Atomically thin MXene/WSe2Schottky heterojunction towards enhanced photogenerated charge carrier.

Two-dimensional materials garner increasing interest in next-generation electronics and optoelectronic devices due to their atomic-thin nature and distinctive physical properties. Building on these advances, we present the successful synthesis of a heterostructure composed of the semi-metallic Ti3C2-MXene and the semiconducting WSe2, in which the atomic layers are vertically aligned. The wet impregnation method effectively synthesizes an atomically thin Ti3C2-MXene/WSe2heterostructure characterized by atomic force microscopy, Raman and time-resolved photoluminescence (TRPL) analysis. In addition, the current-voltage characteristics at the heterostructure reveal the Schottky junction probed by the scanning tunnelling microscopy and the conductive atomic force microscopy tip. The Schottky heterojunction also exhibits enhanced photocatalytic properties by improving the photogenerated charge carriers and inhibiting recombination. This work demonstrates the unique 2D-2D Ti3C2-MXene/WSe2vertical heterojunction possesses superior photon trapping ability and can efficiently transport photogenerated charge carriers to the reaction sites to enhance photocatalysis performance.

A MXene-BiFeO3-ZnO nanocomposite photocatalyst served as a high-performance supercapacitor electrode.

MXenes have attracted considerable attention in the field of energy storage and conversion due to their high surface area, excellent electrical conductivity, and ability to intercalate various ions. However, simultaneously achieving high capacitance, rate capability, cycling stability, and mechanical flexibility is a significant challenge for designing MXene-based supercapacitors. In this article, we explored MXene-BiFeO3-ZnO nanocomposites for both photocatalytic and electric double-layer supercapacitor applications. While the BiFeO3-ZnO nanohybrid heterostructure improves the charge separation properties in nanocomposite photocatalysts, it was applied as an interlayer spacer between the MXene layers to prevent the stacking effect of electrodes in the supercapacitor. Furthermore, the optimization of MXene content in the nanocomposite was established by photocatalytic studies on methylene blue dye, which revealed a maximum of 98.72% degradation under direct sunlight with superior stability. The electrochemical studies on the best composition material reveal a maximum areal capacitance (Ccv) of 142.8 mF cm-2, an energy density (E) of 1.65 µW h cm-2, and a capacitive retention of 99.98% after 8000 cycles at 7 µA cm-2. Additionally, the flexible solid-state supercapacitor fabricated with the same material demonstrates an areal capacitance of 47.6 mF cm-2 and a capacitive retention of 66% after 8000 cycles at 7 µA cm-2, with potential for high-performance flexible supercapacitors.

Iron(II)-α-keto acid complexes of tridentate ligands on gold nanoparticles: the effect of ligand geometry and immobilization on their dioxygen-dependent reactivity.

Two mononuclear nonheme iron(II)-benzoylformate (BF) complexes [(6Me2-Me-BPA)Fe(BF)](ClO4) (1a) and [(6Me3-TPMM)Fe(BF)](ClO4) (1b) of tridentate nitrogen donor ligands, bis((6-methylpyridin-2-yl)methyl)(N-methyl)amine (6Me2-Me-BPA) and tris(2-(6-methyl)pyridyl)methoxymethane (6Me3-TPMM), were isolated and characterized. The structural characterization of iron(II)-chloro complexes indicates that the ligand 6Me2-Me-BPA binds to the iron(II) centre in a meridional fashion, whereas 6Me3-TPMM behaves as a facial ligand. Both the ligands were functionalized with terminal thiol for immobilization on gold nanoparticles (AuNPs), and the corresponding iron(II) complexes [(6Me2-BPASH)Fe(BF)(ClO4)]@C8Au (2a) and [(6Me3-TPMSH)Fe(BF)(ClO4)]@C8Au (2b) were prepared to probe the effect of immobilization on their ability to perform bioinspired oxidation reactions. All the complexes react with dioxygen to display the oxidative decarboxylation of the coordinated benzoylformate, but the complexes supported by 6Me3-TPMM and its thiol-appended ligand display faster reactivity compared to their analogues with the 6Me2-Me-BPA-derived ligands. In each case, an electrophilic iron-oxygen oxidant was intercepted as the active oxidant generated from dioxygen. The immobilized complexes (2a and 2b) display enhanced O2-dependent reactivity in oxygen-atom transfer reactions (OAT) and hydrogen-atom transfer (HAT) reactions compared to their homogeneous congeners (1a and 1b). Furthermore, the immobilized complex 2b displays catalytic OAT reactions. This study supports that the ligand geometry and immobilization on AuNPs influence the dioxygen-dependent reactivity of the complexes.

