Locking and Unlocking the Molecular Spin Crossover Transition

The Fe(II) spin crossover complex [Fe{H₂B(pz)₂}₂(bipy)] (pz = pyrazol‐1‐yl, bipy = 2,2′‐bipyridine) can be locked in a largely low‐spin‐state configuration over a temperature range that includes temperatures well above the thermal spin crossover temperature of 160 K. This locking of the spin state is achieved for nanometer thin films of this complex in two distinct ways: through substrate interactions with dielectric substrates such as SiO₂ and Al₂O₃, or in powder samples by mixing with the strongly dipolar zwitterionic p ‐benzoquinonemonoimine C₆H₂(—? NH₂)₂(—? O)₂. Remarkably, it is found in both cases that incident X‐ray fluences then restore the [Fe{H₂B(pz)₂}₂(bipy)] moiety to an electronic state characteristic of the high spin state at temperatures of 200 K to above room temperature; that is, well above the spin crossover transition temperature for the pristine powder, and well above the temperatures characteristic of light‐ or X‐ray‐induced excited‐spin‐state trapping. Heating slightly above room temperature allows the initial locked state to be restored. These findings, supported by theory, show how the spin crossover transition can be manipulated reversibly around room temperature by appropriate design of the electrostatic and chemical environment.     

Finite Size Effects on the Switching Dynamics of Spin‐Crossover Thin Films Photoexcited by a Femtosecond Laser Pulse

Using ultrafast optical absorption spectroscopy, the room‐temperature spin‐state switching dynamics induced by a femtosecond laser pulse in high‐quality thin films of the molecular spin‐crossover (SCO) complex [Fe(HB(tz)₃)₂] (tz = 1,2,4‐triazol‐1‐yl) are studied. These measurements reveal that the early, sub‐picosecond, low‐spin to high‐spin photoswitching event, with linear response to the laser pulse energy, can be followed under certain conditions by a second switching process occurring on a timescale of tens of nanoseconds, enabling nonlinear amplification. This out‐of‐equilibrium dynamics is discussed in light of the characteristic timescales associated with the different switching mechanisms, i.e., the electronic and structural rearrangements of photoexcited molecules, the propagation of strain waves at the material scale, and the thermal activation above the molecular energy barrier. Importantly, the additional, nonlinear switching step appears to be completely suppressed in the thinnest (50 nm) film due to the efficient heat transfer to the substrate, allowing the system to retrieve the thermal equilibrium state on the 100 ns timescale. These results provide a first milestone toward the assessment of the physical parameters that drive the photoresponse of SCO thin films, opening up appealing perspectives for their use as high‐frequency all‐optical switches working at room temperature.     

Design of Bistable Gold@Spin‐Crossover Core–Shell Nanoparticles Showing Large Electrical Responses for the Spin Switching

A simple chemical protocol to prepare core–shell gold@spin‐crossover (Au@SCO) nanoparticles (NPs) based on the 1D spin‐crossover [Fe(Htrz)₂(trz)](BF₄) coordination polymer is reported. The synthesis relies on a two‐step approach consisting of a partial surface ligand substitution of the citrate‐stabilized Au NPs followed by the controlled growth of a very thin layer of the SCO polymer. As a result, colloidally stable core@shell spherical NPs with a Au core of ca. 12 nm and a thin SCO shell 4 nm thick, are obtained, exhibiting a narrow distribution in sizes. Differential scanning calorimetry proves that a cooperative spin transition in the range 340–360 K is maintained in these Au@SCO NPs, in full agreement with the values reported for pristine 4 nm SCO NPs. Temperature‐dependent charge‐transport measurements of an electrical device based on assemblies of these Au@SCO NPs also support this spin transition. Thus, a large change in conductance upon spin state switching, as compared with other memory devices based on the pristine SCO NPs, is detected. This results in a large improvement in the sensitivity of the device to the spin transition, with values for the ON/OFF ratio which are an order of magnitude better than the best ones obtained in previous SCO devices.     

