Surface Conductivity on Hydrogen-Terminated Nanocrystalline Diamond

Surface Conductivity on Hydrogen-Terminated Nanocrystalline Diamond: Implication of Ordered Water Layers Andrei P. Sommer,* Dan Zhu, and Kai Brühne Institute of Micro and Nanomaterials, UniVersity of Ulm, 89081 Ulm, Germany

CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 11 2298–2301

ReceiVed July 2, 2007; ReVised Manuscript ReceiVed August 20, 2007

ABSTRACT: The electrical conductivity of hydrogenated diamond surfaces was reported in 1989. Whereas the experimental verification of the conductivity is simple, an Ohmmeter is sufficient, a satisfactory explanation of the effect has not been proposed yet. Existing models attempt to explain the effect on the basis of the semiconductor properties of diamond and a water layer adhering to its surface. The central dogma in them is that it is a surface conductivity. Here we show that the conductivity is not restricted to the surface, leaving room for a new understanding of the effect. Our finding not only represents a new paradigm, but provides a platform for the design of smart biomaterials with adjustable biocompatibility, and the production of biosensors and self-sufficient hygrometers for the exploration of water reservoirs on Mars. Surface conductivity on hydrogen-terminated diamond was first reported by Landstrass and Ravi for polycrystalline diamond.1 Several models were proposed to explain the conductivity, which is observed when hydrogen-terminated diamond surfaces are exposed to air. Models attempting to explain the effect range from assuming the action of dissolved hydrogen in the diamond,1 a redox reaction in a superficial water layer by which holes are liberated in the diamond valence band by the transfer of electrons from the valence band to solvated protons (transfer doping model),2,3 hydrogen-induced acceptors in diamond separated from the surface (separation layer model),4 to fluctuations in the hydrogen termination.5 The prevalence of hydrogen in semiconductors and related potentials, caused by its injection into semiconductors, was recognized 20 years ago.6 A principal difference between conventional semiconductor materials and diamond is the extreme inertness of diamond, including a minimal tendency to oxidize when exposed to air.7 This circumstance is of paramount importance in modeling the exclusive surface conductivity of hydrogen-terminated diamond. Diamond appears to be the only material providing direct contact between its surface and the atoms of the terminating agent. Despite ample observational evidence for the surface conductivity of hydrogen-terminated diamond, apparently not correlated with surface roughness,8 the basic properties of the charge converting the insulator diamond into a conductor are still unclear.9 However, the uncertainty is not restricted to the nature of the charge alone. The complexity of the phenomenon, i.e., the charge transfer at the liquid–solid interface, severely challenges the models employed for its description, as reflected by the discussions on the subject.10–12 Confusion is further caused by the amount of data on the conductivity as well as differences in the hydrogen termination parameters, as summarized in a recent review.13 Whereas the implication of the water layer on the effect is clear,14 its characterization is still lacking in the published literature. Attempts to characterize the water layer on hydrogen-terminated diamond by atomic force microscopy (AFM) did not provide an insight, except confirmation of the polarity.15 Recently, several papers have been published on interfacial water layers, including propagation, organization, and differences in viscosity on differently polar surfaces. Ordered water layers form at room temperature on a variety of hydrophobic substrates. They have been identified by us on translucent substrates (polymers), in air,16 and subaquatically.17,18 Their molecular order, induced by unilateral restriction in mobility because of surface contact, was indirectly exposed by monitoring their spontaneous response (i.e., depletion) to 670 nm laser irradiation, a wavelength virtually not absorbed by water. The * Corresponding author. E-mail: [email protected].

