Physical properties of materials derived from diamondoid molecules [electronic resource]
Keywords: Physics

Recent discoveries in novel forms of nanocarbon, from graphene to carbon fiber, have invigorated research into carbon based nanostructures. Until recently, however, this research was almost entirely focused on graphite-like structures while relatively little work was being done on the smaller members of the nano-diamond series, known as the diamondoids. The discovery of large quantities of these diamondoids in petroleum reserves has renewed interest in these unusual molecular nano-particles. We present here a survey of the physical properties of these diamondoids and novel materials which are created from them. We find that they have a number of properties which make them ideal for the study of nanoparticle physics as well as properties which give them exciting potential for a wide range of applications. The diamondoids are small molecules of hydrogen and carbon which have the same rigid cage structure as bulk diamond. The smallest diamondoid, adamantane, has ten carbons that comprise a single diamond cage. Larger and larger diamondoids can be constructed by continuing to add diamond cages onto the molecule, and diamondoids with as many as 11 cages have been detected. These different diamond species can be separated and single morphologies can be isolated to give pure samples of a single shape and size. This allows the production of molecular crystals of diamondoids, which possess new properties owing to the interactions between the nanoparticles in the regular lattice structure. Much work has been done to predict and characterize the basic physical properties of the diamondoids, and we give a summary of these past experiments. These include a number of different spectroscopy and microscopy techniques used on the diamondoids in both the solid state and the gas phase, as well as materials made from chemically functionalized diamondoids. The most studied of these chemically altered diamondoids are the diamondoid-thiols, which are useful for producing single monolayers of diamondoids on metal surfaces. These diamondoid-thiol monolayers have shown great potential for use in various electron emission devices. In this work we focus primarily on solid state diamondoid properties as well as the properties of these diamondoid-thiol self-assembled monolayers. One property discussed is the photoluminescence of the diamondoids in the solid state. This is studied by producing ultra-pure diamondoid crystals and exciting photoluminescence with a UV light source. We find that the diamondoids studied all display similar emission and excitation spectra, which are similar in many ways to the spectra seen from diamondoids in the gas phase as well as other saturated hydrocarbons. However, the diamondoids in the solid state exhibit several unique properties as well. The excitation and emission wavelengths are both red shifted by more then 1 eV compared to the same molecules in the gas phase and are lower than any other saturated hydrocarbon which has been measured. Additionally, the diamondoids exhibit the highest photoluminescence quantum yield ever measured in a saturated hydrocarbon. These unusual properties stem from the strong electronic interactions between the molecule in the solid state. Another solid state property discussed is the dielectric constant, which is studied using microwave impedance measurement. We find that the diamondoids' dielectric constants are significantly lower than that of bulk diamond, as low as 2.46 compared with 5.66 for bulk diamond. This property puts the diamondoids on par with state-of-the-art low-k dielectric materials for use in electronics. Low-k dielectrics are critical for the performance of future microelectronics and the diamondoids are a promising potential material for this application. The ionization potential of the diamondoids in the solid state is also studied. Much like the photoluminescence energies, we find that the ionization energy is greatly reduced in the solid state compared with the gas phase. This is because there are strong quantum confinement effects for the final-state ion in the gas phase that are not present in the bulk crystal. This is another strong indication of electronic interactions between the molecules in the solid state. The ionization potential is also compared with bulk diamond and it is found that the diamondoids are trending slowly towards the bulk value as their size is increased but quantum confinement is still a factor for diamondoids as large as tetramantane (4 cages). An interesting result of these measurements is that the diamondoids appear to have negative electron affinity in the solid state, a property which makes them ideal as electron emitters. The band structure of the diamondoids in the solid state is studied as well, both in theory and experiment. Previous theoretical studies indicate that the lower diamondoids should have a direct band-gap, which is desirable for most optics applications. We extend this study to include several tetramantanes, and find that the more symmetric molecule studied ([121]tetramantane) should have a direct gap while the less symmetric one ([123]tetramantane) should have an indirect gap. We attempt to measure the nature of the gap using resonant inelastic x-ray scattering but the results are inconclusive. We find that the data for the [121]tetramantane are consistent with a direct gap but the data for [123]tetramantane are inconclusive due to severe beam damage to the sample which prevents the collection of sufficient data. We additionally discuss several properties of the self assembled monolayers of diamondoid-thiols. Previous work has shown that these films produce an unusual effect in photoemission experiments where a majority of electrons are emitted in a single sharp peak at low energy with a full width-half max of less than 0.4 eV. We investigate the origin of this unique effect and find that it comes from a combination of negative electron affinity, a property shared with bulk diamond, and an unusually short electron mean free path, a property that appears to be new and unique to the diamondoids. We confirm this short electron mean free path experimentally and perform computer simulations to determine that the short interaction length is a sufficient explanation for the production of the monochromatic peak. We also investigate a technique for making these films more stable, which is a prerequisite for using them in any device application. Our technique is to coat the diamondoid monolayer with a thin film of CsBr, which is a stable, protective over-layer that is relatively transparent to electrons. As an added bonus, this CsBr film also reduces the work function of the emitter, increasing the quantum yield significantly. We find that the combined diamondoid-thiol/CsBr film has a lower energy spread than a typical CsBr emitter but has a longer lifetime than an unprotected diamondoid monolayer emitter. We also discuss initial efforts to characterize the properties of diamondoid monolayers in field emission devices. This is studied by attaching the diamondoid to gold-coated nanowire field emission tips. This experimental setup has the advantage that the same sample can be characterized with and without the diamondoid monolayer present, allowing us to determine precisely the effect of the diamondoid on the work function of the emitter. We find that there is a small reduction in the work function when a lower diamondoid is used, but a huge reduction, as much as 3 eV, when a higher diamondoid is used. This indicates that the diamondoids may be extremely useful for field emission devices is they can be made more stable. Finally, we discuss the origin of the diamondoids in petroleum through a first principles density function theory study of their thermodynamic properties. Through this technique we are able to predict the equilibrium concentration of diamondoids under the conditions where they are known to form. We find that purely random rearrangement reactions occurring at equilibrium in a high pressure, high temperature natural gas field are a sufficient mechanism for explaining the formation of diamondoids and their relative occurrence compared to other molecules and one another.