Nanoarchitectonics of MXene Derived TiO2 /Graphene with Vertical Alignment for Achieving the Enhanced Supercapacitor Performance.

Structural engineering and hybridization of heterogeneous 2D materials can be effective for advanced supercapacitor. Furthermore, architectural design of electrodes particularly with vertical construction of structurally anisotropic graphene nanosheets, can significantly enhance the electrochemical performance. Herein, MXene-derived TiO2 nanocomposites hybridized with vertical graphene is synthesized via CO2 laser irradiation on MXene/graphene oxide nanocomposite film. Instantaneous photon energy by laser irradiation enables the formation of vertical graphene structures on nanocomposite films, presenting the controlled anisotropy in free-standing film. This vertical structure enables improved supercapacitor performance by forming an open structure, increasing the electrolyte-electrode interface, and creating efficient electron transport path. In addition, the effective oxidation of MXene nanosheets by instantaneous photon energy leads to the formation of rutile TiO2 . TiO2 nanoparticles directly generated on graphene enables the effective current path, which compensates for the low conductivity of TiO2 and enables the functioning of an effective supercapacitor by utilizing its pseudocapacitive properties. The resulting film exhibits excellent specific areal capacitance of 662.9 mF cm-2 at a current density of 5 mA cm-2 . The film also shows superb cyclic stability during 40 000 repeating cycles, maintaining high capacitance. Also, the pseudocapacitive redox reaction kinetics is evaluated, showing fast redox kinetics with potential for high-performance supercapacitor applications.

Wafer-Scale Atomic Assembly for 2D Multinary Transition Metal Dichalcogenides for Visible and NIR Photodetection.

The tunable properties of 2D transition-metal dichalcogenide (TMDs) materials are extensively investigated for high-performance and wavelength-tunable optoelectronic applications. However, the precise modification of large-scale systems for practical optoelectronic applications remains a challenge. In this study, a wafer-scale atomic assembly process to produce 2D multinary (binary, ternary, and quaternary) TMDs for broadband photodetection is demonstrated. The large-area growth of homogeneous MoS2, Ni0.06Mo0.26S0.68, and Ni0.1Mo0.9S1.79Se0.21 is carried out using a succinct coating of the single-source precursor and subsequent thermal decomposition combined with thermal evaporation of the chalcogen powder. The optoelectrical properties of the multinary TMDs are dependent on the combination of heteroatoms. The maximum photoresponsivity of the MoS2-, Ni0.06Mo0.26S0.68-, and Ni0.1Mo0.9S1.79Se0.21-based photodetectors is 3.51 × 10-4, 1.48, and 0.9 A W-1 for 532 nm and 0.063, 0.42, and 1.4 A W-1 for 1064 nm, respectively. The devices exhibited excellent photoelectrical properties, which is highly beneficial for visible and near-infrared (NIR) photodetection.

Dual Catalytic and Self-Assembled Growth of Two-Dimensional Transition Metal Dichalcogenides Through Simultaneous Predeposition Process.

The recent introduction of alkali metal halide catalysts for chemical vapor deposition (CVD) of transition metal dichalcogenides (TMDs) has enabled remarkable two-dimensional (2D) growth. However, the process development and growth mechanism require further exploration to enhance the effects of salts and understand the principles. Herein, simultaneous predeposition of a metal source (MoO3 ) and salt (NaCl) by thermal evaporation is adopted. As a result, remarkable growth behaviors such as promoted 2D growth, easy patterning, and potential diversity of target materials can be achieved. Step-by-step spectroscopy combined with morphological analyses reveals a reaction path for MoS2 growth in which NaCl reacts separately with S and MoO3 to form Na2 SO4 and Na2 Mo2 O7 intermediates, respectively. These intermediates provide a favorable environment for 2D growth, including an enhanced source supply and liquid medium. Consequently, large grains of monolayer MoS2 are formed by self-assembly, indicating the merging of small equilateral triangular grains on the liquid intermediates. This study is expected to serve as an ideal reference for understanding the principles of salt catalysis and evolution of CVD in the preparation of 2D TMDs.

Mesoporous Metal Fluoride Nanocomposite Films with Tunable Optical Properties Derived from Precursor Instability.