Efficient and Stable Solid‐State Dye‐Sensitized Solar Cells Based on a High‐Molar‐Extinction‐Coefficient Sensitizer

The high‐molar‐extinction‐coefficient heteroleptic ruthenium dye, cis‐Ru (4,4′‐bis(5‐octylthieno[3,2‐b] thiophen‐2‐yl)‐2,2′‐bipyridine) (4,4′‐dicarboxyl‐2,2′‐bipyridine) (NCS)₂, exhibits an AM 1.5 solar (100 mW cm^?2)‐to‐electric power‐conversion efficiency of 4.6% in a solid‐state dye‐sensitized solar cell (SSDSC) with 2,2′, 7,7′‐tetrakis‐(N,N‐di‐p‐methoxyphenylamine)9,9′‐spirobifluorene (spiro‐MeOTAD) as the organic hole‐transporting material. These SSDSC devices exhibit good durability during accelerated tests under visible‐light soaking for 1000 h at 60 °C. This demonstration elucidates a class of photovoltaic devices with potential for stable and low‐cost power generation. The electron recombination dynamics and charge collection that take place at the dye‐sensitized heterojunction are studied by means of impedance and transient photovoltage decay techniques.     

Magnetic Susceptibility Study of Sub‐Pico‐emu Sample Using a Micromagnetometer: An Investigation through Bistable Spin‐Crossover Materials

A promising and original method to study the spin‐transition in bistable spin‐crossover (SCO) materials using a magnetoresistive multiring sensor and its self‐generated magnetic field is reported. Qualitative and quantitative studies are carried out combining theoretical and experimental approaches. The results show that only a small part of matter dropped on the sensor surface is probed by the device. At a low bias‐current range, the number of detected nanoparticles depends on the amplitude of the current. However, in agreement with the theoretical model, the stray voltage from the particles is proportional to the current squared. By changing both the bias current and the concentration of particle droplet, the thermal hysteresis of an ultrasmall volume, 1 × 10^?4 mm³, of SCO particles is measured. The local probe of the experimental setup allows a highest resolution of 4 × 10^?14 emu to be reached, which is never achieved by experimental methods at room temperature.     

In situ X‐ray Measurements to Probe a New Solid‐State Polycondensation Mechanism for the Design of Supramolecular Organo‐Bridged Silsesquioxanes

A long‐range ordered organic/inorganic material is synthesized from a bis‐silane, (EtO)₃Si? (CH₂)₃? NHCONH? C₆H₄? NHCONH? (CH₂)₃? Si(OEt)₃. This crosslinked sol–gel solid exhibits a supramolecular organization via intermolecular hydrogen bonding interactions between urea groups (? NHCONH? ) and covalent siloxane bonding, ?Si? O? Si?. Time‐resolved in situ X‐ray measurements (coupling small‐ and wide‐angle X‐ray scattering techniques) are performed to follow the different steps involved in the synthetic process. A new mechanism based on the crystallization of the hydrolyzed species followed by their polycondensation in solid state is proposed.     

One‐Pot Exfoliation of Graphitic C3N4 Quantum Dots for Blue QLEDs by Methylamine Intercalation

Here, a simplified synthesis of graphitic carbon nitride quantum dots (g‐C₃N₄‐QDs) with improved solution and electroluminescent properties using a one‐pot methylamine intercalation–stripping method (OMIM) to hydrothermally exfoliate QDs from bulk graphitic carbon nitride (g‐C₃N₄) is presented. The quantum dots synthesized by this method retain the blue photoluminescence with extremely high fluorescent quantum yield (47.0%). As compared to previously reported quantum dots, the g‐C₃N₄‐QDs synthesized herein have lower polydispersity and improved solution stability due to high absolute zeta‐potential (?41.23 mV), which combine to create a much more tractable material for solution processed thin film fabrication. Spin coating of these QDs yields uniform films with full coverage and low surface roughness ideal for quantum dot light‐emitting diode (QLED) fabrication. When incorporated into a functional QLED with OMIM g‐C₃N₄‐QDs as the emitting layer, the LED demonstrates ≈60× higher luminance (605 vs 11 Cd m^?2) at lower operating voltage (9 vs 21 V), as compared to the previously reported first generation g‐C₃N₄ QLEDs, though further work is needed to improve device stability.     

