Light-dependent ionic-electronic conduction in an amorphous octahedral molybdenum cluster thin film

We developed a new environment-sensing device based on the opto-ionic-electronic phenomena of an octahedral molybdenum metal (Mo6) cluster. When the Mo6 cluster is electrochemically deposited on a transparent electrode in an organic solvent containing a trace amount of water, the water permeates the deposited film. During the process, some ligand species that stabilize the frame structure of the Mo6 cluster are substituted with hydroxyl groups, and the negatively charged frame structure of the Mo6 cluster unit is stabilized by hydronium counterions. As a result, the transparent film of the Mo6 cluster fabricated by this method exhibits ionic-electronic mixed conduction of the hydronium ion. The ionic conduction greatly changes depending on the temperature and humidity in the atmosphere, and the electrical conductivity greatly changes depending on the wavelength and intensity of the irradiated light. These unique multisensing properties present new possibilities for environmental sensing applications. Researchers in Japan and France have developed a novel nanomaterial based on thin films with properties sensitive to the environment. One approach to building these nanomaterials is to use so-called atomic clusters, consisting of metal atoms bound to each other, often with accompanying non-metallic atoms. The properties of a material can be altered to suit a specific application by adding functional substances. Motohide Matsuda from Kumamoto University, Thi Kim Ngan Nguyen from the National Institute for Materials Science, Tsukuba, both from Japan, and colleagues created transparent films of indium tin oxide on which they deposited the hexamolybdenum atomic cluster. They investigated the humidity and temperature dependence of the electrical properties of the films, and how their conductivity altered under different light conditions. The results indicated that the innovative nanomaterial could find applications in atmospheric sensors. Photoconductivity response of electrophoretically deposited hexamolybdenum cluster complex.


Introduction
Functional materials based on metals, semiconductors, ceramics, polymers, composites, etc. have provided solutions for societal problems and have led to the development of new technologies for industries 1 . In particular, materials that convert energy, such as thermal, chemical, optical, mechanical, or electrical energy, to one form or another are widely used in everyday situations. Nevertheless, more advanced material properties are required for devices such as piezoelectrics 2 , thermoelectrics 3 , gas sensors 4 , and photodiodes 5 to realize a sustainable and safe society. In recent years, the miniaturization of equipment has resulted in innovative technological advances in various fields 6 . The development of multifunctional materials that can simultaneously detect different types of information using a single material will further expand the applications in sensing, miniaturization, and lightning devices. The effect of light on the electronic properties of ionic conductors near room temperature is a new and impressive topic that was recently discussed on a report on halide perovskites 7 and another on a polymeric ionic liquid 8 . We have focused on metal atom clusters, which are recognized as multifunctional building blocks of nanomaterials 9 , with the aim of designing new smart devices (as a sensor for instance) that can utilize the optoelectronic and opto-ionic properties of such materials. Metal atom clusters, described as "a finite group of metal atoms which are held mainly together, or at least to a significant extent, by bonds directly between metal atoms, even though some nonmetal atoms may also be intimately associated with the cluster" 10 have a variety of interesting properties originating from their simple and unique structures and consisting of a limited number of elements. Among the metal clusters, molybdenum octahedral cluster compounds with the general formula A 2 Mo 6 X i 8 L a 6 (A = Cs + , n-(C 4 H 9 ) 4 N + and X = Cl, Br or I and L = Cl, Br, I, F, OH…, i = inner and a = apical) exhibit photochemical and redox properties due to the delocalization of the valence electrons on the metal centers 11 . The Mo 6 metallic core cluster is stabilized by eight face-capped inner ligands (Br i ) through Mo-Br i bonds, which are dominantly covalent, and six additionally bonded apical ligands (Br a ) through the Mo-Br a linkage, which has strong ionic character 12  cluster-based materials revealed that they exhibit phosphorescence, photovoltaic photocatalytic activity, and photoconductivity under light irradiation 13,14 . The electrical conductivity of the Mo 6 cluster-based material was first described by a three-dimensional (3D) variable-range hopping mechanism based on the coronene radical cation 15 6 Cl i 8 }Cl a 6 ] can conduct protons due the difference in resistance between wet and dry air at room temperature 16 , although neither the origin of the carrier nor the conduction mechanism was clarified. Additionally, Nguyen et al. successfully developed a process for producing transparent homogeneous films based on Nb 6 , Mo 6, and Ta 6 clusters using electrophoretic deposition (EPD) 17 , which offers significant combinations of cost-effectiveness, long-range consistency in film thickness and surface morphology, size-scalability, high deposition rates, and site selectivity. The obtained cluster films exhibited high transmittance in the visible range and strong absorption in the UV and NIR ranges accompanied by a red luminescence emission originating from the Mo 6 clusters 18 . Nguyen et al. used X-ray diffraction (XRD), Fourier transformation infrared spectroscopy (FT-IR), X-ray fluorescence (XRF), and X-ray photoelectron spectroscopy (XPS) to reveal that the Mo 6 films formed by EPD had an amorphous structure, were missing all of the Cs + ions in the original cluster blocks, had H 3 O + ions as alternate counter cations in place of the Cs + ions, exhibited OH − in place of some apical Br − ligands, and had few incorporated water molecules 19 . Therefore, proton conduction was expected for the Mo 6 cluster films formed by EPD; however, their electronic properties have never been evaluated.
In this study, we investigated the humidity and temperature dependence of the electronic properties of a translucent Mo 6 cluster film prepared by EPD. In addition, the conductivity under light irradiation was also studied. Based on the obtained results, the conduction mechanism of the membrane is discussed. Mass spectrometry was used to precisely determine the chemical composition of the Mo 6 cluster films prepared by EPD. In addition, the electronic properties of the film were evaluated in detail by AC impedance measurements and DC measurements, and the influence of light irradiation on the electronic and ionic properties was investigated.