Surface Diffusion Aided by a Chirality Change of Self-Assembled Oligomers under 2D Confinement.

Chirality switching of self-assembled molecular structures is of potential interest for designing functional materials but is restricted by the strong interaction between the embedded molecules. Here, we report on an unusual approach based on reversible chirality changes of self-assembled oligomers using variable-temperature scanning tunneling microscopy supported by quantum mechanical calculations. Six functionalized diazomethanes each self-assemble into chiral wheel-shaped oligomers on Ag(111). At 130 K, a temperature far lower than expected, the oligomers change their chirality even though the molecules reside in an embedded self-assembled structure. Each chirality change is accompanied by a slight center-of-mass shift. We show how the identical activation energies of the two processes result from the interplay of the chirality change with surface diffusion, findings that open the possibility of implementing various functional materials from self-assembled supramolecular structures.

Catalytic Hydrolysis of Thiolates to Alcohols.

A new and efficient catalytic hydrolysis of aliphatic and aromatic thiolates under ambient conditions is presented. Previously, we have demonstrated (Ganguly et al., Inorg. Chem. 2018, 57, 11306-11309) the Co(II) mediated stoichiometric hydrolysis of thiols to produce alcohols/phenols along with a binuclear dicobalt(II)-hydrosulfide complex, [Co2(PhBIMP)(µ2-SH)(DMF)]2+ (1) (PhBIMP is the anion of 2,6 bis[(bis((N-1-methyl-4,5- diphenylimidazoylmethyl) amino)methyl]- 4-methylphenol). In the present work, we have shown that the product of the stoichiometric reaction, 1, may act as an efficient catalyst for the catalytic hydrolysis of a broad range of aliphatic and aromatic thiolates in DMF at room temperature to produce alcohols/phenols. Complex 1 takes up a thiolate (RS-) and a water molecule to generate an active intermediate complex, [Co2(PhBIMP)(µ2-SH)(RS)(H2O)]1+ (2), which, in turn, releases the alcohol/phenol (ROH), hydrosulfide (HS-), and regenerates 1.

Selective oxygenation of C-H and CC bonds with H2O2 by high-spin cobalt(II)-carboxylate complexes.

Four cobalt(II)-carboxylate complexes [(6-Me3-TPA)CoII(benzoate)](BPh4) (1), [(6-Me3-TPA)CoII(benzilate)](ClO4) (2), [(6-Me3-TPA)CoII(mandelate)](BPh4) (3), and [(6-Me3-TPA)CoII(MPA)](BPh4) (4) (HMPA = 2-methoxy-2-phenylacetic acid) of the 6-Me3-TPA (tris((6-methylpyridin-2-yl)methyl)amine) ligand were isolated to investigate their ability in H2O2-dependent selective oxygenation of C-H and CC bonds. All six-coordinate complexes contain a high-spin cobalt(II) center. While the cobalt(II) complexes are inert toward dioxygen, each of these complexes reacts readily with hydrogen peroxide to form a diamagnetic cobalt(III) species, which decays with time leading to the oxidation of the methyl groups on the pyridine rings of the supporting ligand. Intramolecular ligand oxidation by the cobalt-based oxidant is partially inhibited in the presence of external substrates, and the substrates are converted to their corresponding oxidized products. Kinetic studies and labelling experiments indicate the involvement of a metal-based oxidant in affecting the chemo- and stereo-selective catalytic oxygenation of aliphatic C-H bonds and epoxidation of alkenes. An electrophilic cobalt-oxygen species that exhibits a kinetic isotope effect (KIE) value of 5.3 in toluene oxidation by 1 is proposed as the active oxidant. Among the complexes, the cobalt(II)-benzoate (1) and cobalt(II)-MPA (4) complexes display better catalytic activity compared to their α-hydroxy analogues (2 and 3). Catalytic studies with the cobalt(II)-acetonitrile complex [(6-Me3-TPA)CoII(CH3CN)2](ClO4)2 (5) in the presence and absence of externally added benzoate support the role of the carboxylate co-ligand in oxidation reactions. The proposed catalytic reaction involves a carboxylate-bridged dicobalt complex in the activation of H2O2 followed by the oxidation of substrates by a metal-based oxidant.

Carbon-Nanotube-Appended PAMAM Dendrimers Bearing Iron(II) α-Keto Acid Complexes: Catalytic Non-Heme Oxygenase Models.

