Solution-processed Metal Oxide Research Articles

Role of ultrathin Ti3C2Tx MXene layer for developing solution-processed high-performance low voltage metal oxide transistors

Metal oxide transistors have garnered substantial attention for their potential in low-power electronics, yet challenges remain in achieving both high performance and low operating voltages through solution-based fabrication methods. Optimizing interfacial engineering at the dielectric/semiconductor interface is of utmost importance in the fabrication of high-performance thin film transistors (TFTs). In the present article, a bilayer Ti3C2Tx-MXene/SnO2–semiconductor (Tx stands for surface termination) configuration is used to fabricate a high-performance n-type thin film transistor by using an ion-conducting Li-Al2O3 gate dielectric on a p+-Si substrate, where electrical charges are formed and modulated at the Li-Al2O3/SnO2 interface, and Ti3C2Tx-MXene nanosheets serve as the primary electrical charge channel due to their long lateral size and high mobility. A comparative characterization of two distinct TFTs is conducted, one featuring Ti3C2Tx MXene and SnO2 semiconductor layer and the other with SnO2 only. Notably, the TFT with the Ti3C2Tx MXene layer has shown a significant boost in the carrier mobility (10.6 cm2/V s), leading to remarkable improvements in the on/off ratio (1.3 × 105) and subthreshold swing (194 mV/decade), whereas the SnO2 TFT without the Ti3C2Tx MXene layer shows a mobility of 1.17 cm2/V s with 8.1 × 102 on/off ratio and 387 mV/decade subthreshold swing. This investigation provides a possible way toward the development of high-performance, low-voltage TFT fabrication with the MXene/semiconductor combination.

Solution-Processed Metal Oxide Thin-Film Transistor at Low Temperature via A Combination Strategy of H2 O2 -Inducement Technique and Infrared Irradiation Annealing.

Solution processing has emerged as a promising technique for the fabrication of oxide thin-film transistors (TFTs), offering advantages such as low cost, high throughput, and exceptional compositional control. However, achieving reasonable electrical properties typically demands high annealing temperatures in the fabrication process. In addressing this challenge, a novel combination strategy is proposed that involves integrating the H2 O2 inducement technique with infrared (IR) irradiation annealing. The study investigates the effects of precursors and IR irradiation annealing temperatures on the electrical properties of In2 O3 TFTs. It is found that H2 O2 can help accelerate the decomposition of organic residues, while IR irradiation annealing could enhance the film densification. By employing the proposed strategy, metal oxide TFTs consisting of a Zr-Al-O dielectric fabricated at 230°C and an In2 O3 channel layer fabricated at 185°C demonstrated high performance with field-effect mobility = 31.7 cm2 V-1 ·s-1 , threshold voltage = 1.3 V, subthreshold swing = 0.13 V per decade, and on-to-off current ratio = 1.1 × 105 . This work demonstrates the proposed combinational strategy is a general method to fabricate not only metal oxide semiconductorsbut also dielectrics.

Record high external quantum efficiency of 20% achieved in fully solution-processed quantum dot light-emitting diodes based on hole-conductive metal oxides

Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) has been widely used as a hole injection material in quantum dot (QD) light-emitting diodes (QLEDs). However, it degrades the organic materials and electrodes in QLEDs due to its strong hydroscopicity and acidity. Although hole-conductive metal oxides have a great potential to solve this disadvantage, it is still a challenge to achieve efficient and stable QLEDs by using these solution-processed metal oxides. Herein, the state-of-the-art QLEDs fabricated by using hole-conductive MoOx QDs are achieved. The α-phase MoOx QDs exhibit a monodispersed size distribution with clear and regular crystal lattices, corresponding to high-quality nanocrystals. Meanwhile, the MoOx film owns an excellent transmittance, suitable valence band, good morphology and impressive hole-conductivity, demonstrating that the MoOx film could be used as a hole injection layer in QLEDs. Moreover, the rigid and flexible red QLEDs made by MoOx exhibit peak external quantum efficiencies of over 20%, representing a new record for the hole-conductive metal oxide based QLEDs. Most importantly, the MoOx QDs afford their QLEDs with a longer T95 lifetime than these devices made by PEDOT:PSS. As a result, we believe that the MoOx QDs could be used as efficient and stable hole injection materials used in QLEDs.

