Low Extinction Coefficient Research Articles

Optimization of ICPCVD Amorphous Silicon for Optical MEMS Applications

In this paper, we present the optimization of optical and mechanical properties of inductively coupled plasma chemical vapor deposited (ICPCVD) amorphous silicon thin films for fabrication of high-quality optical microelectromechanical systems-based devices operating from visible to short-wave infrared wavelengths (450–3000 nm). Our results indicate that, at relatively high deposition temperatures for plasma CVD, a decrease in the ICP power results in films with lower tensile stress, higher refractive index, and lower extinction coefficient. We show that hydrogen concentration alone is not a sufficient parameter for controlling optical and mechanical quality of the films. In particular, both the hydrogen concentration and the hydrogen-silicon bonding nature together play a vital role in determining the optical and the mechanical quality of the silicon thin films. As a demonstration vehicle, three layer silicon-silicon oxide-silicon-based distributed Bragg reflectors were fabricated for the visible (500–700 nm), near infrared (700–1000 nm), and short-wave infrared (2000–3000 nm) wavelength ranges using an optimized silicon fabrication recipe. The measured optical transmission spectra show close to 90% peak reflectivity. Finally, stress optimization was evaluated by fabricating $270-\mu \mathrm {m}$ diameter circular suspended silicon membranes, which demonstrate a flatness variation on the order of $ nm across the entire lateral dimension. [2015-0029]

Optical and morphological properties of silicon dioxide thin films

The purpose of the present work is to compare the structural and optical properties of the Silicon dioxide films obtained from TMOS Si(OCH3)4), N2O, and NH3 as precursor gases, which with respect to their potential optical applications, have been deposited by using inductively coupled plasma-enhanced chemical-vapor deposition method. The optical property as well as the thickness of the films were analyzed by means of variable angle spectroscopic ellipsometry. Morphological studies were carried out by scanning electron microscopy, and chemical composition characterization was performed with the help of energy dispersive spectroscopy unit coupled with the electron microscope. The type of the substance and the precursor composition used for silicon dioxide synthesis are effective on the chemical composition of the films. The refractive index values of these films advocate their use as high refractive index materials while their low extinction coefficients assure the devices transparency. The work presents deposition rates as well as the films optical properties, chemical composition and morphology regarding the operational parameters of their synthesis. It also provides a comparison of the characteristics of the two competitive precursor compounds.

PbS/CdS/ZnS Quantum Dots: A Multifunctional Platform for In Vivo Near‐Infrared Low‐Dose Fluorescence Imaging

Over the past decade, near‐infrared (NIR)‐emitting nanoparticles have increasingly been investigated in biomedical research for use as fluorescent imaging probes. Here, high‐quality water‐dispersible core/shell/shell PbS/CdS/ZnS quantum dots (hereafter QDs) as NIR imaging probes fabricated through a rapid, cost‐effective microwave‐assisted cation exchange procedure are reported. These QDs have proven to be water dispersible, stable, and are expected to be nontoxic, resulting from the growth of an outer ZnS shell and the simultaneous surface functionalization with mercaptopropionic acid ligands. Care is taken to design the emission wavelength of the QDs probe lying within the second biological window (1000–1350 nm), which leads to higher penetration depths because of the low extinction coefficient of biological tissues in this spectral range. Furthermore, their intense fluorescence emission enables to follow the real‐time evolution of QD biodistribution among different organs of living mice, after low‐dose intravenous administration. In this paper, QD platform has proven to be capable (ex vivo and in vitro) of high‐resolution thermal sensing in the physiological temperature range. The investigation, together with the lack of noticeable toxicity from these PbS/CdS/ZnS QDs after preliminary studies, paves the way for their use as outstanding multifunctional probes both for in vitro and in vivo applications in biomedicine.