Morphology observation of the deposited film
The cross-sectional SEM image of the film deposited on an anodic substrate coated with an ITO layer by EPD is shown in Fig. 1a. The deposited film presented a homogeneous morphology and a relatively flat surface. Its thickness was approximately 1.6 µm.

Ion mobility spectrometry-mass spectrometry (IMS-MS)
The mass spectra dominated by the precursor in acetonitrile solution displayed a single isotope peak corresponding to [{Mo 6 Br i 8 }Br a 6 ] 2-(see Supplementary Fig. 1a). The mass spectra resulting from the dissolution of the scraped films in acetonitrile were more complex. As emphasized in Fig. 1b, they were dominated by doubly charged anions, even though small amounts of the corresponding singly charged ions were also detected (see Supplementary Fig. 1b). In the following section, we focus on the dominant doubly charged species. From the isotope pattern simulations, the different spectral features were tentatively assigned to the ions with a general formula of the form [{Mo 6 Br i 8 }Br a 6-n (OH) a n ] 2-, with n ranging from 0 to 2; other spectral features were attributed to water adducts of the latter ions. As shown in Fig. 1c, all the arrival time distributions (ATDs) of each species were dominated by a single peak, even though the signal-to-noise ratio was relatively low for the [{Mo 6 6 Br i 8 }Br a 6 ] 2anions displayed a slightly broader peak. This may have resulted from the coexistence of different isomers with similar structures. However, it was difficult to isolate distinct populations by operating our instrument in tandem-IMS mode 21 and selecting only parts of the broad ATD peaks. This challenge could be due to coexisting species hidden under the broad ATD feature that undergo (<ms) fast interconversion in the gas phase 22 . The collision cross-sections (CCSs) were determined for the three observed complexes with 0, 1, and 2 substitutions and are listed in Supplementary Table 1. The experimental CCSs were very similar, displaying only a small contraction for each successive substitution (144 → 140 → 137 Å 2 ). The overall similarity was consistent with the absence of major structural rearrangement caused by the substitution. The small but distinguishable CCS contraction was consistent with the difference between the ionic radii of Br − (196 pm) and OH − (134 pm), as reported by Shannon et al. 23 . Interestingly, the CCS for the [{Mo 6 Br i 8 }Br a 6-n (OH) a n ] 2species was similar to that of [{Mo 6 Br i 8 }Br a 6 ] 2-, which suggests that the overall structure of the cluster is conserved upon Br/OH exchange. The mass spectra clearly support our hypothesis that some of the apical Br − ions are exchanged with OH during the EPD process 19 , and IMS indicates that these ligand exchange reactions do not substantially affect the cluster geometry.
Temperature and humidity dependence of electric conductivity for Mo 6

