(680e) Directed Covalent Assembly of Nanodiamonds to Form Continuous Films

Polycrystalline diamond thin films are composed of a mixture of sp3- and sp2-hybridized carbon atoms, inheriting some of the superlative properties from its single crystalline form, such as increased chemical stability with low coefficient of thermal expansion, good thermal conductivity, wide optical transparency, and biocompatibility. These films are most commonly grown via chemical vapor deposition. However the growth process is energy intensive, and requires specialized growth chamber, vacuum conditions, precursor gases, and large power requirement. Further, the growth is restricted to substrates that can resist melting, reaction with process gases, and carbon dissolution at high temperatures. Transfer printing of CVD diamond films onto flexible substrates has been demonstrated; however, this process is limited to feature sizes smaller than 1 mm and the resulting films are prone to breakage upon flexing of the substrate. In this talk, we present a room temperature assembly process for the cyclic attachment of carboxylated ND aggregates (ND-COOH) and a diamine using a carbodiimide cross-linker. The process begins with an amine functionalized surface. In most cases, this can be achieved by hydroxylation with oxygen plasma, followed by silanization with 3-aminopropyltriethoxysilane. The primary amines on the substrate were then reacted with an o-acylisourea active ester form of ND-COOH. This form was realized using 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride. Because the half-life of the acylisourea active ester is close to an hour at 20 °C, the reaction was limited to 30 min. The unreacted active ester groups on the substrate-bound ND-COOH were then quenched with a diamine linker, resulting in an amine-terminated surface. Here, we chose a short-chain length linker, ethylene diamine, to reduce the spacing between ND aggregates. A continous film was achieved by cyclic exposure to freshly activated form of ND-COOH and ethylene diamine.

The morphology of the ND films was examined using atomic form microscopy (AFM) and scanning electron microscopy (SEM), their chemical nature suing X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy, and their thermal transport properties via microfabricated test devices. The effect of solvent media and its pH was studied using deionized water, 10 mM KCl solution, and MES buffers with pH 5.5-7. With deionized water or 1 mM KCl as the reaction medium, when the carbodiimide cross-linker was added to a horn-sonicated ND-COOH suspension in deionized water, its hydrochloride content lowers the pH and leads to particle agglomeration. Further, the particle size distribution in DI water varied greatly. The SEM images of nanodiamond films assembled using DI-water based sol prepared by horn sonication showed higher surface coverage and increased density compared to the films formed by bath sonication.

As seen via SEM and AFM, a pH 6.5 or pH 7 buffer leads to a continuous surface coverage and a similar apparent porosity (~30%), while a pH 6 buffer leads to discontinuous films and more porous films (~65%). Further, the films deposited at pH 7 showed smaller pore sizes and a higher thermal conductivity in comparison to films deposited at pH 6.5. The as-deposited films at pH 7 showed a thermal conductivity as high as 12 ± 2.5 W m^-1 K^-1 at 310 K, which is comparable to that obtained for ultrananocrystalline diamond films obtained via chemical vapor deposition. However, sequential thermal annealing of the nanodiamond films at temperature up to 400 °C led to the aggregation of nanodiamond to segregated islands, loss of surface coverage, and increase in porosity. The thermal conductivity of all the annealed samples was comparable near room temperature regardless of deposition pH. The pH 6.5 and pH 7 samples exhibited a general increase in thermal conductivity with increasing temperature, while pH 6 samples exhibited statistically similar thermal conductivity with increasing temperature. The use of a phonon hopping model to deduce the phonon transfer at the grain-grain boundary indicates that annealing works to homogenize interfaces within the films and reduce sample-to-sample variation but it comes at the expense of less efficient phonon transmission across interfaces. Overall, the results demonstrate a way to achieving porous, low-cost nanocrystalline diamond thin films with tunable film morphology and thermal conductivity for electronics and biomedical applications.