The precisely tailored refractive index of optical materials is the key to utilizing and manipulating light during its propagation through the matrix, thereby improving their application performances. In this paper, mesoporous metal fluoride films with engineered composition (MgF2 :LaF3 ) are demonstrated to achieve finely tunable refractive indices. These films are prepared using a precursor-derived one-step assembly approach via the simple mixing of precursor solutions (Mg(CF3 OO)2 and La(CF3 OO)3 ); then pores are formed simultaneously during solidification owing to the inherent instability of La(CF3 OO)3 . The mesoporous structures are realized through Mg(CF3 OO)2 and La(CF3 OO)3 ions, which interacted with each other based on their electrostatic forces, providing a wide range of refractive indices (from 1.37 to 1.16 at 633 nm). Furthermore, it is systematically several MgF2(1-x) -LaF3(x) layers with different compositions (x = 0.0, 0.3, and 0.5) to form the graded refractive index coating that is optically consecutive between the substrate and the air for broadband and omnidirectional antireflection. An average transmittance of ≈98.03% (400-1100 nm) is achieved with a peak transmittance of ≈99.04% (at 571 nm), and the average antireflectivity is maintained at ≈15.75% even at an incidence of light of 65° (400-850 nm).

Multifunctional-high resolution imaging plate based on hydrophilic graphene for digital pathology.

In the present study, we showed that hydrophilic graphene can serve as an ideal imaging plate for biological specimens. Graphene being a single-atom-thick semi-metal with low secondary electron emission, array tomography analysis of serial sections of biological specimens on a graphene substrate showed excellent image quality with improvedz-axis resolution, without including any conductive surface coatings. However, the hydrophobic nature of graphene makes the placement of biological specimens difficult; graphene functionalized with polydimethylsiloxane oligomer was fabricated using a simple soft lithography technique and then processed with oxygen plasma to provide hydrophilic graphene with minimal damage to graphene. High-quality scanning electron microscopy images of biological specimens free from charging effects or distortion were obtained, and the optical transparency of graphene enabled fluorescence imaging of the specimen; high-resolution correlated electron and light microscopy analysis of the specimen became possible with the hydrophilic graphene plate.

Strategic Customization of Polymeric Nanocomposites Modified by 2D Titanium Oxide Nanosheet for High-k and Flexible Gate Dielectrics.

Organic polymer-based dielectrics with intrinsic mechanical flexibility and good processability are excellent candidates for the dielectric layer of flexible electronics. These polymer films can become even more rigid and electrically robust when modified through cross-linking processes. Moreover, the composites formed by dispersing nanoscale inorganic fillers in a polymer matrix can exhibit further improved polarization property. However, these strategies can be challenging as homogeneous dispersion of nanomaterials in the matrix is difficult to achieve; thus, degradation of electrically insulating properties of nanocomposite layers is often observed. Here, a high-k, pinhole-free, and flexible poly(vinyl alcohol) (PVA)-based nanocomposite dielectric is presented, incorporating 2D TiO2 nanosheets (NSs) for the first time. Despite the attractive dielectric constant, exceptional flexibility, and electrically insulating property of PVA-TiO2 nanocomposites, only few studies on these materials have been reported. The organic/inorganic nanosheet hybrid layer, which reaches an unprecedentedly high dielectric constant of 43.8 (more than four times higher than that of cross-linked PVA), also exhibits an outstanding leakage current density as low as 10-9 A cm-2 . Furthermore, the repeated bending tests for nanocomposite capacitors reveal their capability of operating without any deterioration of their performances even after 1000 iterations of bending cycles at a bending radius of 3 mm.

Understanding the Chemical Composition with Doping Aliovalent Ions, Followed by the Electrochemical Behavior for Surface-Modified Ni-Rich NMC Cathode Materials.

A comparative study of doping aliovalent ions, Zr- or Al-, into Ni-rich Li(Ni,Co,Mn)O2 cathode materials is conducted in terms of the electrochemical properties and chemical analysis, especially on the surface region. The solubility and chemical composition for the given sol-gel treatment matches well with the computational results with which the infinitesimal Zr-coating is identified as exhibiting increased charge capacity with prolonged cycle life. Specifically, the whole process can be understood by the suppressed lithium-ion charge transfer resistance (RCT) during the cycles, which can be facilitated by the decreased NiO formation during the cyclic reactions.