cluster film
The impedance spectra of the cluster film measured at different temperatures under a fixed humidity of 80 RH% are shown in Fig. 2. It was found that the electronic resistance of the cluster film is temperature dependent; as the temperature increases, the semicircular arc decreases, corresponding to a decrease in the electronic resistance.
From the conductivity calculated from Fig. 2a, Arrhenius plots of the conductivity of the cluster film at humidities of 50 RH% and 80 RH% were created and are shown in Fig. 3a. The activation energies (E a ) were calculated from the Arrhenius plots, which were built on the dependence of conductivity on the temperature, represented by the following equation: where A is a constant, T is the temperature, R is the gas constant and E a is the activation energy. The activation energies at relative humidities of 50 RH% and 80 RH% were estimated to be approximately 68 kJ/mol and 50 kJ/ mol, respectively. Similar activation energies were obtained for the Mo 6 cluster films prepared with different deposition times, suggesting that the electronic properties are not affected by the film thickness of this sample size. The impedance spectra of the cluster film measured at different relative humidities are shown in Fig. 2b. The temperature was fixed at 300 K for all the measurements. The electronic resistance of the cluster film was humidity dependence; as the relative humidity decreased, the semicircular arc became larger, indicating an increase in the electronic resistance. The relationship between the electric conductivity of the cluster film and the relative humidity at 300 K is shown in Fig. 3b. The film was prepared at a deposition time of 30 s. The conductivity of the sample prepared at a deposition time of 20 s is shown in Supplementary Fig. 2. The conductivity of the cluster films exponentially changed with increasing humidity, and the conductivities of the two samples prepared by EPD for 30 s and 20 s were similar.
Relaxation-frequency dependence of the Mo 6 cluster film by electric modulus analysis The conductivity σ is given by where n is the carrier concentration, e is the charge of the carrier and μ is the mobility. The carrier in the cluster film was a cation introduced during the deposition process to compensate for the negative charge of the cluster unit. In general, the conductivity of the film depends on the number of H 3 O + and OH − ions created by the hydrolysis reaction during electrophoretic deposition (EPD  19 . We assumed that the cluster film contained stable neutralized components. Moreover, it was recently demonstrated by Saito et al. that water molecules could be easily trapped in the chloride Mo 6 cluster compounds without causing changes to any apical ligands and that these molecules affect the optical properties 24 . For these reasons, the ligand exchange caused by water from humidity could not occur. If this assumption is true, the carrier species and carrier density in the film remain unchanged. Thus, the temperature and humidity dependencies of the conductivity were likely caused by the different mobilities of the carrier. In the present study, the electric modulus M* was calculated by using the relation in Eq. (3), in which j is √−1, ω (= 2πf) is the angular frequency, and C 0 is the geometric capacitance. M* can be separated into real and imaginary parts; the imaginary Part M" is given by Eq. (4), where f represents the frequency.
The frequency dependence of the parameter M" is shown in Fig. 3c, d. A plot of the relation between the value of M" and the frequency is often used to evaluate ion conduction 25 . Figure 3c, d shows the frequency dependence of M" at various c temperatures and d humidities. The frequency at the position of M" max is known as the relaxation frequency. It shifts to a higher frequency with increasing temperature or humidity, indicating that the relaxation processes of the cluster film are affected by the temperature and humidity. The relationship between the f max and relaxation time (τ) can be represented as τ = 1/2πf max ; that is, τ decreases when the temperature and humidity increase. The temperature dependence of the relaxation time is shown in Supplementary Fig. 3. The temperature dependence of the relaxation time is expressed by the following equation: where τ 0 is a material-dependent preliminary factor. The estimated activation energy is approximately 48 kJ/mol, which is almost the same as the activation energy obtained from the Arrhenius plots of the conductivity. The relation between the conductivity and dielectric relaxation is expressed by the following equation: where ε 0 is the dielectric constant of the vacuum and ε s is the static dielectric permittivity 26 . Notably, the conductivity increases when τ decreases in this equation.

Electronic properties of the Mo 6 cluster film under light irradiation characterized by DC measurement
The I-t curve obtained when a DC of 2 V was applied to the cluster film is shown in Fig. 4a. When the DC voltage was applied, the current immediately reached its maximum value and then decreased over time. Moreover, the current continued to flow slowly at a certain constant value after the current reduction almost stopped. When the current became constant, the application of the voltage was stopped. The flowing current was recorded as a negative value, and then it reached 0. Because ions are polarizable, the film is believed to act as an ion conductor 26 . In addition, a low current continued to flow after the abrupt reduction of the current at the initial stage when DC voltage was applied to the film, suggesting that it is due to electron conduction. In amorphous systems at low temperatures, transport is often dominated by hopping conduction. Electrical conduction in such a system is generally achieved through the incoherent transitions of the charge carriers between spatially localized states 27 . Figure 4b shows the I-t curve of the Mo 6 film under irradiation by UV-A, blue and red LED lights when DC was applied. In each case, light irradiation was performed for only 30 s after an elapsed time of 270 s from the start of the DC voltage application. When UV-A and blue light were used, a temporary increase in the current was observed; this increase was not observed when the sample was irradiated with red light. To clarify this behavior, an investigation into the abovementioned phenomenon by light irradiation was carried out by changing the illuminance of UV-A, as shown in Fig. 4c. As shown in Fig. 4c, when a strong light of 950 lx was applied, the current value was obviously increased by approximately 2.6 times that of current resulting form irradiation with weak light of 360 lx.

