Trifluoromethane Sulfonamide Research Articles

Three‐Dimensional (3D) Fluoride Molecular Glue to Improve the SnO2/Perovskite Interface for Efficient Perovskite Solar Cells

AbstractAiming at numerous defects at SnO2/perovskite interface and lattice mismatch in perovskite solar cells (PSCs), we design a kind of three‐dimensional (3D) molecular glue (KBF4‐TFMSA), which is derived from strong intramolecular hydrogen bonding interaction between potassium tetrafluoroborate (KBF4) and trifluoromethane‐sulfonamide (TFMSA). A remarkable efficiency of 25.8 % with negligible hysteresis and a stabilized power output of 25.0 % have been achieved, in addition, 24.57 % certified efficiency of 1 cm2 device is also obtained. Further investigation reveals that this KBF4‐TFMSA can interact with oxygen vacancies and under‐coordinated Sn(IV) from the SnO2, in the meantime, FA+ (NH2−C=NH2+) and K+ cations can be well fixed by hydrogen bonding interaction between FA+ and BF4−, and electrostatic attraction between sulfonyl oxygen and K+ ions, respectively. Thereby, FAPbI3 crystal grain sizes are increased, interfacial defects are significantly reduced while carrier extraction/ transportation is facilitated, leading to better cell performance and excellent stabilities. Non‐encapsulated devices can maintain 91 % of their initial efficiency under maximum‐power‐point (MPP) tracking while continuous illumination (~100 mW cm−2) for 1000 h, and retain 91 % of the initial efficiency after 1000 h “double 60” damp‐heat stability testing (60 °C and 60 %RH (RH, relatively humidity)).

Three-Dimensional (3D) Fluoride Molecular Glue to Improve the SnO2/Perovskite Interface for Efficient Perovskite Solar Cells.

Aiming at numerous defects at SnO2/perovskite interface and lattice mismatch in perovskite solar cells (PSCs), we design a kind of three-dimensional (3D) molecular glue (KBF4-TFMSA), which is derived from strong intramolecular hydrogen bonding interaction between potassium tetrafluoroborate (KBF4) and trifluoromethane-sulfonamide (TFMSA). A remarkable efficiency of 25.8 % with negligible hysteresis and a stabilized power output of 25.0 % have been achieved, in addition, 24.57 % certified efficiency of 1 cm2 device is also obtained. Further investigation reveals that this KBF4-TFMSA can interact with oxygen vacancies and under-coordinated Sn(IV) from the SnO2, in the meantime, FA+ (NH2-C=NH2 +) and K+ cations can be well fixed by hydrogen bonding interaction between FA+ and BF4 -, and electrostatic attraction between sulfonyl oxygen and K+ ions, respectively. Thereby, FAPbI3 crystal grain sizes are increased, interfacial defects are significantly reduced while carrier extraction/ transportation is facilitated, leading to better cell performance and excellent stabilities. Non-encapsulated devices can maintain 91 % of their initial efficiency under maximum-power-point (MPP) tracking while continuous illumination (~100 mW cm-2) for 1000 h, and retain 91 % of the initial efficiency after 1000 h "double 60" damp-heat stability testing (60 °C and 60 %RH (RH, relatively humidity)).

Preparation of polyelectrolyte based on bottle brush Polyethylene glycol‐Poly(N‐isopropylacrylamide) copolymers

AbstractThe increased demand for high energy‐density batteries led to using lithium metal (Li) anode. However, safety issues associated with Li anode encouraged the researchers to use less reactive and safer electrolytes with higher electrochemical stability. High ionic conductivity (σ), wider electrochemical stability window, and higher mechanical strength alongside the ability to self‐heal possible structural damages were at the center of focus for developing efficient solid electrolytes. In this regard, we developed a group of solid polymer electrolytes (SPEs) with high σ and electrochemical stability. In this regard, bottle brush polyethylene glycol (bbPEG) was synthesized using a chain transfer reaction. Polynipaam (PNIPAAM) blocks were also incorporated into the structure to prepare a block copolymer based on PNIPAM‐bbPEG‐PNIMAM represented by the ABA structure. The block copolymer was then blended with polyvinylidene fluoride (PVDF) with the block copolymer to PVDF weight ratio of 7:2% and 25% bis(trifluoromethane) sulfonamide (TFSI) salt was added and the final mixture was UV‐cured. The results indicated that the final SPE possessed high ionic conductivity with a relatively high lithium‐ion transference number () of 0.416 and the σ of 1.265 × 10−5 S/cm at 25°C.