Attachable piezoelectric nanogenerators using collision-induced strain of vertically grown hollow MoS2 nanoflakes.

Piezoelectric materials convert external mechanical force into electrical energy, due to the breaking of the centrosymmetry of the atomic structure. Piezoelectricity-based nano-generators (PNGs) based on two-dimensional transition metal dichalcogenides (TMDs) can generate electrical energy stably by the piezoelectric effect at their nanoscale thickness. However, the commercialization of TMD-based PNGs is limited by their poor piezoelectric performance and microscale energy harvesting. Here, we present the first centimeter-scale PNGs based on molybdenum disulfide (MoS2) nanosheets with vertically grown hollow MoS2 nanoflakes (v-MoS2 NFs) obtained by chemical vapor deposition for energy harvesting from human motions. The collision of v-MoS2 NFs with a preferred odd-atomic-layer number and their 2H antiparallel phase leads to efficient electrical energy generation during the bending movement. Further, basal MoS2 films with v-MoS2 NFs are transferred onto flexible substrates via conventional polymer-assisted methods for the fabrication of attachable and wearable piezoelectric power generators.

Self-assembly of core-shell structures driven by low doping limit of Ti in LiCoO2: first-principles thermodynamic and experimental investigation.

Introducing additives is a general method of performance improvement in materials engineering, but details regarding whether the additive is doped in the host crystal or present as a secondary phase are usually examined from experimental experience, with a systematic theoretical prediction lacking, which sometimes causes controversy on the role of additives. In this study, the dopability of Ti in crystalline LiCoO2 (LCO) is investigated by a first-principles simulation method, and the doping limit is quantitatively calculated. The probability of Ti substitution for Co is examined and related to point-defect formation in LCO as a function of the general experimental variables of temperature and gas-phase partial pressures, enabling practical use of the theoretical model for real experiments. It was found that Ti substitution for Co, accompanied by the formation of a Li vacancy, is the most probable Ti doping form in LCO, but the doping limit is very low and most Ti would segregate into secondary phases. The theoretical prediction showed good agreement with the experimental results. Based on theoretical predictions, particles having LCO cores and Ti-rich shells are obtained from a simple sol-gel route followed by one-step firing without additional surface treatment. The high-voltage cyclability of LCO is greatly improved. The method demonstrated in this study may be a useful tool for screening suitable coating or doping elements for various material systems and provide a guide for designing simple spontaneous coating processes, as in this study.

Chemically Functionalized, Well-Dispersed Carbon Nanotubes in Lithium-Doped Zinc Oxide for Low-Cost, High-Performance Thin-Film Transistors.

Surface-functionalized carbon nanotubes (CNTs) are introduced into lithium-doped ZnO thin-film transistors (TFTs) as an alternative to the conventional incorporation of an expensive element, indium. The crucial role of surface functionalization of CNTs is clarified with the demonstration of indium-free ZnO-based TFTs with a field-effect mobility of 28.6 cm(2) V(-1) s(-1) and an on/off current ratio of 9 × 10(6) for low-cost, high-performance electronics.

Improved impedance characteristics of all-water-processable triple-stacked hole-selective layers in solution-processed OLEDs.

We herein report an investigation of the device performance capabilities and impedance characteristics of solution-processed organic light-emitting devices (OLEDs) with all-water-processable triple-stacked hole-selective layers (HSLs) on an indium-tin-oxide (ITO) anode, fabricated using a simple coating technique. Highly smooth and homogeneous triple-stacked layers were deposited via horizontal-dip- (H-dip-) coating using aqueous dispersions of graphene oxide (GO), molybdenum oxide (MoO3), and poly(ethylenedioxy thiophene):poly(styrene sulfonate) ( PEDOT: PSS). From the triple-stacked GO/MoO3/ PEDOT: PSS HSLs used as hole-injection layers (HILs) in the OLEDs, which outperform a conventional single HIL of PEDOT: PSS, it was found that OLEDs with triple-stacked HILs exhibited characteristic impedance properties, including low parallel resistance with trap-free space-charge-limited conductivity. Furthermore, it was shown that the relaxation frequency of a sample OLED with triple-stacked GO/MoO3/ PEDOT: PSS HILs was much higher than that of a reference device with a single PEDOT: PSS HIL. These impedance behaviors indicate that carrier (hole) injection in the sample OLED is more efficient than that in any of the other devices tested here. The results presented here clarify that the triple-stacked GO/MoO3/ PEDOT: PSS layers can act as efficient HILs on an ITO anode, representing a remarkable advance in relation to the mass production of high-performance solution-processable OLEDs.

