefficient light emission

Quantum Dots A challenge to improving synthesis methods for superefficient light-emitting semiconductor nanoparticles is that current analytical methods cannot measure efficiencies above 99%. Hanifi et al. used photothermal deflection spectroscopy to measure very small nonradiative decay components in quantum dot photoluminescence. The method allowed them to tune the synthesis of CdSe/CdS quantum dots so that the external luminescent efficiencies exceeded 99.5%. This is important for applications that require an absolute minimum amount of photon energy to be lost as heat, such as photovoltaic luminescent concentrators. Science , this issue p. [1199][1] [1]: /lookup/doi/10.1126/science.aat3803

Redefining near-unity luminescence in quantum dots with photothermal threshold quantum yield.

A variety of optical applications rely on the absorption and reemission of light. The quantum yield of this process often plays an essential role. When the quantum yield deviates from unity by significantly less than 1%, applications such as luminescent concentrators and optical refrigerators become possible. To evaluate such high performance, we develop a measurement technique for luminescence efficiency with sufficient accuracy below one part per thousand. Photothermal threshold quantum yield is based on the quantization of light to minimize overall measurement uncertainty. This technique is used to guide a procedure capable of making ensembles of near-unity emitting cadmium selenide/cadmium sulfide (CdSe/CdS) core-shell quantum dots. We obtain a photothermal threshold quantum yield luminescence efficiency of 99.6 ± 0.2%, indicating nearly complete suppression of nonradiative decay channels.

Growth and characterization of β-Ga2O3 thin films on different substrates

β-Ga2O3 thin films were grown on the substrates of sapphire, GaN, and single crystals of β-Ga2O3, using plasma-assisted molecular beam epitaxy. By varying deposition conditions, pure-phase epitaxial β-Ga2O3 thin films were obtained, and the crystal quality of the as-grown films was optimized. A systematic characterization and a detailed analysis were performed on the films, including the nucleation process, surface morphology, crystal quality, thermal stability, as well as electrical and optical properties. Optical absorption was investigated using photothermal deflection spectroscopy, which provides detailed information about sub-gap optical absorption. Photocurrent measurements indicated a pronounced persistent photo-conductivity of β-Ga2O3. A blue-UV emission with an energy of 3–3.5 eV was observed by cathodoluminescence spectroscopy. The Fermi level position of the as-grown film was determined based on temperature-dependent electrical conductivity measurements. It is proposed that oxygen vacancies in the film form a defect band at around Ec-0.8 eV that pins the Fermi level and is related to the observed photocurrent and cathodoluminescence characteristics.

Atmospherically induced defects in (FASnI3)0.6(MAPbI3−3xCl3x)0.4 perovskites

The effects of brief atmospheric exposure are assessed for mixed composition perovskites comprised of a combination of formamidinium tin iodide (FASnI3) and methylammonium lead iodide + chloride (MAPbI3−3xCl3x) as well as MA0.7FA0.3PbI3 perovskite for use as absorber layers in high-efficiency thin film solar cells. Photothermal deflection spectroscopy (PDS) is employed to probe the relative defect density of all films by way of extracting the Urbach energy characterizing the width of the sub-bandgap absorption tail. Exposure to atmosphere for ~3 min prior to PDS measurement is found to degrade mixed composition films such that exposed films had an average increase of 36% in Urbach energies compared to those that are fully kept out of atmosphere through immersion in inert C6F14 liquid. By contrast, the same exposure to atmosphere induced no change in Urbach energy for MA0.7FA0.3PbI3. These results demonstrate the influence of ambient exposure on perovskites, particularly those containing Sn, and highlight the fact that handling procedure itself throughout all fabrication and subsequent characterization steps influences the measured results.

