This work reports on the electrochemical behaviour of Fe and Zn based metal-organic framework (MOF) compounds, which are "doped" with chiral molecules, namely: cysteine and camphor sulfonic acid. Their electrochemical behaviour was thoroughly investigated via "solid-state" electrochemical measurements, exploiting an "ad hoc" tailored experimental set-up: a paste obtained by carefully mixing the MOF with graphite powder is deposited on a glassy carbon (GC) surface. The latter serves as the working electrode (WE) in cyclic voltammetry (CV) measurements. Infrared (IR), X-ray diffraction (XRD) and absorbance (UV-Vis) techniques are exploited for a further characterization of the MOFs' structural and electronic properties. The experimental results are then compared with DFT based quantum mechanical calculations. The electronic and structural properties of the MOFs synthesized in this study depend mainly on the type of metal center, and to a minor extent on the chemical nature of the dopant.

Metal-organic frameworks (MOFs) have attracted increasing scientific interest due to unique features including high specific surface areas, exceptional porosity, high crystallinity and tuneable pore size [1]. In fact, the opportunity to achieve porous materials with high modularity and diverse functionality make MOFs suitable candidate for solid-state materials. In recent years, scientists conducted intense research in the production of chiral MOFs (CMOFs). The attractiveness of chiral MOFs is due to their specific application including chiral enantioselective recognition, enantioselective separation, asymmetric catalysis, and sensing. Chirality within MOFs can occur in each of the components, whether linker, metal node, or even guest molecules [2]. MIL-53(Fe) is the most common iron-based MOF. This class of compound is obtained by a combination between iron(III) cations and 1,4-dicarboxylic acid consists of three-dimensional networks which contain FeO6 hexagonal chains and dicarboxylate anions. MIL-53(Fe) shows significant advantages compared with other MOFs, which include chemical stability, the presence of nontoxic and widely available metals [3]. MIL-53(Fe) derivatives containing chiral molecules (cysteine and camphorsulfonic acid) were characterized by IR, XRD and Electron Spectroscopy. The electrochemical behaviour of the synthesized MOFs was characterized on solid state using a glassy carbon (GC) electrode and by making a paste with graphite powder. Cyclic voltammetry (CV) measurements show different redox behaviour depending on the type of molecules present inside the framework. The experimental results were integrated and related to the properties obtained using DFT based quantum mechanical calculations. References [1] Q. Wang, D. Astruc, Chem. Rev. 2020, 120, 1438–1511 [2] Z. Sharifzadeh, K. Berijani, A. Morsali, Coordination Chemistry Reviews 2021, 445, 214083 [3] F. Millange, R. I. Walton, Israel Journal of Chemistry 2018, 58, 1019–1035

Molecular spintronics or spin‐based electronics, which utilizes both the spin degrees of freedom and electron charge, has become a hot topic in modern science. Since the introduction of spintronics in 1988, many efforts have been devoted to controlling spin‐polarized current using an external magnetic field, leading to the implementation of commercial solid‐state devices based on the giant magnetoresistance effect. In molecular spintronics, much progress has been achieved with organic molecules, but the role played by chiral molecules is yet to be explored in detail, while it promises to play a role in the future. It has been proved that the interaction of electrons with chiral molecules is spin specific, as supported by several experimental tools, and by theoretical studies. This effect is named “chiral‐induced spin selectivity” (CISS). CISS is based on the fact that chiral molecules exhibit spin‐specific transport properties, and hence can be used as a substitute for ferromagnetic materials. Here, recent spin‐dependent electrochemistry results are highlighted, where chiral molecules are immobilized on a ferromagnetic electrode. Practical applications of the CISS effect, for spin control of charge transport in complex molecular architectures, and in the water‐splitting process are also reviewed.

