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.

Highly-smooth Mo(CO)₄(2,2′-bipy) thin film showing a color gradient depending on the thickness can be obtained by vapor-phase ligand exchange reaction.

Fabrication of organometallic complexes (OMCs) into high-quality thin films would enable observation of previously-unseen chemical reactions, and allow elucidation of properties that are difficult to detect in other phases than thin films. In addition, the thin films would have various applications in fields such as electronics, photoelectronics, optics, photonics, magnets and spintronics.^1–6 Most attempts to obtain high-quality OMC films 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 high-quality OMC films is sought.Direct formation of OMC films 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 films. Also, physical vapor deposition methods to pre-synthesize 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 film remains a challenge.Chemical vapor deposition (CVD) is an effective method to produce large-scale two-dimensional materials^8,9 and to form films of polymers¹⁰ 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 films 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 films. It is useful for electrocatalytic CO₂ reduction¹² and ligand exchange reaction.¹³ We exploited a vapor-phase ligand exchange reaction between hexacarbonylmolybdenum(0) (Mo(CO)₆) and 2,2′-bipyridine (2,2′-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′-bipy) thin film for the first time. We then used the Mo(CO)₄(2,2′-bipy) film in organometallic thin-film devices and measured their electrical properties.High-quality OMC thin films were prepared using single-step CVD. The metal precursor was Mo(CO)₆ and the ligand precursor was 2,2′-bipy. The goal was to obtain Mo(CO)₄(2,2′-bipy) 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′-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′-bipy powder was placed at the center. This arrangement exploits the temperature gradient in the furnace, and the higher vaporization temperature of 2,2′-bipy than of Mo(CO)₆. A SiO₂/Si target substrate was placed downstream from the center of the tube to collect product efficiently. The quartz tube was flushed using Ar, then the tube furnace was heated from room temperature to the target temperature at 10 °C min⁻¹ (Fig. 1b). After 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.Open in a separate windowFig. 1(a) Experimental scheme of the CVD system for the synthesis of Mo(CO)₄(2,2′-bipy) thin film. (b) Molecular structure Mo(CO)₄(2,2′-bipy) obtained by vapor-phase ligand exchange reaction between Mo(CO)₆ and 2,2′-bipy.The resulting large-scale Mo(CO)₄(2,2′-bipy) film was highly uniform and had no notable physical defects or chunks (Fig. 2). We confirmed that the product did not undergo thermal decomposition at operating temperature (123 °C) (Fig. S2 and Table S1^†). The surface of the resulting film was examined using a tapping-mode atomic force microscope (AFM). To measure the thickness of the resulting film, we etched away the film except for a part that was covered using Kapton tape as a shadow mask. The film was treated using CF₄ at flow rate of 40 sccm and O₂ plasma at 5 sccm with power of 150 W power, respectively. Etching time was determined depending on film thickness, from 30 to 90 min. After 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′-bipy) film was highly smooth and uniform surface (root mean square roughness r_RMS = 0.267 nm, Fig. 2b), which is similar to that of a bare SiO₂/Si substrate (r_RMS = 0.249 nm, Fig. S3b^†). A scanning electron microscopy (SEM) image (Fig. S4^†) of the obtained film shows uniform and homogenous surfaces over a large area. Energy-dispersive electron energy loss spectroscopy (EELS) analysis confirmed 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 films.Open in a separate windowFig. 2(a) Optical microscopy image, photograph (inset), and (b) AFM image of the obtained Mo(CO)₄(2,2′-bipy) thin film. (c) Raman spectra of the Mo(CO)₄(2,2′-bipy) powder (purple), and two precursors (Mo(CO)₆ (green), 2,2′-bipy (coral blue)). (d) FT-IR spectrum of Mo(CO)₄(2,2′-bipy) thin film showing four carbonyl stretching bands.The chemical structure of the resulting film was confirmed by Raman and FT-IR analyses (Fig. 2d). For a direct comparison of the chemical structure of the resulting film with a reference, we separately synthesized Mo(CO)₄(2,2′-bipy) bulk powder by using a microwave-assisted synthesis method described elsewhere¹⁴ (ESI^†). The obtained powder was red (Fig. S6^†) and its structure as confirmed by ¹H-NMR was identical to that reported in the reference paper.