Tuning the Electrical Properties of Graphene via Nitrogen Plasma-Assisted Chemical Modification.

The control in electrical properties of graphene is essentially required in order to realize graphenebased nanoelectronics. In this study, N-doped graphene was successfully obtained via nitrogen plasma treatment. Graphene was synthesized on copper foil using thermal chemical vapor deposition. After N2 plasma treatment, the G-band of the graphene was blueshifted and the intensity ratio of 2D- to G-bands decreased with increasing the plasma power. Pyrrolic-N bonding configuration induced by N2 plasma treatment was studied by X-ray photoelectron spectroscopy. Remarkably, electrical characterization including Hall measurement and I-V characteristics of the N-doped graphene exhibit semiconducting behavior as well as the n-type doping effect.

Multiarray Gas Sensors Using Ternary Combined Ti3C2Tx MXene-Based Nanocomposites.

This paper reports chemiresistive multiarray gas sensors through the synthesized ternary nanocomposites, using a one-pot method to integrate two-dimensional MXene (Ti3C2Tx) with Ti-doped WO3 (Ti-WO3/Ti3C2Tx) and Ti3C2Tx with Pd-doped SnO2 (Pd-SnO2/Ti3C2Tx). The gas sensors based on Ti-WO3/Ti3C2Tx and Pd-SnO2/Ti3C2Tx exhibit exceptional sensitivity, particularly in detecting 70% at 1 ppm acetone and 91.1% at 1 ppm of H2S. Notably, our sensors demonstrate a remarkable sensing performance in the low-ppb range for acetone and H2S. Specifically, the Ti-WO3/Ti3C2Tx sensor demonstrates a detection limit of 0.035 ppb for acetone, and the Pd-SnO2/Ti3C2Tx sensor shows 0.116 ppb for H2S. Simultaneous measurements with Ti-WO3/Ti3C2Tx- and Pd-SnO2/Ti3C2Tx-based sensors enable the evaluation of both the concentration and type of unknown target gases, such as acetone or H2S. Furthermore, density functional theory calculations are performed to clarify the role of Ti and Pd doping in enhancing the performance of Ti-WO3/Ti3C2Tx and Pd-SnO2/Ti3C2Tx nanocomposites. Theoretical simulations contribute to a deeper understanding of the doping effects, providing essential insights into the mechanisms underlying the enhanced gas response of the gas sensors. Overall, this work provides valuable insights into the gas-sensing mechanisms and introduces a novel approach for high-performance multiarray gas sensing.

Edge-Rich 3D Structuring of Metal Chalcogenide/Graphene with Vertical Nanosheets for Efficient Photocatalytic Hydrogen Production.

Constructing pertinent nanoarchitecture with abundant exposed active sites is a valid strategy for boosting photocatalytic hydrogen generation. However, the controllable approach of an ideal architecture comprising vertically standing transition metal chalcogenides (TMDs) nanosheets on a 3D graphene network remains challenging despite the potential for efficient photocatalytic hydrogen production. In this study, we fabricated edge-rich 3D structuring photocatalysts involving vertically grown TMDs nanosheets on a 3D porous graphene framework (referred to as 3D Gr). 2D TMDs (MoS2 and WS2)/3D Gr heterostructures were produced by location-specific photon-pen writing and metal-organic chemical vapor deposition for maximum edge site exposure enabling efficient photocatalytic reactivity. Vertically aligned 2D Mo(W)S2/3D Gr heterostructures exhibited distinctly boosted hydrogen production because of the 3D Gr caused by synergetic impacts associated with the large specific surface area and improved density of exposed active sites in vertically standing Mo(W)S2. The heterostructure involving graphene and TMDs corroborates an optimum charge transport pathway to rapidly separate the photogenerated electron-hole pairs, allowing more electrons to contribute to the photocatalytic hydrogen generation reaction. Consequently, the size-tailored heterostructure showed a superior hydrogen generation rate of 6.51 mmol g-1 h-1 for MoS2/3D graphene and 7.26 mmol g-1 h-1 for WS2/3D graphene, respectively, which were 3.59 and 3.76 times greater than that of MoS2 and WS2 samples. This study offers a promising path for the potential of 3D structuring of vertical TMDs/graphene heterostructure with edge-rich nanosheets for photocatalytic applications.