Hydrogen is the ecologically ideal energy vector. Efficient photo-electrochemical production of hydrogen from water could be the optimal solution to the energy storage problems related to renewable sources. However, in the water splitting reaction the electric potential required to initiate the process significantly exceeds the thermodynamic limit. By controlling the spins of the electrons that are transferred from the solution to the anode, and ensuring that they are coaligned, the threshold voltage for the process can in theory be decreased to that of the thermodynamic voltage. In the present study, by using TiO2 anodes coated with chiral materials, we explore what are the effects of having a spin-polarized current on water electrolysis. The spin-polarization arises from exploiting what is known as Chiral Induced Spin Selectivity effect by using chiral molecules as spin filters. When using chiral molecules instead of a non-chiral analogue, the hydrogen production from water is enhanced, the threshold voltage is reduced and the by-product formation of hydrogen peroxide is suppressed.

The production of hydrogen through water splitting in a photoelectrochemical cell suffers from an overpotential that limits the efficiencies. In addition, hydrogen-peroxide formation is identified as a competing process affecting the oxidative stability of photoelectrodes. We impose spin-selectivity by coating the anode with chiral organic semiconductors from helically aggregated dyes as sensitizers; Zn-porphyrins and triarylamines. Hydrogen peroxide formation is dramatically suppressed, while the overall current through the cell, correlating with the water splitting process, is enhanced. Evidence for a strong spin-selection in the chiral semiconductors is presented by magnetic conducting (mc-)AFM measurements, in which chiral and achiral Zn-porphyrins are compared. These findings contribute to our understanding of the underlying mechanism of spin selectivity in multiple electron-transfer reactions and pave the way toward better chiral dye-sensitized photoelectrochemical cells.

We show that in an electrochemical cell, in which the photoanode is coated with chiral molecules, the overpotential required for hydrogen production drops remarkably, as compared with cells containing achiral molecules. The hydrogen evolution efficiency is studied comparing seven different organic molecules, three chiral and four achiral. We propose that the spin specificity of electrons transferred through chiral molecules is the origin of a more efficient oxidation process in which oxygen is formed in its triplet ground state. The new observations are consistent with recent theoretical works pointing to the importance of spin alignment in the water-splitting process.

We report on a systematic investigation of temperature dependent current–voltage (I–V) characteristics of Pd/ZnO Schottky barrier diodes in the 30–300K temperature range. The ideality factor was observed to decrease with increase in temperature, whilst the barrier height increases with increase in temperature. The observed trend has been attributed to barrier inhomogeneities, which results in a distribution of barrier heights at the interface. Using the dependence of saturation current values on temperature, we have calculated the Richardson constant (A⁎) which was investigated in the two distinct temperature regions: 140–200K and 210–300K and values of 3×10−12 and 3×10−9Acm−2K−2 were obtained, respectively. A mean barrier height of 0.97eV was obtained in the 140–300K temperature range. Applying the barrier height inhomogeneities correction, the value of A⁎ was obtained from the modified Richardson plots as 39.43 and 39.03Acm−2K−2 in the 140–200K and 210–300K temperature range. The modified Richardson constant (A⁎⁎) has proved to be strongly affected by barrier inhomogeneities and dependent on contact quality.

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We report on the space charge spectroscopy studies performed on thermally treated melt-grown single crystal ZnO. The samples were annealed in different ambients at 700 °C and also in oxygen ambient at different temperatures. A shallow donor with a thermal activation enthalpy of 27 meV was observed in the as-received samples by capacitance-temperature, CT scans. After annealing the samples, an increase in the shallow donor concentrations was observed. For the annealed samples, E27 could not be detected and a new shallow donor with a thermal activation enthalpy of 35 meV was detected. For samples annealed above 650 °C, an increase in acceptor concentration was observed which affected the low temperature capacitance. Deep level transient spectroscopy revealed the presence of five deep level defects, E1, E2, E3, E4, and E5 in the as-received samples. Annealing of the samples at 650 °C removes the E4 and E5 deep level defects, while E2 also anneals-out at temperatures above 800 °C. After annealing at 700 °C, the T2 deep level defect was observed in all other ambient conditions except in Ar. The emission properties of the E3 deep level defect are observed to change with increase in annealing temperature beyond 800 °C. For samples annealed beyond 800 °C, a decrease in activation enthalpy with increase in annealing temperature has been observed which suggests an enhanced thermal ionization rate of E3 with annealing.