Bilayer channel structure to improve the stability of solution-processed metal oxide transistors under AC stress

In this work, we report solution-processed bilayer channel thin film transistors (TFTs) based on solution-processed amorphous oxide semiconductors (AOSs) that fulfill both superb electrical performance and device stability against applied alternating current (AC) drain bias stress. Acceptor-like states generation via hot carrier effect (HCE) is a representative device degradation mechanism suggested for AOS-based TFTs when subjected to AC bias stress, featuring threshold voltage (Vth) shift and on-current (Ion) lowering. As excess electric field and accumulated carriers in the AOS channel cause HCE, alleviating carrier accumulation is a key solution for mitigating HCE. Bilayer channel TFTs comprising two different AOSs could work as a countermeasure to overcome HCE since proper energy band alignment of two channels creates a conduction band difference that act as the energy barrier for carriers. In this regard, we employed amorphous indium-gallium-zinc oxide (IGZO) and zinc-tin oxide (ZTO) as bottom and top layer, respectively, for bilayer channel TFT. Designed bilayer channel showed a conduction band difference of 0.18 eV, and fabricated TFT based on this architecture exhibited high mobility over to 5.9 cm2/Vs with slight Vth shift of 0.1 V and Ion lowering of 1.7% for 1000 s against applied AC stress, demonstrating device stability under AC drain bias stress.

Solution-processed high entropy metal oxides as dielectric layers with high transmittance and performance and application in thin film transistors

High entropy metal oxides (HEMOs) are emerging materials with unique structures and functional properties, but it's barely researched as dielectric materials. This paper investigates solution-processed HEMOs as the dielectric material. Six HEMOs systems were compared and the role of various metal oxides in the high entropy system is revealed. It turns out that the (HfMgTiYZr)Ox system has the most excellent dielectric properties with a low leakage current density of 2.9 × 10−7A/cm2@2MV/cm and a high dielectric constant of 13.0. Moreover, the transmittance of the films is over 88% in the range of the visible light. The high performances of the HEMOs derive from the complex internal structure and the single solid solution phase. Applying (HfMgTiYZr)Ox as dielectric layers, the thin film transistors attain an Ion/off of 6.3 × 108, a mobility of 14.2cm2/vs, a ss of 0.34 V, and a Vth of 1.3 V. In this paper, we demonstrate that HEMOs have good dielectric properties and can be used in thin film transistors which implies a promising future in the application of electronic devices.

Solution-processed pristine metal oxides as electron-transporting materials for perovskite solar cells

In recent years, perovskite material-based photovoltaic devices have attracted great attention of researchers because of an expeditious improvement in their efficiency from 3.8% to over 25%. The electron transport layer (ETL), which functions for the extraction and transportation of photogenerated electrons from active perovskite material to the electrodes, is a vital part of these perovskite solar cells (PSCs). The optoelectronic properties of these electron transport layer materials also have an impact on the performance of these perovskite solar cells, and for commercialized flexible perovskite solar cells, low-temperature and solution-processable electron transport layers having high stability and suitable optoelectronic properties are needed. In this regard, the solution-processable films of different metal oxides have been largely investigated by many research groups. So, this review summarizes the optoelectronic properties of the different metal oxide-based electron transport layers and the development in the performance of the perovskite solar cells, which have solution-processable metal oxides as electron transport layers.

The impact of nickel doping on metal-oxide network in solution-processed indium zinc oxide transistors

Solution-processed nickel zinc oxide (IZO) thin-film transistors (TFTs) are of great interest as core elements for future display technology based on the low-cost manufacture. It still remains a challenge to ensure stability in the off-current state of IZO using a dopant that is more resource-friendly than Ga in solution processing. In this study, we suggest a solution-based doping process of the IZO semiconductors with a Ni carrier suppressor and investigated the effects of Ni on their transistor operation. Owing to similar size of Ni2+ to Zn2+, an appropriate concentration of Ni partially substituted Zn sites and the strong bonding force of Ni with O reduces hydroxide groups on the surface bonded to Zn, whereas excess Ni addition rather inhibited the formation of smooth thin film. The resulting Ni-doped IZO TFT devices from an optimized Ni concentration exhibited higher performances with significantly reduced off-state currents while maintaining high on-state currents, compared to IZO TFTs. This approach enriches the current family of solution-processed metal oxide semiconductors by providing another option for resource-friendly carrier suppressor, which can contribute to the inexpensive and simple solution process for fabricating TFTs.