Electronic properties of the Mo 6 cluster film under light irradiation characterized by AC impedance measurements
Impedance measurements of the cluster film were performed under UV-A, blue and red light irradiation with illuminances of 280 lx, 100 klx, and 35 klx, respectively. The photon flux densities of these illuminances were roughly similar under the conditions. Figure 4d shows the impedance plots for UV-A LED light irradiation. The semicircular arc of the impedance was recorded to be slightly larger than that before irradiation, representing a decrease in the conductivity. Figure 4e shows the increasing rate of the impedance when the UV-A, blue, and red LED lights were irradiated. An increase in the impedance was clearly observed when the samples was irradiated with UV-A and blue light, while no significant changes were observed with red light, as expected.

Discussion
A schematic of the structure of the Mo 6 cluster in the film proposed from these results is shown in Fig. 5a. When the Cs 2 Mo 6 Br 14 powder is dispersed in methylethylketone (MEK), it dissociates into Cs + and [{Mo 6 Br i 8 } Br a 6 ] 2ions, and the two Br apical ligands at the maximum are simultaneously replaced with OH. The sites of the replaced Br apical ligands are random. Water molecules present in the MEK solvent generate H + by electrolysis on the electrode surface during the application of an electric field and then combine with H 2 O to form H 3 O + . Thus, H 3 O + ion acts as a counter cation to neutralize the negatively charged Mo 6 cluster unit deposited on the substrate. As a result, a new amorphous network is formed by the disordered arrangement of the modified Mo 6 clusters. In addition, the H 3 O + counter cations coordinate with the substituted OH groups at the Br a sites by hydrogen bonding, and many water molecules undergo similar hydrogen bonding interactions. Indeed, previous experimental and simulation results clearly demonstrated the existence of HO − H* − OH bridges between adjacent cluster units, which seems to favor the vehicle diffusion model 16 .
Based on the temperature-and humidity-dependent conductivity, it was clarified that the conduction mechanism is related to the water molecule content and mobility. According to the E a of the Mo 6 cluster film, which is approximately 50-70 kJ/mol, the conduction mechanism of the cluster film is proposed to be the vehicle mechanism by which H 3 O + moves 28 . Schematic illustrations of the proposed conduction mechanism are shown in Fig. 5b. Under high humidity conditions, more water molecules are incorporated into the film; they surround the hydronium ion coordinated to the apical OH ligands of the Mo 6 cluster, weaken the hydrogen bond, and allow the hydronium ion to easily move within the networks. As a result, the activation energy decreases at higher humidities. From the viewpoint of temperature, it is obvious that the conductivity increases with increasing temperature, corresponding to an increase in the mobility of the H 3 O + ion.
As a result of the DC measurement, a large increase in the current due to light irradiation was confirmed. The measurement of the photoluminescence excitation (PLE) of the cluster film 14 revealed that the Mo 6 cluster is excited by light in the wavelength range of 370 and 470 nm, resulting in red light emission. These observations confirm that the electronic properties of the cluster film can be modified by UV-A and blue light irradiation. The Mo 6 cluster releases one electron when it is excited by light irradiation, and as a result, the Mo 6 cluster, which possesses 23 electrons, becomes a powerful oxidizing agent 29 . The excited electrons probably become free electrons and flow through the film when an electric field is applied.
As a result of the AC impedance measurement, the impedance increased by 6% and 7% when the sample was exposed to UV-A and blue irradiation, respectively. These highly reproducible phenomena are reversible, as observed in Fig. 4d for UV-A irradiation. The conductivity reduced by light irradiation was restored to the initial state after 1 hour of equilibration. Because the Mo 6 cluster exhibits photocatalytic properties, water molecules and/or hydronium ions contained in the film decompose in the photoreaction 13 , resulting in decreased ionic conductivity. This mechanism is assumed to be possible due to the statistical distribution of ions located between the apical hydroxyl groups of adjacent clusters. Moreover, when carrier reduction occurs, the negative charge of the cluster unit decreases, and Mo 6 Br 12 (H 2 O) 2 can form locally, as reported by Sleight et al. 30 The efficiency of the ionic conductivity of the Mo 6 cluster can be enhanced by the cluster complex containing an apical hydroxyl group with the formula [{Mo 6 X i 8 }L a 6-x-y (OH) a x (H 2 O) a y ] y−2 (X = L = Cl, Br, I; x + y ≤ 6). In addition, the control of the intensity of the H 3 O + ion created by hydrolysis during electrophoretic deposition also contributes to the increase in ionic conductivity. Similar clusters with Cl and I as inner ligands were also tested. Very recently, A. Renaud  Based on these experiments, the dependence of the humidity, irradiated light strength, and irradiation wavelength on the electronic properties of the Mo 6 cluster film was demonstrated for the first time. Taking into account the most advantageous characteristics of the Mo 6 cluster, such as the large Stokes shift, long lifetime and high red luminescent efficiency, the EPD film is a promising multifunctional device for humidity and UV-light sensing.