Promoting Cathodic Kinetics and Anodic Stability in Practical Room-Temperature Sodium-Sulfur Batteries with Bifunctional Electrolytes.

The development of room-temperature (RT) sodium-sulfur (Na-S) batteries is severely hindered due to the slow kinetics of the S cathode and the instability of the Na-metal anode. To overcome this, we introduced a dual-functional electrolyte cosolvent, trifluoromethanesulfonamide (TFMSA). Short-chain Na2Sx (1 ≤ x ≤ 2) can be effectively dissolved due to the strong H-S bond interaction between TFMSA and sulfides, which changes the S conversion process, thereby effectively enhancing the conversion kinetics of the cathode. Meanwhile, TFMSA can generate a stable solid electrolyte interphase on the Na-metal surface to protect it from soluble polysulfide attack. Therefore, the RT Na-S batteries using the ether electrolyte show a high initial discharge capacity of 896.6 mAh g-1 and a capacity retention rate of 73% after 150 cycles at 0.2C, and the pouch cell also demonstrates its practical performance. This work proposes a dual-functional electrolyte cosolvent selection principle to inspire the practical application of high-performance RT Na-S batteries.

Use of genomic prediction to screen sorghum B-lines in hybrid testcrosses.

Use of trifluoromethanesulfonamide (TFMSA), a male gametocide, increases the opportunities to identify promising B-lines because large quantities of F1 seed can be generated prior to the laborious task of B-line sterilization. Combining TFMSA technology with genomic selection could efficiently evaluate sorghum B-lines in hybrid combination to maximize the rates of genetic gain of the crop. This study used two recombinant inbred B-line populations, consisting of 217 lines, which were testcrossed to two R-lines to produce 434 hybrids. Each population of testcross hybrids were evaluated across five environments. Population-based genomic prediction models were assessed across environments using three different cross-validation (CV) schemes, each with 70% training and 30% validation sets. The validation schemes were as follows: CV1-hybrids chosen randomly for validation; CV2-B-lines were randomly chosen, and each chosen B-line had one of the two corresponding testcross hybrids randomly chosen for the validation; and CV3-B-lines were randomly chosen, and each chosen B-line had both corresponding testcross hybrids chosen for the validation. CV1 and CV2 presented the highest prediction accuracies; nonetheless, the prediction accuracies of the CV schemes were not statistically different in many environments. We determined that combining the B-line populations could improve prediction accuracies, and the genomic prediction models were able to effectively rank the poorest 70% of hybrids even when genomic prediction accuracies themselves were low. Results indicate that combining genomic prediction models and TFMSA technology can effectively aid breeders in predicting B-line hybrid performance in early generations prior to the laborious task of generating A/B-line pairs.

Chemical emasculation in cowpea (Vigna unguiculata (L.) Walp.) and dicotyledonous model species using trifluoromethanesulfonamide (TFMSA)