response has been observed exclusively on hydrophobic substrates. Importantly, the experiments performed by us to explore the nature of nanoscopic water layers on hydrophobic substrates indicated some unexpected properties, including the capability to spread, submerge smooth barriers placed onto the substrates, and push sessile drops several millimeters in diameter across the substrates, reflecting stability against evaporation at room temperature and an unusual fluidity.18 The corresponding physical picture is that of a highly organized, compact, nanoscopic water layer. AFM measurements performed at room temperature in air, suggested for nanoscopic water layers on hydrophilic substrates, viscosity values exceeding that of bulk water by a factor of 6–7,19 whereas water vapor condensing on a hydrophilic tip presented icelike properties at hydrophobic surfaces.20 As an example for nanocrystalline diamond21 deposited on silicon wafers via a high-temperature chemical vapor deposition (CVD) process,22 and emphasis on the prevalence of nanoscopic water layers on hydrophobic substrates, we analyze the surface conductivity on hydrogen-terminated diamond in air. Considering the functional versatility of diamond, receiving increasing importance as a biomaterial on all scales, including nanoparticles,23 and the nanoscopic water layers, recognized as key determinants of biocompatibility,24 a synoptic analysis of the two appears to be a highly promising enterprise. We used four different types of substrates based on nanocrystalline diamond. The thickness of the diamond layer was typically 1 µm. Differences were found exclusively in the surface chemistry and were realized by termination with fluorine, oxygen, and hydrogen; fluorine and oxygen via plasma treatment by reactive ion etching, hydrogen via hot filament CVD.25 Surface conductivity (resistance) tests were conducted under ambient conditions (25.7 °C, relative humidity 55%) using a digital multimeter (Voltcraft, M-3860M, Hirschau, Germany, range 40 MΩ) and validated by the use of an integrating digital multimeter (PREMA, 316, Mainz, Germany). We used spring-loaded platinum electrodes (purity 99.99%), applied in a two-point configuration and acting as Schottky contacts. Surface conductivity was detectable only on hydrogenterminated diamond, confirming the earlier findings reported by others. Previously, we demonstrated that the height of nanoscopic water layers deposited from the air onto hydrophobic surfaces could be instantly modulated with light intensities as low as the solar constant.16 In view of this result and the reported dependence of the contact potential difference between hydrogen-terminated diamond and the cantilever of an AFM on variations in illumination,26 we carried out the measurements in dim light. To check the effect of humidity on the surface conductivity, we monitored the resistance before, during, and after breathing onto the diamond samples. The resistance increased upon breathing in a time on the

10.1021/cg070610b CCC: $37.00  2007 American Chemical Society Published on Web 11/07/2007

Communications

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Figure 1. (a) Self-explanatory representation of the contact positions on the surface of hydrogen-terminated diamond (1 and 2) and silicon side (3 and 4). (b) Illustration of the principle of probing the presence of a conductive water layer on the diamond samples.

order of 1 s from 1.186 MΩ to a maximum between 1.985 and 2.010 MΩ (electrode configuration 1–2 in Figure 1a) and returned with a time constant of approximately 2 min to the initial value. The peak value depended on the performance of the experimenter. Alternatively, the impact of humidity on the conductivity was demonstrated by holding a sterile tissue wetted with ultra pure water close to the surface of the sample, thus excluding the possibility that the effect was caused by expired carbon dioxide or by an increase in temperature associated with the process of expiration. Exposure to nitrogen gas had no effect. To ascertain that the observed proportionality between humidity and resistance was indeed related to the surface conductivity, we applied the platinum electrodes across the diamond–silicon junction (points 1–3 in Figure 1a) and increased the humidity by breathing upon the sample. The response was a manifest increase in resistance, from 0.454 to 0.772 MΩ. For clarification, we additionally measured the resistances between poles 3 and 4 and poles 4 and 2 (Figure 1a) and obtained values of 0.020 and 0.730 MΩ, respectively. Values were regarded as stable when there was no change during an observational period of 30 min. To probe the current path, we placed a rectangular bar consisting of polished polymethyl methacrylate (PMMA) onto the hydrogenated diamond surface and measured the resistance across the bar (Figure 1b). There was no change in the previously detected resistance. Application of glue (an epoxy-based resin) between PMMA bar and the substrate did not interrupt the conductivity. Increasing the mechanical stress on the electrodes, i.e., by the use of a screw clamp, resulted in an immediate increase in conductivity, corresponding to a drop in resistance, for instance, from 1.186 to 0.510 MΩ. To check effects of surface contaminants on the surface conductivity, we produced distinct fields (rings) of a nonconductive material on hydrogen-terminated diamond. For this, we placed drops of a water-based suspension containing 60 nm nanospheres (Duke Scientific, Palo Alto, CA) of 10 µL each onto a sample. To slow the process of evaporation, we placed the sample into a partially sealed evaporation chamber. Figure 2 shows a representative light microscopy image of a ring on hydrogen-terminated diamond, produced in 20 h. The mechanism of ring formation has been described in our earlier work.27 Inspection by scanning electron microscopy (SEM) performed on similar samples revealed a thin nanoclay film within the rings, and a practically nanosphere-free area outside, in agreement with our previous study.28 Subsequently, we checked the resistance inside the rings and outside: inside the conductivity was destroyed, apparently by an insulating layer consisting of nanospheres; outside, even at a distance on the order of 1 mm from the ring wall, the surface maintained its conductivity. Interestingly, samples kept under ambient conditions for 1 year, and even visibly contaminated ones (fingerprints and dust, as well as spots of iron oxide, potassium permanganate, and calcium phosphate crystallized on the hydrogenated diamond from slowly dried water drops containing these substances), maintained their conductivity, including the reported characteristics. The only