¹⁴ Raman spectra (Fig. 2c) were obtained from precursors, product film, and reference powder. The carbonyl stretching band was observed at 1900–2000 cm⁻¹, and several bipyridine stretching bands were observed at 1100–1600 cm⁻¹.¹⁵ The stretching bands of the aromatic rings of 2,2′-bipy (asterisks) and the carbonyl stretching bands (triangle) were clearly resolved. The vibrational band of Mo(CO)₄(2,2′-bipy) film (red) is almost identical with the reference Mo(CO)₄(2,2′-bipy) powder (purple).To identify chemical species observed in Raman spectra, we conducted density functional theory (DFT) calculations using Dmol³ modules in Material Studio program packages (details in ESI^†). The six representative bands observed at 1170.8, 1263.5, 1322.0, 1535.5, 1563.9 and 1600.1 cm⁻¹ in the Raman spectrum of Mo(CO)₄(2,2′-bipy) film were confirmed as molybdenum-coordinated bipyridine ligand stretching bands (red), which are shifted and split from the original stretching bands of bipyridine precursor (1147.6, 1236.4, 1303.4, 1574.1, and 1590.2 cm⁻¹, blue). Also, the carbonyl stretching band shifted from 2005.9 cm⁻¹ to 1888.2 cm⁻¹ because to the new coordination bonds between molybdenum and 2,2′-bipy ligand has a weaker π-accepting ability than the previously-bonded carbonyl ligands.¹⁶ The DFT calculations generated Raman vibrational modes of precursors and film (ESI Videos^†). Also, we measured the FT-IR spectrum of Mo(CO)₄(2,2′-bipy) film by using an attenuated total reflection (ATR) mode (650–4000 cm⁻¹). The spectrum (Fig. 2d) shows carbonyl stretching bands at 2014, 1898, 1866, and 1824 cm⁻¹, which correspond respectively to A^1a₁, B₁, A^1b₁, and B₂ vibrational modes of carbonyl ligands interacting to the six-coordinated molybdenum complex; this result is a good match to reference data.¹⁷For further characterization, we measured ¹H-NMR of dissolved Mo(CO)₄(2,2′-bipy) film in CDCl₃. The NMR peaks (Fig. S7,^† triangles) match well with the reference NMR peaks of Mo(CO)₄(2,2′-bipy) powder,¹⁴ and also confirmed the presence of small amount of 2,2′-bipy precursor (squares). Grazing-incidence wide-angle X-ray scattering (GI-WAXS) measurements (Fig. S8^†) we confirmed the amorphous structure of the resulting film.One of the big advantages of CVD is that the thickness of the resulting film can be controlled easily by changing the amount of precursors. Mo(CO)₄(2,2′-bipy) films were formed on SiO₂/Si substrate by using different amounts of precursors; the films 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^†).Open in a separate windowFig. 3(a) Photograph of Mo(CO)₄(2,2′-bipy) films showing a continuous color gradient depending on the thickness. The rightmost sample is a bare SiO₂/Si substrate. (b) I–V characteristic curve of Mo(CO)₄(2,2′-bipy) thin film; inset: photograph of the fabricated device. (c) I–V characteristics at rt ≤ T ≤ 100 °C for DC voltage −40 to 40 V.To exploit the geometrical advantage of the thin film for device fabrication, field-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 × 10⁻⁹ S, and the highest conductivity was 1.99 × 10⁻⁵ S m⁻¹. The linear characteristic of I–V curve is a sign of ohmic contact between film and electrode, as a consequence of the uniform and smooth surface of Mo(CO)₄(2,2′-bipy) film. The electrical conductance of Mo(CO)₄(2,2′-bipy) film 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′-bipy) film. To compare the electrical property of film with reference powder, Mo(CO)₄(2,2′-bipy) powder was pelletized and fabricated on a SiO₂/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′-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 film.^18,19In summary, we synthesized large-scale, highly-uniform, smooth, and thickness-controllable Mo(CO)₄(2,2′-bipy) thin films 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 film geometry that is advantageous for various electrical and optical device applications. FET devices that use Mo(CO)₄(2,2′-bipy) thin film exhibit semiconducting behaviour. We believe that these results provide insights that will guide development of novel strategies to synthesize various OMC films for use in various electrical and optical applications.     