A one-step method to prepare carbon and dielectric composited films featuring a skeleton–skin structure

A one-step and cost-efficient strategy to prepare carbon and dielectric composites with a skeleton–skin structure by a sol–gel method has been proposed. For the model system in the present work, i.e. C–TiO2 composite films, the insight of the wet chemical mechanisms in sols was illustrated by the electrical compensation theory. The structure and carbon crystallization evolutions with annealing process were investigated, demonstrating the nano carbon with an average sp2 cluster size ranging from 1.1nm to 1.4nm. The resultant composite films turn out to be rutile TiO2 skeleton covered by carbon with intimate interfaces. Benefited from this firm composite structure containing nano carbon, the film possesses tunable and outstanding electrical and optical performance. By appropriate handling annealing temperature, the film could present a high extinction coefficient qualified for solar selective absorber, or show a relatively low extinction coefficient and resistivity (about 10−2Ωcm).

Low-temperature preparation of high-n TiO2 thin film on glass by pulsed laser deposition

Single-phase rutile-type TiO2 thin films with a high refractive index (n) and a low extinction coefficient (k) prepared on glass are expected to improve the performance of anti-reflection coatings. In this study, TiO2 thin films were prepared by the pulsed laser deposition (PLD) method at temperatures ranging from room temperature to 600°C under an oxygen partial pressure of 1–9Pa or a 10−5Pa vacuum, and their crystal structure, microstructure and optical properties were investigated. A single-phase rutile-type TiO2 thin film was successfully prepared on a glass substrate by depositing at room temperature in a vacuum followed by post-annealing at 450°C in air. A nanocrystalline oxygen-deficient phase in the as-deposited films plays an important role in the formation of the single rutile phase during post-annealing. The single-phase rutile-type TiO2 thin films showed excellent optical properties, with n=3.14 and k<0.05 at λ=400nm.

Electrical and optical properties of Si-doped indium tin oxides as transparent electrode and anti-reflection coating for solar cells

We have studied the electrical and optical properties of Si-doped indium tin oxides (ITSOs) as transparent electrodes and anti-reflection coatings for Si-based solar cells. The ITSO thin films were obtained by co-sputtering of ITO and SiO2 targets under target power control. The resistivity of the ITSO thin films deposited at 0.625 in terms of power ratio (ITO/SiO2) were 391 Ωcm. In this condition, the ITSO thin films showed very high resistivity compared to sputted pure ITO thin films (1.08 × 10−3 Ωcm). However, refractive index of ITSO thin films deposited at the same condition at 500 nm is somewhat lowered to 1.97 compared to ITO thin films (2.06). The fabricated graded refractive index AR coatings using ITO, ITSO, and SiO2 thin films kept over 80% of transmittance regardless of their thickness varing from 97 nm to 1196 nm because of their low extinction coefficient. As the AR coating with graded refractive indices using ITO, ITSO, and SiO2 layers was applied to general silicon-based solar cell, the current level increased nearly twice more than that of bare silicon solar cell without AR coating.

Regiochemistry and substituent effects on pyrylogen and thiopyrylogen stability and electronic character

A physical organic analysis of a collection of 57 data sets generated by B3LYP/6-311+G(2d,p) geometry optimizations and TD-DFT calculations is presented. The analysis engenders recommendations for syntheses of new pyrylogen electron-transfer sensitizers that are potentially optimized for low energy absorbance and high extinction coefficients. It is argued that bathochromically shifted intense absorbance is especially useful to avoid competitive absorbance by electron-transfer substrates and for use at the low concentrations likely achievable in experimentally attractive nonpolar solvents.

Effect of the nanocrystalline structure type on the optical properties of TiO2:Nd (1 at.%) thin films

Titanium dioxide thin films, each doped with the same amount of neodymium (1at.%) were deposited by Low Pressure Hot Target Reactive Sputtering and High Energy Reactive Magnetron Sputtering processes in order to obtain anatase and rutile thin film structures respectively. The microstructure and phase composition were analyzed using the transmission electron microscopy method including high resolution electron microscopy imaging. The measurements of the optical properties showed, that both prepared thin films were transparent in the visible light range and had a low extinction coefficient of ca. 3⋅10−3. The thin film with the anatase structure had a lower cut-off wavelength and refractive index and a higher value of optical energy band gap as-compared to the TiO2:Nd coating with the rutile structure. Simultaneously, more efficient photoluminescence emission was observed for the rutile thin films.

