Photochemical Functionalization of Hydrogen-Terminated Diamond Surfaces:  A Structural and Mechanistic Study

Abstract

Hydrogen-terminated diamond surfaces can be covalently modified with molecules bearing a terminal vinyl (C=C) group via a photochemical process using sub-band-gap light at 254 nm. We have investigated the photochemical modification of hydrogen-terminated surfaces of nanocrystalline and single-crystal diamond (111) to help understand the structure of the films and the underlying mechanism of photochemical functionalization. A comparison of the rates of photochemical modification of single-crystal diamond and nanocrystalline diamond films shows no significant difference in reactivity, demonstrating that the modification process is not controlled by grain boundaries or other structures unique to polycrystalline films. We find that both single-crystal and polycrystalline hydrogen-terminated diamond samples exhibit negative electron affinity and are functionalized at comparable rates, while oxidized surfaces with positive electron affinity undergo no detectable reaction. Gas chromatography-mass spectrometry (GC-MS) analysis shows the formation of new chemical products in the liquid phase that are formed only when the alkenes are illuminated in direct contact with H-terminated diamond, while control experiments with other surfaces and in the dark show no reaction. Our results show that the functionalization is a surface-mediated photochemical reaction and suggest that modification is initiated by the photoejection of electrons from the diamond surfaces into the liquid phase.

Diamond‐based DNA sensors: surface functionalization and read‐out strategies

“…The photo-attachment is indeed effective on undoped diamond [62], and because of this electron transport through the bulk can already be excluded. No direct cleavage of the C-H bonds under 254 nm illumination has been observed by Nichols et al [63]. The H-terminated diamond surface exhibits upward band bending near the surface.…”

“…In the paper by Nichols et al [63], two different pathways were envisaged for electronic ejection into the alkene containing liquid phase: either excitation from surface states, slightly above the VB, to the CB, followed by diffusion and emission, or direct photo-emission from the VB to the vacuum level, without passing through the conduction band. In their article, they also presented the energy levels of the alkene molecule TFAAD used in the route of Fig.…”

“…These states have sub-second life-time; from them, electrons can tunnel into unoccupied π*-states of liquid phase alkene molecules. Nichols et al derive that only 1 out of 1000 photo-excited electrons results in surface bonding [63]; Shin et al report a quantum efficiency of 1 in 1600 [62].…”

Covalent modification of graphitic carbon substrates by non-electrochemical methods

“…The surface of hydrogen-plasma treated GC has been shown to have sp 3 diamond-like character [76,77] and so reaction at these sites is expected. For H-terminated diamond, it is proposed that grafting is initiated by photoemission of electrons from the surface to the liquid phase, forming radical anions that abstract H atoms from the surface [78]. The surface radical can react with the alkene, as shown in Fig.…”

Silicon and Silicon-Related Surfaces for Biosensor Applications

“…Functional molecules, including DNA could be grafted in this way to diamond surfaces [Knickerbocker, et al, 2003] and the hybridization with complementary DNA strands could be monitored on the surfaces. In this case, the surface reactivity differs from that of silicon: the reaction initiation on diamond and amorphous carbon surfaces is due to their negative electron affinity [Nichols, et al, 2005] Upon irradiation with sub-band gap wavelengths, electrons are ejected from the surface into the surrounding alkenes, causing the formation of charged reactive species in the liquid close to the diamond surface [Wang, et al, 2007].…”