Figure 2. Crystalline ring formed by polystyrene nanospheres on hydrogen-terminated diamond. The ring was formed by a slowly evaporating water drop containing the nanospheres. The surface conductivity inside the ring was zero; the zone external to the ring maintained its conductivity. Diameter of the ring: 4 mm.

difference between old and virgin samples (1 day old)—facilitating their discrimination—was the time constant of the response (increase in resistance and subsequent relaxation) to a specific stimulus (humidity). Signals were significantly shorter for virgin samples. This effect, together with the observed influence of the mechanical stress exerted on the contacts on the conductivity, may be a good explanation for the divergence in the data published on surface conductivity on hydrogenated diamond. The presently accepted theory of the conductivity on hydrogenterminated diamond assumes that the charge, transported parallel to the diamond surface, consists of holes, which are liberated by the transfer of electrons from the valence band in the diamond (donor) into the dissociated water molecules in the water layer (acceptor). Generalizing this model, we may expect that an increase of the capacity on the acceptor side, realized by an increase in the thickness of the water layer, should be accompanied by an increase in surface conductivity. Surprisingly, we observe exactly the opposite: increasing the thickness of the water layer on the diamond resulted in a spontaneous increase in resistance, corresponding to a drop in conductivity. Thus, we are justified in expecting that the reason for the conductivity of the hydrogen-terminated diamond is partly, or completely different from the mechanisms postulated in the accepted theories.2–5,29,30 Noting that humidity is a prerequisite for surface conductivity, we turn our attention to the principal difference between the structure of the nanoscopic water layer on hydrogen-terminated diamond and on other surfaces. Intrinsic to hydrogenated diamond is the triple coincidence: solid surface, pronounced hydrophobicity and unilateral affinity to water molecules. Understanding this aspect is essential for a comprehensive analysis of the conductivity effect. For this, it is instructive to exploit the information available on the nature of water layers adhering to hydrophobic surfaces in air, i.e., polymers, which are prone to corrosion and unilateral affinity to molecules is only temporarily realizable. On hydrogen-terminated diamond the order of the interfacial water molecules is not imposed by only spatial restriction at the solid surface16–18 but also chemically (see Figure 3 in ref 2 and Figure 9 in ref 3), i.e., induced by the alignment of at least one layer of water molecules, mediated by the hydrogen-termination