Efficient improvements in the performance of Ru(II) π-expanded terpyridyl dyes in dye-sensitized solar cells: A theoretical study

Abstract To overcome the shortcomings of traditional Ru dyes in low molar extinction coefficient ( ϵ ) of the metal-to-ligand charge transfer band in the near-infrared region, we designed and studied two Ru dyes, HIS-2a and HIS-2b derived from HIS-2 . The intrinsic causes for varying short-circuit photocurrent density ( J sc ) and open-circuit photovoltage ( V oc ) have been systematically investigated on the dyes and dye/(TiO 2 ) 38 systems including the conduction band energy shift, electronic structures, light harvesting efficiency, electron injection driving force, electronic coupling, dye regeneration driving force and so on. Those results suggest that, compared with HIS-2 and HIS-2b , HIS-2a has red-shifted absorption spectrum and increased transition intensity in NIR region, which would lead to a larger J sc . At the same time, the conduction band energy of HIS-2a shifts little compared with other dyes, which will not have much effect on V oc . Hence, we could speculate that HIS-2a is a promising candidate with enhanced J sc and a more efficient Ru(II) π-expanded terpyridyl dye. We also hope that this study will provide theoretical guidelines for the synthesis of new Ru dyes.

Using Plant Phenotypic Plasticity to Improve Crop Performance and Stability Regarding Climatic Uncertainty: A Computational Study on Sunflower

A crop can be represented as a biotechnical system in which components are either chosen (cultivar, management) or given (soil, climate) and whose combination generates highly variable stress patterns and yield responses.In this study, we used modeling and simulation to predict the crop phenotypic plasticity resulting from the interaction of plant traits, climatic variability and management actions [1].We relied on numerical experiment to explore how virtual plant material, designed by recombining 8 plant traits within bounds observed in cultivated genetic diversity, performed in a large target population of cropping environments. We used optimization methods to search for trait combinations that matched desired crop specifications, in this case increasing the crop yield expectation and avoiding low yield (third quantile).Results showed a trade-off between crop performance and stability, with different optimal trait combinations among the best solutions. There was little variability in the value of three plant traits (late maturity, low leaf area distribution, conservative stomatal conductance regulation) among the optimal solutions, indicating that these traits had a major value in the tested population of environments. On the other hand, the five other traits displayed variability in optimal values, which was identified as the basis of the performance/stability trade-off. Four plant types, corresponding to different strategies of resource use were identified in the optimal solutions. These plant types ranged from a conservative resource management strategy (early flowering, low light extinction coefficient) to a more productive one. Results suggested that key traits were responsible the cultivar global adaptation capacity whereas secondary traits allowed distinctive resource use strategies that were more adapted to specific conditions.We argue that process-based modeling and simulation could be integrated in trait-based breeding approaches as a way to screen plant phenotypic plasticity in large population of environments before proceeding to phenotyping approach in field conditions.

Calcium manganate: A promising candidate as buffer layer for hybrid halide perovskite photovoltaic-thermoelectric systems

We have systematically studied the feasibility of CaMnO3 thin film, an n-type perovskite, to be utilized as the buffer layer for hybrid halide perovskite photovoltaic-thermoelectric device. Locations of the conduction band and the valence band, spontaneous polarization performance, and optical properties were investigated. Results indicate the energy band of CaMnO3 can match up well with that of CH3NH3PbI3 on separating electron-hole pairs. In addition, the consistent polarization angle helps enlarge the open circuit voltage of the composite system. Besides, CaMnO3 film shows large absorption coefficient and low extinction coefficient under visible irradiation, demonstrating high carrier concentration, which is beneficial to the current density. More importantly, benign thermoelectric properties enable CaMnO3 film to assimilate phonon vibration from CH3NH3PbI3. All the above features lead to a bright future of CaMnO3 film, which can be a promising candidate as a buffer layer for hybrid halide perovskite photovoltaic-thermoelectric systems.