Hybridization plays an indispensable role in creating the diversity associated with plant evolution and genetic improvement of crops. Production of hybrids requires control of pollination and avoidance of self-pollination for species that are predominantly autogamous. Hand emasculation, male sterility genes or male gametocides have been used in several plant species to induce pollen sterility. However, in cowpea (Vigna unguiculata (L.) Walp), a self-pollinated cleistogamous dryland crop, only hand emasculation is used, but it is tedious and time-consuming. In this study, male sterility was effectively induced in cowpea and two dicotyledonous model species (Arabidopsis thaliana (L.) Heynh. and Nicotiana benthamiana Domin) using trifluoromethanesulfonamide (TFMSA). Pollen viability assays using Alexander staining showed that 30 ml of 1000 mg/l TFMSA with two-time treatments of one-week interval at the early stage of the reproductive phase under field or greenhouse conditions induced 99% pollen sterility in cowpea. TFMSA treatment induced non-functional pollen in diploid A. thaliana at two-time treatment of 10 ml of 125–250 mg/l per plant and N. benthamiana at two-time treatment of 10 ml of 250–1000 mg/l per plant. TFMSA-treated cowpea plants produced hybrid seeds when used as the female parent in crosses with non-treated plants used as male parents, suggesting that TFMSA had no effect on female functionality in cowpea. The ease of TFMSA treatment and its effectiveness to induce pollen sterility in a wide range of cowpea genotypes, and in the two model plant species tested in this study, may expand the scope of techniques for rapid pollination control in self-pollinated species, with potential applications in plant breeding and plant reproduction science.

In Situ Polymerization of Fluorinated Polyacrylate Copolymer Solid Electrolytes for High-Voltage Lithium Metal Batteries at Room Temperature

Poly(ethylene oxide) is a promising solid polymer electrolyte (SPE) matrix. However, its low ionic conductivity, narrow electrochemical window, and sluggish interfacial charge transfer limit its use in solid-state batteries at room temperature. Herein, we report the in situ polymerization of poly(2,2,3,4,4,4-hexafluorobutyl acrylate-copolymer-2-(2-(2-methoxyethoxy) ethoxy)ethyl acrylate) (P(HFBA-co-MEA)) with lithium bis(trifluoromethane sulfonamide) (LiTFSI) to form a fluorinated SPE (P(HFBA-co-MEA)/LiTFSI). When the molar ratio of HFBA to MEA is 1:1, P(HFBA1:1-co-MEA) exhibits a lower lithium-ion absorption energy (−4.602 eV) than that of PMEA (−4.372 eV), providing a fast transport pathway for lithium ions. The electron-absorbing properties of the −C–F group can reduce the lone-pair electron density around the ether-oxygen group and increase the electrochemical window of P(HFBA1:1-co-MEA)/LiTFSI to 4.9 V. In addition, P(HFBA1:1-co-MEA)/LiTFSI demonstrates excellent compatibility with lithium anodes, and the Li|P(HFBA1:1-co-MEA)/LiTFSI|LiNi0.5Co0.3Mn0.2O2 battery has a capacity retention of 62.3% after 170 charge–discharge cycles at 0.1C.

Improved Electrical Performance of InAlN/GaN High Electron Mobility Transistors with Post Bis(trifluoromethane) Sulfonamide Treatment

An enhancement of the electrical performance of the InAlN/GaN high electron mobility transistors (HEMTs) is demonstrated by the incorporation of post bis(trifluoromethane) sulfonamide (TFSI) treatment. The surface treatment of TFSI solution results in the increase of 2DEG electron mobility from 1180 to 1500 cm2/Vs and thus a reduction of on-state resistance and an increase in transconductance. The results indicate that the positive charge of H+ will decrease the polarization charges of the InAlN barrier under the access region due to the converse piezoelectric effect, leading to the reduced polarization Coulomb field (PCF) scattering in InAlN/GaN HEMT. This offers a possible way to improve the electron mobility and device performance of InAlN/GaN HEMTs for further application.

Elastomeric Liquid-Free Conductor for Iontronic Devices.