Low‐Temperature Soft‐Cover Deposition of Uniform Large‐Scale Perovskite Films for High‐Performance Solar Cells

Large‐scale high‐quality perovskite thin films are crucial to produce high‐performance perovskite solar cells. However, for perovskite films fabricated by solvent‐rich processes, film uniformity can be prevented by convection during thermal evaporation of the solvent. Here, a scalable low‐temperature soft‐cover deposition (LT‐SCD) method is presented, where the thermal convection‐induced defects in perovskite films are eliminated through a strategy of surface tension relaxation. Compact, homogeneous, and convection‐induced‐defects‐free perovskite films are obtained on an area of 12 cm², which enables a power conversion efficiency (PCE) of 15.5% on a solar cell with an area of 5 cm². This is the highest efficiency at this large cell area. A PCE of 15.3% is also obtained on a flexible perovskite solar cell deposited on the polyethylene terephthalate substrate owing to the advantage of presented low‐temperature processing. Hence, the present LT‐SCD technology provides a new non‐spin‐coating route to the deposition of large‐area uniform perovskite films for both rigid and flexible perovskite devices.     

Intramolecular Locked Dithioalkylbithiophene‐Based Semiconductors for High‐Performance Organic Field‐Effect Transistors

New 3,3′‐dithioalkyl‐2,2′‐bithiophene ( SBT )‐based small molecular and polymeric semiconductors are synthesized by end‐capping or copolymerization with dithienothiophen‐2‐yl units. Single‐crystal, molecular orbital computations, and optical/electrochemical data indicate that the SBT core is completely planar, likely via S(alkyl)?S(thiophene) intramolecular locks. Therefore, compared to semiconductors based on the conventional 3,3′‐dialkyl‐2,2′‐bithiophene, the resulting SBT systems are planar (torsional angle <1°) and highly π‐conjugated. Charge transport is investigated for solution‐sheared films in field‐effect transistors demonstrating that SBT can enable good semiconducting materials with hole mobilities ranging from ≈0.03 to 1.7 cm² V^?1 s^?1. Transport difference within this family is rationalized by film morphology, as accessed by grazing incidence X‐ray diffraction experiments.     

Polyarene‐Functionalized Fullerenes in Carbon Nanotubes: Towards Controlled Geometry of Molecular Chains

Mechanisms for controlling the assembly of molecular arrays in carbon nanotubes via alteration of the size and geometry of the functional groups attached to the molecules inserted into the nanotubes are studied. As model compounds, a series of structurally related fullerenes functionalized with polyaryl groups (C₆₀X, where X is a polyaryl group) of various lengths are synthesized to explore this effect. These molecules are inserted into single‐walled carbon nanotubes (SWNTs) under mild conditions to prevent their decomposition and to form C₆₀X@SNWT structures. The molecular chains thus formed are studied by high‐resolution transmission electron microscopy, X‐ray diffraction, and Raman spectroscopy, revealing that the functional groups increase the interfullerene separation proportionally with the size of X. However, the functional groups themselves appear to adopt various orientations with respect to each other and exhibit intermolecular π–π interactions within the cavities of the carbon nanotubes. All these effects create a distribution of observed interfullerene separations in nanotubes, which are examined by theoretical simulations and interpreted in terms of molecular geometries and intermolecular interactions.     

Revealing the Cooperative Relationship between Spin,Energy, and Polarization Parameters toward Developing High‐Efficiency Exciplex Light‐Emitting Diodes