Determination of Dissolved Iron Redox Species in Freshwater Sediment using DGT Technique Coupled to BDS

In this work we have developed a novel method for determination of iron redox species by the use of diffusive gradients in thin-film (DGT) technique coupled to photothermal beam deflection spectroscopy (BDS). The combination of both methods achieved low limit of detection (LOD) of 0.14 μM for Fe (II) ions. The total Fe concentration determined in the Vrtojbica river sediment (Slovenia, Rožna Dolina, 5000 Nova Gorica) was 49.3 μgL-1. The Fe (II) and Fe (III) concentration amounted to 12.8 μgL-1 and 39.9 μgL-1, respectively. Such an approach opens new opportunities for monitoring the content of iron species in natural waters and sediments and provides highly sensitive chemical analysis and an accurate qualitative and quantitative characteristic of the materials under study.

Probing charge transfer states at organic and hybrid internal interfaces by photothermal deflection spectroscopy

In organic and hybrid photovoltaic devices, the asymmetry required for charge separation necessitates the use of a donor and an acceptor material, resulting in the formation of internal interfaces in the device active layer. While the core objective of these interfaces is to facilitate charge separation, bound states between electrons and holes may form across them, resulting in a loss mechanism that diminishes the performance of the solar cells. These interfacial transitions appear in organic systems as charge transfer (CT) states and as bound charge pairs (BCP) in hybrid systems. Despite being similar, the latter are far less investigated. Herein, we employ photothermal deflection spectroscopy and pump-push-probe experiments in order to determine the characteristics and dynamics of interfacial states in two model systems: an organic P3HT:PCBM and hybrid P3HT:ZnO photovoltaic layer. By controlling the area of the internal interface, we identify CT states between 1.4 eV and 1.8 eV in the organic bulk-heterojunction (BHJ) and BCP between 1.1 eV and 1.4 eV in the hybrid BHJ. The energetic distribution of these states suggests that they not only contribute to losses in photocurrent, but also significantly limit the possible maximum open circuit voltage obtainable from these devices.

Numerical and experimental investigations of pressure-driven gas flow in hollow-core photonic crystal fibers.

A comprehensive understanding of gas flow in long hollow-core photonic crystal fibers (HC-PCFs) is critical for evaluating their sensing performance for low-concentration gases, especially in terms of response time. The aim of this paper is to numerically and experimentally investigate the pressure-driven gas flow dynamics in a relatively long HC-PCF-based gas sensor. The gas flow in the core of a 1.1m long HC-PCF was numerically modeled to examine the gas sensing response time in terms of the time for the gas to fill the core (gas filling time). The model was validated against the experimental results of continuous-wave modulated photothermal spectroscopy. The model was then used to analyze the effects of gas inlet pressure, core diameter, fiber length, and gas type on the gas flow field and gas filling time. The results revealed that a lower gas filling time was achieved as the pressure difference between the inlet and outlet increased, the core diameter increased, and/or the core length decreased. The developed numerical model provides valuable information such as cross-sectional velocity profiles and gas flow rates that cannot be readily obtained from simpler analytical models.

Structural disorder and in-gap states of Mg-implanted GaN films evaluated by photothermal deflection spectroscopy

Both structure of the valence band maximum (VBM) and deep-level defects for box-profile Mg-ion implanted (4 × 1019 cm−3) GaN samples are characterized by photothermal deflection spectroscopy (PDS). Compared with the results evaluated by positron annihilation spectroscopy, the variations caused by the thermal annealing is discussed with respect to Urbach energy, defect levels in the band gap, and photoluminescence. Forming Urbach tail gradually at the VBM at the 1000 °C annealing, vacancy-type defects clusters without drastic decrease of deep-level defects. Green and yellow luminescence emits slightly as the PDS signal at the deep-level is decreased by the annealing at 1100 °C due to the decrease of non-radiative recombination centers. The Urbach energy is improved by the further annealing so that the luminescence becomes intense due to the energy transfer through the phonon from Urbach region. The shift of Fermi level towards the valence band at the 1300 °C annealing, the sign of p-conduction, is confirmed from the valence band spectra. Compared to the Mg-doped GaN with p-conduction, it is considered that the improvement of Urbach energy is crucial for p-type activation.