Hydrogen-terminated Surface Research Articles

Boron-Doped Diamond as a Quasi-Reference Electrode.

As a working electrode, boron-doped diamond (BDD) has been studied in detail in electrochemical processes because of its superior electrochemical properties. However, these characteristics have rarely been mentioned when BDD is used as a quasi-reference electrode (QRE). Herein, we conducted a systematic investigation on BDD electrodes, with different boron-doping levels (1 and 0.1%) and different surface terminations (hydrogen and oxygen) for their application as a QRE. A BDD electrode with 1% boron and a hydrogen-terminated surface achieved the best stability. Its open-circuit potential (OCP) exhibited less than 100 mV of potential drift over 6000 s and showed a minuscule half-wave potential difference (E1/2) of 0.0037 V in 0.1 mM K3[Fe(CN)6]/1 M KCl solution before and after the OCP measurement. Based on these observations, anions are found to contribute to the potential, which we preliminarily speculate as related to the capacitance caused by electrostatic adsorption on the positively charged hydrogen-terminated surface. The repeatability of measurement was verified through continuous cyclic voltammetry tests in 0.1 mM K3[Fe(CN)6]/1 M KCl, showing a maximum E1/2 difference of 0.042 V. The contribution of the redox couples was excluded, and the repeatability was considered to originate from its surface stability. Finally, a linear response of the optimized BDD as a QRE was validated (R2 > 0.99) by determination of free chlorine and dopamine concentrations, respectively. These results consolidate the existing fundamental research on BDD electrodes and promote the possibility of its application as a QRE in harsh environments or in vivo biological monitoring.

Influence of confinement and surface functionalization effects on fluorescent properties of the impurity–vacancy complexes in ultrasmall nanodiamonds

We have done a thorough ab-initio research of vacancy–impurity complexes (including nitrogen, silicon and germanium) in 0.65 nm thick diamond nanocrystal and compared the results with those obtained for the complexes, located in diamond nanofilms of the same thickness and bulk crystals. While we consider only hydrogen-terminated and bare surfaces of nanoparticles and nanofilms , we are able to investigate the influence of confinement effects onto formation energy and electron structure of the impurity levels in the vicinity of band gap. It was found, that formation energy of these centers in nanoobjects (nanofilms and nanoparticles) is 2 eV less than that of bulk crystals. This implies preferable formation of these centers during growth close to the surface or in ultrasmall nanodiamonds. Confinement effects also significantly affect the band structure of nanoobjects. Beside the well-known effect of the band gap widening, they also lead to “popping up” of filled impurity levels over the valence band maximum. For negatively charged NV− center, located in nanodiamond, this results in interference of the upper, partially filled impurity level with the surface states, bringing about the splitting of this impurity level. We predict, that this effect would manifest itself in intermittent luminescence of NV− doped ultrasmall nanodiamonds with size below 5 nm.

Hydrogen-Terminated Diamond MOSFETs Using Ultrathin Glassy Ga2O3 Dielectric Formed by Low-Temperature Liquid Metal Printing Method

The p-type surface conductivity of hydrogen-terminated diamond (H-diamond) provides a viable approach toward diamond-based wide-bandgap metal-oxide-semiconductor field-effect transistors (MOSFETs) for high-power and high-frequency electronics. A facile, low-cost, and low-temperature method to form gate dielectrics on diamond that also preserves the integrity of hydrogen-termination is highly desirable for high-performance diamond surface electronics with process flexibility and high yield. In this work, we demonstrate a p-channel diamond MOSFET with an ultrathin glassy Ga2O3 dielectric layer derived from liquid metal. A liquid metal printing method was employed to transfer an amorphous Ga2O3 layer over the desired active p-channel region of H-diamond at low temperature, allowing the protection and preservation the hydrogen-terminated surface while also forming an efficient gate dielectric. The results of this work suggest that the liquid metal method can provide an efficient, low-cost, and high-yield pathway to form high-quality dielectrics for diamond-based transistors.