We report on the studies carried out on hydrogen peroxide treated melt-grown, bulk single crystal ZnO samples. Results show the existence of two shallow donors in the as-received ZnO samples with energy levels (37.8±0.3)meV that has been suggested as Zni related and possibly H-complex related and (54.5±0.9)meV, which has been assigned to an Al-related donor. Annealing studies performed on the hydrogen peroxide treated samples reveal the existence of a conductive channel in the samples in which new energy levels have been observed, Zn vacancies, related to the Group I elements, XZn. The surface donor volume concentration of the conductive channel was calculated from a theory developed by Look (2007) [1]. Results indicate an increase in the surface volume concentration with increasing annealing temperature from 60×1017cm−3 at 200°C to 4.37×1018cm-3 at 800°C.

We report on the temperature dependence of the capture cross-section of the E3 deep level defect observed in single crystal ZnO samples. Temperature dependent deep level transient spectroscopy reveals an increase in the DLTS peak height with an increase in the rate window frequency for the E3 level which is a proof that the E3 deep level has a temperature activated capture cross-section. However the observed capture rate is not constant during the filling pulse but depends on the occupancy of the defect itself. This phenomenon is in contradiction with what is expected of an ideal deep level.

We have systematically investigated the effects of high-temperature annealing on ZnO and ZnO devices using current voltage, deep level transient spectroscopy (DLTS) and Laplace DLTS measurements. Current–voltage measurements reveal the decrease in the quality of devices fabricated on the annealed samples, with the high-temperature annealed samples yielding devices with low barrier heights and high reverse currents. DLTS results indicate the presence of three prominent defects in the as-received samples. Annealing the ZnO samples at 300 °C, 500 °C, and 600 °C in Ar results in an increase in reverse leakage current of the Schottky contacts and an introduction of a new broad peak. After 700 °C annealing, the broad peak is no longer present, but a new defect with an activation enthalpy of 0.18 eV is observed. Further annealing of the samples in oxygen after Ar annealing causes an increase in intensity of the broad peak. High-resolution Laplace DLTS has been successfully employed to resolve the closely spaced energy levels.

Effects of annealing ZnO in hydrogen, oxygen, and argon have been investigated using deep level transient spectroscopy (DLTS) and Laplace-DLTS (LDLTS) measurements. Current-voltage (IV) measurements indicate a decrease in zero–bias barrier height for all the annealed samples. Conventional DLTS measurements reveal the presence of three prominent peaks in the un-annealed and annealed samples. A new peak with an activation enthalpy of 0.60 eV has been observed in the H2 annealed samples, while an estimated energy level of 0.67 eV has been observed in Ar annealed samples. O2 annealing does not introduce new peaks but causes a decrease in the concentration of the E3 peak and an increase in concentration of the E1 peak. The concentrations of all the intrinsic defects have decreased after H2 and Ar annealing; with Ar annealing giving peaks with the lowest concentrations. The E2 peak anneals out after annealing ZnO in Ar and H2 at 300 °C. From the annealing behaviour of E3, we have attributed to transition metal ion related defects, while E4 has been explained as a defect, whose formation favours oxygen deficient conditions. Laplace DLTS has successfully been employed to resolve the closely spaced energy levels in the E4 peak, splitting it into three peaks with energy levels, 0.68 eV, 0.58 eV, and 0.50 eV below the minimum of the conduction band for the Ar annealed sample.