High Performing Inverted Flexible Perovskite Solar Cells via Solution Phase Deposition of Yttrium-Doped SnO2 Directly on Perovskite

Solution processing of flexible perovskite solar cells (f-PSCs) provides an avenue for scalable, high-throughput printing of lightweight, scalable, and cost-effective flexible solar cells. However, the deposition of fully solution-processed metal oxide charge transport layers on perovskites has been limited by solvent incompatibilities and high processing temperatures for metal oxide nanoparticles. In this study, we present high-performance, inverted f-PSCs from the direct deposition of yttrium-doped SnO2 nanoparticles functionalized with acetate on top of perovskite as an ink in anhydrous ethanol via blade coating. Yttrium doping improved device performance by improving the charge extraction with a decreased series resistance leading to improvements in the open-circuit voltage and fill factor. The champion power conversion efficiency for 0.1 cm2 devices increased from 14.3% for undoped SnO2 to 16.5% with 2% Y:SnO2 doping, which is unprecedented for f-PSCs on ITO-PET substrates employing SnO2 as an ETL.

Aqueous electrolyte-gated solution-processed metal oxide transistors for direct cellular interfaces.

Biocompatible field-effect-transistor-based biosensors have drawn attention for the development of next-generation human-friendly electronics. High-performance electronic devices must achieve low-voltage operation, long-term operational stability, and biocompatibility. Herein, we propose an electrolyte-gated thin-film transistor made of large-area solution-processed indium-gallium-zinc oxide (IGZO) semiconductors capable of directly interacting with live cells at physiological conditions. The fabricated transistors exhibit good electrical performance operating under sub-0.5 V conditions with high on-/off-current ratios (>107) and transconductance (>1.0 mS) over an extended operational lifetime. Furthermore, we verified the biocompatibility of the IGZO surface to various types of mammalian cells in terms of cell viability, proliferation, morphology, and drug responsiveness. Finally, the prolonged stable operation of electrolyte-gated transistor devices directly integrated with live cells provides the proof-of-concept for solution-processed metal oxide material-based direct cellular interfaces.

High-k and high-temperature-resistant polysilsesquioxane: Potential for solution-processed metal oxide semiconductor transistors operating at low voltage

High-k and high-temperature-resistant polysilsesquioxane: Potential for solution-processed metal oxide semiconductor transistors operating at low voltage

Detection of acetone vapours using solution-processed tin oxide thin-film transistors

Abnormal concentrations of volatile organic compounds (VOCs) in human breathe can be used as disease-specific biomarkers for the non-invasive diagnosis of medical conditions, such as acetone for diabetes. Solution-processed bottom gate top contact metal oxide thin-film transistors (TFTs) are used to detect acetone vapours, as part of a proof-of-concept study. The effect of increasing annealing temperature (T) and channel length (L) on electrical and sensing performance are explored. Drain current (Ids) increases following exposure as acetone undergoes a redox reaction with the adsorbed oxygen species on the semiconductor surface, which results in free electrons being released back into the conduction band. Responsivity (R) is maximized at negative bias (Vgs < 0). For L = 50 μm, the peak R of the TFT annealed at 450 °C is three times greater than that of the TFT annealed at 350 °C, with Vgs = − 37.5 V and − 33 V, respectively.Graphical abstract

High-performance and low-power source-gated transistors enabled by a solution-processed metal oxide homojunction

Cost-effective fabrication of mechanically flexible low-power electronics is important for emerging applications including wearable electronics, artificial intelligence, and the Internet of Things. Here, solution-processed source-gated transistors (SGTs) with an unprecedented intrinsic gain of ~2,000, low saturation voltage of +0.8 ± 0.1 V, and a ~25.6 μW power consumption are realized using an indium oxide In2O3/In2O3:polyethylenimine (PEI) blend homojunction with Au contacts on Si/SiO2. Kelvin probe force microscopy confirms source-controlled operation of the SGT and reveals that PEI doping leads to more effective depletion of the reverse-biased Schottky contact source region. Furthermore, using a fluoride-doped AlOx gate dielectric, rigid (on a Si substrate) and flexible (on a polyimide substrate) SGTs were fabricated. These devices exhibit a low driving voltage of +2 V and power consumption of ~11.5 μW, yielding inverters with an outstanding voltage gain of >5,000. Furthermore, electrooculographic (EOG) signal monitoring can now be demonstrated using an SGT inverter, where a ~1.0 mV EOG signal is amplified to over 300 mV, indicating significant potential for applications in wearable medical sensing and human-computer interfacing.