Controlled nanostructuring of TiO2 nanoparticles: a sol–gel approach

The ever increasing interest in titanium oxide (titania) is motivated by its applications in solar cells, biomaterials and photo-catalytic activities. Nanocrystalline titania is preferred in these applications due to chemical stability, mechanical hardness, high refractive index and excellent transmission in the visible region. Titania exists in three different crystallographic phases i.e. anatase, rutile and brookite, amongst which brookite is the most difficult to synthesize. Anatase and rutile crystallize in tetragonal phase whereas brookite has orthorhombic phase. In the present work, titania nanoparticles are synthesized following sol–gel approach. TiCl4 is used as precursor and ammonia as a gelation agent. pH of the sol is varied in the range of 1–11. Nanostructures and hollow core titania nanoparticles with mean diameter of 120 and 70 nm respectively have been synthesized without the use of any hard/soft template. At pH 1 the nanoparticles show amorphous behavior whereas increasing the pH induces crystallinity in nanoparticles. The presence of (020), (202) and (321) confirms the formation of pure brookite phase at a low temperature of 60 °C. The presence of absorption bands in fourier transform infrared spectroscopy in the range of 450–700 cm−1 correspond to infrared active mode of Ti–O–Ti stretching indicating the formation of titania. Detailed Spectroscopic analyses indicate that these nanoparticles are highly transmitting in the visible and infrared region with band gap in the range of 2.96–3.03 eV. Cauchy Model used for fitting the experimental spectroscopic data gives a high value of refractive index with low extinction coefficient.

Properties of humic substances isolated from different natural sources

The purpose of study was to determine properties of humic substances by combination of modern and traditional methods. Humic substances isolated from three different sources from Mongolia such as peloid from Lake Gurban nuur, coal from Baganuur deposit and oil shale from Shine khudag deposit. On the basis of determination H/C and O/C atomic ratios in humic substances by elemental analysis, confirmed existing of aromatic structures in the molecules and oxidized functional groups. Have been studied the structure of humic substances by spectral method. For example infrared spectrums showed that humic substances are characterizing with poly-structural components, with different quantity in the samples. Light adsorption of samples in the UV-Vis region, a decrease on the absorption intensity with an increase of the wave length was observed (Fig. 2). The high ratio Н/С, attributed to stretching of C=C bond of aromatic rings in IR spectrums, the high content of functional groups, lower extinction coefficients, confirmed that aromatic fragments to prevail than aliphatic chain fragments in structure of all studied HS. DOI: http://dx.doi.org/10.5564/mjc.v14i0.199 Mongolian Journal of Chemistry 14 (40), 2013, p51-56

Composite photothermal platform of polypyrrole-enveloped Fe₃O₄ nanoparticle self-assembled superstructures.

Photothermal nanoplatforms with small size, low cost, multifunctionality, good biocompatibility and in particular biodegradability are greatly desired in the exploration of novel diagnostic and therapeutic methodologies. Despite Fe3O4 nanoparticles (NPs) have been approved as safe clinical agents, the low molar extinction coefficient and subsequent poor photothermal performance shed the doubt as effective photothermal materials. In this paper, we demonstrate the fabrication of polypyrrole (PPy)-enveloped Fe3O4 NP superstructures with a spherical morphology, which leads to a 300-fold increase in the molar extinction coefficient. The basic idea is the optimization of Fe3O4 electronic structures. By controlling the self-assembly of Fe3O4 NPs, the diameters of the superstructures are tuned from 32 to 64 nm. This significantly enhances the indirect transition and magnetic coupling of Fe ions, thus increasing the molar extinction coefficient of Fe3O4 NPs from 3.65 × 10(6) to 1.31 × 10(8) M(-1) cm(-1) at 808 nm. The envelopment of Fe3O4 superstructures with conductive PPy shell introduces additional electrons in the Fe3O4 oscillation system, and therewith further enhances the molar extinction coefficient to 1.12 × 10(9) M(-1) cm(-1). As a result, the photothermal performance is greatly improved. Primary cell experiments indicate that PPy-enveloped Fe3O4 NP superstructures are low toxic, and capable to kill Hela cells under near-infrared laser irradiation. Owing to the low cost, good biocompatibility and biodegradability, the PPy-enveloped Fe3O4 NP superstructures are promising photothermal platform for establishing novel diagnostic and therapeutic methods.

