Isolation and structure of higher diamondoids, nanometer-sized diamond molecules

Spatially resolved electronic and vibronic properties of single diamondoid molecules

Diamondoids are a unique form of carbon nanostructure best described as hydrogen-terminated diamond molecules. Their diamond-cage structures and tetrahedral sp3 hybrid bonding create new possibilities for tuning electronic bandgaps, optical properties, thermal transport and mechanical strength at the nanoscale. The recently discovered higher diamondoids have thus generated much excitement in regards to their potential versatility as nanoscale devices. Despite this excitement, however, very little is known about the properties of isolated diamondoids on metal surfaces, a very relevant system for molecular electronics. For example, it is unclear how the microscopic characteristics of molecular orbitals and local electron-vibrational coupling affect electron conduction, emission and energy transfer in the diamondoids. Here, we report the first single-molecule study of tetramantane diamondoids on Au(111) using scanning tunnelling microscopy and spectroscopy. We find that the diamondoid electronic structure and electron-vibrational coupling exhibit unique and unexpected spatial correlations characterized by pronounced nodal structure across the molecular surfaces. Ab initio pseudopotential density functional calculations reveal that much of the observed electronic and vibronic properties of diamondoids are determined by surface hydrogen terminations, a feature having important implications for designing future diamondoid-based molecular devices.

Experimental determination of the ionization potentials of the first five members of the nanodiamond series

The ionization potentials of size- and isomer-selected diamondoids (nanodiamond containing one to five crystal cages) have been measured by means of total-ion-yield spectroscopy. We find a monotonic decrease of the ionization potential with increasing diamondoid size. This experimental result is compared to recent theoretical predictions and comparable investigations on related carbon clusters, the fullerenes, which show isomer effects to be stronger than size dependence.

Monochromatic Electron Photoemission from Diamondoid Monolayers

We found monochromatic electron photoemission from large-area self-assembled monolayers of a functionalized diamondoid, [121]tetramantane-6-thiol. Photoelectron spectra of the diamondoid monolayers exhibited a peak at the low-kinetic energy threshold; up to 68% of all emitted electrons were emitted within this single energy peak. The intensity of the emission peak is indicative of diamondoids being negative electron affinity materials. With an energy distribution width of less than 0.5 electron volts, this source of monochromatic electrons may find application in technologies such as electron microscopy, electron beam lithography, and field-emission flat-panel displays.

Functionalized nanodiamonds part 3: thiolation of tertiary/bridgehead alcohols

[reaction: see text] Treatment of acyclic as well as polycyclic tertiary mono- and dihydroxy hydrocarbon derivatives with thiourea in the presence of hydrobromic and acetic acid represents a convenient one-step route to the respective tertiary thiols and dithiols. This procedure was used for the preparation of diamondoid thiols of diamantane, triamantane, [121]tetramantane, and others that are prospective nanoelectronic materials.

Raman spectroscopy of diamondoids

A selection of diamondoid hydrocarbons, from adamantane to [121321] heptamantane, have been analysed by multi-wavelength laser Raman spectroscopy. Spectra were assigned using vibrational frequencies and Raman intensities were calculated by employing the B3LYP functional and the split valence basis set of Schafer, Horn and Ahlrichs with polarisation functions on carbon atoms. The variation of the spectra and associated vibrational modes with the structure and symmetry of the molecules are discussed. Each diamondoid was found to produce a unique Raman spectrum, allowing for easy differentiation between molecules. Using the peak assignments derived from the calculations we find that the low frequency region of the spectra, corresponding to CCC-bending/CC-stretching modes, is particularly characteristic of the geometric shape of the diamondoid molecules.

Functionalized Nanodiamonds Part I. An Experimental Assessment of Diamantane and Computational Predictions for Higher Diamondoids

The structures, strain energies, and enthalpies of formation of diamantane 1, triamantane 2, isomeric tetramantanes 3-5, T(d)-pentamantane 6, and D(3d)-hexamantane 7, and the structures of their respective radicals, cations, as well as radical cations, were computed at the B3LYP/6-31G* level of theory. For the most symmetrical hydrocarbons, the relative strain (per carbon atom) decreases from the lower to the higher diamondoids. The relative stabilities of isomeric diamondoidyl radicals vary only within small limits, while the stabilities of the diamondoidyl cations increase with cage size and depend strongly on the geometric position of the charge. Positive charge located close to the geometrical center of the molecule is stabilized by 2-5 kcal mol(-1). In contrast, diamondoid radical cations preferentially form highly delocalized structures with elongated peripheral C-H bonds. The effective spin/charge delocalization lowers the ionization potentials of diamondoids significantly (down to 176.9 kcal mol(-1) for 7). The reactivity of 1 was extensively studied experimentally. Whereas reactions with carbon-centered radicals (Hal)(3)C(*) (Hal=halogen) lead to mixtures of all possible tertiary and secondary halodiamantanes, uncharged electrophiles (dimethyldioxirane, m-chloroperbenzoic acid, and CrO(2)Cl(2)) give much higher tertiary versus secondary selectivities. Medial bridgehead substitution dominates in the reactions with strong electrophiles (Br(2), 100 % HNO(3)), whereas with strong single-electron transfer (SET) acceptors (photoexcited 1,2,4,5-tetracyanobenzene) apical C(4)-H bridgehead substitution is preferred. For diamondoids that form well-defined radical cations (such as 1 and 4-7), exceptionally high selectivities are expected upon oxidation with outer-sphere SET reagents.