Radiation effect of X-ray with 100 kGy dose on the electrical properties of MESFET based on hydrogen-terminated diamond surface conductivity

The irradiation effect of X-ray on the electrical properties of Schottky-barrier diode (SBD) and metal-semiconductor field-effect transistors (MESFET) based on the surface conductivity of hydrogen-terminated single-crystal diamond (SCD) epilayers was investigated. The Ohmic contact was formed by a Pd/Ti/Au multilayer and the Schottky metal was Al thin film for the fabrication of the diamond SBDs and MESFETs. The X-ray irradiation was performed with a dose of 100 kGy. It was observed that both the forward current of the SBDs and the drain current of the MESFETs experienced a reduction after the X-ray irradiation. The type of the single-crystal diamond substrate had an obvious effect on the radiation properties. For the MESFETs on the type-Ib SCD substrate, the variation of the drain currents as the irradiation was inhomogeneous across the devices. For the MESFETs on the type-IIa SCD substrate, the reduction of the drain currents is more uniform and the threshold voltage changed little upon X-ray irradiation. The partial oxidation in the air of the exposure area in the device and the edge of the Al gate may be responsible for the degradation of the device performance under X-ray irradiation. The passivation technique with radiation-robustness is needed for diamond devices based on the surface conductivity of diamond.

Macro-superlubric triboelectric nanogenerator based on tribovoltaic effect

<h2>Summary</h2> Reducing the coefficient of friction (COF) and wear rate is significant for triboelectric nanogenerators (TENGs). Here, we devised a new macroscale superlubric triboelectric nanogenerator (SL-TENG) that can withstand high-contact stress with ultralow COF (<0.01) and ultralow wear rate based on a hydrogenated diamond-like carbon (DLC) film from the tribological perspective. Specifically, the SL-TENG produced an output with the peak short-circuit current of approximately 60 nA and a power density of up to 5.815 W/m² during the friction of <i>DLC</i> versus <i>steel ball</i> with a Hertz contact stress of 1.37 GPa. Moreover, first-principle calculations demonstrate that the ultralow friction on DLC is mainly due to the hydrogen-terminated surface. The SL-TENG can generate direct current (DC) output caused by tribovoltaic effect and can light up several light-emitting diodes (LEDs) without rectification. Online superlubric friction monitoring and early warning can be realized, owing to the close correspondence between tribology and triboelectricity.

Photoelectrochemical and Nanogravimetric Study of Electrolytic Transformation of Silicon-Oxide Interface

While silica is chemically stable in most acid electrolytes, its properties could significantly change due to electrolyte absorption. We demonstrate the photoelectrochemical and nanogravimetric features that allow identification of electrolytic transformation of Si-SiO2 interface. To that end, photoresponsivity of p-type silicon with ultrathin thermal oxide layer (10 nm) was studied. The responses were compared with those of hydrogen-terminated Si surface. The quartz crystal nanogravimetry (QCN) characterized the transformation process with nanogram resolution in situ and in real time. In neutral solution (pH 7), electrolyte absorption along with some dissolution of the silica layer has been detected by the QCN. No silica dissolution indications were observed in an acid electrolyte; its absorption was about one third of the dry oxide mass. Other discussed phenomena included photoelectron generation, recombination, and charging of the oxide film. The observed effects could be generalized to other Si/oxide systems and this has been demonstrated with HfO2 and Al2O3 ultrathin layers produced by the atomic layer deposition (ALD). The obtained results enable us to properly understand photoresponsivity, passivity, and degradation of Si/oxide electrodes for applications in devices of solar energy conversion.