Poly(amidoamine) dendrimers grafted on carbon nanotubes have been appended with iron(II)-α-keto acid (benzoylformate) complex of polypyridyl ligand to design artificial non-heme oxygenase model. This nano-enzyme was applied for selective catalytic oxidation of organic molecules. Although the carbon nanotubes serve as a robust heterogeneous platform, the amine terminals of dendrimers provide catalysts binding sites and the amide bonds provide a necessary second coordination sphere similar to the enzymatic polypeptide chains. Such a hybrid design prevented the deactivation of the primary active sites leading to 8 times faster oxidative decarboxylation rates than those of its homogeneous analogue. An electrophilic iron(IV)-oxo intermediate has been intercepted, which catalyzes the selective oxidation of alcohols to aldehydes and incorporates single oxygen atoms into sulfides and olefins by using aerial oxygen with multiple turnover numbers. The catalyst was consecutively regenerated three times by mild chemical treatment and showed negligible loss of activity.

Oxidizing Ability of a Dioxygen-Activating Nonheme Iron(II)-Benzilate Complex Immobilized on Gold Nanoparticles.

An iron(II)-benzilate complex [(TPASH)FeII(benzilate)]ClO4@C8Au (2) (TPASH = 11-((6-((bis(pyridin-2-ylmethyl)amino)methyl)pyridin-2-yl)methoxy)undecane-1-thiol) immobilized on octanethiol stabilized gold nanoparticles (C8Au) of core diameter less than 5 nm has been prepared to evaluate its reactivity toward O2-dependent oxidations compared to a nonimmobilized complex [(TPA-O-Allyl)FeII(benzilate)]ClO4 (1a) (TPA-O-Allyl = N-((6-(allyloxymethyl)pyridin-2-yl)methyl)(pyridin-2-yl)- N-(pyridin-2-ylmethyl)methanamine). X-ray crystal structure of the nonimmobilized complex 1a reveals a six-coordinate iron(II) center in which the TPA-O-Allyl acts as a pentadentate ligand and the benzilate anion binds in monodentate fashion. Both the complexes (1a and 2) react with dioxygen under ambient conditions to form benzophenone as the sole product through decarboxylation of the coordinated benzilate. Interception studies reveal that a nucleophilic iron-oxygen intermediate is formed in the decarboxylation reaction. The oxidants from both the complexes are able to carry out oxo atom transfer reactions. The immobilized complex 2 not only performs faster decarboxylation but also exhibits enhanced reactivity in oxo atom transfer to sulfides. Importantly, the immobilized complex 2, unlike 1a, displays catalytic turnovers in sulfide oxidation. However, the complexes are not efficient to carry out cis-dihydroxylation of alkenes. Although the immobilized complex yields a slightly higher amount of cis-diol from 1-octene, restricted access of dioxygen and substrates at the coordinatively saturated metal centers of the complexes likely makes the resulting iron-oxygen species less active in oxygen atom transfer to alkenes. The results implicate that surface immobilized nonheme iron complexes containing accessible coordination sites would exhibit better reactivity in O2-dependent oxygenation reactions.

Generic and Scalable Method for the Preparation of Monodispersed Metal Sulfide Nanocrystals with Tunable Optical Properties.

A rational synthetic method that produces monodisperse and air-stable metal sulfide colloidal quantum dots (CQDs) in organic nonpolar solvents using octyl dithiocarbamic acid (C8DTCA) as a sulfur source, is reported. The fast decomposition of metal-C8DTCA complexes in presence of primary amines is exploited to achieve this purpose. This novel technique is generic and can be applied to prepare diverse CQDs, like CdS, MnS, ZnS, SnS, and In2S3, including more useful and in-demand PbS CQDs and plasmonic nanocrystals of Cu2S. Based on several control reactions, it is postulated that the reaction involves the in situ formation of a metal-C8DTCA complex, which then reacts in situ with oleylamine at slightly elevated temperature to decompose into metal sulfide CQDs at a controlled rate, leading to the formation of the materials with good optical characteristics. Controlled sulfur precursor's reactivity and stoichiometric reaction between C8DTCA and metal salts affords high conversion yield and large-scale production of monodisperse CQDs. Tunable and desired crystal size could be achieved by controlling the precursor reactivity by changing the reaction temperature and reagent ratios. Finally, the photovoltaic devices fabricated from PbS CQDs displayed a power conversion efficiency of 4.64% that is comparable with the reported values of devices prepared with PbS CQDs synthesized by the standard methods.

Simultaneous observation of surface- and edge-states of a 2D topological insulator through scanning tunneling spectroscopy and differential conductance imaging.