Photostimulated Pyrothermoelectric Coupling in Two-Dimensional Tin Monoselenide Enabling Zero-Biased Multimodal Transducers.

Despite the advancement of the Internet of Things (IoT) and portable devices, the development of zero-biased sensing systems for the dual detection of light and gases remains a challenge. As an emerging technology, direct energy conversion driven by intriguing physical properties of two-dimensional (2D) materials can be realized in nanodevices or a zero-biased integrated system. In this study, we unprecedentedly attempted to exploit the photostimulated pyrothermoelectric coupling of two-dimensional SnSe for use in zero-biased multimodal transducers for the dual detection of light and gases. We synthesized homogeneous, large-area 6 in SnSe multilayers via a rational synthetic route based on the thermal decomposition of a solution-processed single-source precursor. Zero-biased SnSe transducers for the dual monitoring of light and gases were realized by exploiting the synergistic coupling of the photostimulated pyroelectric and thermoelectric effects of SnSe. The extracted photoresponsivity at 532 nm and NO2 gas responsivity of the SnSe-based transducers corresponded to 1.07 × 10-6 A/W and 13263.6% at 0 V, respectively. To bring universal applicability of the zero-biased SnSe transducers, the wide operation bandwidth photoelectrical properties (visible to NIR) and dynamic current responses toward two NO2/NH3 gases were systematically evaluated.

Artificial Q-Grader: Machine Learning-Enabled Intelligent Olfactory and Gustatory Sensing System.

Portable and personalized artificial intelligence (AI)-driven sensors mimicking human olfactory and gustatory systems have immense potential for the large-scale deployment and autonomous monitoring systems of Internet of Things (IoT) devices. In this study, an artificial Q-grader comprising surface-engineered zinc oxide (ZnO) thin films is developed as the artificial nose, tongue, and AI-based statistical data analysis as the artificial brain for identifying both aroma and flavor chemicals in coffee beans. A poly(vinylidene fluoride-co-hexafluoropropylene)/ZnO thin film transistor (TFT)-based liquid sensor is the artificial tongue, and an Au, Ag, or Pd nanoparticles/ZnO nanohybrid gas sensor is the artificial nose. In order to classify the flavor of coffee beans (acetic acid (sourness), ethyl butyrate and 2-furanmethanol (sweetness), caffeine (bitterness)) and the origin of coffee beans (Papua New Guinea, Brazil, Ethiopia, and Colombia-decaffeine), rational combination of TFT transfer and dynamic response curves capture the liquids and gases-dependent electrical transport behavior and principal component analysis (PCA)-assisted machine learning (ML) is implemented. A PCA-assisted ML model distinguished the four target flavors with >92% prediction accuracy. ML-based regression model predicts the flavor chemical concentrations with >99% accuracy. Also, the classification model successfully distinguished four different types of coffee-bean with 100% accuracy.

Facile fabrication of gas sensors based on molybdenum disulfide nanosheets and carbon nanotubes by self-assembly.

The rising importance of gas detection has prompted rigorous research on flexible and transparent high-performance gas sensors. We demonstrated a sensor for NO2 detection at room temperature, in which our device was fabricated via screen printing on a flexible substrate, and MoS2 and single-walled carbon nanotube (SWCNT) were coated on a specific area by the self-assembly method. This fabrication process is rapid, facile, and cost-effective. The proposed sensor enables precise and stable NO2 gas sensing from 50 ppb to 100 ppm. This method should also be applicable to the selective detection of other gases.

Wafer-Scale Production of Two-Dimensional Tin Monoselenide: Expandable Synthetic Platform for van der Waals Semiconductor-Based Broadband Photodetectors.