Fluorine-terminated diamond (110) surfaces for nitrogen-vacancy quantum sensors

Diamond (110) surface is one of the low-index diamond faces but its effects on nitrogen-vacancy (NV) based quantum sensor remain unclear. The fluorine, hydrogen, nitrogen, and oxygen-terminated diamond (110) surfaces used for the NV centers are proposed here, and their electronic properties are investigated based on first-principles calculations. The oxygen-terminated diamond (110) surface has inter-bandgap states and surface electron spins, the nitrogen-terminated (110) surface has inter-band gap states, and the hydrogen-terminated (110) surface has negative electron affinity. Thus, these three surfaces may not be suitable for shallow NV centers. The fluorine-terminated diamond (110) surface has positive electron affinity, no surface related inter bandgap states, and no surface electron spins, so it is a promising candidate for NV-based quantum sensors.

Origin of two-dimensional hole gas formation on Si-treated diamond surfaces: Surface energy band diagram perspective

The Si terminal is considered to be able to form two-dimensional hole gas as hydrogen terminal, however, further confirmation of theory and experiments is necessary. In this study, the energy band diagram for the type IIa diamond surface after Si surface treatment was investigated experimentally. The results revealed a Fermi level of 0.2 eV above the valance band maximum, work function of 4.89 eV, and electron affinity of χ=-0.38eV. The energy band diagram of the Si-treated surface was similar to that of a hydrogen-terminated surface, which possesses the suitable chemical potential window facilitating electrons transfer from valance band to the aqueous adsorbed layer. Maybe this is the origin of the currently reported C–Si diamond two-dimensional hole gas channel. In addition, the conductivity of Si-treated surface varying with vacuum degree was observed, and the Hall measurement results showed that hole mobility and concentration are 69 cm2/V∙S and 3.92E + 10 cm−2 respectively.

Water-Soluble Silicon Quantum Dots toward Fluorescence-Guided Photothermal Nanotherapy

We report carboxy-terminated silicon quantum dots (SiQDs) that exhibit high solubility in water due to the high molecular coverage of surface monolayers, bright light emission with high photoluminescence quantum yields (PLQYs), long-term stability in the PL property for monitoring cells, less toxicity to the cells, and a high photothermal response. We prepared water-soluble SiQDs by the thermal hydrosilylation of 10-undecenoic acid on their hydrogen-terminated surfaces, provided by the thermal disproportionation of triethoxysilane hydrolyzed at pH 3 and subsequent hydrofluoric etching. The 10-undecanoic acid-functionalized SiQDs (UA:SiQDs) showed long-term stability in hydrophilic solvents including ethanol and water (pH 7). We assess their interaction with live cells by means of cellular uptake, short-term toxicity, and, for the first time, long-term cytotoxicity. Results show that UA:SiQDs are potential candidates for theranostics, with their good optical properties enabling imaging for more than 18 days and a photothermal response having a 25.1% photothermal conversion efficiency together with the direct evidence of cell death by laser irradiation. UA:SiQDs have low cytotoxicity with full viability of up to 400 μg/mL for the short term and a 50% cell viability value after 14 days of incubation at a 50 μg/mL concentration.

A monolithic, back-gated diamond field-effect transistor for tunable color centers

We present an innovative design for a monolithic field effect transistor, where all components consist of the wide-bandgap material diamond. The back gate-electrode is realized by a buried, degenerately boron-doped diamond (resistivity < 10−2 Ω cm), while the dielectric material is made of lightly nitrogen-doped diamond. The 2DHG on the hydrogen-terminated surface serves as the conductive channel of the transistor. We discuss the band structure of this device, the function of each individual component and show the sample preparation routine. Furthermore, we investigate the electrical tunability of the 2DHG and the optical tunability of NV-centers in a first proof-of-principle sample. Additionally, we use the field effect to manipulate the charge state of color centers in the nitrogen-doped film. This vertical and monolithic device structure opens up a range of applications, not only in the diamond semiconductor and quantum information technology, but also for sensing applications where the back-gating is advantageous or where an all-diamond layer sequence is beneficial.

