Undoped, high-quality diamond is, under almost all circumstances, one of the best insulators known. However, diamond covered with chemically bound hydrogen shows a pronounced conductivity when exposed to air. This conductivity arises from positive-charge carriers (holes) and is confined to a narrow near-surface region. Although several explanations have been proposed, none has received wide acceptance, and the mechanism remains controversial. Here, we report the interactions of hydrogen-terminated, macroscopic diamonds and diamond powders with aqueous solutions of controlled pH and oxygen concentration. We show that electrons transfer between the diamond and an electrochemical reduction/oxidation couple involving oxygen. This charge transfer is responsible for the surface conductivity and also influences contact angles and zeta potentials. The effect is not confined to diamond and may play a previously unrecognized role in other disparate systems.

http://science.sciencemag.org/content/sci/318/5855/1424.full.pdf

Charge Transfer Equilibria Between Diamond and an Aqueous Oxygen Electrochemical Redox Couple

Conductive diamond possesses unique features as compared to other solid electrodes, such as a wide electrochemical potential window, a low and stable background current, relatively rapid rates of electron-transfer for soluble redox systems without conventional pretreatment, long-term responses, stability, biocompatibility, and a rich surface chemistry. Conductive diamond microcrystalline and nanocrystalline films, structures and particles have been prepared using a variety of approaches. Given these highly desirable attributes, conductive diamond has found extensive use as an enabling electrode across a variety of fields encompassing chemical and biochemical sensing, environmental degradation, electrosynthesis, electrocatalysis, and energy storage and conversion. This review provides an overview of the fundamental properties and highlights recent progress and achievements in the growth of boron-doped (metal-like) and nitrogen and phosphorus-doped (semi-conducting) diamond and hydrogen-terminated undoped diamond electrodes. Applications in electroanalysis, environmental degradation, electrosynthesis electrocatalysis, and electrochemical energy storage are also discussed. Diamond electrochemical devices utilizing micro-scale, ultramicro-scale, and nano-scale electrodes as well as their counterpart arrays are viewed. The challenges and future research directions of conductive diamond are discussed and outlined. This review will be important and informative for chemists, biochemists, physicists, materials scientists, and engineers engaged in the use of these novel forms of carbon.

Conductive diamond: synthesis, properties, and electrochemical applications

We report a new scheme for attachment of functionalized organic molecules to polycrystalline diamond films. In this scheme, ultraviolet light is used to cause a local reaction between a hydrogen-terminated diamond surface and organic molecules present as a thin overlayer liquid film. Comparison of functionalized alkenes and alkanes shows that alkenes attach more efficiently. By attaching organic molecules with suitable protecting groups and then deprotecting after attachment to the surface, it is possible to prepare diamond surfaces terminated with carboxylic acid groups or with primary amine groups. These functional groups may serve as an attractive starting point for further chemical modification of diamond surfaces.

Photochemical Functionalization of Diamond Films

Functionalization of diamond surfaces holds considerable promise from both fundamental and applied research aspects. This review summarizes briefly the state of the art of chemical, photochemical and electrochemical strategies for the grafting of different organic functionalities on diamond. Depending on the sought-after application and the desired physical property of diamond, halogenated, aminated, carboxylated and oxidized diamond surfaces have been proposed. After a brief introduction, the review is primarily divided into two parts, presenting chemical functionalisation strategies used on oxygen-terminated diamond, followed by methods used for the formation of C–C, C–X and C–N bonds on hydrogen-terminated diamond.

Different strategies for functionalization of diamond surfaces

By forming a highly stable Al2O3 gate oxide on a C-H bonded channel of diamond, high-temperature, and high-voltage metal-oxide-semiconductor field-effect transistor (MOSFET) has been realized. From room temperature to 400 °C (673 K), the variation of maximum drain-current is within 30% at a given gate bias. The maximum breakdown voltage (VB) of the MOSFET without a field plate is 600 V at a gate-drain distance (LGD) of 7 μm. We fabricated some MOSFETs for which VB/LGD > 100 V/μm. These values are comparable to those of lateral SiC or GaN FETs. The Al2O3 was deposited on the C-H surface by atomic layer deposition (ALD) at 450 °C using H2O as an oxidant. The ALD at relatively high temperature results in stable p-type conduction and FET operation at 400 °C in vacuum. The drain current density and transconductance normalized by the gate width are almost constant from room temperature to 400 °C in vacuum and are about 10 times higher than those of boron-doped diamond FETs.

C-H surface diamond field effect transistors for high temperature (400 °C) and high voltage (500 V) operation

Diamond field effect transistors have operated in electrolyte solution for the first time Since the hydrogen-terminated diamond surfaces are stable enough for the use as an electrochemical electrode, the diamond surface channels are exposed to the electrolyte in the transistor structure A perfect pinch-off and saturated current-voltage characteristics have been obtained for bias voltages within the potential window The threshold voltages are almost constant in electrolytes with different pH values of 7-13, indicating pH insensitiveness of the hydrogen-terminated diamond surface Based on this pH insensitive surface, ion selective regions can be fabricated to form transistor-based biosensors

Electrolyte-Solution-Gate FETs Using Diamond Surface for Biocompatible Ion Sensors

Ozone-treated channel diamond field-effect transistors

Cl- sensitive biosensor used electrolyte-solution-gate diamond FETs.

Effect of iodide ions on the hydrogen-terminated and partially oxygen-terminated diamond surface

Diamond field-effect transistors (FETs) operate in electrolyte solutions having a wide pH range of 1–13. The FETs have been fabricated using a p-type surface conductive layer, where the diamond surface is exposed directly to the electrolyte solutions. From the drain current-gate voltage (Ids–Vgs) characteristics of the FETs, it appears that the threshold voltages of the FETs are independent of the pH value of the solution. In Cl- ionic solutions, however, the threshold voltages shift approximately 30 mV with a one-order-of-magnitude change of molar concentration of Cl- ions. This sensitivity of the FET to Cl- ion's concrentration is observed in the 10-1–10-6 M range of potassium chloride (KCl) solutions.

Toshikatsu Sakai

Papers

Electrolyte-Solution-Gate FETs Using Diamond Surface for Biocompatible Ion Sensors

Ozone-treated channel diamond field-effect transistors

Cl- sensitive biosensor used electrolyte-solution-gate diamond FETs.

Effect of iodide ions on the hydrogen-terminated and partially oxygen-terminated diamond surface

Effect of Cl- ionic solutions on electrolyte-solution-gate diamond field-effect transistors