Association between metabolic syndrome and bone loss at various skeletal sites in postmenopausal women: a 3-year retrospective longitudinal study

Summary

Although the presence of metabolic syndrome (MetS) and increasing numbers of MetS components were associated with attenuated bone loss at various skeletal sites in postmenopausal women, this beneficial effect of MetS on bone mass can be mainly explained by higher mechanical loading in the affected subjects.

Introduction

Previous cross-sectional epidemiological studies reported the inconsistent results regarding the combined effects of MetS on bone mass. In our present report, we performed a large, longitudinal study to evaluate MetS in relation to annualized bone mineral density (BMD) changes in postmenopausal Korean women.

Methods

The study cohort consisted of 1,218 postmenopausal women who had undergone comprehensive routine health examinations with an average follow-up interval of 3 years. The BMD at the lumbar spine and proximal femur sites was measured with dual-energy X-ray absorptiometry using the same equipment at baseline and at follow-up.

Results

Following adjustment for age, baseline BMD, and lifestyle factors, the women with MetS had 21.7, 17.0, 26.7, and 31.1 % less bone loss at the total femur, femur neck, trochanter, and lumbar spine, respectively, compared with MetS-free women (P?=?0.004 to 0.041). Consistently, the rates of bone loss at all skeletal sites were linearly attenuated with increasing numbers of MetS components (P?=?0.004 to <0.001). Importantly, when weight and height were added as confounding factors, the differences and trends of annualized BMD changes according to the MetS status disappeared.

Conclusion

Our current results indicate that the beneficial effects of MetS on bone mass can be mainly explained by higher mechanical loading in the affected subjects. Consequently, MetS per se may not be a meaningful concept for predicting future bone loss and for explaining associations between osteoporosis and cardiovascular diseases.     

Organ culture stability of the intervertebral disc: Rat versus rabbit

There is a need to develop mechanically active culture systems to better understand the role of mechanical stresses in intervertebral disc (IVD) degeneration. Motion segment cultures that preserve the native IVD structure and adjacent vertebral bodies are preferred as model systems, but rapid ex vivo tissue degeneration limits their usefulness. The stability of rat and rabbit IVDs is of particular interest, as their small size makes them otherwise suitable for motion segment culture. The goal of this study was to determine if there are substantial differences in the susceptibility of rat and rabbit IVDs to culture‐induced degeneration. Lumbar IVD motion segments were harvested from young adult male Sprague–Dawley rats and New Zealand White rabbits and cultured under standard conditions for 14 days. Biochemical assays and safranin‐O histology showed that while glycosaminoglycan (GAG) loss was minimal in rabbit IVDs, it was progressive and severe in rat IVDs. In the rat IVD, GAG loss was concomitant with the loss of notochordal cells and the migration of endplate (EP) cells into the nucleus pulposus (NP). None of these changes were evident in the rabbit IVDs. Compared to rabbit IVDs, rat IVDs also showed increased matrix metalloproteinase‐3 (MMP‐3) and sharply decreased collagen type I and II collagen expression. Together these data indicated that the rabbit IVD was dramatically more stable than the rat IVD, which showed culture‐related degenerative changes. Based on these findings we conclude that the rabbit motion segments are a superior model for mechanobiologic studies. © 2013 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 31: 838–846, 2013     

Influence of Temporal Variation in Wall Shear Stress on Intima‐Media Thickening in Arteriovenous Fistulae

