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1.
Nat Commun ; 11(1): 3611, 2020 Jul 17.
Artigo em Inglês | MEDLINE | ID: mdl-32681047

RESUMO

Temperature measurement at the nanoscale is of paramount importance in the fields of nanoscience and nanotechnology, and calls for the development of versatile, high-resolution thermometry techniques. Here, the working principle and quantitative performance of a cost-effective nanothermometer are experimentally demonstrated, using a molecular spin-crossover thin film as a surface temperature sensor, probed optically. We evidence highly reliable thermometric performance (diffraction-limited sub-µm spatial, µs temporal and 1 °C thermal resolution), which stems to a large extent from the unprecedented quality of the vacuum-deposited thin films of the molecular complex [Fe(HB(1,2,4-triazol-1-yl)3)2] used in this work, in terms of fabrication and switching endurance (>107 thermal cycles in ambient air). As such, our results not only afford for a fully-fledged nanothermometry method, but set also a forthcoming stage in spin-crossover research, which has awaited, since the visionary ideas of Olivier Kahn in the 90's, a real-world, technological application.

2.
Nanoscale ; 2020 Apr 05.
Artigo em Inglês | MEDLINE | ID: mdl-32248213

RESUMO

A main goal of molecular electronics is to relate the performance of devices to the structure and electronic state of molecules. Among the variety of possibilities that organic, organometallic and coordination chemistries offer to tune the energy levels of molecular components, spin crossover phenomenon is a perfect candidate for elaboration of molecular switches. The reorganization of the electronic state population of the molecules associated to the spin crossover can indeed lead to a significant change in conductivity. However, molecular spin crossover is very sensitive to the environment and can disappear once the molecules are integrated into devices. Here, we show that the association of ultra-small 1.2 nm platinum nanoparticles with FeII triazole-based spin crossover coordination polymers leads to self-assemblies, extremely well organized at the sub-3 nm scale. The quasi-perfect alignment of nanoparticles observed by transmission electron microscopy, in addition to specific signature in infrared spectroscopy, demonstrates the coordination of the long-chain molecules with the nanoparticles. Spin crossover is confirmed in such assemblies by X-ray absorption spectroscopic measurements and shows unambiguous characteristics both in magnetic and charge transport measurements. Coordinating polymers are therefore ideal candidates for the elaboration of robust, well-organized, hybrid self-assemblies with metallic nanoparticles, while maintaining sensitive functional properties, such as spin crossover.

3.
Adv Mater ; : e2000987, 2020 Apr 17.
Artigo em Inglês | MEDLINE | ID: mdl-32301210

RESUMO

The thermally induced spin-crossover (SCO) phenomenon in transition metal complexes is an entropy-driven process, which has been extensively studied through calorimetric methods. Yet, the excess heat capacity associated with the molecular spin-state switching has never been explored for practical applications. Herein, the thermal damping effect of an SCO film is experimentally assessed by monitoring the transient heating response of SCO-coated metallic microwires, Joule-heated by current pulses. A damping of the wire temperature, up to 10%, is evidenced on a time scale of tens of microseconds due to the spin-state switching of the molecular film. Fast heat-charging dynamics and negligible fatigability are demonstrated, which, together with the solid-solid nature of the spin transition, appear as promising features for achieving thermal energy management applications in functional devices.

4.
J Phys Condens Matter ; 32(26): 264002, 2020 Mar 02.
Artigo em Inglês | MEDLINE | ID: mdl-32120350

RESUMO

The complex dielectric permittivity of a series of spin crossover complexes, with variable ligand stoichiometry [Fe(Htrz)1+y-x (trz)2-y (NH2trz) x ](BF4) y ·nH2O, has been investigated as a function of temperature in a wide frequency range. In each compound, a substantial drop of the conductivity and permittivity is evidenced when going from the low spin to the high spin state, albeit with decreasing amplitude for increasing ligand substitution (i.e. for increasing x). The deconvolution of the dielectric spectra using the Havriliak-Negami equation allowed to extract the dipole and conductivity relaxation times, their distributions as well as the dielectric strengths in both spin states. Remarkably, no clear correlation appears between the conductivity changes and the lattice properties (Debye temperature) in the dilution series. We rationalize these results by considering the dimensionality of the system (1D), wherein the charge transport occurs most likely by hopping along the [Fe(Rtrz)3] n n+ chains.

5.
J Phys Condens Matter ; 32(21): 214010, 2020 Feb 07.
Artigo em Inglês | MEDLINE | ID: mdl-32032965

RESUMO

Multilayer crossbar junctions composed of ITO/[Fe(HB(1,2,4-triazol-1-yl)3)2]/M (with M = Al or Ca) were fabricated and investigated for their resistance switching properties. Current-voltage-temperature maps revealed ON/OFF resistance ratios as high as 400, with the ON and OFF states defined, respectively, as the low-resistance, low spin state and the high-resistance, high spin state of the spin crossover layer. Similar results were obtained with Al and Ca cathodes indicating that the charge transport in the insulating spin crossover film is at the origin of the resistance switching instead of electron injection at the electrodes. The reproducibility and stability of the device properties were also studied.

