We present a novel technology for transferring glassy carbon microstructures, originally fabricated on a silicon wafer through a high-temperature process, to a polymeric flexible substrate such as polyimide. This new transfer technique addresses a major barrier in Carbon-MEMS technology whose widespread use so has been hampered by the high-temperature pyrolysis process (⩾900 °C), which limits selection of substrates. In the new approach presented, patterning and pyrolysis of polymer precursor on silicon substrate is carried out first, followed by coating with a polymer layer that forms a hydrogen bond with glassy carbon and then releasing the ensuing glassy carbon structure; hence, transferring it to a flexible substrate. This enables the fabrication of a unique set of glassy carbon microstructures critical in applications that demand substrates that conform to the shape of the stimulated/actuated or sensed surface. Our findings based on Fourier transform infared spectroscopy on the complete electrode set demonstrate-for the first time-that carbonyl groups on polyimide substrate form a strong hydrogen bond with hydroxyl groups on glassy carbon resulting in carboxylic acid dimers (peaks at 2660 and 2585 cm −1 ). This strong bond is further confirmed by a tensile test that demonstrated an almost perfect bond between these materials that behave as an ideal composite material. Further, mechanical characterization shows that ultimate strain for such a structure is as high as 15% with yield stress of ~20 MPa. We propose that this novel technology not only offers a compelling case for the widespread use of carbon-MEMS, but also helps move the field in new and exciting directions.

No abstract

With increasing innovations in lithographically patterned glassy carbon (GC) microstructures supported on a flexible polyimide substrate, interest in understanding the nature and strength of the interface between these two materials has come to the forefront. However, although polyimide and glassy carbon have been both extensively studied independently, interface bonds between them in a composite structure have not been investigated. The work presented here investigates the interaction between GC and polyimide at their interface by comparing the infrared spectrum of the composite microstructures (GC bonded to polyimide) to the spectra of both materials alone. A significant difference in the hydroxyl and anhydride peaks between the polyimide, carbon, and composite was found, indicating the presence of not only hydrogen bonding, but also covalent bonding in the composite microstructure. Functionalization of GC through oxygen plasma etching and annealing above 200 • C was observed to result in the formation of additional anhydrides. We submit that the presence of these sets of strong covalent bonds opens vast opportunities for a wider usage of variety of robust GC microstructures supported on polyimide substrate.

In previous work in our lab, the redox-dependent binding behavior of a phenylene-diamine urea, UHH, was investigated in the presence of different guests in CH2Cl2. The urea functionality contains two good H-donors in the two urea NH bonds (a DD motif) that are capable of H-bonding with two appropriately spaced H-acceptors (AA motif). An example is 1,8-naphthyridine, naph, which contains two pyridine-type N’s appropriately spaced and orientated to H-bond to the two NH’s in UHH. 1H NMR studies indicated a modest Kbinding of 30 M-1 in CH2Cl2 between UHH and naph. The expectation was that oxidation of the phenylenediamine couple in UHH would increase the acidity of one of the NH’s leading to stronger H-bonding and a negative shift in the observed redox potential upon addition of naph. However, instead what happened was that the current for the oxidation increased with little change in peak potential. This can be explained by proton transfer from the UHH radical cation to the naphthyridine. The resulting neutral radical will be immediately oxidized by a second electron leading to the increase in current. The proton transfer will change the H-bonding motif from AA-DD to AD-DA, which is inherently weaker because of unfavorable secondary interactions. In addition because of the two electron oxidation, both binding partners in the resulting complex, (naphH+)(UH+), will be positively charged. Therefore it is not surprising that the H-bonding does not strengthen upon oxidation. In this study the phenylenediamine couple is replaced by a ferrocene, FcUHH, which is only capable of a one electron oxidation. Cyclic voltammetry experiments will be run with FcUHH and naphthyridine to see if eliminating the possibility of the second electron transfer prevents proton transfer and results in the expected stronger H-bonding to the oxidized form.

In previous work in our lab, the redox-dependent binding behavior of a phenylenediamine urea, UHH, was investigated in the presence of different guests in CH2Cl2. The urea functionality contains two good H-donors in the two urea NH bonds (a DD motif) that are capable of H-bonding with two appropriately spaced H-acceptors (AA motif). Examples are the cyclic diamide PZD (see Figure) and 1,8-naphthyridine, naph. 1H NMR studies indicated a modest Kbinding of 100 M-1 with PZD and 30 M-1 with naph in CH2Cl2. The expectation was that oxidation of the phenylenediamine couple in UHH to the radical cation would increase the acidity of one of the NH’s leading to stronger H-bonding with the guests and a negative shift in the observed redox potential upon addition of these guests. A small negative shift was observed with PZD, but further research indicated that the actual overall reaction occurring upon oxidation of UHH was not the simple 1 electron oxidation to the radical cation expected, but rather 2 electron oxidation of one UHH accompanied by proton transfer to a reduced UHH to give UH+ and HUHH+ as shown in the Figure. Addition of PZD does not change this overall reaction. An alternative explanation of the potential shift is that PZD is actually H-bonding to the protonated HUHH+, which would also be a stronger H-donor. Interestingly, similar magnitude shifts are observed upon oxidation of the ferrocene-containing urea, FcUHH. Since Fc is only capable of being oxidized by 1 electron, oxidation of the Fc also makes a +1 charge, which appears to have a similar effect on the H-bonding ability of the urea as does protonation. In contrast to the PZD, addition of naph causes the current for the oxidation of UHH to increase with little change in peak potential. This can be explained by proton transfer from the UHH radical cation to the naphthyridine. The resulting neutral radical will be immediately oxidized by a second electron leading to the increase in current. The proton transfer will change the H-bonding motif from AA-DD to AD-DA, which is inherently weaker because of unfavorable secondary interactions. In addition, because of the two electron oxidation, both binding partners in the resulting complex, (naphH+)(UH+), will be positively charged. Therefore it is not surprising that the H-bonding does not strengthen upon oxidation. It will be of interest to compare this behavior to that of FcUHH and naph. Cyclic voltammetry experiments will be run with FcUHH and naphthyridine to see if eliminating the possibility of the second electron transfer prevents proton transfer and results in the expected stronger H-bonding to the oxidized form. Figure 1