Highly efficient diamond electromechanical transducer based on released metal–oxide–semiconductor structure

We propose and demonstrate an efficient, integrated, and customizable metal–oxide–semiconductor (MOS) actuator capable of active on-chip driving and tuning microelectromechanical resonators. A single-crystal diamond mechanical resonator with a hydrogen-terminated surface was utilized for demonstration. In this actuator, the electrical field applied to the gate tunes the width of the depletion region of the MOS capacitor on the cantilever and induces an actuation force. The proposed actuator overcomes the drawbacks of conventional actuators, such as the growth of piezoelectric materials, formation of p–n junctions, high dc voltages, and nanoscale air gaps. The actuator has various merits, such as low-power dissipation (∼pW), low-voltage operation (∼mV), and a tailored amplitude through a low dc bias of less than 1 V. The proposed actuator is universally applicable in all semiconductors compatible with complementary metal–oxide–semiconductor.

Oxygenation of Diamond Surfaces via Hummer’s Method

Oxygen bonded with diamond surfaces impacts important properties such as electrical conductivity, Schottky barrier height, field emission, and chemical reactivity. Though processes such as thermal, hydrogen plasma, etc., are efficient in oxidizing the hydrogen-terminated diamond surfaces, the oxidation of pristine diamond surfaces through wet chemical treatments is still in its infancy. Herein, we investigated the efficacy of Hummer’s method, one of the most celebrated chemical oxidation procedures to convert graphite to graphene oxide, to oxidize the pristine diamond surfaces. We attempted to oxidize both microcrystalline diamond powders and polycrystalline diamond wafers. Due to the presence of an acidic oxidative environment and the formation of strong oxidizing agents such as Mn2O7 and MnO3+ during the course of the reaction, Hummer’s method is found to be very effective in oxidizing the pristine diamond surfaces. The degree of oxygen termination is validated through various spectroscopic and surface probe measurements. Microcrystalline diamond powder is more prone to oxidation to polycrystalline diamond wafers due to excess surface area, and many facets with different dangling bond densities are exposed to the oxidizing medium. The experimental observations are endorsed through molecular dynamics simulations.

Effect of oxygen terminated surface of boron-doped diamond thin-film electrode on seawater salinity sensing

Tremendous demands for highly sensitive and stable seawater salinometers have motivated intensive research on advanced electrode materials. Boron-doped diamond (BDD) is attractive in terms of its high mechanical stability and chemical inertness, but is usually hindered by its low double-layer capacitance (Cdl) for seawater salinity detection. Here, inspired by the principle of oxygen-terminated BDD electrode endowing higher Cdl than hydrogen-terminated surface, we introduce the oxygen terminated surface by oxygen plasma or reactive ion etch (RIE), and the fabricated oxygen terminated BDD electrodes demonstrate high sensitivity and long-term stability in seawater salinity detection comparing with the hydrogen terminated BDD electrodes. Significantly, the as-fabricated O-BDD-RIE electrodes not only show remarkable enhanced response even better than the commercial platinum black electrodes but also display long-time stability which is weekly verified by continuous monitor for 90 days. The outstanding performance of the oxygen terminated BDD electrodes can be ascribed to the enhancement of C-O surface functional group on Cdl. In addition, a comprehensive analysis of effective electroactive surface area (EASA) and Cdl proves that the surface oxygen is the major factor for the improved Cdl. In summary, the excellent oxygen terminated BDD electrodes promise potential application in seawater salinity detection.

Combined electrical resistivity-electron reflectivity measurements for evaluating the homogeneity of hydrogen-terminated diamond surfaces

The homogeneity of hydrogen termination on single-crystal CVD diamond surfaces exposed to air was studied with a new approach that encloses a combination of sheet resistance measurements, performed using four-point-probe method in Van Der Pauw configuration, and electron reflectivity measurements performed at intermediate kinetic energy to derive surface sensitive information on the chemical terminations (inelastic mean free path <0.8 nm). The coherence between the 2-dimensional spatial distributions of sheet resistance and electron reflectivity maps, as well as their similar dynamic range and consistency with an electron energy loss analysis, demonstrates that the proposed combined approach allows for a reliable and easy evaluation of both the effectiveness and homogeneity of the hydrogen-termination process of a diamond surface.