In situphotogenerated hydroxyl radicals in the reaction atmosphere for the accelerated crystallization of solution-processed functional metal oxide thin films

We propose a disruptive method to accelerate the crystallization at low temperatures of functional metal oxide films whereby hydroxyl radicals (•OH) are photogeneratedin situfrom the atmosphere where solution-deposited layers are UV-irradiated.

Efficient and stable organic solar cells based on all-solution-processed metal oxide transport layers

Doping of solution-processed molybdenum oxide hole transport layers enabled the efficient and stable organic solar cells.

Readily Accessible Metallic Micro-Island Arrays for High-Performance Metal Oxide Thin-Film Transistors.

Thin-film transistors using metal oxide semiconductors are essential in many unconventional electronic devices. Nevertheless, further advances will be necessary to broaden their technological appeal. Here, a new strategy is reported to achieve high-performance solution-processed metal oxide thin-film transistors (MOTFTs) by introducing a metallic micro-island array (M-MIA) on top of the MO back channel, where the MO is a-IGZO (amorphous indium-gallium-zinc-oxide). Here Al-MIAs are fabricated using honeycomb cinnamate cellulose films, created by a scalable breath-figure method, as a shadow mask. For IGZO TFTs, the electron mobility (µe ) increases from ≈3.6 cm2 V-1 s-1 to near 15.6 cm2 V-1 s-1 for optimal Al-MIA dimension/coverage of 1.25 µm/51%. The Al-MIA IGZO TFT performance is superior to that of controls using compact/planar Al layers (Al-PL TFTs) and Au-MIAs with the same channel coverage. Kelvin probe force microscopy and technology computer-aided design simulations reveal that charge transfer occurs between the Al and the IGZO channel which is optimized for specific Al-MIA dimensions/surface channel coverages. Furthermore, such Al-MIA IGZO TFTs with a high-k fluoride-doped alumina dielectric exhibit a maximum µe of >50.2 cm2 V-1 s-1 . This is the first demonstration of a micro-structured MO semiconductor heterojunction with submicrometer resolution metallic arrays for enhanced transistor performance and broad applicability to other devices.

Performance Enhancement of Solution-Processed SixSnyO TFTs using Solution Combustion Synthesis