Preparation and Reactivity of [D3d]-Octahedrane: The Most Stable (CH)12 Hydrocarbon

The synthesis of the (CH)12 hydrocarbon [D(3d)]-octahedrane (heptacyclo[6.4.0.0(2,4).0(3,7).0(5,12).0(6,10).0(9,11)]dodecane) 1 and its selective functionalization retaining the hydrocarbon cage is described. The B3LYP/6-311+G* strain energy of 1 is 83.7 kcal mol(-1) (4.7 kcal mol(-1) per C-C bond) which is significantly higher than that of the structurally related (CH)16 [D(4d)]-decahedrane 2 (75.4 kcal mol(-1); 3.1 kcal mol(-1) per C-C bond) and (CH)20 [I(h)]-dodecahedrane 3 (51.5 kcal mol(-1); 1.7 kcal mol(-1) per C-C bond); the heats of formation for 1-3 computed according to homodesmotic equations are 52, 35, and 4 kcal mol(-1). Catalytic hydrogenation of 1 leads to consecutive opening of the two cyclopropane rings to give C2-bisseco-octahedrane (pentacyclo[6.4.0.0(2,6).0(3,11).0(4,9)]dodecane) 16 as the major product. Although 1 is highly strained, its carbon skeleton is kinetically quite stable: Upon heating, 1 does not decompose until above 180 degrees C. The B3LYP/6-31G* barriers for the S(R)2 attack of the tBuO. and Br3C. radicals on a carbon atom of one of the cyclopropane fragments (Delta(298) = 27-28 kcal mol(-1)) are higher than those for hydrogen atom abstraction. The latter barriers are virtually identical for the abstraction from the C1-H and C2-H positions with the tBuO. radical (DeltaG(298) = 17.4 and 17.9 kcal mol(-1), respectively), but significantly different for the reaction at these positions with the Br3C. radical (DeltaG(298) = 18.8 and 21.0 kcal mol(-1)). These computational results agree well with experiments, in which the chlorination of 1 with tert-butyl hypochlorite gave a mixture of 1- and 2-chlorooctahedranes (ratio 3:2). The bromination with carbon tetrabromide under phase-transfer catalytic (PTC) conditions (nBu4NBr/NaOH) selectively gave 1-bromooctahedrane in 43 % isolated yield. For comparison, the PTC bromination was also applied to 2,4-dehydroadamantane yielding 54 % 7-bromo-2,4-dehydroadamantane.

Molecular limits to the quantum confinement model in diamond clusters

The electronic structure of monodispersed, hydrogen-passivated diamond clusters (diamondoids) in the gas phase has been studied with x-ray absorption spectroscopy. The data show that the bulk-related unoccupied states do not exhibit any quantum confinement. Additionally, density of states below the bulk absorption edge appears, consisting of features correlated to CH and CH2 hydrogen surface termination, resulting in an effective redshift of the lowest unoccupied states. The results contradict the commonly used and very successful quantum confinement model for semiconductors, which predicts increasing band edge blueshifts with decreasing particle size. Our findings indicate that in the ultimate size limit for nanocrystals a more molecular description is necessary.

Electron emission from diamondoids: a diffusion quantum Monte Carlo study

We present density-functional theory (DFT) and quantum Monte Carlo (QMC) calculations designed to resolve experimental and theoretical controversies over the optical properties of H-terminated C nanoparticles (diamondoids). The QMC results follow the trends of well-converged plane-wave DFT calculations for the size dependence of the optical gap, but they predict gaps that are 1-2 eV higher. They confirm that quantum confinement effects disappear in diamondoids larger than 1 nm, which have gaps below that of bulk diamond. Our QMC calculations predict a small exciton binding energy and a negative electron affinity (NEA) for diamondoids up to 1 nm, resulting from the delocalized nature of the lowest unoccupied molecular orbital. The NEA suggests a range of possible applications of diamondoids as low-voltage electron emitters.

Ultradispersity of diamond at the nanoscale

Nanometre-sized diamond has been found in meteorites, protoplanetary nebulae and interstellar dusts, as well as in residues of detonation and in diamond films. Remarkably, the size distribution of diamond nanoparticles seems to be peaked around 2-5 nm, and to be largely independent of preparation conditions. We have carried out ab initio calculations of the stability of nanodiamond as a function of surface hydrogen coverage and of size. We have found that at about 3 nm, and for a broad range of pressures and temperatures, particles with bare, reconstructed surfaces become thermodynamically more stable than those with hydrogenated surfaces, thus preventing the formation of larger grains. Our findings provide an explanation of the size distribution of extraterrestrial and of terrestrial nanodiamond found in ultradispersed and ultracrystalline diamond films. They also provide an atomistic structural model of these films, based on the topology and structure of 2-3-nm dimond clusters consisting of a diamond core surrounded by a fullerene-like carbon network.