Smooth and large scale organometallic complex film by vapor-phase ligand exchange reaction

A simple and reliable method for the formation of smooth and large-scale organometallic complex thin films was developed. We applied chemical vapor deposition (CVD) for this. From the vapor-phase reaction of Mo(CO)6 and 2,2′-bipyridine, large-scale and highly smooth Mo(CO)4(2,2′-bipy) films were obtained. Regardless of the thickness, they show a high smoothness and stability in ambient conditions. Chemical structure and composition of the resulting film were confirmed by 1H-NMR, Raman, FT-IR spectroscopy and elemental analyses. Smooth and uniform surface of the resulting films was characterized using AFM. We believe that our method will provide great opportunities for the fundamental studies of traditional organometallic complexes and their applications by taking advantages of thin film geometry.

Fabrication of organometallic complexes (OMCs) into highquality thin lms would enable observation of previouslyunseen chemical reactions, and allow elucidation of properties that are difficult to detect in other phases than thin lms. In addition, the thin lms would have various applications in elds such as electronics, photoelectronics, optics, photonics, magnets and spintronics. [1][2][3][4][5][6] Most attempts to obtain highquality OMC lms have tried to coat pre-synthesized target complexes. 7 However, these methods were mostly unsuccessful and if successful were not easily applied to general OMCs. Therefore, a new reliable method for the formation of highquality OMC lms is sought.
Direct formation of OMC lms on a target substrate is a promising method because of their good reactivity and potentially interesting electrical and optical properties. OMCs have been synthesized in solution phase, but this method is more appropriate for obtaining three-dimensional structures than for lms. Also, physical vapor deposition methods to presynthesize OMCs are also not appropriate due to their low vapor pressure and the possibility of decomposition. Hence, development of efficient synthesis methods to obtain large-scale and uniform OMC lm remains a challenge.
Chemical vapor deposition (CVD) is an effective method to produce large-scale two-dimensional materials 8,9 and to form lms of polymers 10 and metal-organic frameworks. 11 Here, we report a rapid and highly efficient CVD method that uses in situ vapor-phase chemical reactions of precursors to synthesize highly-uniform thin lms of OMCs. This method facilitates direct reaction of precursors without any disturbance by solvent or impurities, and therefore induces facile formation of pure, highly-uniform and smooth OMC lms. It is useful for electrocatalytic CO 2 reduction 12 and ligand exchange reaction. 13 We exploited a vapor-phase ligand exchange reaction between hexacarbonylmolybdenum(0) (Mo(CO) 6 ) and 2,2 0 -bipyridine (2,2 0 -bipy). The reaction was conducted in a CVD system within 5 min to yield highly-uniform, smooth, and large-scale Mo(CO) 4 (2,2 0 -bipy) thin lm for the rst time. We then used the Mo(CO) 4 (2,2 0 -bipy) lm in organometallic thin-lm devices and measured their electrical properties.
High-quality OMC thin lms were prepared using single-step CVD. The metal precursor was Mo(CO) 6 and the ligand precursor was 2,2 0 -bipy. The goal was to obtain Mo(CO) 4 (2,2 0bipy) by a reaction that proceeds well in solution phase. 12,13 Considering the vaporization temperature of Mo(CO) 6 (94.45 C) and 2,2 0 -bipy (113.75 C) (Fig. S1 †), we chose 123 C as a target temperature for efficient and simultaneous evaporation of precursors. In the CVD system for vapor-phase organometallic reaction (Fig. 1a), Mo(CO) 6 powder was placed 13.5 cm upstream from the center. And 2,2 0 -bipy powder was placed at the center. This arrangement exploits the temperature gradient in the furnace, and the higher vaporization temperature of 2,2 0bipy than of Mo(CO) 6 . A SiO 2 /Si target substrate was placed downstream from the center of the tube to collect product efficiently. The quartz tube was ushed using Ar, then the tube furnace was heated from room temperature to the target temperature at 10 C min À1 (Fig. 1b). Aer 5 min of reaction at target temperature, the power to the furnace was turned off and the sample was allowed to cool passively to room temperature.