Electrochemical route to bio-compatible fluorine-terminated diamond surface

In this communication, we propose a novel method to electrochemically modify hydrogen-terminated diamond surface in MISFETs with LiF dielectric at room temperature. The chemical reaction is 2F- + C–H+2h+→C–F + HF. According to measurement results of X-ray photoelectron spectroscopy on diamond, C–F bonds were produced after electrochemistry modification. A model was proposed to explain the electrochemistry modification on MISFETs. In this model, the theoretical electrolysis voltage of F− was calculated to be −3.8V, which was consistent with the experimental value of -5V.

Electron emission from H-terminated diamond enhanced by polypyrrole grafting

Electron emission plays an important role in diverse applications, from cold cathodes to chemical processes (solvated electrons, water purification), energy generation (thermionic or dye-sensitized solar cells), and even cancer treatment. Here we show that by surface treatment using electrochemically grown polypyrrole the secondary-electron emission and photoelectron emission from boron-doped diamond is enhanced even above the intensity of electron emission from the hydrogen-terminated surface with negative electron affinity. This enhancement is stable in air for at least one month and it persists also in vacuum after thermal annealing. Scanning electron microscopy, Kelvin probe force microscopy, total photoelectron yield spectroscopy as well as surface mapping by Auger and secondary ion mass spectroscopies are used to characterize and correlate the surface electronic and chemical properties. A model of the electron emission enhancement is provided.

Formation and Evaluation of Al2O3 Layer By Direct ALD on Epitaxial SiGe

In recent years, SiGe and Ge have drawn considerable attention as a high mobility channel material toward next-generation complementary metal-oxide-semiconductor (CMOS) circuits. In particular, by inducing strain into Ge using Ge/SiGe hetero-structure, highly enhanced mobility which greatly exceed the bulk mobility have been reported [1]. In order to obtain high mobility, it is necessary to generate larger strain. As one of the methods, strained SiGe on Ge-on-Si is expected. In addition, in order to realize a high mobility SiGe channel MOSFET, it is necessary to form the high-quality gate stacks and interfaces between a gate insulator film and a SiGe surface. In fact, the use of atomic layer deposition (ALD) or plasma oxidation has provided a high quality interface between Ge and the insulator film [2, 3]. However, unlike Si, wet cleaning of Ge or hydrofluoric acid treatment does not provide a high-quality hydrogen-terminated surface, and there is concern about surface oxidation and impurity adsorption. Therefore, we have attempted direct ALD on epitaxially grown Ge and reported that the Ge/Al2O3 interface properties are improved [4]. In the case of SiGe, a high-quality hydrogen-terminated surface cannot be obtained by wet cleaning or hydrofluoric acid treatment. For this reason, oxidation or impurity adsorption on the SiGe surface is feared. In this study, for the application to strained SiGe channel MOSFET, an Al2O3 film was formed using ALD on epitaxially grown SiGe as a gate stack structure on SiGe, and the interface structure was evaluated.Two types of samples were prepared by two methods. An n-type Ge (100) substrate was used in this study. The Ge substrate was chemically cleaned with NH4OH solution and followed by a HF dip and de-ionized water rinse. The wet-cleaned Ge substrate was loaded into an MBE chamber and subjected to thermal cleaning at 600 °C for 10 min. Subsequently, a 50 nm thick SiGe layer (called epi-SiGe hereafter) was hetero-epitaxially grown on the Ge substrate at 300 °C. The as-grown sample was subsequently transferred to the vacuum-connected ALD chamber and an Al2O3 film was deposited by ALD (Sample (A)). For comparison, after the epitaxial growth of the SiGe, the sample was once taken out from the chamber and exposed to the atmosphere for 5 min. The sample was then reloaded into the ALD chamber and an Al2O3 film was deposited (sample (B)). ALD of Al2O3 films was carried out for 20 cycles with precursors of TMA and H2O at a substrate temperature of 300 °C. Chemical bonding states were examined by x-ray photoelectron spectroscopy (XPS) using an ESCA-300 manufactured by Scienta Instruments AB.According to figs.1 (a) and (b), it was confirmed that the GeOx peak of the sample (A) transferred in a vacuum was significantly smaller than that of the sample (B) once exposed to the atmosphere. In particular, almost no Ge4+ peak [5] arising from Ge in GeO2 was observed, indicating that oxidation of Ge was suppressed by transport in vacuum. It is considered that the GeOx peak in the sample (A) is derived from the interface between Al2O3 and SiGe. According to figs.1 (c) and (d), no Si4+ component (binding energy range 102eV to 104eV) is found in the Si 2p 3/2 spectrum of the sample (A) transferred in a vacuum. In summary, XPS analyses reveal that formation of GeO2 and SiO2 by natural oxidation can be almost avoided by ALD on the epitaxial SiGe, and it is indicated that the Al2O3/SiGe interface created does not contain the interfacial GeO2 layer which is present for the sample exposed to the air before ALD. These results clearly indicate that the direct ALD on epitaxial SiGe is very promising way to highly improve the SiGe MOSFET performances.AcknowledgmentsThis work was partly supported by the MEXT Supported Program for the Strategic Research Foundation at Private Universities 2015–2019, a Grant-in-Aid for Scientific Research from MEXT and Interdisciplinary Research Center for Nano Science and Technology in Tokyo City University.

