A BN-Doped U-Shaped Heteroacene as a Molecular Floating Gate for Ambipolar Charge Trapping Memory.

Two wide-band gap U-shaped polycyclic aromatic hydrocarbons with/without boron and nitrogen (BN-) doping (BN-1 and C-1) were synthesized to tune the electronic features to suit the performance requirements for organic field-effect transistor memory (OFET-NVM). The chemical structures were characterized by scanning tunneling microscopy and single-crystal diffraction. Owing to the electron-donor effect of N and the high electron affinity of B, the BN-1-based OFET-NVM displays large ambipolar memory windows and an enhanced charge storage density compared to C-1 and most reported small molecules. A novel supramolecular system formed from BN-1 and PMMA contributes to fabricating uniform films with homogeneous microstructures, which serve as a two-in-one tunnelling dielectric and charge-trapping layer to realize long-term charge retention and reliable endurance. Our results demonstrate that both BN doping and supramolecular engineering are crucial for the charge trapping of OFET-NVM.

Direct Layer-by-Layer Growth of Crystalline Ag-TCNQ Thin Films on Functionalized Au Substrate: How Critical Is the pH of Ag(I) Solution?

Wet-chemical fabrication of a crystalline Ag-TCNQ (TCNQ = 7,7,8,8-tetracyanoquinodimethane) thin film on non-Ag substrate is challenging whereby the chemistry was powered by photon energy and/or electrical energy. We report for the first time, direct chemical growth of a Ag-TCNQ thin film on a functionalized Au substrate by employing the layer-by-layer (LbL) approach at ambient reaction conditions. Various Ag(I) salt precursors previously realized to be unsuitable for the fabrication of Ag-TCNQ thin films on non-Ag substrates ultimately gave rise to dense and uniform thin films of Ag-TCNQ. The crucial knob regulating the direct formation of the thin films of Ag-TCNQ was identified to be the pH of the respective Ag(I) solutions.

Emergent Interface in Heterostructured Thin Films of Cu(II) and Cu(I) Coordination Polymers.

In this work we report fabrication of high-quality AB- and BA-type heterostructured thin films of cubic Cu(II) (A-type) and tetragonal Cu(I) (B-type) coordination polymers (CPs) on the functionalized Au substrate by the layer-by-layer method. Successful growth of Cu(I)-CP on top of Cu(II)-CP was assigned to be due to the interfacial reduction reaction (IRR). Notably, electrical transport measurements across AB- and BA-type heterostructured thin films revealed rectification of current in opposite directions. We have attributed such an interestingly new observation to the formation of a well-defined interface of Cu(II)-CP and Cu(I)-CP resembling a p-n junction-a hitherto unreported phenomenon that is anticipated to open enormous opportunities for the heterostructured thin films of CPs, likewise celebrated interfaces of oxide heterostructures.

Perspective on the Interfacial Reduction Reaction.

Chemical reactions involving oxidation and reduction processes at interfaces may vary from those in conventional liquid-phase or solid-phase reactions and could influence the overall outcome. This article primarily features a study on metal-ligand coordination at the solid-liquid interface. Of particular mention is the spontaneous reduction of Cu(II) to Cu(I) at a solid-liquid interface without the need of any extraneous reducing agent, unlike in the liquid-phase reaction whereby no reduction of Cu(II) to Cu(I) took place. As a consequence of the interfacial reduction reaction (IRR), thin films of Cu-TCNQ (tetracyanoquinodimethane) and Cu-HCF (hexacyanoferrate) were successfully deposited onto a thiol-functionalized Au substrate via a layer-by-layer (LbL) method. IRR is anticipated to be useful in generating new functional and stimuli-responsive materials, which are otherwise difficult to achieve via conventional liquid-phase reactions.

Embedding S = 1/2 Kagome-like Lattice in Reduced Graphene Oxide.

An elegant platform to explore frustrated magnetism is the kagome spin lattice. In this work, clinoatacamite, a naturally occurring S = 1/2 kagome-like antiferromagnetic insulator, is synthesized in water at ambient pressure for the first time from a cuprous chloride (CuCl) precursor whereby Cu(I) was spontaneously oxidized to Cu(II) in the form of clinoatacamite [Cu2(OH)3Cl] with a simultaneous reduction of graphene oxide (GO) to reduced graphene oxide (rGO) in one pot. A stable nanocomposite of phase-pure clinoatacamite nanocrystals embedded in the rGO matrix was isolated. The clinoatacamite-rGO nanocomposite was determined to be magnetically active with a markedly enhanced coercive field of ∼2500 Oe at 5 K as well as electronically active with a conductivity value of ∼200 S·m-1 at 300 K. Our results illustrate an avenue of combining exotic magnetic and electronic lattices without impeding their individual characteristics and synergistically generating a new class of magnetic semiconductors.