For a long time, the potential application of gel-based ionic devices was limited by the problem of liquid leakage or evaporation. Here, we utilized amorphous, irreversible and reversible cross-linked polyTA (PTA) as a matrix and lithium bis(trifluoromethane sulfonamide) (LiTFSI) as an electrolyte to prepare a stretchable (495%) and self-healing (94%) solvent-free elastomeric ionic conductor. The liquid-free ionic elastomer can be used as a stable strain sensor to monitor human activities sensitively under extreme temperatures. Moreover, the prepared elastic conductor (TEOA0.10-PTA@LiTFSI) was also considered an electrode to assemble with self-designed repairable dielectric organosilicon layers (RD-PDMS) to develop a sustainable triboelectric nanogenerator (SU-TENG) with outstanding performance. SU-TENG maintained good working ability under extreme conditions (-20 °C, 60 °C, and 200% strain). This work provided a low-cost and simple idea for the development of reliable iontronic equipment for human-computer interaction, motion sensing, and sustainable energy.

Improving the electrochemical performance of lithium-sulfur batteries by interface modification with a bifunctional electrolyte additive

Lithium-sulfur (Li-S) batteries with sulfur cathode and lithium anode are considered as a promising candidate in the next generation of high specific energy secondary batteries. However, the scalable application of Li-S batteries is limited by the unstable electrode/electrolyte interfaces. Herein, a novel bifunctional electrolyte additive, trifluoromethane sulfonamide (TFMSA), is introduced to stabilize both electrode/electrolyte interfaces to enhance the electrochemical performance of Li-S batteries. The TFMSA molecule can not only alleviate the passivation of the sulfur cathode surface, but also suppress Li dendrites growth and prevent the lithium anode from the corrosion of polysulfides. As a result, the assembled Li-S battery with TFMSA-containing electrolyte displays excellent long-cycle performance and rate capability. Moreover, the Li||Li symmetric cell using TFMSA electrolyte can stably cycle for 1200 h at 1 mA cm−2 and 1 mAh cm−2. The Li-S pouch cell with TFMSA also exhibits outstanding performance, indicating the prospect of practical application. This work sheds light on an innovative idea for interface modification via an electrolyte additive to overcome the inherent issues of Li-S batteries.

Analysis of effects of trifluoromethanesulfonamide in sorghum inbred lines and their derived hybrids

AbstractIn sorghum [Sorghum bicolor (L.) Moench], development of male‐sterile versions (A‐lines) of the seed parents (B‐lines) is a tedious and time‐consuming process limiting opportunities for the improvement of new seed parents. Assessment of the value of a potential new seed parent requires testcross hybrid evaluations, but because of male fertility of these lines, conventional methods are inadequate to generate hybrid seed required for testing. Mechanisms to reduce the number of lines that require sterilization would be beneficial. The chemical gametocide trifluoromethanesulfonamide (TFMSA) was evaluated for its potential applicability to generate testcross hybrid seed. To study the effect on inbred line productivity, foliar applications of 25 mg of TFMSA were made to a select set of male‐fertile B‐lines and were compared to the corresponding untreated male‐sterile A‐line. Trifluoromethanesulfonamide induced near‐complete male‐sterility and allowed for testcross hybrid seed production across a range of genotypes. Comparison to untreated B‐lines indicated an average 3‐d delay in flowering and a 13‐cm reduction in plant height. Furthermore, average seed weight per panicle (productivity) was reduced by 8 g. Comparison of testcross hybrids produced by TFMSA induced male‐sterile lines and cytoplasmic male‐sterile lines did not reveal any significant and meaningful differences between the hybrids derived from both sterility systems. Moreover, inbred lines produced hybrids that ranked consistently in both systems. These results show TFMSA can affect productivity of treated inbred lines, but not the derived hybrid, which demonstrates that TFMSA can be used to produce hybrids for evaluation prior to investment in sterilizing of these lines.