Excellent c-Si surface passivation by low-temperature atomic layer deposited titanium oxide

In this work, we demonstrate that thermal atomic layer deposited (ALD) titanium oxide (TiOx) films are able to provide a—up to now unprecedented—level of surface passivation on undiffused low-resistivity crystalline silicon (c-Si). The surface passivation provided by the ALD TiOx films is activated by a post-deposition anneal and subsequent light soaking treatment. Ultralow effective surface recombination velocities down to 2.8 cm/s and 8.3 cm/s, respectively, are achieved on n-type and p-type float-zone c-Si wafers. Detailed analysis confirms that the TiOx films are nearly stoichiometric, have no significant level of contaminants, and are of amorphous nature. The passivation is found to be stable after storage in the dark for eight months. These results demonstrate that TiOx films are also capable of providing excellent passivation of undiffused c-Si surfaces on a comparable level to thermal silicon oxide, silicon nitride, and aluminum oxide. In addition, it is well known that TiOx has an optimal refractive index of 2.4 in the visible range for glass encapsulated solar cells, as well as a low extinction coefficient. Thus, the results presented in this work could facilitate the re-emergence of TiOx in the field of high-efficiency silicon wafer solar cells.

Thermal properties of TiO2 films fabricated by atomic layer deposition

Thin, amorphous, high index, dense, low scattering & absorption (low extinction coefficient) and optical grade TiO2 films of various thicknesses are prepared by Atomic layer deposition (ALD) technique and investigated in terms of thermo-optic coefficient (dn/dT) and temperature dependent density (dρ/dT). The dρ/dT and dρ/dT are calculated by modeling ellipsometric experimental data by developing appropriate optical model such as Cauchy Model. The modeled data was further modeled with Lorentz-Lorenz relation under least-square approach. The dn/dT of TiO2 films shows negative values for thin and positive values for relatively thicker films and reveals no significant changes in dn/dT and dρ/dT when film thickness increases more than ~ 200 nm. The coefficient values are calculated for a wavelength range of 380–1800 nm. The average values of (dn/dT)60nm= −4.2 ± 0.7 × 10−5°C−1 and (dρ/dT)60nm= −6.6 ± 1.1 × 10−5gcm−3 °C−1 at wavelength of 640 nm. The reported coefficients values are measured and calculated for TiO2 film of thickness t = 60 nm and described in detail.

Free-carrier absorption and Burstein–Moss shift effect on quantum efficiency in heterojunction silicon solar cells

Effects of the carrier concentration on the dielectric function of indium tin oxide (ITO) films were investigated by spectroscopic ellipsometry using Tauc–Lorentz and Drude oscillator terms. The real and imaginary parts of dielectric function values increased with increasing carrier concentration. Samples with lower refractive indices had (440) preferable crystallographic orientations. A more significant Burstein–Moss (B–M) effect leads to a lower extinction coefficient (k) value in the short-wavelength regions, while more Free-carrier absorption (FCA) leads to a higher k value in the near-infrared region. The electrical properties obtained from optical studies, such as the carrier concentration and the carrier mobility, were compared with those obtained by van der Pauw measurements. The observed discrepancies between optically and electrically obtained values were due to carrier transport and grain boundaries. ITO films with a widened band gap energy due to the B–M shift are key to improve quantum efficiency (QE) at short-wavelengths in heterojunction silicon (HJ) solar cells. The reduced QE at long-wavelengths is attributed to optical scattering in the ITO films due to FCA. Despite widening the band gap, the short-circuit current density (Jsc) decreased from 36.27 mA/cm2 to 34.17 mA/cm2 with increasing carrier concentration from 6.15 × 1020 cm−3 to 9.84 × 1020 cm−3. This result demonstrates that the FCA effect may affect Jsc more in HJ solar cells. The ITO film, prepared at the lowest carrier concentration, was used as an antireflection layer to fabricate HJ solar cells via optimization of the passivated layer. It resulted in a cell efficiency of 19.12% and an open-circuit voltage of 702 mV.