Materials and methods
Preparation of cluster film by EPD Cs 2 Mo 6 Br 14 powder was synthesized from MoBr 2 and CsBr by a solid-state method at high temperature 17 . The Cs 2 Mo 6 Br 14 powder was dissolved in reagent grade MEK (99.5%, Kishida Chemical Co., Ltd.) at a concentration of 5 g/L with agitation by a magnetic stirrer until a transparent solution was obtained 19 . ITO glass and a stainless steel sheet were connected to a DC power supply (PD56-10AD, KENWOOD) and served as the anodic substrate and cathode electrode, respectively. The ITO glass was washed with distilled water and ethanol by ultrasonication before use. EPD was carried out at a constant voltage of 15 V for 30 s. The Mo 6 cluster film deposited on the ITO glass substrate was characterized after drying in air for 24 h.

Mass spectrometry
IMS-MS measurements were performed using a homemade tandem drift tube combined with a quadrupole-timeof-flight mass spectrometer (Maxis Impact Bruker, Bremen, Germany) described elsewhere in detail 32,33 . To perform the IMS-MS measurements, the cluster film was first scraped from the ITO substrate, dissolved in 1 mL of acetonitrile, and then diluted in the same solvent to a final concentration of 50 μmol/L. For comparison, solutions of the precursor powder were prepared in acetonitrile at the same concentration. The solutions were directly injected into an electrospray source using a syringe pump (120 µL/ h) and analyzed in negative ion mode. The electrospray ions were periodically injected into a 79-centimeter-long drift tube filled with helium at a pressure of 4 Torr and maintained at 298 K. Drift voltages ranging from 250 to 500 V were applied across the tube to allow mobility separation. Mass-resolved ATDs were finally obtained by recording the mass spectra as a function of the arrival time of the ions at the end of the drift tube. Absolute CCSs were obtained without calibration by measuring the arrival times as a function of the inverse drift field based on the Mason-Schamp equation 20 . Following this procedure, the uncertainty of the absolute value of the CCS was estimated to be 2%.

AC impedance and DC measurement
The electrical conductivity of the cluster films was measured by the AC impedance method, which was carried out in the frequency range of 1-10 6 Hz under an applied voltage of 0.1 V using a potentio-galvanostat (IVIUMSTAT, Ivium Technologies). The electric circuit was assembled using a micrometer; the electrode was in contact with the cluster film at a constant force of 5.2 N. In addition, to maintain a constant test environment, the measurements were carried out in a thermohygrostat (SH-222, Espec Corp.) at fixed temperatures in the range of 300-350 K and a fixed relative humidity in the range of 20-80 RH% (Supplementary Fig. 4). The electrical conductivity was expressed by the following equation: where σ is the electrical conductivity, l is the thickness of the film, A is the area of the sample (0.09 π cm 2 ), and R is the total resistance measured by electrochemical impedance spectroscopy (EIS). The Nyquist diagram was fitted by using a Randles equivalent circuit, as depicted in Supplementary Fig. 4.

Irradiation conditions
For the electrical conductivity measurement of the cluster film under UV-A or visible light irradiation, both the DC and AC impedance methods were applied. The AC impedance method was performed under the previously mentioned conditions. The DC measurements were performed under an applied voltage of 2 V (V 1 ) using a measurement system assembled as shown in Supplementary Fig. 4. LEDs with fixed wavelengths of 390-395 nm (UV-A), 465-475 nm (blue), and 660 nm (red) were used for light irradiation. In Supplementary  Fig. 4, R1 is the shunt resistance, and V 2 is the voltage applied to the LEDs. The illuminance was measured using a luminometer (MT-912, URCERI). The measurements were carried out at 300 K and 50 RH%.