Direct Exfoliation of Conductive MoS2 Using Peroxide for Solid State Sensor and Catalytic Applications

Two-dimensional (2D) materials have attracted much attention over the last decade due to their high performance in nanoelectronic devices. The discovery of graphene opened up many opportunities to investigate and explore other 2D materials. There has been a drive to expand the toolbox of 2D materials to also include insulators and semiconductors with a variety of bandgaps. As a result, a wide range of materials have been discovered or predicted, with molybdenum disulfide (MoS2) being particularly popular. Using the semiconducting phase of MoS2 (2H-MoS2) requires a relatively high voltage to get sufficient conductivity due to the presence of a band gap. However, for applications in batteries, supercapacitors, electrocatalysts and solar cells, a substantially increased conductivity is required in order to achieve reasonable currents.1 The most common source of conductive MoS2 is metallic MoS2 (1T-MoS2) that has been prepared via the lithium intercalation process, which requires inert atmosphere processing and safety procedures.2 Hence, there is a desire to develop a safer and more efficient process to yield conductive MoS2. Defects play a very important role in modulating the electrical properties of MoS2. Sonication of MoS2 in an appropriate solvent results in many disordered structural defects. The most common defects on MoS2 are sulfur defects. These defects increase the energy level of the gap state and eventually deteriorate the device performance. Thiol based molecules are commonly used to reduce the number of sulfur defects on MoS2. Other molecules such as oxygen or organic super acids like bis(trifluoromethane) sulfonamide (TFSI) have also been reported to passivate the surface defect.3 Past research has mainly focused on the theoretical study of defective MoS2 and how to utilize those defects for improving photoluminescent efficiency. However, those defects can also be utilized to improve the conductivity of MoS2 as a safer alternative for applications in batteries, supercapacitors, solar cells, electrocatalyst and sensors.Conductive MoS2 (c-MoS2) can be used as active material for low-cost solid-state chemiresistive pH sensors.4 In chemiresistive sensors, conductivity changes are observed based on direct interactions between the active material and the analyte.5 Even though chemiresistive pH sensors based on exfoliated graphene, carbon nanotubes, or graphitic materials are available, their sensing response is limited to less than 20%.6 On the other hand, MoS2 has attracted great attention as a promising electrocatalyst for the hydrogen evaluation reaction (HER) because of abundant active sites at edge sites and on the basal plane for facilitating hydrogen production. Water splitting is the simplest and most convenient method to generate hydrogen. In industrial applications, an electrocatalyst is commonly used to accelerate the HER and reduce the overpotential. Even though 2H-MoS2 has good catalytic activity at the edge sites, its low electrical conductivity limits the achievable current density, resulting in a high Tafel value and making it unsuitable for practical application in HER.In this work, we show a simple and effective way to prepare few layer c-MoS2 under ambient conditions using 0.06 vol% aqueous hydrogen peroxide. We have demonstrated that the bulk conductivity of the conductive MoS2 that we prepared is up to seven orders of magnitude higher than that of the semiconducting phase of MoS2. The samples were also characterized with Hall measurements, X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy which showed that hydrogen molybdenum bronze (HxMoO3) and substoichiometric MoO3−y help tune the conductivity of the nanometer-scale thin films without impacting the sulfur-to-molybdenum ratio. C-MoS2 was further functionalized with thiols to determine the number of residual reactive sites. An important goal of our work is to control the conductivity of the MoS2 thin films in safe and facile ways that enable their application in low-cost chemiresistive sensors in liquid environments. We fabricated chemiresistive pH sensors with centimeter channel lengths while maintaining low measurement voltages. We further measured the catalytic activity of c-MoS2 films in 0.5 M H2SO4 electrolyte solution with three electrode systems using linear sweep voltammetry (LSV) which showed a lower Tafel value at 10 mA/cm2 current density. The lower Tafel value demonstrated that c-MoS2 has potential to use as catalyst for HER. Our study furthers the understanding of conductive forms of MoS2, and also opens up a new pathway for next generation electronic and energy conversion devices.

Effect of anion identity on ion association and dynamics of sodium ions in non-aqueous glyme based electrolytes-OTf vs TFSI.