A 2D form of Bi2Se3 which acts as a topological insulator was grown through colloidal synthesis method. The surface-states and edge-states of the nanoplates were simultaneously probed through scanning tunneling spectroscopy (STS). At the interior, density of states (DOS) revealed the location of conduction and valence band edges. The DOS at the edges, on the other hand, brought out gapless conducting states along with a Dirac point at a non-zero value below the Fermi energy representing the Dirac cone of a 2D topological insulator. In differential tunnel conductance (dI/dV), images are recorded at different voltages and the two sections of the topological insulator can be viewed selectively or simultaneously with a clear contrast in illumination. Upon increasing the 2D-nanoplates thickness, the material turned into a 3D topological insulator with gapless surface states.

Current Rectification through Vertical Heterojunctions between Two Single-Layer Dichalcogenides (WSe2|MoS2 pn-Junctions).

We form junctions between two single layers of p-type WSe2 and n-type MoS2 in both sequences. The WSe2|MoS2 and MoS2|WSe2 junctions of ultimate thickness limit exhibit current rectification when characterized vertically with a scanning tunneling microscope (STM) tip. The direction of rectification in the pn-junction is opposite to that of the np-junction, confirming occurrence of the rectification to be due to the junctions themselves. From scanning tunneling spectroscopy (STS) and correspondingly the density of states (DOS), we locate the conduction and valence band edges (CB and VB, respectively) of the materials inferring their single-layer and 2H phase configuration. Band edges of the semiconductors form a type-II band alignment resulting in current rectification. In junctions of WSe2 and MoS2 with single layers having a partial overlap, we map band edges along different points on individual semiconductors and the overlapped region (junction). The results have provided experimental evidence of current rectification through van der Waals vertical heterojunctions between two single layers.

Differential conductance (dI/dV) imaging of a heterojunction-nanorod.

Through scanning tunneling spectroscopy, we envisage imaging a heterostructure, namely a junction formed in a single nanorod. While the differential conductance spectrum provides location of conduction and valence band edges, dI/dV images record energy levels of materials. Such dI/dV images at different voltages allowed us to view p- and n-sections of heterojunction nanorods and more importantly the depletion region in such a junction that has a type-II band alignment. Viewing of selective sections in a heterojunction occurred due to band-bending in the junction and is correlated to the density of states spectrum of the individual semiconductors. The dI/dV images recorded at different voltages could be used to generate a band diagram of a pn junction.

Heterodimers formed through a partial anionic exchange process: scanning tunneling spectroscopy to monitor bands across the junction vis-à-vis photoinduced charge separation.

We report controlled formation of heterodimers and their charge separation properties. CdS|CdTe heterodimers were formed through an anionic exchange process of CdS nanostructures. With control over the duration of the anionic exchange process, bulk|dot, bulk|bulk, and then dot|bulk phases of the semiconductors could be observed to have formed. A mapping of density of states as derived from scanning tunneling spectroscopy (STS) brought out conduction and valence band-edges along the nanostructures and heterodimers. The CdS|CdTe heterodimers evidenced a type-II band-alignment between the semiconductors along with the formation of a depletion region at the interface. The width (of the depletion region) and the energy-offset at the interface depended on the size of the semiconductors. We report that the width that is instrumental for photoinduced charge separation in the heterodimers has a direct correlation with the performance of hybrid bulk-heterojunction solar cells based on the nanostructures in a polymer matrix.

Improvement in PbS-based Hybrid Bulk-Heterojunction Solar Cells through Band Alignment via Bismuth Doping in the Nanocrystals.

We introduce dopants in lead sulfide (PbS) quantum dots (QDs) in forming hybrid bulk-heterojunction (BHJ) solar cells. Because an increase in the content of bismuth as dopants in PbS QDs transforms the intrinsic p-type semiconductor into an n-type one, the band alignment between a conjugated polymer and the doped QDs changes upon doping affecting performance of BHJ solar cells. From scanning tunneling spectroscopy (STS) of the doped QDs, we observe a shift in their Fermi energy leading to formation of a type II band alignment in the polymer:doped-QD interface. We also show that the dopants improve electron-conduction process through the QDs. With the dopants controlling both band alignments at the interface and the conduction process, we show that the dopant concentration in QDs influences open-circuit voltage unfavorably and short-circuit current in a beneficial manner. The device performance of hybrid BHJ solar cells is hence maximized at an optimum concentration of bismuth in PbS QDs.

p-i-n heterojunctions with BiFeO3 perovskite nanoparticles and p- and n-type oxides: photovoltaic properties.