Experimental studies to reveal the cooperative relationship between spin, energy, and polarization through intermolecular charge‐transfer dipoles to harvest nonradiative triplets into radiative singlets in exciplex light‐emitting diodes are reported. Magneto‐photoluminescence studies reveal that the triplet‐to‐singlet conversion in exciplexes involves an artificially generated spin‐orbital coupling (SOC). The photoinduced electron parametric resonance measurements indicate that the intermolecular charge‐transfer occurs with forming electric dipoles (D^+?→A^??), providing the ionic polarization to generate SOC in exciplexes. By having different singlet‐triplet energy differences (ΔE_ST) in 9,9′‐diphenyl‐9H,9′H‐3,3′‐bicarbazole (BCzPh):3′,3′″,3′″″‐(1,3,5‐triazine‐2,4,6‐triyl)tris(([1,1′‐biphenyl]‐3‐carbonitrile)) (CN‐T2T) (ΔE_ST = 30 meV) and BCzPh:bis‐4,6‐(3,5‐di‐3‐pyridylphenyl)‐2‐methyl‐pyrimidine (B3PYMPM) (ΔE_ST = 130 meV) exciplexes, the SOC generated by the intermolecular charge‐transfer states shows large and small values (reflected by different internal magnetic parameters: 274 vs 17 mT) with high and low external quantum efficiency maximum, EQE_max (21.05% vs 4.89%), respectively. To further explore the cooperative relationship of spin, energy, and polarization parameters, different photoluminescence wavelengths are selected to concurrently change SOC, ΔE_ST, and polarization while monitoring delayed fluorescence. When the electron clouds become more deformed at a longer emitting wavelength due to reduced dipole (D^+?→A^??) size, enhanced SOC, increased orbital polarization, and decreased ΔE_ST can simultaneously occur to cooperatively operate the triplet‐to‐singlet conversion.     

One‐Step Solution Phase Growth of Transition Metal Dichalcogenide Thin Films Directly on Solid Substrates

Ultrathin transition metal dichalcogenides (TMDs) have exotic electronic properties. With success in easy synthesis of high quality TMD thin films, the potential applications will become more viable in electronics, optics, energy storage, and catalysis. Synthesis of TMD thin films has been mostly performed in vacuum or by thermolysis. So far, there is no solution phase synthesis to produce large‐area thin films directly on target substrates. Here, this paper reports a one‐step quick synthesis (within 45–90 s) of TMD thin films (MoS₂, WS₂, MoSe₂, WSe₂, etc.) on solid substrates by using microwave irradiation on a precursor‐containing electrolyte solution. The numbers of the quintuple layers of the TMD thin films are precisely controllable by varying the precursor's concentration in the electrolyte solution. A photodetector made of MoS₂ thin film comprising of small size grains shows near‐IR absorption, supported by the first principle calculation, exhibits a high photoresponsivity (>300 mA W^?1) and a fast response (124 µs). This study paves a robust way for the synthesis of various TMD thin films in solution phases.     

Controllable,Wide‐Ranging n‐Doping and p‐Doping of Monolayer Group 6 Transition‐Metal Disulfides and Diselenides

Developing processes to controllably dope transition‐metal dichalcogenides (TMDs) is critical for optical and electrical applications. Here, molecular reductants and oxidants are introduced onto monolayer TMDs, specifically MoS₂, WS₂, MoSe₂, and WSe₂. Doping is achieved by exposing the TMD surface to solutions of pentamethylrhodocene dimer as the reductant (n‐dopant) and “Magic Blue,” [N(C₆H₄‐p‐Br)₃]SbCl₆, as the oxidant (p‐dopant). Current–voltage characteristics of field‐effect transistors show that, regardless of their initial transport behavior, all four TMDs can be used in either p‐ or n‐channel devices when appropriately doped. The extent of doping can be controlled by varying the concentration of dopant solutions and treatment time, and, in some cases, both nondegenerate and degenerate regimes are accessible. For all four TMD materials, the photoluminescence intensity; for all four materials the PL intensity is enhanced with p‐doping but reduced with n‐doping. Raman and X‐ray photoelectron spectroscopy (XPS) also provide insight into the underlying physical mechanism by which the molecular dopants react with the monolayer. Estimates of changes of carrier density from electrical, PL, and XPS results are compared. Overall a simple and effective route to tailor the electrical and optical properties of TMDs is demonstrated.     