Sodium-based rechargeable battery technologies are being pursued as an alternative to lithium, in part due to the relative abundance of sodium compared to lithium. Despite their low dielectric constant, glyme-based electrolytes are particularly attractive for these sodium-based batteries due to their ability to chelate with the sodium ion and their high electrochemical stability. While the glyme chain length is a parameter that can be tuned to modify solvation properties, charge transport behavior, reactivity, and ultimately battery performance, anion identity provides another tunable variable. Trifluoromethanesulfonate (triflate/OTf) and bis(trifluoromethane)sulfonamide (TFSI) are chemically similar anions, which are often used in battery electrolytes for lithium-based batteries. In this paper, molecular simulations are used to examine the differences in ion association and charge transport between sodium salts of these two anions at different salt concentrations in glymes with the increasing chain length. The use of the modified force field developed for NaOTf in glymes for the NaTFSI electrolytes was validated by comparing the TFSI-sodium ion radial distribution functions to the results from ab initio molecular dynamics simulations on 1.5M NaTFSI in diglyme. While the ion association behavior as a function of salt concentration showed similar trends for both NaOTf and NaTFSI in tetraglyme and triglyme electrolytes, the dominant solvation structures for the two sets of electrolytes are distinctly different in the monoglyme and diglyme cases. The conductivity is impacted by both the ion association behavior in these electrolytes and the non-vehicular or hopping transport of the anions in these systems.

Simultaneous increases in electrical conductivity and work function of ionic liquid treated PEDOT:PSS: In-depth investigation and thermoelectric application

The electrical conductivity of poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) should be at least comparable to that of transparent conductive oxide electrode for the practical implementation in the electronics industry. In this report, a clever post-treatment method for enhancing both electrical property and work function of the PEDOT:PSS film is studied for application in electronics requiring high work-function organic conducting materials. An ionic liquid that has strong electron-withdrawing group, bis(trifluoromethane)sulfonamide (TFSI), was applied to the PEDOT:PSS film during post-treatment. TFSI molecules were distributed throughout the resulting PEDOT:PSS film, which allowed the film to increase its electrical conductivity and work function, as confirmed by photoemission spectroscopy. The enhancement of the work function after TFSI treatment led to low hole injection barriers at various organic semiconductor/PEDOT:PSS interfaces. Moreover, the TFSI treatment was also found to be effective in changing the electronic properties of a composite of carbon nanotubes and PEDOT:PSS (denoted as MWPE). Finally, the effects of TFSI treatment on the thermoelectronic performance of the PEDOT:PSS and MWPE films were discussed, suggesting potential applicability of TFSI-treated PEDOT:PSS in organic thermoelectrics without inorganic fillers.

Dipole-assisted carrier transport in bis(trifluoromethane) sulfonamide-treated O-ReS2 field-effect transistor

We demonstrate the dipole-assisted carrier transport properties of bis(trifluoromethane)sulfonamide (TFSI)-treated O-ReS2 field-effect transistors. Pristine ReS2 was compared with defect-mediated ReS2 to confirm whether the presence of defects on the interface enhances the interaction between O-ReS2 and TFSI molecules. Prior to the experiment, density functional theory (DFT) calculation was performed, and the result indicated that the charge transfer between TFSI and O-ReS2 is more sensitive to external electric fields than that between TFSI and pristine ReS2. After TFSI treatment, the drain current of O-ReS2 FET was significantly increased up to 1,113.4 times except in the range of −0.32−0.76 V owing to Schottky barrier modulation from dipole polarization of TFSI molecules, contrary to a significant degradation in device performance in pristine ReS2 FET. Moreover, in the treated O-ReS2 device, the dipole direction was highly influenced by the voltage sweep direction, generating a significant area of hysteresis in I–V and transfer characteristics, which was further verified by the surface potential result. Furthermore, the dipole state was enhanced according to the wavelength of the light source and photocurrent. These results indicate that TFSI-treated ReS2 FET has large potential for use as next-generation memristor, memory, and photodetector.