Amorphous oxide semiconductors thin-film transistors (AOs TFTs) have gained interest over the years because of their advantages over a-Si and poly-Si transistors.In particular, tin oxide is used as a semiconductor material because of its abundance, low toxicity, and high intrinsic mobility. Performance-wise, tin oxide TFTs need improvement because the reported average mobility remains < 10 cm2/Vs.1 Previous research used silicon doping to reduce tin oxide’s oxygen vacancies and improve its performance as a semiconductor channel.Currently, SixSnyO TFTs are fabricated using vacuum process, but solution processing is important to realize large-scale and high-throughput electronics. Sol-gel method is one of the conventional routes to produce solution-processed metal oxides. However, it requires high temperature (≤500°C) to facilitate film densification which is not compatible with flexible substrates having low degradation temperatures. Among the approaches for low-temperature TFT fabrication, solution combustion synthesis (SCS) has garnered significant attention because of its simplicity and versatility.2 SCS uses the energy released by combustion to help the formation of more M-O-M networks in the film. Thus, there is no need to use high temperature to condense and densify the film. This study attempts to produce SixSnyO TFTs through solution processing for the first time. We investigated the effect of solution combustion synthesis to the quality of SixSnyO films. Next, we optimized the aging time and oxidizer to fuel ratio (ammonium nitrate to urea) to determine the films with better quality. For aging time optimization, SixSnyO precursors were stirred at RT for 24-h, 48-h, and 72-h. Lastly, we compared the TFT performance in terms transfer characteristics of SixSnyO TFTs fabricated by sol-gel and SCS annealed at the same temperature. Secondary ion mass spectrometry (SIMS) analysis was used to compare the carbon impurities present in both SixSnyO films. The sol-gel film contained higher amounts of carbon impurities, about an order of magnitude higher as shown in Figure 1. This proves that SCS provides extra energy to help degrade the carbon-containing compounds in the film. Carbon atoms are considered impurities that can introduce defects in the SixSnyO film. These defects are detrimental to the performance characteristics of the fabricated TFT device. In addition, the thickness of the films was determined using X-ray reflectivity. The SCS film is 15.70 nm-thick while the sol-gel film is 18.88-nm thick. Furthermore, optimization of the SixSnyO films via SCS revealed that oxidizer to fuel ratio affects the roughness and amount of M-O bonds formed in the film. The 3:1 ratio failed to produce a film while other ratios were successful in producing films. Although the film with 1:1 ratio has root mean square roughness (Rq) value of 0.380 nm, it contains depressions and voids. On the other hand, the film using a 1:3 ratio showed a uniform and even surface with Rq value of 0.377 nm. In addition, characterization of the two films through X-ray photoelectron spectroscopy (XPS) showed higher percentage of M-O bonds in the O1s spectrum (530.5 eV) for the film with 1:3 ratio. The values are 45.9% and 59.3% for 1:1 and 1:3 ratios, respectively. The amount of oxygen vacancy and H- containing compounds also decreased in the 1:3 film.SixSnyO TFTs were fabricated from both SCS and sol-gel methods. Both TFTs showed high off-current and highly negative Vth, as shown in Figure 1. Therefore, both TFTs still contain several defects which suggests that 3% Si doping may be insufficient and must be increased to produce a significant effect. The sol-gel fabricated TFT also showed lower drain current. Therefore, the low annealing temperature at 300 ˚C is insufficient to densify the film and reduce impurities. On the other hand, SCS-assisted fabrication showed higher mobility at 3.14 cm2/Vs – confirming that SCS provided additional energy to degrade impurities and improve the M-O network in the SixSnyO channel. In conclusion, we were able to produce SixSnyO TFTs with improved mobility using solution combustion synthesis. However, the high V th and high off-current issues need to be addressed by increasing silicon dopant addition and exploring other fuel options. But overall, we can confirm that SCS is beneficial in improving the quality of SixSnyO films annealed at 300 ˚C. In the future, this low temperature fabrication is important to realize flexible SixSnyO TFT devices.AcknowledgementsThis research was supported by NAIST foundation grant.References Chang, H., et al. ACS Applied Electronic Materials, 3(11), 4943–4949.Carlos, E., et al. Chemistry - A European Journal, 26(42), 9099–9125. Figure 1

Core–shell carbon-polymer quantum dot passivation for near infrared perovskite light emitting diodes

High-performance perovskite light-emitting diodes (PeLEDs) require a high quality perovskite emitter and appropriate charge transport layers to facilitate charge injection and transport within the device. Solution-processed n-type metal oxides represent a judicious choice for the electron transport layer (ETL); however, they do not always present surface properties and energetics compatible with the perovskite emitter. Moreover, the emitter itself exhibits poor nanomorphology and defect traps that compromise the device performance. Here, we modulate the surface properties and interface energetics between the tin oxide (SnO2) ETL with the perovskite emitter by using an amino functionalized difluoro{2-[1-(3,5-dimethyl-2H-pyrrol-2-ylidene-N)ethyl]-3,5-dimethyl-1H-pyrrolato-N}boron compound and passivate the defects present in the perovskite matrix with carbon-polymer core–shell quantum dots inserted into the perovskite precursor. Both these approaches synergistically improve the perovskite layer nanomorphology and enhance the radiative recombination. These properties resulted in the fabrication of near-infrared PeLEDs based on formamidinium lead iodide (FAPbI3) with a high radiance of 92 W sr−1 m−2, an external quantum efficiency (EQE) of 14%, reduced efficiency roll-off and prolonged lifetime. In particular, the modified device retained 80% of the initial EQE (T80) for 33 h compared to 6 h of the reference cell.

