Fabrication of Transparent Thin Film of Octahedral Molybdenum Metal Clusters by Electrophoretic Deposition

Abstract

Thin films of octahedral molybdenum metal clusters (Mo 6) were fabricated on indium tin oxide (ITO) glass slides by the electrophoretic deposition (EPD) technique. The solubility of the Cs 2 Mo 6 Br 14 cluster compounds, which are composed of [Mo 6 Br i 8 Br a 6 ] 2− units associated with two cesium cations, in different solvents was investigated in order to achieve a welldispersed colloidal solution of the Mo 6 cluster units that was stable during the following EPD process. Distilled water, ethanol, 1-propanol, acetone, methyl-ethyl ketone (MEK) and acetyl acetone were selected as the dispersing media of the Cs 2 Mo 6 Br 14 cluster compounds. Homogeneous, transparent Mo 6 cluster films with prominent luminescent properties were obtained by the EPD when dissolving Cs 2 Mo 6 Br 14 into the MEK and acetyl acetone solutions. The obtained Mo 6 cluster films were examined by investigating the thickness (color 3D laser spectroscopy), the morphology (FE-SEM), the Br/Mo ratio (XRF), and the crystallographic parameters variation (XRD). The optical emission properties were characterized by UV-Vis spectroscopy, FL spectroscopy and micro PL spectroscopy. The thickness of the Mo 6 cluster films changed depending on the kind of the solvents. The atomic ratio of the Mo and Br elements in the Mo 6 cluster-based unit were relatively maintained during the dispersion and deposition in the MEK solution. The UV-Vis absorption spectra exhibited a high transparency in the visible light range (500-850 nm) accompanied by many peaks due to light interference in the wavelength range from 400 nm to 2000 nm. The emission intensity of the Mo 6 cluster film deposited in MEK slightly decreased during continuous irradiation of 325 nm laser light for 600s.

Electrophoretic Coating of Octahedral Molybdenum Metal Clusters for UV/NIR Light Screening

“…It should be mentioned that the Cs + cations were not detected in all the Mo 6 films without or with the PDMS probably due to the migration of the dissociated Cs + toward the cathode, the counter electrode, during the EPD. This phenomenon has been similarly demonstrated in a previous study [31]. Based on this result, the chemical structure of the Mo 6 Most importantly, the PDMS coating on the surface of the Mo 6 cluster film does not significantly affect the thickness of the original Mo 6 film.…”

“…The Mo 6 film with the maximum thickness was obtained at 40 s (~2 µm), but the homogeneity and the light transmittance of the deposited film slightly decreased with the deposition time. The decrease in the thickness beginning at 40 s has been explained in our previous report [31]. It can be suggested that, at 40 s, the 2 µm-thick film acts as a resistance layer which makes a significant reduction of the electric field which affect to the electrophoresis of the free clusters.…”

“…The nanocluster powder was added to reagent grade MEK (99%, Nacalai Tesque, Inc., Kyoto, Japan) at a concentration of 5 g/L, then stirred until obtaining a transparent yellow suspension (Figure 1c) [31]. The zeta potential and electronic conductivity of the prepared Cs 2 Mo 6 Br 14 cluster suspension were measured by a zeta-potential analyzer (Zetasizer Nano Z, Malvern Instruments Ltd., Malvern, UK), which were about −(10 ± 2) mV and (0.32 ± 0.70) mS/cm, respectively.…”

“…However, one strong restriction of this method is the requirement of a high concentration of the polymer matrix in order to stabilize enough Mo 6 clusters on the glass surface [30]. In order to overcome this problems, our previous study investigated the processability of the octahedral Mo 6 cluster film by the EPD process by dissolving the Mo 6 cluster precursors (Cs 2 Mo 6 Br i 8 Br a 6 ) in different liquid media [31] and concluded that the MEK solution was the most suitable edium in order to obtain transparent, homogeneous, and thin Mo 6 films. The strong advantage of the EPD process is that we can easily fabricate the pure Mo 6 cluster film containing no counter cations at room temperature that create a new Mo 6 cluster network exhibiting prominent luminescent stabilization [31].…”

“…In order to overcome this problems, our previous study investigated the processability of the octahedral Mo 6 cluster film by the EPD process by dissolving the Mo 6 cluster precursors (Cs 2 Mo 6 Br i 8 Br a 6 ) in different liquid media [31] and concluded that the MEK solution was the most suitable edium in order to obtain transparent, homogeneous, and thin Mo 6 films. The strong advantage of the EPD process is that we can easily fabricate the pure Mo 6 cluster film containing no counter cations at room temperature that create a new Mo 6 cluster network exhibiting prominent luminescent stabilization [31]. The key work is to fabricate the Mo 6 cluster film with no counter cations by depositing the [Mo 6 Br i 8 Br a 6−x ] x−2 (L = H 2 O or OH) anion on ITO conductive substrate by applying an electric field [32].…”