Two‐Dimensional Mott Insulators in SrVO3 Ultrathin Films

Strongly correlated oxides that undergo a metal‐insulator transition (MIT) are a subject of great current interest for their potential application to future electronics as switches and sensors. Recent advances in thin film technology have opened up new avenues to tailor MIT for novel devices beyond conventional CMOS scaling. Here, dimensional‐crossover‐driven MITs are demonstrated in high‐quality epitaxial SrVO₃ (SVO) thin films grown by a pulsed electron‐beam deposition technique. Thick SVO films (∼25 nm) exhibit metallic behavior with the electrical resistivity following the T² law corresponding to a Fermi liquid system. A temperature driven MIT is induced in SVO ultrathin films with thicknesses below 6.5 nm. The transition temperature T_MIT is at 50 K for the 6.5 nm film, 120 K for the 5.7 nm film and 205 K for the 3 nm film. The emergence of the observed MIT can be attributed to the dimensional crossover from a three‐dimensional metal to a two‐dimensional Mott insulator, as the resulting reduction in the effective bandwidth W opens a band gap at the Fermi level. The magneto‐transport study of the SVO ultrathin films also confirm the observed MIT is due to the electron‐electron interactions other than disorder‐induced localization.     

Tribological properties of Ta‐C‐N and Ta‐N thin films

The amorphous Ta‐C‐N and Ta‐N thin films were deposited using magnetron sputtering on silicon wafer under the similar condition. The as‐prepared thin films were characterized using scanning electron microscope (SEM), optical profiling system, nano‐indentation and friction test instruments. The results show that, compared with the Ta‐N thin film, the Ta‐C‐N thin film has higher nano‐hardness (9.45 GPa) and elastic modulus (225.71 GPa). Furthermore, the lower friction coefficient and wear rate of the Ta‐C‐N thin film are 0.238 and 5.94×10^–6 mm^–3· N^–1·m^–1, respectively. The wear surface of Ta‐C‐N thin film is smoother than that of the Ta‐N thin film. Therefore, it shows better anti‐wear properties.     

Atomically Precise Prediction of 2D Self‐Assembly of Weakly Bonded Nanostructures: STM Insight into Concentration‐Dependent Architectures

A joint experimental and computational study is reported on the concentration‐dependant self‐assembly of a flat C₃‐symmetric molecule on a graphite surface. As a model system a tripodal molecule, 1,3,5‐tris(pyridin‐3‐ylethynyl)benzene, has been chosen, which can adopt either C_3h or C_s symmetry when planar, as a result of pyridyl rotation along the alkynyl spacers. Density functional theory (DFT) simulations of 2D nanopatterns with different surface coverage reveal that the molecule can generate different types of self‐assembled motifs. The stability of fourteen 2D patterns and the influence of concentration are analyzed. It is found that ordered, densely packed monolayers and 2D porous networks are obtained at high and low concentrations, respectively. A concentration‐dependent scanning tunneling microscopy (STM) investigation of this molecular self‐assembly system at a solution/graphite interface reveals four supramolecular motifs, which are in perfect agreement with those predicted by simulations. Therefore, this DFT method represents a key step forward toward the atomically precise prediction of molecular self‐assembly on surfaces and at interfaces.     

SERS property of Co complex on boron nitride nanosheets/silver nanoparticles aggregates and photophysical characterisation of interactions among Co complex ion-exchanged within zirconium phosphate

ABSTRACT

The Co(II) complex of [Co(bpy)₂(NO₃)]Cl·3H₂O, bpy = 2,2'-bipyridine, has been synthesised by the solution crystallisation method. The boron nitride nanosheets/silver nanoparticles/[Co(bpy)₂(NO₃)]Cl·3H₂O nanoaggregate was prepared in order to better analyse the adsorption orientation of the cobalt complex on the surface of silver nanoparticles. The result of the surface-enhanced Raman scattering measurement indicates that the molecular plane of the cobalt complex presents a tilted orientation with respect to the surface of silver nanoparticles. The luminescence property of [Co(bpy)₂(NO₃)]⁺ two-dimensional arrangements within the layers of zirconium phosphate (ZrP) was also investigated. Steady-state luminescence spectra of the [Co(bpy)₂(NO₃)]⁺-exchanged ZrP materials show an increase in the luminescence intensity.     