2300 Crystal Growth & Design, Vol. 7, No. 11, 2007 on the diamond. This picture suggests that the order of the molecules in the water layer implicated in the conductivity of hydrogenated diamond is superior to similar layers on any alternative substrate. The implication of the order might be in principle direct (conduction within the nanoscopic water layer, normal to the sample surface) and/or indirect (facilitation of the transport of holes in the diamond subsurface). In the direct mode, conductivity could be related to the presence of excess protons, in accord with the high mobility of excess protons in water.31 Proton transfer depends on the water structure,32 which is probably highly ordered on hydrogenterminated diamond. Thus, interpretation of the conductivity in hydrogenated diamond becomes a major challenge. In the elementary act, a proton is hopping from a hydronium ion (H3O+) to a neighboring water molecule. For the proton transfer to occur, it was postulated that the water molecule closest to the hydronium ion, i.e., shortest oxygen–oxygen distance (maximum order), is the most likely candidate to which the proton can be transferred.33 In hydrogen-terminated diamond, conductivity could operate according to the mechanism of the aforementioned transfer doping model.2,3 Actually, the model needs a spectacular Fermi level shift into the valence band of the diamond, which is thought to require a perfect termination with hydrogen and a defect-free subsurface.13 It is worth noting that diamond is the only known material that exhibits this kind of surface conductivity. There are uncertainties in our apparently straightforward conductivity experiments, which should be mentioned: the thickness and stability of the ordered water layer. However, it is plausible that with increasing distance from the polarizing site (hydrogen), order and stability will decrease. The analysis of supramolecular systems, i.e., of the chemistry and collective behavior of organized ensembles of molecules, proved fruitful in the study of interfacial water, even in an ultrahigh vacuum, revealing distinct regions of coexistence of disordered and crystalline water.34 If indeed the interface with aligned and oriented H2O molecules is equivalent with the interlayer in which excess protons have the highest mobility,33 we could expect that although conductivity will strongly depend on the integrity of this layer, an excess of less-ordered water molecules (e.g., produced by exposure to humidity) will disturb the ordered phase and result in a decrease in conductivity. The reported increase in the resistance on hydrogenterminated diamond in response to an increase in humidity might be looked upon as a confirmation of this expectation. Interpreted on the basis of the donor–acceptor system, the increase in resistance with humidity is an anomalous effect and in conflict with the concept of the transfer doping model. According to it, the conductivity should be boosted or kept constant (saturation) by a thicker water layer. However, although a nanoscopic water layer conducting protons parallel to the surface of the sample—analogous to the subsurface conduction in the transfer doping model—30 might appear reasonable on the basis of the order induced by the hydrogenation to the water molecules, a consideration of the results of the experiments performed by us (images a and b in Figure 1) force us to exclude this possibility. Clearly, a water-layer-mediated surface conductivity is in partial conflict with the measured increase in resistance between poles 1 and 3 (Figure 1a) as a response to an increase in humidity. Similarly, the PMMA bar did not interrupt the surface conductivity (Figure 1b). To encircle the phenomenon, we now compare RX (sum of the resistances measured between the poles 1–3, 3–4, and 4–2) with RS (value measured between poles 1 and 2), that is 1.186 MΩ: RX ) R1,3 + R3,4 + R4,2 ) 0.454 MΩ + 0.020 MΩ + 0.730 MΩ ) 1.204 MΩ RS is practically equal to RX. It is unlikely that the representative identity is simply a coincidence. Actually, it provides an explanation for our own findings as well as the findings of other groups. For constant humidity levels, the correlation between RX and RS was found to be generally valid. Probably, a transient increase in humidity and the associated propagation of the water film on the surface of the hydrogenated diamond enhances the concentration

Communications of the water molecules in the space between electrodes and diamond, thereby reducing the conductivity locally. Alternatively, it could be argued that the transient coexistence of a preexistent nanoscopic water layer (ordered) with a less-ordered phase (surplus humidity) caused a decrease in the preexistent order, thereby impeding the aforementioned transfer of protons across an ordered layer of water molecules. It is clear that the surface conductivity on hydrogenated diamond is a highly complex phenomenon, involving the simultaneous interplay of several parameters. Remarkably, the relationship between humidity and resistance observed by us contradicts recent laboratory experiments, suggesting a reciprocal interdependence, i.e., a decrease in resistance upon an increase in humidity.35 Hydrogen-terminated nanocrystalline diamond surfaces represent a combination of inertness with hydrophobicity, thereby offering a unique platform for the systematic investigation of nanoscopic water layers. The interface spanned by the hydrogen and water molecules can be characterized by competing bonds, spatial constraint, and polarization. Because of the insignificant oxidation of the diamond surface, this combination is presumably unrealizable on other materials. Exploiting the information ascertained on the nanoscopic water layers, we demonstrated that humidity affects the organization of the water molecules on hydrogenated diamond. The importance of our finding includes the discovery of a system that facilitates direct access to the nature of nanoscopic water layers on surfaces, including biologically relevant surfaces. Biologically, water layers represent a dynamic element of paramount importance, especially with regard to the transfer of information between cells during first contact events. Their understanding will lead to an extension of existing models and inspire the design of smart biomaterials that permit controlling the key determinants of biocompatibility, including inertness, hardness, elasticity, surface chemistry, surface charge, microstructure, nanostructure, and the structure of the nanoscopic water layers,24 which, as we recently demonstrated, persists even in an aqueous environment.18 The most prominent smart biomaterials involve engineered surfaces that exhibit dynamic changes in interfacial properties, such as wettability, in response to an electrical potential.36 The list of practical applications for hydrogenated nanocrystalline diamond seems endless, reaching from smart biomaterials, devices for pulmonary function tests, and simple biosensors operating under extreme atmospheric conditions to robust self-sufficient resistive humidity sensors, as could be needed to explore hidden water reservoirs on Mars.

Acknowledgment. A.P.S. is grateful to Horst-Dieter Försterling for fruitful discussions.

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