6.
Dalton Trans ; 48(45): 16853-16856, 2019 Dec 07.
Artigo em Inglês | MEDLINE | ID: mdl-31693038

RESUMO

The post-synthetic reaction between p-anisaldehyde and the spin-crossover compound [Fe(NH2-trz)3](NO3)2 was explored, obtaining different degrees of transformation from 23% to full conversion by varying the reaction time. The post-synthetic SCO complexes obtained were studied by magnetometry, powder X-ray diffraction (PXRD), elemental analysis, solid state NMR and IR and compared with the corresponding compounds obtained by direct synthetic routes, revealing new spin crossover properties.

7.
J Phys Chem Lett ; 10(23): 7391-7396, 2019 Dec 05.
Artigo em Inglês | MEDLINE | ID: mdl-31714791

RESUMO

We use broad-band dielectric spectroscopy to investigate the spin-state dependence of electrical properties of the [Fe(Htrz)2(trz)](BF4) spin crossover complex. We show that the Havriliak-Negami theory can fully describe the variation of the complex dielectric permittivity of the material across the pressure-temperature phase diagram. The analysis reveals three dielectric relaxation processes, which we attribute to electrode/interface polarization, dipole relaxation, and charge transport relaxation. The contribution of the latter appears significant to the dielectric strength. Remarkably, the permittivity and conductivity changes between the high spin and low spin states are amplified at the corresponding relaxation frequencies.

8.
Small ; 15(47): e1903892, 2019 Nov.
Artigo em Inglês | MEDLINE | ID: mdl-31617319

RESUMO

Thin films of the molecular spin-crossover complex [Fe(HB(1,2,4-triazol-1-yl)3 )2 ] undergo spin transition above room temperature, which can be exploited in sensors, actuators, and information processing devices. Variable temperature viscoelastic mapping of the films by atomic force microscopy reveals a pronounced decrease of the elastic modulus when going from the low spin (5.2 ± 0.4 GPa) to the high spin (3.6 ± 0.2 GPa) state, which is also accompanied by increasing energy dissipation. This technique allows imaging, with high spatial resolution, of the formation of high spin puddles around film defects, which is ascribed to local strain relaxation. On the other hand, no clustering process due to cooperative phenomena was observed. This experimental approach sets the stage for the investigation of spin transition at the nanoscale, including phase nucleation and evolution as well as local strain effects.

9.
Adv Mater ; 31(25): e1901361, 2019 Jun.
Artigo em Inglês | MEDLINE | ID: mdl-31034107

RESUMO

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.

10.
Chem Commun (Camb) ; 55(33): 4769-4772, 2019 Apr 18.
Artigo em Inglês | MEDLINE | ID: mdl-30924832

RESUMO

A series of spin-crossover (SCO) coordination nanoparticles (ca. 60 nm) with the general formulae [Fe(Htrz)1+y-x(trz)2-y(NH2trz)x](BF4)y·nH2O (x = 0, 0.1, 0.2 and 0.3) were synthesized in concentrated solutions without using any surfactant or polymer. The nanoparticle powders were investigated by transmission electron microscopy, powder X-ray diffraction, magnetometry, calorimetry, Raman/IR spectroscopies, elemental analysis and 57Fe Mössbauer spectrometry. Remarkably, the latter revealed a large decrease of the lattice stiffness when incorporating a small amount of amino-triazole ligand, reflected by the drop of the Debye temperature from 285 K (x = 0) to 205 K (x = 0.3). This collapse of the lattice cohesion was attributed to a reorganization of the supramolecular interactions between the Fe-triazole chains. This effect on the SCO properties is also discussed.

11.
J Phys Chem Lett ; 10(7): 1511-1515, 2019 Apr 04.
Artigo em Inglês | MEDLINE | ID: mdl-30883126

RESUMO

Spin-crossover nanomaterials have been actively studied in the past decade for their potential technological applications in sensing, actuating, and information processing devices. Unfortunately, an increasing number of the metallic centers become inactive at reduced sizes, presumably due to surface effects, limiting their switching ability and thus the scope of applications. Here we report on the investigation of "frozen" metallic centers in nanoparticles (2-80 nm size) of the spin-crossover compound Fe(pyrazine)[Ni(CN)4]. Magnetic measurements reveal both high-spin and low-spin residual fractions at atmospheric pressure. A pressure-induced transition of the high-spin residue is observed at around 1.5 GPa by synchrotron Mössbauer spectroscopy. We show that it is equivalent to a downshift of the transition temperature by ca. 400 K due to the size reduction. Unexpectedly, small-angle neutron scattering experiments demonstrate that these high-spin residual centers are not confined to the surface, which contradicts general theoretical considerations.