Unlocking the potential of voltage control for high rate zirconium and hafnium oxide deposition by reactive magnetron sputtering

Target voltage (VT) feedback-based closed-loop gas flow control is one of the most cost-effective and well-known methods to regulate reactive oxide thin film sputtering processes. However, it can be difficult to perform due to target voltage non-linearity, changes in signal direction and drifts in the case of Zr and Hf sputtering in an Ar/O2 ambient. We show in this paper that the use of the ratio of VT over the O* (oxygen plasma emission, or alternatively oxygen partial pressure) as a feedback signal in a closed-loop control, enables stable operation anywhere in the ‘transition’ region (the region between ‘metal’ and ‘fully poisoned’ sputtering target states) as well as extending into the ‘forbidden’ parts of it. The result of using VT over the O* ratio as the main feedback signal is a stable process exhibiting constant sputter target state and partial pressure of oxygen as well as high deposition rate (e.g. 6.4 Å/s for ZrO2 and 9.2 Å/s for HfO2 at DT–S = 20 cm by pulse-DC sputtering). Such a process is able to deliver ZrO2 and HfO2 coatings that exhibit a low extinction coefficient in the near UV (0.0002 for HfO2 and 0.001 for ZrO2 at 350 nm) and a high refractive index (2.05 for HfO2 and 2.16 for ZrO2 at 1000 nm), without any ion/plasma assistance, at low substrate temperature (20–50 °C) and no nitrogen addition.

Fabrication of submicron-scale rectangular bar of transparent In–Ga–Zn–O: A study of the possible application of transparent In–Ga–Zn–O optical waveguide

We studied the optical properties of In–Ga–Zn–O (IGZO) films and found a very low extinction coefficient of the films. For the potential application of the films, we propose an optical waveguide device made of IGZO. We have succeeded in producing a submicron-scale rectangular-bar structure of IGZO using our newly developed dry etching process. Simulation results showed an ∼5 dB/cm propagation loss of a 400 × 400 nm2 square optical waveguide device of amorphous IGZO at a wavelength of 1.55 µm, when a standard deviation of ∼4 nm and a correlation length of ∼100 nm of sidewall roughness were achieved.

Judicious Design of Indoline Chromophores for High-Efficiency Iodine-Free Dye-Sensitized Solar Cells

Indoline photosensitizers exhibit impressive short-circuit photocurrent but generally low molar extinction coefficient and rapid charge recombination, which limits their application in thin-film dye-sensitizerd solar cells (DSCs). Here, we incorporate a new dithieno[3,2-b:2',3'-d]pyrrole (DTP) segment (i.e., dihexyloxy-triphenylamine (DHO-TPA) substituted DTP) as the conjugated π-linker to construct a series of high molar absorption coefficient indoline dyes (XW69, XW70, and XW71) for DSCs employing a cobalt(II/III) redox electrolyte. Interestingly, this DTP linker is demonstrated as an efficient building block, not only slowing down the kinetics of charge recombination of titania electrons with tris(1,10-phenanthroline)cobalt(III) ions but also making a great contribution to the light absorption properties in comparison with the dihexylaniline substituted DTP. With respect to the dihexyloxy-triphenylamine dye (XW68), these new indoline dyes exhibit stronger light-harvesting and thus better power conversion efficiency of DSCs made from thin titania films. Benefitting from the bulky rigidity of the donor and π-conjugation unit, the XW70 dye displays a promising conversion efficiency as high as 8.78%, with a short-circuit current density (J(SC)) of 13.3 mA cm(-2), open-circuit voltage (V(OC)) of 943 mV, and fill factor (FF) of 0.70 under AM 1.5 illumination (100 mW cm(-2)). Furthermore, the effect of light irradiation on these dyes adsorbed on nanocrystalline TiO2 films was investigated, proving the photostability of these indoline chromophores. Our work has valued the feasibility of judicious design of indoline chromophores to obtain organic photosensitizers for high-efficiency iodine-free DSCs made from thin titania films.