The variation in electrical characteristics of Au/n-Ge (100) Schottky contacts have been systematically investigated as a function of temperature using current–voltage (I−V) measurements in the temperature range 140–300K. The I–V characteristics of the diodes indicate very strong temperature dependence. While the ideality factor n decreases, the zero-bias Schottky barrier height (SBH) (ΦB) increases with the increasing temperature. The I–V characteristics are analyzed using the thermionic emission (TE) model and the assumption of a Gaussian distribution of the barrier heights due to barrier inhomogeneities at the metal–semiconductor interface. The zero-bias barrier height ΦB vs. 1/2kT plot has been used to show the evidence of a Gaussian distribution of barrier heights and values of ΦB=0.615eV and standard deviation σs0=0.0858eV for the mean barrier height and zero-bias standard deviation have been obtained from this plot, respectively. The Richardson constant and the mean barrier height from the modified Richardson plot were obtained as 1.37Acm−2K−2 and 0.639eV, respectively. This Richardson constant is much smaller than the reported of 50Acm−2K−2. This may be due to greater inhomogeneities at the interface.

Iridium (Ir) Schottky barrier diodes were deposited on bulk grown (1 0 0) Sb-doped n-type germanium by using the electron beam deposition system. Electrical characterization of these contacts using current-voltage (I-V) and capacitance-voltage (C-V) measurements was performed under various annealing conditions. The variation of the electrical properties of these Schottky diodes can be attributed to combined effects of interfacial reaction and phase transformation during the annealing process. Thermal stability of the Ir/n-Ge (1 0 0) was observed up to annealing temperature of 500 degrees C. Furthermore, structural characterization of these samples was performed by using a scanning electron microscopy (SEM) at different annealing temperatures. Results have also revealed that the onset temperature for agglomeration in a 20 nm Ir/n-Ge (1 0 0) system occurs between 600 and 700 degrees C. (C) 2011 Elsevier B.V. All rights reserved.

Ruthenium (Ru) Schottky contacts were fabricated on n-Ge (1 0 0) by electron beam deposition. Current-voltage (I-V), deep level transient spectroscopy (DLTS), and Laplace-DLTS techniques were used to characterise the as-deposited and annealed Ru/n-Ge (1 0 0) Schottky contacts. The variation of the electrical properties of the Ru samples annealed between 25 degrees C and 575 degrees C indicates the formation of two phases of ruthenium germanide. After Ru Schottky contacts fabrication, an electron trap at 0.38 eV below the conduction band with capture cross section of 1.0 x 10(-14) cm(-2) is the only detectable electron trap. The hole traps at 0.09, 0.15, 0.27 and 0.30 eV above the valence band with capture cross sections of 7.8 x 10(-13) cm(-2), 7.1 x 10(-13) cm(-2), 2.4 x 10(-13) cm(-2) and 6.2 x 10(-13) cm(-2), respectively, were observed in the as-deposited Ru Schottky contacts. The hole trap H(0.30) is the prominent single acceptor level of the E-centre, and H(0.09) is the third charge state of the E-centre. H(0.27) shows some reverse annealing and reaches a maximum concentration at 225 degrees C and anneals out after 350 degrees C. This trap is strongly believed to be V-Sb-2 complex formed from the annealing of V-Sb defect centre. (C) 2011 Elsevier B.V. All rights reserved.

A systematic investigation to check the quality of Pd Schottky contacts deposited on ZnO has been performed on electron beam (e-beam) deposited and resistively/thermally evaporated samples using current-voltage, IV, and conventional deep level transient spectroscopy (DLTS) measurements. Room temperature IV measurements reveal the dominance of pure thermionic emission on the resistively evaporated contacts, while the e-beam deposited contacts show the dominance of generation recombination at low voltages, <0.30 V, and the dominance of pure thermionic emission at high voltages, greater than 0.30 V. The resistively evaporated contacts have very low reverse currents of the order of 10−10 A at a reverse voltage of 1.0 V whereas the e-beam deposited contacts have reverse currents of the order of 10−6 A at 1.0 V. Average ideality factors have been determined as (1.43 ± 0.01) and (1.66 ± 0.02) for the resistively evaporated contacts and e-beam deposited contacts, respectively. The IV barrier heights have been calculated as (0.721 ± 0.002) eV and (0.624 ± 0.005) eV for the resistively evaporated and e-beam deposited contacts, respectively. Conventional DLTS measurements reveal the presence of three prominent defects in both the resistive and e-beam contacts. Two extra peaks with energy levels of 0.60 and 0.81 eV below the conduction band minimum have been observed in the e-beam deposited contacts. These have been explained as contributing to the generation recombination current that dominates at low voltages and high leakage currents. Based on the reverse current at 1.0 V, the degree of rectification, the dominant current transport mechanism and the observed defects, we conclude that the resistive evaporation technique yields better quality Schottky contacts for use in solar cells and ultraviolet detectors compared to the e-beam deposition technique. The 0.60 eV has been identified as possibly related to the unoccupied level for the doubly charged oxygen vacancy, Vo2+.