12.
J Am Chem Soc ; 140(31): 10034-10042, 2018 08 08.
Artigo em Inglês | MEDLINE | ID: mdl-29996053

RESUMO

Ammonia borane hydrolysis is considered as a potential means of safe and fast method of H2 production if it is efficiently catalyzed. Here a series of nearly monodispersed alloyed bimetallic nanoparticle catalysts are introduced, optimized among transition metals, and found to be extremely efficient and highly selective with sharp positive synergy between 2/3 Ni and 1/3 Pt embedded inside a zeolitic imidazolate framework (ZIF-8) support. These catalysts are much more efficient for H2 release than either Ni or Pt analogues alone on this support, and for instance the best catalyst Ni2Pt@ZiF-8 achieves a TOF of 600 molH2·molcatal-1·min-1 and 2222 molH2·molPt-1·min-1 under ambient conditions, which overtakes performances of previous Pt-base catalysts. The presence of NaOH boosts H2 evolution that becomes 87 times faster than in its absence with Ni2Pt@ZiF-8, whereas NaOH decreases H2 evolution on the related Pt@ZiF-8 catalyst. The ZIF-8 support appears outstanding and much more efficient than other supports including graphene oxide, active carbon and SBA-15 with these nanoparticles. Mechanistic studies especially involving kinetic isotope effects using D2O show that cleavage by oxidative addition of an O-H bond of water onto the catalyst surface is the rate-determining step of this reaction. The remarkable catalyst activity of Ni2Pt@ZiF-8 has been exploited for successful tandem catalytic hydrogenation reactions using ammonia borane as H2 source. In conclusion the selective and remarkable synergy disclosed here together with the mechanistic results should allow significant progress in catalyst design toward convenient H2 generation from hydrogen-rich substrates in the close future.

13.
J Am Chem Soc ; 140(28): 8970-8979, 2018 07 18.
Artigo em Inglês | MEDLINE | ID: mdl-29920196

RESUMO

Molecular spin crossover complexes are promising candidates for mechanical actuation purposes. The relationships between their crystal structure and mechanical properties remain, however, not well understood. In this study, combining high pressure synchrotron X-ray diffraction, nuclear inelastic scattering, and micromechanical measurements, we assessed the effective macroscopic bulk modulus ( B = 11.5 ± 1.5 GPa), Young's modulus ( Y = 10.9 ± 1.0 GPa), and Poisson's ratio (ν = 0.34 ± 0.04) of the spin crossover complex [FeII(HB(tz)3)2] (tz = 1,2,4-triazol-1-yl). Crystal structure analysis revealed a pronounced anisotropy of the lattice compressibility, which was correlated with the difference in spacing between the molecules as well as by the distribution of the stiffest C-H···N interactions in different crystallographic directions. Switching the molecules from the low spin to the high spin state leads to a remarkable drop of the Young's modulus to 7.1 ± 0.5 GPa both in bulk and thin film samples. The results highlight the application potential of these films in terms of strain (ε = -0.17 ± 0.05%), recoverable stress (σ = -21 ± 1 MPa), and work density ( W/V = 15 ± 6 mJ/cm3).

14.
Chemistry ; 24(48): 12686-12694, 2018 Aug 27.
Artigo em Inglês | MEDLINE | ID: mdl-29863753

RESUMO

Click ferrocenyl-terminal dentromers, a family of arene-cored dendrimers with triple branching (9-Fc, 27-Fc, 81-Fc, and 243-Fc), reduce AuIII to ferricinium dentromer-stabilized Au nanoparticles (AuNPs). Cyclic voltammetry studies in CH2 Cl2 show reversible CV waves with some adsorption for the 243-Fc dentromer and the number of redox groups found, 255±25, by using the Bard-Anson method, is close to the theoretical number of 243. The dentromers reduce aqueous HAuCl4 to water-soluble ferricinium chloride dentromer-stabilized AuNPs with core sizes between 30 and 47 nm. These triazolylferricinium dentromer-stabilized AuNPs are reduced by cobaltocene to cobalticinium chloride and ferrocene dentromer weakly stabilized AuNPs together with a redshift of the AuNP plasmon. The weakness of the AuNP stabilization is characterized by dentromer extraction with CH2 Cl2 along with irreversible AuNP agglomeration for the 9, 27, and 81-ferrocenyl dentromer, with only the 243-ferrocenyl dentromer-AuNP withstanding this process. Altogether, this demonstrates the electronic switching of the dentromer-mediated AuNP stabilization.

