Novel bis(biphenyl)-capped polyynes have been synthesized to investigate the modulation of the electronic and optical properties of sp-hybridized carbon-atom wires (CAWs) capped with {\pi}-conjugated ...\(sp^{2}\) endgroups. Raman and Surface Enhanced Raman spectroscopy (SERS) investigation of these systems and Density Functional Theory (DFT) calculations reveal structural changes from polyyne-like with alternating single-triple bonds towards cumulene-like with more equalized bonds as a consequence of the charge transfer occurring when wires interact with metallic nanoparticles. While polyynes have semiconducting electronic properties, a more equalized system tends to a cumulene-like structure characterized by a nearly metallic behavior. The possibility to drive a semiconductor-to-metal transition has been investigated by systematic DFT calculations on a series of CAWs capped with different conjugated endgroups revealing that the modulation of the structural, electronic and vibrational properties of the sp-carbon chain towards the metallic wire cannot be simply obtained by using extended {\pi}-conjugated \(sp^{2}\) carbon endgroups, but require a suitable chemical design of the endgroup and control of charge transfer. These results provide useful guidelines for the design of novel \(sp-sp^2\) hybrid carbon nanosystems with tunable properties, where graphene-like and polyyne-like domains are closely interconnected. The capability to tune the final electronic or optical response of the material makes these hybrid \(sp-sp^2\) systems appealing for a future all-carbon-based science and technology.
The kinetic mechanism of the electroless deposition of NiP from hypophosphite solutions was investigated from first principles with the intent of determining a set of elementary reactions that could ...effectively identify the most relevant steps of this complex process. The outcome of the computational approach is discussed in terms of experimental results obtained by electrochemical methods and surface analysis techniques 1,2. Simulations were performed with Density Functional Theory DFT using the B3LYP exchange and correlation functionals 3, the 6-311+G(d,p) basis set for P,C,H and O atoms 4, and the Stuttgart effective core potential basis sets for Ni 5). The interaction of Ni++ and H2PO2- with the Ni surface was studied using Ni clusters of different sizes (from 4 to 12 atoms). Solvation in water was modeled using the implicit polarizable continuum model 6; calculations were performed using the G09 computational suite 7. Simulations were initially devoted to the identification of the first reaction steps that lead to the adsorption of Ni++ on the Ni surface and to its successive reduction by means of the hypophosphite anion. Though several theories were proposed to describe this key step in the literature, our search was guided by the original insight published several years ago 8, according to which the reduction of Ni++ requires the formation of an intermediate species containing the NiOH group. Such species would then take active part in the transfer of the OH group to H2PO2-, thus favoring its oxidation to H2PO3-. In addition, it was imposed that the searched elementary mechanism satisfies the experimental observation, obtained with marked deuterium species, that each molecule of hydrogen that is evolved contextually to the deposition of NiP contains two H atoms coming from two H2PO2- anions. The possibility that NiOH+ is formed in solution prior to deposition was investigated determining computationally the pKa of Ni++. Results revealed that the formation of NiOH+ is not favored in the pH ranges in which NiP is usually deposited. A similar result was obtained when considering the possibility that NiOH+, after being formed in solution, is stabilized through the formation of a complex with H2PO2-. A thermodynamically favored set of reactions was instead found when it was investigated the possibility that NiOH is formed at the surface contextually with a Ni-H2PO2 complex. It was in fact found that the following set of reactions has a globally slightly negative (≈ -5 ± 5 kcal/mol) free energy change:
R1) Ni
++
+ surf ® Ni
ads
++
R2) Ni
ads
++
+ 2 H
2
O + H
2
PO
2
-
® NiOH(H
2
PO
2
)
ads
+ H
3
O
+
The minimum energy structure of the surface NiOH(H2PO2)ads species optimized at the B3LYP level on a Ni8 cluster in implicit water. The two reactions set R1-R2 provides a mechanism for the initial step of reduction of Ni++, thermodynamically viable and consistent with the hypotheses formulated above. The results of this analysis suggest therefore that the initial step of Ni++ reduction during NiP deposition involves a concerted mechanism in which adsorbed Ni++ contextually reacts with H2PO2- and a water molecule. This mechanism can be also supported by electrochemical measurements, indicating the need of OH funtionalization of the catalytic surface in order to have the reaction of hyposphite. In order to understand completely the adsorption phenomena on the electrode surface during the electroless deposition process,
in situ
-Raman analyses were performed. The purpose of this experimental section is to discern the different complexes adsorbed on the electrode surface while the reaction proceeds. These tests were carried out at different temperature, starting from the ambient temperature to the effective process temperature. The effect of each complexing agent dissolved into electroless NiP bath on complexes adsorbed on the electrode surface and the comparison with the DFT computational analysis will be presented.
References
1 P.L. Cavallotti, L. Magagnin, Influence of added elements on ACD electroless NiP, ECS Transactions, 50 (53) 1-8 (2013).
2 P.L. Cavallotti, L. Magagnin, C. Cavallotti, Influence of added elements on autocatalytic chemical deposition electroless NiP, Electrochim. Acta (2013), http://dx.doi.org/10.1016/j.electacta.2013.09.083.