Achieving current rectification ratios ≥ 105 across thin films of coordination polymer.

Downsizing coordination polymers (CPs) to thin film configurations is a prerequisite for device applications. However, fabrication of thin films of CPs including metal-organic frameworks (MOFs) with reasonable electrical conductivity is challenging. Herein, thin film fabrication of a Cu(ii)-CP employing a layer-by-layer method is demonstrated whereby a self-assembled monolayer on Au was used as the functionalized substrate. Growth of the Cu(ii)-CP at the solid-liquid interface generated open-metal Cu(ii) sites in the thin film which were susceptible to activation by molecular dopant molecules. A significant enhancement in in-plane electrical conductivity and an unheralded cross-plane current rectification ratio (exceeding 105 both at room-temperature and at an elevated temperature) were achieved. Such a remarkable rectification ratio was realized, similar to those of commercial Si rectifier diodes. This phenomenon is attributed to the formation of an electronic heterostructure in the molecularly doped thin film. Molecular doping additionally transformed the interfacial properties of thin films from hydrophilic to highly hydrophobic.

Metamagnetism in Nanosheets of CoII-MOF with TN at 26 K and a Giant Hysteretic Effect at 5 K.

Herein, we have synthesized at room-temperature two-dimensional nanosheets of a MOF comprised of cobalt(II) ion with benzenedicarboxylic acid  ligand, which exhibited unusual magnetic properties. Direct-current magnetic susceptibility revealed an antiferromagnetic (AFM) transition at 26 K (Néel temperature,  TN) followed by a canting of the spin moments along with the concomitant appearance of a sigmoidal-shaped magnetization versus field ( M- H) curve at 15 K. Such a canted AFM ordering led to nonzero remnant magnetization with a remarkably high coercive field of ∼10 kOe at 5 K. Metamagnetism was further substantiated by the alternating-current magnetic susceptibility measurements.

Spontaneous Reduction of Copper(II) to Copper(I) at Solid-Liquid Interface.

Oxidation and reduction reactions are of central importance in chemistry as well as vital to the basic functions of life and such chemical processes are generally brought about by oxidizing and reducing agents, respectively. Herein, we report the discovery of an interfacial reduction reaction (IRR) - without the use of any external reducing agent. In course of metal-ligand coordination, spontaneous reduction of Cu(II) to Cu(I) at a solid-liquid interface was observed-unlike in a liquid-phase reaction where no reduction of Cu(II) to Cu(I) was occurred. High-quality thin films of a new coordination network compound bearing a Fe(II)-CN-Cu(I) link were fabricated by IRR and employed for efficient electro-catalysis in the form of oxygen reduction reaction. Also, thermally activated reversible structural phase transition modulated the electron transport property in thin film. This work unveils the importance of chemical reactions at solid-liquid interfaces that can lead to the development of new functional thin film materials.

Thermally Driven Resistive Switching in Solution-Processable Thin Films of Coordination Polymers.

Metal-organic coordination polymers (CPs) downsized to thin films with controllable electrical conductivity are promising for electronic device applications. Here we demonstrate, for the first time, thermally driven resistive switching in thin films of semiconducting CPs consisting of silver ion and tetracyanoquinodimethane ligand (Ag-TCNQ). High-quality and highly hydrophobic thin films of Ag-TCNQ were fabricated through a layer-by-layer approach upon sacrificing a predeposited layer of Cu-TCNQ on a thiolated Au substrate. Reversible switching between the high-resistance state (HRS) at 300 K and the low-resistance state (LRS) at 400 K with an enhancement factor of as high as ∼106 in the electrical resistance was realized. The phenomenon is attributed to the alternation of the Schottky barrier at the metal-semiconductor interface by thermal energy and not due to the formation of a conductive filament. Our discovery of thermally driven resistive switching as well as sacrificial growth of CP thin films on an organically modified substrate holds promise for the development of solution-processable nonvolatile memory devices.

Selective Sensing of Metal Ions and Nitro Explosives by Efficient Switching of Excimer-to-Monomer Emission of an Amphiphilic Pyrene Derivative.

An amphiphilic pyrene derivative exhibiting unusually stable excimer emission due to strong aggregation is presented. The aggregated system served as an intelligent sensor for metal ions and nitro explosives in aqueous media. The excimer displayed excellent selectivity toward Cu2+ among the tested cations. The observation was interpreted on the basis of chelation of metal ions involving the hydroxyl and amino groups of two molecules, leading to the ligand-to-metal charge-transfer (CT) process. The excimer was further applied for the cell imaging of Cu2+ ions. Also, while treating the excimer with various nitro explosives, it displayed efficient 2,4,6-trinitrophenol sensing, corroborating mainly the CT process from pyrene to the analyte due to intercalation of the analyte within pyrene.