Arteriovenous fistula (AVF) failure is mainly due to venous stenosis characterized by significant amount of intima‐media thickening (IMT), probably in the presence of negative (inward) remodeling. Our hypothesis is that the longitudinal changes in wall shear stress (WSS) within different configurations of AVF can influence remodeling factors (changes in luminal diameter (ΔD_h) and IMT) during its maturation process. D_h is an equivalent diameter for a noncircular conduit. A total of six AVFs with curved (C‐AVF; n = 3) and straight (S‐AVF; n = 3) configurations were created between the femoral artery and vein of three pigs, bilaterally. CT scans and ultrasounds were utilized to calculate local WSS at 2D (D: days), 7D, and 28D postsurgery. For each AVF, IMT was measured at four regions along the vein using morphometric analyses. At these regions, repeated measurements of WSS and luminal diameter of each AVF were obtained over time. The ΔD_h between 7D and 28D was significantly larger for C‐AVF than for S‐AVF (2.27 ± 0.67 mm vs. 0.02 ± 0.55 mm; p < 0.05). Also, at 28D the amount of IMT in C‐AVF (77.46 ± 7.10 units) was significantly greater (p < 0.05) when compared with S‐AVF (53.71 ± 8.23 units). These structural changes were accompanied by significantly different gradients of WSS over time (τ′) for C‐AVF (?0.56 ± 0.60 dyne/cm²/day) in comparison with S‐AVF (0.71 ± 0.39 dyne/cm²/day). Negative τ′ for C‐AVF corresponded to reduction in WSS level over time resulting in a physiological level of WSS at 28D (4.08 ± 5.08 dyne/cm²). In contrast, a positive τ′ for S‐AVF was associated with the increase in WSS levels over time causing high levels of WSS at 28D (36.68 ± 5.32 dyne/cm²). The decrease in WSS levels for the C‐AVF over time was associated with outward remodeling of the venous wall (favorable to maturation). In contrast, for S‐AVF, the increase in WSS levels over time was associated with inward remodeling and subsequently, venous stenosis. Thus, temporal gradients of WSS, which could be altered by the surgical configuration of AVF, may provide important information on the remodeling behavior of AVFs. Identification of an optimal AVF configuration, which results in a temporal decrease in WSS and an outward remodeling of the venous wall, may reduce AVF maturation failure.     

Xanthohumol ameliorates 2,3,7,8‐tetrachlorodibenzo‐p‐dioxin–induced cellular toxicity in cultured MC3T3‐E1 osteoblastic cells

2,3,7,8‐tetrachlorodibenzo‐p‐dioxin (TCDD) is an environmental contaminant. Xanthohumol is a prenylated flavonoid found in hops (Humulus lupulus) and beer. The aim of the current study was to explore the role of xanthohumol in modulating the toxicity of TCDD in MC3T3‐E1 osteoblastic cells. In cells treated with TCDD alone, intracellular Ca²⁺ concentrations, mitochondrial membrane potential disruption, reactive oxygen species production, cardiolipin peroxidation, nitric oxide release and cytochrome P450 1A1 expression were significantly increased. TCDD treatment increased the mRNA levels of extracellular signal‐regulated kinase 1 and nuclear factor kappa B, and significantly decreased the level of protein kinase B (AKT) in MC3T3‐E1 osteoblastic cells. However, the presence of xanthohumol alleviated the pathological effects of TCDD. In addition, xanthohumol treatment significantly increased the expression of genes associated with osteoblast differentiation (alkaline phosphatase, osteocalcin, osteoprotegerin and osterix). We conclude that xanthohumol has a beneficial influence and may antagonize TCDD toxicity in osteoblastic cells.     

Health service utilization,unmet healthcare needs,and the potential of telemedicine services among Korean expatriates

Background

With the significant growth of migration and expatriation, facilitated by increased global mobility, the number of Koreans living abroad as of 2016 is approximately 7.4 million (15% of the Korean population). Healthcare utilization or health problems, especially among expatriates in developing countries, have not been well researched despite the various health risks these individuals are exposed to. Consequently, we identified the health utilization patterns and healthcare needs among Korean expatriates in Vietnam, Cambodia, and Uzbekistan.

Methods

This cross-sectional survey examined 429 Korean expatriates living in Vietnam (n?=?208), Cambodia (n?=?60), and Uzbekistan (n?=?161) who had access to the Internet and were living abroad for at least 6?months. A 67-item questionnaire was used, and feedback was received via an online survey program. Stepwise logistic regression analyses were performed to evaluate factors associated with unmet healthcare needs and preferences of certain type of telemedicine.

Results

We found that 45.5% (195/429) of respondents had used medical services in their country of stay. Among those who visited health institutions >?3 times, the most popular choice was general hospitals (39.4%, 15/38); however, they initially visited Korean doctors’ or local doctors’ offices. The most essential criteria for healthcare service facilities was a “skilled professional” (39.3%, 169/429), 42% wanted a health program for chronic disease management, and 30% wanted specialized internal medicine. A substantial number wanted to access telemedicine services and were willing to pay for this service. They were particularly interested in experts’ second opinion (61.5%, 264/429) and quick, 24-h medical consultations (60.8%, 261/429). Having unmet healthcare needs and being younger was strongly associated with all types of telemedicine networks.

Conclusions

Nearly half of the expatriates in developing countries had unmet healthcare needs. Telemedicine is one potential solution to meet these needs, especially in developing countries.