Solution-Processed Silicon Doped Tin Oxide Thin Films and Thin-Film Transistors Based on Tetraethyl Orthosilicate

Recently, tin oxide (SnO2) has been the preferred thin film material for semiconductor devices such as thin-film transistors (TFTs) due to its low cost, non-toxicity, and superior electrical performance. However, the high oxygen vacancy (VO) concentration leads to poor performance of SnO2 thin films and devices. In this paper, with tetraethyl orthosilicate (TEOS) as the Si source, which can decompose to release heat and supply energy when annealing, Si doped SnO2 (STO) films and inverted staggered STO TFTs were successfully fabricated by a solution method. An XPS analysis showed that Si doping can effectively inhibit the formation of VO, thus reducing the carrier concentration and improving the quality of SnO2 films. In addition, the heat released from TEOS can modestly lower the preparation temperature of STO films. By optimizing the annealing temperature and Si doping content, 350 °C annealed STO TFTs with 5 at.% Si exhibited the best device performance: Ioff was as low as 10−10 A, Ion/Ioff reached a magnitude of 104, and Von was 1.51 V. Utilizing TEOS as an Si source has a certain reference significance for solution-processed metal oxide thin films in the future.

Water-Processed Ultrathin Crystalline Indium-Boron-Oxide Channel for High-Performance Thin-Film Transistor Applications.

Thin-film transistors (TFTs) made of solution-processable transparent metal oxide semiconductors show great potential for use in emerging large-scale optoelectronics. However, current solution-processed metal oxide TFTs still suffer from relatively poor device performance, hindering their further advancement. In this work, we create a novel ultrathin crystalline indium–boron–oxide (In-B-O) channel layer for high-performance TFTs. We show that high-quality ultrathin (~10 nm) crystalline In-B-O with an atomically smooth nature (RMS: ~0.15 nm) could be grown from an aqueous solution via facile one-step spin-coating. The impacts of B doping on the physical, chemical and electrical properties of the In2O3 film are systematically investigated. The results show that B has large metal–oxide bond dissociation energy and high Lewis acid strength, which can suppress oxygen vacancy-/hydroxyl-related defects and alleviate dopant-induced carrier scattering, resulting in electrical performance improvement. The optimized In-B-O (10% B) TFTs based on SiO2/Si substrate demonstrate a mobility of ~8 cm2/(V s), an on/off current ratio of ~106 and a subthreshold swing of 0.86 V/dec. Furthermore, by introducing the water-processed high-K ZrO2 dielectric, the fully aqueous solution-grown In-B-O/ZrO2 TFTs exhibit excellent device performance, with a mobility of ~11 cm2/(V s), an on/off current of ~105, a subthreshold swing of 0.19 V/dec, a low operating voltage of 5 V and superior bias stress stability. Our research opens up new avenues for low-cost, large-area green oxide electronic devices with superior performance.

Optimization of Dual-Fuel Combustion Synthesis for Rapid Formation of Solution-Processed Metal-Oxide Thin-Film Transistors

Solution processing of metal-oxide semiconductors has received significant attention in various fields of electronics owing to its advantages such as simple fabrication process, large-area scalability, and facile stoichiometric tunability. However, the conventional sol–gel route requires a relatively long annealing time to obtain a low-defect film with high density and sufficient amount of metal–oxygen–metal bonding state, which prevents implementation in cost-effective continuous manufacturing. Here, we report rapid formation of solution-processed oxide semiconductors by employing a dual-fuel-based solution combustion synthesis route. In particular, by optimizing the ratio of dual fuels of acetylacetone and 1,1,1-trifluoro-acetylacetone (molar ratio of 7:3), high-performance indium–gallium–zinc oxide (IGZO) thin-film transistors (TFTs) could be fabricated at 350 °C with the annealing time as short as 5 min (In:Ga:Zn = 0.68:0.1:0.22). Based on spectroscopic analysis, it was found that the dual fuels enabled rapid formation of the metal–oxygen–metal lattice structure with low defective oxygen bonding states. The IGZO TFTs fabricated with an optimized fuel ratio exhibited average field-effect mobilities of 1.11 and 3.69 cm2 V–1 s–1 with annealing times of 5 and 20 min, respectively (averaged in 9∼12 devices). Also, in the case of the 5 min annealed device, the threshold voltage was −0.48 ± 1.96 V, showing enhancement-mode operation. Furthermore, the device showed good stability against both positive gate bias stress and negative gate bias stress conditions with small threshold voltage shifts of −1.28 and – 1.28 V in 5760 s, respectively.