Defect Mediated Small Molecular Doping of Graphene

AbstractIt is shown that the improved electrical properties of graphene on organic small‐molecule‐based surface doping is dominated by the doping effect of these materials at the defect sites on graphene. It is hypothesized that this may be due to the healing effect of semi‐bulk substitution‐like doping at the defect sites. In this study, defects are intentionally created with varying densities on graphene by exposing it to trifluoro methane (CHF3) plasma for different time periods. The films are then doped by solution spin coating of bis(trifluoromethane)sulfonamide (TFSA). The measured change in electrical conductivity is proportional to the defect density of the graphene, implying a healing effect of these organic small molecular dopants. First principles calculations are performed to capture this mechanism, and the results conform with the experimental observations. The introduction of defects leads to a modulation of the workfunction while adversely affecting the charge carrier transport properties. It is shown that subsequent TFSA doping aids in healing the defective sites thereby improving conduction while maintaining the changed workfunction. The two processes act in tandem for the enhanced electrical properties of graphene.

Improving the electrical performance of monolayer top-gated MoS2 transistors by post bis(trifluoromethane) sulfonamide treatment

Due to the large surface to volume ratio of two-dimensional (2D) materials, the electrical performance of MoS2 is very sensitive to extrinsic ambient conditions. Post treatments are favorable to improve or recover the electrical performance of MoS2. In this work, the influence of post forming gas annealing (FGA) and bis(trifluoromethane) sulfonamide (TFSI) treatment on the electrical performance of monolayer top-gated MoS2 transistors is investigated. A negative shift of threshold voltage and an improvement in electron mobility are observed for both the post FGA and TFSI treatment. However, post TFSI treatment is more effective than the post FGA treatment in improving the electron mobility and gate controllability. Linear transmission-line-model measurement results indicate that the post FGA treatment is preferable in reducing the contact resistance, while post TFSI treatment is more effective in reducing the sheet resistance of MoS2. Raman and x-ray photoelectron spectroscopy reveal that water desorbs from MoS2 after the FGA and TFSI treatment and S vacancies are introduced into MoS2 after FGA. The results show that post TFSI treatment can be a simple and effective approach to improve the electrical property of MoS2 and may be used for other S-containing 2D transition metal dichalcogenides.

Tuning the Conductivity of Molybdenum Disulfide (MoS2) Thin Films through Defect Engineering

Two-dimensional (2D) materials have attracted much attention over the last decade due to their high performance in nanoelectronic devices. The discovery of graphene opened up many opportunities to investigate and explore other 2D materials. There has been a drive to expand the toolbox of 2D materials to also include insulators and semiconductors with a variety of bandgaps. As a result, a wide range of materials have been discovered or predicted,1 with molybdenum disulfide (MoS2) being particularly popular. The semiconducting phase of MoS2 (2H-MoS2) is one of the most commonly studied among the transition metal dichalcogenides.2 It has a thickness dependent band gap which has drawn attention for field-effect transistors (FET) where a high on/off current ratio is desired.3-5 However, for applications in batteries,6supercapacitors,7 and solar cells,8 a substantially increased conductivity is required in order to achieve reasonable currents. Using 2H-MoS2 requires a relatively high voltage to get sufficient conductivity due to the presence of a band gap. The most common source of conductive MoS2 is metallic MoS2 (1T-MoS2) that has been prepared via the lithium intercalation process, which requires inert atmosphere processing and safety procedures.9 Hence, there is a desire to develop a safer and more efficient process to yield conductive MoS2. Defects plays a very important role in modulating the electrical properties of MoS2. Sonication of MoS2 in an appropriate solvent results in many disordered structural defects. The most common defects on MoS2 are sulfur defects.10 These defects increase the energy level of the gap state and eventually deteriorate the device performance. Thiol based molecules are commonly used to reduce the number of sulfur defects on MoS2.11 Other molecules such as oxygen or organic super acids like bis(trifluoromethane) sulfonamide (TFSI) have also been reported to passivate the surface defect.12,13 Past research has mainly focused on the theoretical study of defective MoS2 and how to utilize those defects for improving photoluminescent efficiency. However, those defects can also be utilized to improve the conductivity of MoS2 as a safer alternative for applications in batteries, supercapacitors, solar cells and sensors.In this work, we show a simple and effective way to prepare few layer conductive MoS2 under ambient conditions. We have demonstrated that the sheet resistance of the conductive MoS2 that we prepared is up to five orders of magnitude higher than that of the semiconducting phase of MoS2, depending on the dopant concentration. The samples were also characterized with Hall measurements, X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. An important goal of our work is to control the conductivity of the MoS2 thin films in safe and facile ways that enable their application in low-cost chemiresistive sensors in liquid environments. We fabricated chemiresistive pH sensors with centimeter channel lengths while maintaining low measurement voltages. Our study furthers the understanding of conductive forms of MoS2, and also opens a new pathway for next generation electronic devices.