Current-voltage (IV) and capacitance-voltage (CV) measurement techniques have successfully been employed to study the effects of annealing highly rectifying Pd/ZnO Schottky contacts. IV results reveal a decrease in the contact quality with increasing annealing temperature as confirmed by a decrease in the zero bias barrier height and an increase in the reverse current measured at -1.5 V. An average barrier height of (0.77 +/- 0.02) eV has been calculated by assuming pure thermionic emission for the as-deposited material and as (0.56 +/- 0.03) eV after annealing at 550 degrees C. The reverse current has been measured as (2.10 +/- 0.01) x 10(-10) A for the as-deposited and increases by 5 orders of magnitude after annealing at 550 degrees C to (1.56 +/- 0.01) x 10(-5) A. The depletion layer width measured at -2.0 V has shown a strong dependence on thermal annealing as it decreases from 1.09 mu m after annealing at 200 degrees C to 0.24 mu m after annealing at 500 degrees C, resulting in the modification of the dopant concentration within the depletion region and hence the current flowing through the interface from pure thermionic emission to thermionic field emission with the donor concentrations increasing from 6.90 x 10(15) cm(-3) at 200 degrees C to 6.06 x 10(16) cm(-3) after annealing at 550 degrees C. This increase in the volume concentration has been explained as an effect of a conductive channel that shifts closer to the surface after sample annealing. The series resistance has been observed to decrease with increase in annealing temperature. The Pd contacts have shown high stability up to an annealing temperature of 250 degrees C as revealed by the IV and CV characteristics after which the quality of the contacts deteriorates with increase in annealing temperature. (C) 2011 Elsevier B.V. All rights reserved.

Hydrothermal grown n-type ZnO samples have been investigated by deep level transient spectroscopy (DLTS), thermal admittance spectroscopy (TAS), temperature dependent Hall effect (TDH) measurements, and secondary ion mass spectrometry (SIMS) after thermal treatments up to 1500 °C, in order to study the electrical properties of samples with different lithium content. The SIMS results showed that the most pronounced impurities were Li, Al, Si, Mg, Ni, and Fe with concentrations up to ∼5×1017 cm−3. The Li concentration was reduced from ∼1017 cm−3 in as-grown samples to ∼1015 cm−3 for samples treated at 1500 °C, while the concentration of all the other major impurities appeared stable. The results from DLTS and TAS displayed at least five different levels having energy positions of Ec−20 meV, Ec−55 meV, Ec−0.22 eV, Ec−0.30 eV, and Ec−0.57 eV (Ec denotes the conduction band edge), where the Ec−55 meV level is the dominant freeze out level for conduction electrons in samples treated at temperatures <1300 °C, while higher annealing temperatures revealed the shallower (Ec−20 meV) level. The TDH measurements showed a pronounced increase in the electron mobility for the heat treated samples, where a peak mobility of 1180 cm2/V s was reached for a sample treated at 1300 °C. The results provide strong evidence that Li in hydrothermal ZnO is almost exclusively in the substitutional configuration (LiZn), supporting theoretical predictions that the formation of LiZn prevails over Li on the interstitial site for Fermi level positions at and above the middle of the band gap.