Substrate Selection for Full Exploitation of Organic Semiconductor Films: Epitaxial Rubrene on β‐Alanine Single Crystals

Rubrene (RUB) is one of the most studied organic semiconductors because, in the orthorhombic single‐crystal phase, it exhibits a record exciton diffusion length and one of the highest charge carrier mobilities ever reported. Here, thin films of oriented crystalline RUB are successfully grown in vacuum on millimeter‐sized (010)‐β‐alanine (β‐ala) single crystals with a step‐growth protocol, exploiting organic epitaxy. The experimental characterization demonstrates that these RUB films grow in the orthorhombic polymorph with the (100)_RUB plane in contact with the (010)_β‐ala surface and with precise azimuthal orientations. A complementary study of the RUB(100)/β‐ala(010) interface, performed by computational simulations, confirms the epitaxial relations expected by considering the molecular scale corrugations of the surfaces. Moreover, thanks to the wide transparency region of β‐ala, the RUB absorption bands in the UV range are directly detected for the first time. Finally, removal of the water‐soluble substrate enables the integration of the films in field effect transistors as high quality active organic layers. The characteristics of such RUB‐based devices confirm the quality and versatility of epitaxial thin films for use in organic electronics.     

Exposing {001} Crystal Plane on Hexagonal Ni‐MOF with Surface‐Grown Cross‐Linked Mesh‐Structures for Electrochemical Energy Storage

Hexagonal nickel‐organic framework (Ni‐MOF) [Ni(NO₃)₂·6H₂O, 1,3,5‐benzenetricarboxylic acid, 4‐4′‐bipyridine] is fabricated through a one‐step solvothermal method. The {001} crystal plane is exposed to the largest hexagonal surface, which is an ideal structure for electron transport and ion diffusion. Compared with the surrounding rectangular crystal surface, the ion diffusion length through the {001} crystal plane is the shortest. In addition, the cross‐linked porous mesh structures growing on the {001} crystal plane strengthen the mixing with conductive carbon, inducing preferable conductivity, as well as increasing the area of ion contact and the number of active sites. These advantages enable the hexagonal Ni‐MOF to exhibit excellent electrochemical performance as supercapacitor electrode materials. In a three‐electrode cell, specific capacitance of hexagonal Ni‐MOF in the 3.0 m KOH electrolyte is 977.04 F g^?1 and remains at the initial value of 92.34% after 5,000 cycles. When the hexagonal Ni‐MOF and activated carbon are assembled into aqueous devices, the electrochemical performance remains effective.     

Thiazole Imide‐Based All‐Acceptor Homopolymer: Achieving High‐Performance Unipolar Electron Transport in Organic Thin‐Film Transistors

High‐performance unipolar n‐type polymer semiconductors are critical for advancing the field of organic electronics, which relies on the design and synthesis of new electron‐deficient building blocks with good solubilizing capability, favorable geometry, and optimized electrical properties. Herein, two novel imide‐functionalized thiazoles, 5,5′‐bithiazole‐4,4′‐dicarboxyimide (BTzI) and 2,2′‐bithiazolothienyl‐4,4′,10,10′‐tetracarboxydiimide (DTzTI), are successfully synthesized. Single crystal analysis and physicochemical study reveal that DTzTI is an excellent building block for constructing all‐acceptor homopolymers, and the resulting polymer poly(2,2′‐bithiazolothienyl‐4,4′,10,10′‐tetracarboxydiimide) (PDTzTI) exhibits unipolar n‐type transport with a remarkable electron mobility (μ_e) of 1.61 cm² V^?1 s^?1, low off‐currents (I_off) of 10^?10?10^?11 A, and substantial current on/off ratios (I_on/I_off) of 10⁷?10⁸ in organic thin‐film transistors. The all‐acceptor homopolymer shows distinctive advantages over prevailing n‐type donor?acceptor copolymers, which suffer from ambipolar transport with high I_offs > 10^?8 A and small I_on/I_offs < 10⁵. The results demonstrate that the all‐acceptor approach is superior to the donor?acceptor one, which results in unipolar electron transport with more ideal transistor performance characteristics.