We formed p-i-n heterojunctions based on a thin film of BiFeO3 nanoparticles. The perovskite acting as an intrinsic semiconductor was sandwiched between a p-type and an n-type oxide semiconductor as hole- and electron-collecting layer, respectively, making the heterojunction act as an all-inorganic oxide p-i-n device. We have characterized the perovskite and carrier collecting materials, such as NiO and MoO3 nanoparticles as p-type materials and ZnO nanoparticles as the n-type material, with scanning tunneling spectroscopy; from the spectrum of the density of states, we could locate the band edges to infer the nature of the active semiconductor materials. The energy level diagram of p-i-n heterojunctions showed that type-II band alignment formed at the p-i and i-n interfaces, favoring carrier separation at both of them. We have compared the photovoltaic properties of the perovskite in p-i-n heterojunctions and also in p-i and i-n junctions. From current-voltage characteristics and impedance spectroscopy, we have observed that two depletion regions were formed at the p-i and i-n interfaces of a p-i-n heterojunction. The two depletion regions operative at p-i-n heterojunctions have yielded better photovoltaic properties as compared to devices having one depletion region in the p-i or the i-n junction. The results evidenced photovoltaic devices based on all-inorganic oxide, nontoxic, and perovskite materials.

Band mapping across a pn-junction in a nanorod by scanning tunneling microscopy.

We map band-edges across a pn-junction that was formed in a nanorod. We form a single junction between p-type Cu2S and n-type CdS through a controlled cationic exchange process of CdS nanorods. We characterize nanorods of the individual materials and the single junction in a nanorod with an ultrahigh vacuum scanning tunneling microscope (UHV-STM) at 77 K. From scanning tunneling spectroscopy and correspondingly the density of states (DOS) spectra, we determine the conduction and valence band-edges at different points across the junction and the individual nanorods. We could map the band-diagram of nanorod-junctions to bring out the salient features of a diode, such as p- and n-sections, band-bending, depletion region, albeit interestingly in the nanoscale.

Aligned magnetic domains in p- and n-type ferromagnetic nanocrystals and in pn-junction nanodiodes.

We form pn- and np-junctions between monolayers of p- and n-type nanocrystals that exhibit current rectification in the nanodiodes when characterized with a scanning tunneling microscope (STM) tip. With the use of ferromagnetic nanocrystals, we study the effect of mutual alignment of magnetization vectors on current rectification in the junction between the two nanocrystals. We show that when the magnetization vectors of the p- and of the n-type nanocrystals are parallel to each other (and both facing toward the apex of the STM tip) tunneling current in both bias modes increases with correspondingly a higher rectification ratio. This is in contrast to the parameters of the nanodiodes in which magnetization vectors of the components are unaligned or randomized. To analyze the results, we record scanning tunneling spectroscopy of the monolayer of the components having magnetization vectors aligned or unaligned to locate their valence and conduction band edges and to determine the effect of the alignment on the band edges. Upon alignment of the magnetization vectors of the nanocrystals in a monolayer, the conduction band edge of the p-type and valence band edge of the n-type semiconductor shift towards the Fermi energy leading to a change in energy levels of the pn-junctions and accounting for the improved parameters of the nanodiodes.

Molecular rectifiers based on donor/acceptor assemblies: effect of orientation of the components' magnetic moments.

In forming donor/acceptor assemblies that act as molecular rectifiers, we have introduced magnetic organic molecules as electron-donating and electron-accepting moieties. We have oriented the magnetic moment of the donor and acceptor components separately and immobilized them (and their moments) so that the molecular assemblies that act as rectifiers could be formed with moments mutually parallel or anti-parallel to each other. We have characterized the molecular assemblies formed on an electrode with a scanning tunneling microscope tip. Such donor/acceptor assemblies with a control over the orientation of moments of the components provided unique systems to study the effect of the nature of alignment on molecular rectifiers. We have observed that the rectification ratio increased in junctions with moments of the components being parallel to each other. The improvement in the rectification ratio has been explained in terms of an efficient electron-transfer process in a moment-aligned junction between the donor and acceptor moieties.

Magnetic moment assisted layer-by-layer film formation of a Prussian Blue analog.

We formed magnetic moment assisted layer-by-layer (LbL) films of a Prussian Blue analogue (PB). We applied an external magnetic field to each monolayer of PB to orient the magnetic moment of the compound perpendicular to the substrate. Aligned moments or orientation of the magnetic compounds themselves were immobilized in each monolayer, so that the moments could augment formation of the subsequent monolayers of LbL adsorption process. We hence could form multilayered LbL films of PB molecules with their magnetic moments oriented perpendicular to the substrate. We also formed LbL films of the compound with their moments oriented parallel to the substrate and facing one particular direction. We have measured conductivity and dielectric constant of the two types of films and compared the parameters with that of conventional LbL films deposited without orienting magnetic moments of the molecules.