3 A. D. Becke, J. Chem. Phys. 98, 1372. (1993), C.T. Lee, W.T. Yang, R. G. Parr, Phys. Rev. B, 37, 785 (1988).
4 K. Raghavachari, J. S. Binkley, R. Seeger, and J. A. Pople, J. Chem. Phys. 72, 650 (1980).
5 P. Fuentealba, H. Preuss, H. Stoll, L.v. Szentpaly, Chem. Phys. Lett., 89, 418 (1982).
6 B. Mennucci, E. Cances, J. Tomasi, J. Phys. Chem. B 101, 10506 (1997).
7 M. J. Frisch et al., Gaussian 09, Revision A.1; Gaussian, Inc.: Wallingford CT, 2009.
8 P. Cavallotti, G. Salvago, Electrochimica Metallorum III, 239 (1968).
Recently, carbon nanomaterials have become a hot topic within the field of materials science because they have the potential to provide solutions to technological and environmental challenges in ...various areas. Among these materials, graphene is rapidly emerging as one of the most exciting materials for electro catalysts in electrochemical applications such as super-capacitors, Li-ion batteries, solar cells, and biosensors, due to its unique structure and electronic properties. Applications of graphene in biology are very appealing due to the possibility to have a conductive substrate prone to be modified to strictly interact with specific molecules if properly functionalized 1.Although a basic energy source for humans, dysfunctions in the metabolism of glucose are becoming more and more common. These metabolic diseases are called diabetes. At the beginning of the new millennium, 172 million people were affected by diabetes and, according to estimation, that number would have risen to 366 million in 2030 2. However, this latter number has already been reached and new estimation for 2035 foresee 595 million cases 3, with diabetes becoming the 7th cause of death 4. In this work, deposition of graphene on thin and flexible electroplated copper surface by chemicalvapor deposition (CVD) has been researched for bottom-up glucose sensor building. Copper samples covered with graphene have then been used as electrodesin glucose sensors and their performances have been compared with that of bare copper.CVD is one of the most promising fabrication methods of graphene; this method is probably the best candidate to be used in the electronic industry, due to the high-quality and low defects layers it can yield. Electrodeposited copper substrates have been used as catalysts for the atmospheric pressure CVD process. These substrates have been used as deposited, or further processed to tailor their surface properties: for example, the effect of annealing in a reducing atmosphere and electropolishing have been researched. Graphene/FLG surface was then decorated with gold nanoparticles via immersion plating, to enhance sensor sensitivity. Finally, inkjet printing of copper oxide nanoparticles was implemented to test the properties of the so obtained nanohybrid. To assess the quality of grown graphene layers, Raman spectroscopy was employed, due to well-known graphene/FLG Raman fingerprints. Further characterization with a scanning electron microscope (SEM) and with X-ray photoemission spectroscopy (XPS) and UV photoemission spectroscopy (UPS) has been performed.
1 Sharma, N., Ojha, H., Bharadwaj, A., Pathak, D.P., Sharma, R.K., 2015. RSC Adv. 5 (66), 53381–53403.
2
Wild, S., Roglic, G., Green, A., Sicree, R. & King, H. 2004, "Global Prevalence of Diabetes: Estimates for the year 2000 and projections for 2030
", Diabetes care, vol.27, no. 5, pp. 1047-1053.
3 http://www.expo2015.orgmagazineitlifestylediabete–tutti-i-numeri-in-italia-e-nelmondo.html
4 Tian, K., Prestgard, M. & Tiwari, A. 2014, "A review of recent advances in non-enzymatic systems", vol. 41, pp. 100-118.
Graphene, thanks to its novel properties, has proved to be a very good material for electronics applications, spacing from MEMS to biosensing technology. In the last few years chemical vapor ...deposition of organic precursors onto transition metal supports revealed to be a trustworthy method for high quality graphene deposition, thanks to catalytic properties of the already cited metals. In this work, we selected nickel as the suitable transition metal substrate for graphene growth via chemical vapor deposition, which has been carried out starting from a gaseous methane and hydrogen process stream. In fact, nickel-carbon interaction mechanisms are well known and suitable for the effort of controlling the quantity of organic material that is able to grow onto the metal surface. Obtained samples were analyzed by Raman spectroscopy; spectra pointed out a total absence of the D peak, whose presence is generally accounted for graphene bad transport properties due to many domain boundaries; on the other side, Raman study revealed a higher than one intensity ratio between G peak and 2D band. The succeeding step consisted in the application of the so developed methodology in order to produce graphene coated devices. We based our approach on already existing works 1,2 but tried to exploit the properties of the galvanic displacement process in order to cover monocrystalline silicon wafers with a thin nickel film. First, we tuned the displacement process in order to fit our needs, performing the plating at different times, fixing our attention to film compactness and adhesion to silicon. Then chemical vapor deposition was employed and samples were analyzed to understand the degree of reproducibility of the process. Again, Raman spectroscopy, AFM, SEM and XPS were employed in order to understand graphene quality achieved. Raman spectra were very similar to those of the freestanding nickel foils, while XPS performed onto the same substrate yielded some coherent results: in those points were the nickel signal was higher, and so carbon signal was lower, we ended up with 1.3 nm few layered graphene thickness. Recollecting graphite interplanar distances, which was roughly 0.34 nm, 3 to 4 graphene sheets were deposited. By atomic force microscopy we were able to highlight graphene sheets stacked onto the sample surface. Microstructured, polycrystalline, silicon wafer were coated by the carbonaceous layer using the same chemical vapor deposition process already explained. Raman spectroscopy was again the main characterization technique used to gather information about the deposit: the intensity ratio I(G)/I(2D) was higher than one. In conclusion, we managed to deposit good quality, few layered, graphene onto silicon substrate by using a transition metal thin film as a buffer layer for the growth. A first attempt has been made to understand if graphene layers were grown also between the metal film and silicon; this would represent a first concern towards a very cost efficient and reproducible graphene coated semiconductors.
1 D.Q. McNerny et al.; Sci. Rep. 4, 5049; DOI:10.1038/srep05049 (2014).
2 Yu Yao et al. / Energy Procedia 38 ( 2013 ) 807 – 815.
