PerspectiveChemistry

Surface-Conducting Diamond

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Science  30 Nov 2007:
Vol. 318, Issue 5855, pp. 1391-1392
DOI: 10.1126/science.1151314

On page 1424 of this issue, Chakrapani et al. (1) prove experimentally that an insulator—diamond—can become a metal by charge transfer from the solid to the liquid (2). A similar type of charge transfer has long been known to occur in solid/solid junctions used in ultrafast transistors (3). The effect should find application, for example, in chemical and biological sensors.

In solid/solid junctions, electrons migrate from the semiconductor with the larger band gap (a band of “forbidden” energies that cannot be occupied by electrons) into the semiconductor with the lower band gap, thereby losing energy and becoming trapped because they cannot climb back over the interfacial barrier. This motion separates the negatively charged electrons from their positively charged donors and is one of the reasons for the ultrahigh mobility of electrons in this two-dimensional system.

Chakrapani et al. now prove that a similar phenomenon takes place at the solid/liquid interface, based on the migration of carriers from the solid (diamond) into the liquid (electrolyte). The authors show unambiguously that electrons from diamond transfer into redox electronic states of the electrolyte. This transfer generates a highly conductive surface layer in diamond, which originates from missing electrons (termed “holes”).

In 1989, Landstrass and Ravi showed that diamond can be reversibly transformed from insulating to metallic (3). The work attracted much attention and controversy. Two groups developed electrochemical models to explain the observations (4, 5), but both teams focused on water layers adsorbed in the diamond surface. These layers are ill-defined and their electrochemical properties are thus difficult to pin down. Therefore, the electrochemical basis for the observed effect has not been understood until the present work.

The migration (or tunneling) of electrons from diamond into the electrolyte requires that empty electronic states exist in the liquid and that they can be reached without having to cross a substantial energy barrier. In semiconductors, the valence band is filled with electrons and therefore can be considered the main electron source in this process. However, most semiconductors have a valence-band maximum lower in energy than corresponding electrochemical levels (5). To reverse this scenario, a strong electronic surface-dipole layer is required. In the case of diamond, the properties of this dipole layer—and hence the electron affinity of the diamond surface—can be tuned by using hydrogen and oxygen atoms to terminate carbon bonds at the surface (6).

With changing termination from oxygen to hydrogen, the valence-band maximum shifts continuously to higher energies (see the figure). This continuous variation of the surface energy can be used to tune the energetic levels of diamond with respect to redox molecular levels at the solid/liquid interface. It is comparable to the band-gap engineering at the solid/solid interface of semiconductors and will be important for the optimization of bioelectronic devices; diamond is a highly promising material for such devices, because it provides strong bonding to biomolecules like DNA and proteins (7).

Transfer doping.

The energy of valence-band electrons can be increased with respect to the electrolyte by changing the surface coverage from oxygen to hydrogen. As shown experimentally by Chakrapani et al., valence-band electrons transfer into the electrolyte if their energy is above that of electronic states of the electrolyte that can be occupied (Fermi energy, dashed red line). This transfer gives rise to surface conductivity.

The surface conductivity properties generated by this “transfer doping” are interesting, because the holes in diamond are confined to a fairly deep yet very narrow energy well (8, 9). Theoretically, holes can propagate in this well with ultrahigh mobilities, but measured mobilities are orders of magnitude lower than predicted (10). The low mobility is a result of the strong, highly efficient scattering in electronic fields that arises from ions in the liquid.

The surface conductivity increases or decreases depending on the detailed properties of the liquid. This effect can thus be used to measure properties of liquids in contact with diamond, for example, using in-plane gate transistor structures in which a hydrogen-terminated diamond surface is the gate of the sensor (11). Such devices can detect variations of pH close to the Nernst limit (59 mV/pH) and show promise as fast and sensitive chemical and biological sensors (12).

Diamond biosensors—based on transfer doping and related effects—are currently developed by an increasing number of research teams for cancer screening, human DNA decoding, and use as ultrasensitive enzyme/protein detectors. The future of such applications will strongly depend on the chemical stability especially of hydrogen-terminated diamond in aqueous environments, where solids like silicon and gold degrade rapidly. However, C-H bonds are chemically more stable than, for example, Si-H bonds, and experiments on DNA-functionalized surfaces of diamond, silicon, and gold have shown that carbon is more stable than all other transducers (13).

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