Boryslav A. Tkachenko

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.

Diamondoids: functionalization and subsequent applications of perfectly defined molecular cage hydrocarbons

The term “diamondoid” describes cage hydrocarbon molecules that are superimposable on the diamond lattice. Diamondoids that are formally built by face-fusing of adamantane units, namely diamantane, triamantane, tetramantane, etc., have fascinated chemists since the beginning of the last century. The functionalization of these perfectly defined (C,H)-molecules is described here. Thus, diamondoid halides and diamondoid alcohols are first rank precursors for amino and phosphine-substituted diamondoids that have proved to be highly useful in therapeutic applications and metal catalysis, respectively. The extent of functionalization and polyfunctionalization achieved for adamantane and diamantane, and the synthesis and applications of the resulting organohybrids are illustrated, revealing their high potential in fields such as organocatalysis, polymers, molecular electronics and mechanics.

Origin of the monochromatic photoemission peak in diamondoid monolayers.

Recent photoemission experiments have discovered a highly monochromatized secondary electron peak emitted from diamondoid self-assembled monolayers on metal substrates. New experimental data and simulation results are presented to show that a combination of negative electron affinity and strong electron-phonon scattering is responsible for this behavior. The simulation results are generated using a simple Monte Carlo transport algorithm. The simulated spectra recreate the main spectral features of the measured ones.

Near-Edge X-ray Absorption Fine Structure Spectroscopy of Diamondoid Thiol Monolayers on Gold

Diamondoids, hydrocarbon molecules with cubic-diamond-cage structures, have unique properties with potential value for nanotechnology. The availability and ability to selectively functionalize this special class of nanodiamond materials opens new possibilities for surface modification, for high-efficiency field emitters in molecular electronics, as seed crystals for diamond growth, or as robust mechanical coatings. The properties of self-assembled monolayers (SAMs) of diamondoids are thus of fundamental interest for a variety of emerging applications. This paper presents the effects of thiol substitution position and polymantane order on diamondoid SAMs on gold using near-edge X-ray absorption fine structure spectroscopy (NEXAFS) and X-ray photoelectron spectroscopy (XPS). A framework to determine both molecular tilt and twist through NEXAFS is presented and reveals highly ordered diamondoid SAMs, with the molecular orientation controlled by the thiol location. C 1s and S 2p binding energies are lower in adamantane thiol than alkane thiols on gold by 0.67 +/- 0.05 and 0.16 +/- 0.04 eV, respectively. These binding energies vary with diamondoid monolayer structure and thiol substitution position, consistent with different degrees of steric strain and electronic interaction with the substrate. This work demonstrates control over the assembly, in particular the orientational and electronic structure, providing a flexible design of surface properties with this exciting new class of diamond nanoparticles.

[123]Tetramantane: Parent of a New Family of σ-Helicenes¶

We present a new type of sigma-helical structure based on a diamondoid (nanodiamond) framework, C(2)-symmetric [123]tetramantane, whose (+) and (-) isomers could be enantioseparated by HPLC techniques. Bromination of the enantiopure hydrocarbon led to the isolation of (+)-7-bromo-[123]tetramantane, which could be crystallized and subjected to X-ray structure analysis. Using the anomalous dispersion, we have identified this compound as the P isomer for the hydrocarbon moiety. Experimental and computed optical rotatory dispersion (ORD) and vibrational circular dichroism (VCD) spectra independently and in agreement with the X-ray structure analysis gave M-(-) as the configuration of the second eluted parent hydrocarbon isomer.

Electronic structure tuning of diamondoids through functionalization

We investigated the changes in electronic structures induced by chemical functionalization of the five smallest diamondoids using valence photoelectron spectroscopy. Through the variation of three parameters, namely functional group (thiol, hydroxy, and amino), host cluster size (adamantane, diamantane, triamantane, [121]tetramantane, and [1(2,3)4]pentamantane), and functionalization site (apical and medial) we are able to determine to what degree these affect the electronic structures of the overall systems. We show that unlike, for example, in the case of halobenzenes, the ionization potential does not show a linear dependence on the electronegativity of the functional group. Instead, a linear correlation exists between the HOMO-1 ionization potential and the functional group electronegativity. This is due to localization of the HOMO on the functional group and the HOMO-1 on the diamondoid cage. Density functional theory supports our interpretations.

The influence of a single thiol group on the electronic and optical properties of the smallest diamondoid adamantane

At the nanoscale, the surface becomes pivotal for the properties of semiconductors due to an increased surface-to-bulk ratio. Surface functionalization is a means to include semiconductor nanocrystals into devices. In this comprehensive experimental study we determine in detail the effect of a single thiol functional group on the electronic and optical properties of the hydrogen-passivated nanodiamond adamantane. We find that the optical properties of the diamondoid are strongly affected due to a drastic change in the occupied states. Compared to adamantane, the optical gap in adamantane-1-thiol is lowered by approximately 0.6 eV and UV luminescence is quenched. The lowest unoccupied states remain delocalized at the cluster surface leaving the diamondoids negative electron affinity intact.

Reactivities of the Prism‐Shaped Diamondoids [1(2)3]Tetramantane and [12312]Hexamantane (Cyclohexamantane)

Various functional groups have been incorporated into the structures of the naturally occurring diamondoids [1(2)3]tetramantane and [12312]hexamantane (cyclohexamantane), which represent hydrogen-terminated prism-shaped nanodiamonds. The selectivities of the C-H substitutions in [1(2)3]tetramantane depend on the reagent employed and give products substituted at either central (through bromination) or peripheral (through nitroxylation and photo-oxidation) positions. The hydrogen-coupled electron-transfer mechanism of C-H nitroxylation with the model electrophile NO(2)(+)...HNO(3) was verified computationally at the B3PW91 and MP2 levels of theory by utilizing the 6-31G(d) and cc-pVDZ basis sets. The thermodynamically controlled nitroxylation/isomerization of [1(2)3]tetramantane allows the preparation of peripherally trisubstituted derivatives, which were transformed into tripod-like nanodiamond building blocks. The bromination of cyclohexamantane selectively gives the 2-bromo derivative, reproducing the chemical behavior of the {111} surface of the hydrogen-terminated diamond.

Diamondoid-modified DNA

We prepared novel C5-modified triphosphates and phosphoramidites with a diamondoid functionally linked to the nucleobase. Using primer extension experiments with different length templates we investigated whether the modified triphosphates were enzymatically incorporated into DNA and whether they were further extended. We found that all three modified nucleotides can be incorporated into DNA using a single-nucleotide incorporation experiment, but only partially using two templates that demand for multiple incorporation of the modified nucleotides. The modified phosphoramidites were introduced into oligonucleotides utilizing DNA synthesizer technology. The occurring oligonucleotide structures were examined by circular dichroism (CD) and melting temperature (T(m)) measurements and were found to adapt similar helix conformations as their unmodified counterparts.

Diamondoid Coating Enables Disruptive Approach for Chemical and Magnetic Imaging with 10 nm Spatial Resolution

Diamondoids are unique molecular nano-materials with diamond structure and fascinating properties such as negative electron affinity and short electron mean free paths. A thin layer of diamondoids deposited on a cathode is able to act as an electron monochromator, reducing the energy spread of photo-emitted electrons from a surface. This property can be applied effectively to improve the spatial resolution in x-ray photoemission electron microscopy (X-PEEM), which is limited by chromatic aberration of the electron optics. In this paper, we present X-PEEM measurements reaching the technological relevant spatial resolution of 10 nm without the need of expensive and complex corrective optics. Our results provide a simple approach to image surface chemical and magnetic information at nanometer scales by employing diamondoids.