Sensitive Voltammetric Detection of Chloroquine Drug by Applying a Boron-Doped Diamond Electrode

In this research, a boron-doped diamond (BDD) electrode has been explored to detect the chloroquine drug. The electrochemical performance of BDD electrode towards the irreversible anodic response of chloroquine was investigated by subjecting this electrode to the cathodic (−0.5 A cm−2 by 180 s, generating a predominantly hydrogen-terminated surface) and anodic (+0.5 A cm−2 by 30 s, oxygen-terminated surface) pretreatments. The cathodically pretreated BDD electrode ensured a better-defined anodic peak and higher current intensity. Thus, by applying the cathodically pretreated BDD electrode and square-wave voltammetry (SWV), the analytical curve was linear from 0.01 to 0.25 µmol L−1 (correlation coefficient of 0.994), with sensitivity and limit of detection of 12.2 µA L µmol−1 and 2.0 nmol−1, respectively. This nanomolar limit of detection is the lowest recorded so far with modified and unmodified electrodes.

Investigation of charge carrier trapping in H-terminated diamond devices

Surfaces and interfaces can dominate charge carrier transport dynamics in electronic devices, impeding realization of a material's full potential. Here, we investigate transport in a two-terminal diamond device comprising a conductive channel defined by a hydrogen-terminated diamond surface, bridging two TiC contacts. The surface charge distribution was imaged by monitoring the photoluminescence of nitrogen vacancy centers incorporated below the active device layer. A strong charge accumulation near the TiC contact/H-terminated channel interface is observed and is discussed in terms of deviation from Ohmic behavior evident in the DC electrical measurements. Small voltage steps applied to the device result in current transients due to carrier trapping at the contact/diamond interface. This gives rise to dynamic negative capacitance at low AC frequencies and is discussed in detail.

Annealing Temperature on Contact Properties between Nickel Film and Hydrogen-Terminated Single Crystal Diamond

Ohmic contact of nickel on hydrogen-terminated single-crystal diamond film was investigated with an annealing temperature ranging from room temperature to 750 °C in hydrogen atmosphere. Nickel film was deposited on a hydrogen-terminated single-crystal diamond surface with gold film in order to protect it from oxidation. Contact properties between nickel and hydrogen-terminated single crystal diamond were measured by a circular transmission line model. The lowest specific contact resistivity was 7.82 × 10−5 Ω cm2 at annealing temperature of 750 °C, indicating good ohmic contact, which reveals improved thermal stability by increasing temperature.