A synthetic platform for industrially applicable two-dimensional (2D) semiconductors that addresses the paramount issues associated with large-scale production, wide-range photosensitive materials, and oxidative stability has not yet been developed. In this study, we attained the 6 in. scale production of 2D SnSe semiconductors with spatial homogeneity using a rational synthetic platform based on the thermal decomposition of solution-processed single-source precursors. The long-range structural and chemical homogeneities of the 2D SnSe layers are manifested using comprehensive spectroscopic analyses. Furthermore, the capability of the SnSe-based photodetectors for broadband photodetection is distinctly verified. The photoresponsivity and detectivity of the SnSe-based photodetectors are 5.89 A W-1 and 1.8 × 1011 Jones at 532 nm, 1.2 A W-1 and 3.7 × 1010 Jones at 1064 nm, and 0.14 A W-1 and 4.3 × 109 Jones at 1550 nm, respectively. The minimum rise times for the 532 and 1064 nm lasers are 62 and 374 µs, respectively. The photoelectrical analysis of the 5 × 5 SnSe-based photodetector array reveals 100% active devices with 95.06% photocurrent uniformity. We unequivocally validated that the air and thermal stabilities of the photocurrent yielded from the SnSe-based photodetector are determined to be >30 d in air and 160 °C, respectively, which are suitable for optoelectronic applications.

One-Dimensional π-d Conjugated Coordination Polymer Intercalated MXene Compound for High-Performance Supercapacitor Electrode.

MXenes possess the characteristics required for high-performance supercapacitors, such as high metallic conductivity and electrochemical activity, but their full potential remains unrealized owing to their tendency to self-restack when fabricated into an electrode. Designing an MXene interlayer with an effective intercalant has, therefore, become an important criterion to alleviate the restacking issue while also synergistically interact with MXene to further improve its electrochemical activity. This study reports the intercalation of 1D π-d conjugated coordination polymer (Ni-BTA) within the Ti3 C2 Tx nanosheet for the fabrication of a highly efficient supercapacitor electrode. Ni-BTA, which consists of a nickel center and 1,2,4,5-benzenetetramine (BTA) organic chain, is uniformly intercalated by direct synthesis on the abundant oxygen terminals on the Ti3 C2 Tx nanosheet surface. The intercalated Ni-BTA acts as an effective charge carrier transportation pathway through its 1D stretched delocalized π-d electrons while participating in pseudocapacitive activity with the Ni centers. As a result, the Ni-BTA/MXene film exhibits excellent rate performance and a gravimetric specific capacitance of 264.4 F g-1 at 5 mV s-1 . This magnitude is retained up to 94.6% after 10 000 cycles. The present study provides insights into the design of MXene interlayers for the fabrication of highly robust and stable supercapacitors.

Spectro-Microscopic Perceptions into Oxidation Behavior of Large-Scale Molybdenum Disulfide and its Photoelectrical Correlation.

Despite the encouraging properties and research of 2D MoS2 , an ongoing issue associated with the oxidative instability remains elusive for practical optoelectronic applications. Thus, in-depth understanding of the oxidation behavior of large-scale and homogeneous 2D MoS2 is imperative. Here the structural and chemical transformations of large-area MoS2 multilayers by air-annealing with altered temperature and time via combinatorial spectro-microscopic analyses (Raman spectroscopy, X-ray photoelectron spectroscopy, and atomic force microscopy) are surveyed. The results gave indications pertaining to temperature- and time-dependent oxidation effects: i) heat-driven elimination of redundant residues, ii) internal strain stimulated by the formation of MoO bonds, iii) deterioration of the MoS2 crystallinity, iv) layer thinning, and v) morphological transformation from 2D MoS2 layers to particles. Photoelectrical characterization of the air-annealed MoS2 is implemented to capture the link between the oxidation behavior of MoS2 multilayers and their photoelectrical properties. The photocurrent based on MoS2 air-annealed at 200 °C is assessed to be 4.92 µA, which is 1.73 times higher than that of pristine MoS2 (2.84 µA). The diminution in the photocurrent of the photodetector based on MoS2 air-annealed above 300 °C in terms of the structural, chemical, and electrical conversions induced by the oxidation process is further discussed.