15.
Phys Chem Chem Phys ; 20(14): 9139-9145, 2018 Apr 04.
Artigo em Inglês | MEDLINE | ID: mdl-29577144

RESUMO

Using optical microscopy we studied the vacuum pressure dependence (0.1-1000 mbar) of the nucleation and growth dynamics of the thermally induced first-order spin transition in a single crystal of the spin-crossover compound [Fe(HB(tz)3)2] (tz = 1,2,4-triazol-1-yl). A crossover between a quasi-static hysteresis regime and a temperature-scan-rate-dependent kinetic regime is evidenced around 5 mbar due to the change of the heat exchange coupling between the crystal and its external environment. Remarkably, the absorption/dissipation rate of latent heat was identified as the key factor limiting the switching speed of the crystal.

16.
Adv Mater ; 30(8)2018 Feb.
Artigo em Inglês | MEDLINE | ID: mdl-29315914

RESUMO

Spin crossover particles of formula [Fe{(Htrz)2 (trz)}0.9 (NH2 -trz)0.3 ](BF4 )1.1 and average size of 20 nm ± 8 nm are homogeneously dispersed in poly(vinylidene fluoride-co-trifluoro-ethylene), P(VDF-TrFE), and poly(vinylidene fluoride) (PVDF) matrices to form macroscopic (cm-scale), freestanding, and flexible nanocomposite materials. The composites exhibit concomitant thermal expansion and discharge current peaks on cycling around the spin transition temperatures, i.e., new "product properties" resulting from the synergy between the particles and the matrix. Poling the P(VDF-TrFE) (70-30 mol%) samples loaded with 25 wt% of particles in 18 MV m-1 electric field results in a piezoelectric coefficient d33 = -3.3 pC N-1 . The poled samples display substantially amplified discharges and altered spin transition properties. Analysis of mechanical and dielectric properties reveals that both strain (1%) and permittivity (40%) changes in the composite accompany the spin transition in the particles, giving direct evidence for strong electromechanical couplings between the components. These results provide a novel route for the deployment of molecular spin crossover materials as actuators in artificial muscles and generators in thermal energy harvesting devices.

17.
Adv Mater ; 30(5)2018 Feb.
Artigo em Inglês | MEDLINE | ID: mdl-29171924

RESUMO

Nanoscale spin crossover materials capable of undergoing reversible switching between two electronic configurations with markedly different physical properties are excellent candidates for various technological applications. In particular, they can serve as active materials for storing and processing information in photonic, mechanical, electronic, and spintronic devices as well as for transducing different forms of energy in sensors and actuators. In this progress report, a brief overview on the current state-of-the-art of experimental and theoretical studies of nanomaterials displaying spin transition is presented. Based on these results, a detailed analysis and discussions in terms of finite size effects and other phenomena inherent to the reduced size scale are provided. Finally, recent research devices using spin crossover complexes are highlighted, emphasizing both challenges and prospects.

18.
Adv Mater ; 29(46)2017 Dec.
Artigo em Inglês | MEDLINE | ID: mdl-29067720

RESUMO

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.

19.
J Am Chem Soc ; 139(33): 11610-11615, 2017 08 23.
Artigo em Inglês | MEDLINE | ID: mdl-28763209

RESUMO

Non-noble metal nanoparticles are notoriously difficult to prepare and stabilize with appropriate dispersion, which in turn severely limits their catalytic functions. Here, using zeolitic imidazolate framework (ZIF-8) as MOF template, catalytically remarkably efficient ligand-free first-row late transition-metal nanoparticles are prepared and compared. Upon scrutiny of the catalytic principles in the hydrolysis of ammonia-borane, the highest total turnover frequency among these first-row late transition metals is achieved for the templated Ni nanoparticles with 85.7 molH2 molcat-1 min-1 at room temperature, which overtakes performances of previous non-noble metal nanoparticles systems, and is even better than some noble metal nanoparticles systems. Mechanistic studies especially using kinetic isotope effects show that cleavage by oxidative addition of an O-H bond in H2O is the rate-determining step in this reaction. Inspired by these mechanistic studies, an attractive and effective "on-off" control of hydrogen production is further proposed.

20.
J Phys Chem Lett ; 8(13): 3147-3151, 2017 Jul 06.
Artigo em Inglês | MEDLINE | ID: mdl-28635283

RESUMO

We report on the effect of hydrostatic pressure on the electrical conductivity and dielectric permittivity of the [Fe(Htrz)2(trz)](BF4) (Htrz = 1H-1,2,4,-triazole) spin crossover complex. Variable-temperature and -pressure broad-band impedance spectrometry revealed a piezoresistive effect of more than 1 order of magnitude for pressures as low as 500 bar, associated with a large pressure-induced hysteresis of 1700 bar. The origin of the piezoresistive effect has been attributed to the pressure-induced spin state switching in the complex, and the associated P,T phase diagram was determined.

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