The resulting large-scale Mo(CO) 4 (2,2 0 -bipy) lm was highly uniform and had no notable physical defects or chunks (Fig. 2). We conrmed that the product did not undergo thermal decomposition at operating temperature (123 C) ( Fig. S2 and Table S1 †). The surface of the resulting lm was examined using a tapping-mode atomic force microscope (AFM). To measure the thickness of the resulting lm, we etched away the lm except for a part that was covered using Kapton tape as a shadow mask. The lm was treated using CF 4 at ow rate of 40 sccm and O 2 plasma at 5 sccm with power of 150 W power, respectively. Etching time was determined depending on lm thickness, from 30 to 90 min. Aer the process, the measured thickness was 38.6 nm (Fig. S3a †); the height distribution measured by AFM showed that the Mo(CO) 4 (2,2 0 -bipy) lm was highly smooth and uniform surface (root mean square roughness r RMS ¼ 0.267 nm, Fig. 2b), which is similar to that of a bare SiO 2 /Si substrate (r RMS ¼ 0.249 nm, Fig. S3b †). A scanning electron microscopy (SEM) image (Fig. S4 †) of the obtained lm shows uniform and homogenous surfaces over a large area. Energy-dispersive electron energy loss spectroscopy (EELS) analysis conrmed the presence of Mo, C, N, and O (Fig. S5 †).
These results demonstrate that CVD is a highly efficient approach to obtain highly-uniform OMC lms.
The chemical structure of the resulting lm was conrmed by Raman and FT-IR analyses (Fig. 2d). For a direct comparison of the chemical structure of the resulting lm with a reference, we separately synthesized Mo(CO) 4 (2,2 0 -bipy) bulk powder by using a microwave-assisted synthesis method described elsewhere 14 (ESI †). The obtained powder was red (Fig. S6 †) and its structure as conrmed by 1 H-NMR was identical to that reported in the reference paper. 14 Raman spectra (Fig. 2c) were obtained from precursors, product lm, and reference powder. The carbonyl stretching band was observed at 1900-2000 cm À1 , and several bipyridine stretching bands were observed at 1100-1600 cm À1 . 15 The stretching bands of the aromatic rings of 2,2 0 -bipy (asterisks) and the carbonyl stretching bands (triangle) were clearly resolved. The vibrational band of Mo(CO) 4 (2,2 0 -bipy) lm (red) is almost identical with the reference Mo(CO) 4 (2,2 0 -bipy) powder (purple).
One of the big advantages of CVD is that the thickness of the resulting lm can be controlled easily by changing the amount of precursors. Mo(CO) 4 (2,2 0 -bipy) lms were formed on SiO 2 /Si substrate by using different amounts of precursors; the lms showed a continuous color gradient that depended on the thickness (Fig. 3a). The thickness can be controlled from the range of tens of nanometers to micron scale; all surfaces were highly smooth (Fig. S9 †).  To exploit the geometrical advantage of the thin lm for device fabrication, eld-effect transistor (FET) electronic devices with a channel length of 20 mm (Fig. 3b, inset) were fabricated (ESI and Fig. S10 †). The I ds -V ds curve (Fig. 3b) of the resulting device indicated that the highest electrical conductance was 1.90 Â 10 À9 S, and the highest conductivity was 1.99 Â 10 À5 S m À1 . The linear characteristic of I-V curve is a sign of ohmic contact between lm and electrode, as a consequence of the uniform and smooth surface of Mo(CO) 4 (2,2 0 -bipy) lm. The electrical conductance of Mo(CO) 4 (2,2 0 -bipy) lm increased as the temperature was increased from room temperature to 100 C (Fig. 3c); this result shows the semiconducting nature of the Mo(CO) 4 (2,2 0 -bipy) lm. To compare the electrical property of lm with reference powder, Mo(CO) 4 (2,2 0 -bipy) powder was pelletized and fabricated on a SiO 2 /Si substrate. The pellet was rough and thick, so we used silver paste as an adhesive electrode. The I-V characteristic curve (Fig. S11 †) of Mo(CO) 4 (2,2 0 -bipy) pellet exhibits non-ohmic current-voltage characteristics between the pellet and the electrode, and eventually failed to measure the electrical property of the complex. These results demonstrate that the intrinsic properties of organometallic materials requires synthesis of uniform and smooth organometallic lm. 18,19 In summary, we synthesized large-scale, highly-uniform, smooth, and thickness-controllable Mo(CO) 4 (2,2 0 -bipy) thin lms by vapor-phase ligand exchange reaction that exploits chemical vapor deposition (CVD). Our strategy facilitates the vapor-phase reaction of precursors without any disturbance of solvent or impurities, and also yields a suitable smooth lm geometry that is advantageous for various electrical and optical device applications. FET devices that use Mo(CO) 4 (2,2 0 -bipy) thin lm exhibit semiconducting behaviour. We believe that these results provide insights that will guide development of novel strategies to synthesize various OMC lms for use in various electrical and optical applications.

Conflicts of interest
There are no conicts to declare.