In-depth study of the chemical/electronic structures of two-dimensional molybdenum disulfide materials with sub-micrometer-resolution scanning photoelectron microscopy

The limitation of x-ray photoelectron spectroscopy (XPS) carried out with an in-lab source can be overcome by synchrotron-based scanning photoelectron microscopy (SPEM) that integrates the XPS technique with scanning microscopy, enabling the analysis of the chemical/electronic structure of a material with sub-micrometer spatial resolution. Using this analytical method, the changes in the structural properties of two-dimensional (2D) molybdenum disulfide (MoS2) materials according to the synthetic method used were studied. In addition to the monolayer form of a 2D MoS2 material (MoS2 monolayer), multilayered 2D MoS2 materials and MoS2 monolayers doped with the widely used p-type dopants, gold chloride (AuCl3) and bis(trifluoromethane)sulfonamide (TFSI), were prepared by a controlled synthetic process. Through comparative analysis of the SPEM data with those obtained by other techniques such as Raman spectroscopy, auger electron spectroscopy (AES), and atomic force microscopy (AFM), the variation in the chemical/electronic structures of MoS2 2D materials depending on the synthetic process was clarified. From SPEM data acquired from locations where AuCl3 or TFSI dopant molecules were present, all the MoS2 chemical states were confirmed to have relatively lower binding energies than those of an as-grown MoS2 monolayer. These differences are related to the effects of p-type doping on the MoS2 2D material. Separately, the increase in the MoS2 layer number is manifested in the form of brightness difference in the SPEM data. Meanwhile, all the binding energies of the MoS2 chemical states including the onset of the valence band maximum are slightly lower in the bright regions than in the dark regions, and these binding energy differences are presumably due to the change in the bandgap change based on the MoS2 layer number.

Ion-Conducting Redox-Active Polymer Gels Based on Stable Nitroxide Radicals.

Redox-active polymer networks based on stable nitroxide radicals are a very promising class of materials to be used in the so-called organic radical batteries. In order to obtain fast-charging and high power electrodes, however, excellent ionic conductivity inside the electrode material is required to allow easy diffusion of ions and fast redox reactions. In this contribution, we investigated redox-active poly(2,2,6,6-tetramethylpiperidinyloxy-4-yl methacrylate) chains cross-linked through ionic liquid-like 1,2,3-triazolium groups. Different networks were prepared in which the amount of cross-linker and the counter-anion associated to the 1,2,3-triazolium group were varied. The ionic conductivities of the different polymer networks were first measured in the solid state by electrochemical impedance spectroscopy at different temperatures, and an increased ionic conductivity was measured when 1,2,3-triazolium groups were present in the network. The effects of the chemical nature of the counterions associated to the 1,2,3-triazolium groups and of the crosslinking density were then studied. The best ionic conductivities were obtained when bis (trifluoromethane)sulfonamide (TFSI) counter-anions were used, and when the crosslinking density of the TFSI-containing gel was higher. Finally, those ion-conducting gels were loaded with free LiTFSI and the transference number of lithium ions was accordingly measured. The good ionic conductivities and lithium ions transference numbers measured for the investigated redox-active gels make them ideal candidates for application as electrode materials for either organic radical batteries or pseudo-capacitors energy storage devices.