Strong and Tunable Spin-Orbit Coupling in a Two-Dimensional Hole Gas in Ionic-Liquid Gated Diamond Devices.

Hydrogen-terminated diamond possesses due to transfer doping a quasi-two-dimensional (2D) hole accumulation layer at the surface with a strong, Rashba-type spin-orbit coupling that arises from the highly asymmetric confinement potential. By modulating the hole concentration and thus the potential using an electrostatic gate with an ionic-liquid dielectric architecture the spin-orbit splitting can be tuned from 4.6-24.5 meV with a concurrent spin relaxation length of 33-16 nm and hole sheet densities of up to 7.23 × 10(13) cm(-2). This demonstrates a spin-orbit interaction of unprecedented strength and tunability for a 2D hole system at the surface of a wide band gap semiconductor. With a spin relaxation length that is experimentally accessible using existing nanofabrication techniques, this result suggests that hydrogen-terminated diamond has great potential for the study and application of spin transport phenomena.

Spin-orbit interaction in a two-dimensional hole gas at the surface of hydrogenated diamond.

Hydrogenated diamond possesses a unique surface conductivity as a result of transfer doping by surface acceptors. Yet, despite being extensively studied for the past two decades, little is known about the system at low temperature, particularly whether a two-dimensional hole gas forms at the diamond surface. Here we report that (100) diamond, when functionalized with hydrogen, supports a p-type spin-3/2 two-dimensional surface conductivity with a spin-orbit interaction of 9.74 ± 0.1 meV through the observation of weak antilocalization effects in magneto-conductivity measurements at low temperature. Fits to 2D localization theory yield a spin relaxation length of 30 ± 1 nm and a spin-relaxation time of ∼ 0.67 ± 0.02 ps. The existence of a 2D system with spin orbit coupling at the surface of a wide band gap insulating material has great potential for future applications in ferromagnet-semiconductor and superconductor-semiconductor devices.

Direct observation of phonon emission from hot electrons: spectral features in diamond secondary electron emission.

In this work we use high-resolution synchrotron-based photoelectron spectroscopy to investigate the low kinetic energy electron emission from two negative electron affinity surfaces of diamond, namely hydrogenated and lithiated diamond. For hydrogen-terminated diamond electron emission below the conduction band minimum (CBM) is clearly observed as a result of phonon emission subsequent to carrier thermalization at the CBM. In the case of lithiated diamond, we find the normal conduction band minimum emission peak is asymmetrically broadened to higher kinetic energies and argue the broadening is a result of ballistic emission from carriers thermalized to the CBM in the bulk well before the onset of band-bending. In both cases the spectra display intensity modulations that are the signature of optical phonon emission as the main mechanism for carrier relaxation. To our knowledge, these measurements represent the first direct observation of hot carrier energy loss via photoemission.

Towards wafer-size graphene layers by atmospheric pressure graphitization of silicon carbide.

Graphene, a single monolayer of graphite, has recently attracted considerable interest owing to its novel magneto-transport properties, high carrier mobility and ballistic transport up to room temperature. It has the potential for technological applications as a successor of silicon in the post Moore's law era, as a single-molecule gas sensor, in spintronics, in quantum computing or as a terahertz oscillator. For such applications, uniform ordered growth of graphene on an insulating substrate is necessary. The growth of graphene on insulating silicon carbide (SiC) surfaces by high-temperature annealing in vacuum was previously proposed to open a route for large-scale production of graphene-based devices. However, vacuum decomposition of SiC yields graphene layers with small grains (30-200 nm; refs 14-16). Here, we show that the ex situ graphitization of Si-terminated SiC(0001) in an argon atmosphere of about 1 bar produces monolayer graphene films with much larger domain sizes than previously attainable. Raman spectroscopy and Hall measurements confirm the improved quality of the films thus obtained. High electronic mobilities were found, which reach mu=2,000 cm (2) V(-1) s(-1) at T=27 K. The new growth process introduced here establishes a method for the synthesis of graphene films on a technologically viable basis.

Hydrogen-terminated diamond electrodes. I. Charges, potentials, and energies.

Hydrogen-terminated intrinsic diamond is a most unusual insulator since in contact with air or electrolyte it develops a hole accumulation layer just below the surface. When immersed in electrolyte the hole concentration responds to ionic charges and potentials and this response can be monitored by measuring the conductance of the hole accumulation layer without interference form the bulk conductivity. This feature has been widely used for chemical sensors in the form of solution-gate field effect transistors (SGFET). Here we analyze the charge and the potential profiles in the diamond and the electrolyte, as well as the static differential capacitance of the diamond electrode under controlled potential conditions. From this analysis we derive expressions for the transfer characteristics of diamond-based SGFET's that faithfully describe experimental data presented here as well. This holds in particular for the threshold region of the transfer characteristics that can only be modeled if the unusual semiconducting properties of the diamond electrode are taken into account properly. From fits to our data we derive (among other things) a value of chi=-0.50+/-0.02eV for the electron affinity of the hydrogen-terminated diamond surface in contact with aqueous electrolyte.

Hydrogen-terminated diamond electrodes. II. Redox activity.

One of the most attractive features of diamond is its robust p -type surface conductivity that develops spontaneously under atmospheric conditions on hydrogen-terminated samples. An electrochemical charge transfer between diamond and an air-borne redox couple has been suggested to be responsible for the spontaneous appearance of surface-near holes. We present direct proof for the redox activity of the diamond surface by measuring pH -dependent open circuit potentials and quasistatic polarization curves for hydrogen-terminated and partially oxidized diamond electrodes. Under open circuit conditions we find in fact a mixed (or corrosion) potential that is consistent with the simultaneous equilibriation of the electrode versus both the hydrogen-hydronium and the oxygen-hydroxyl redox couple. Our data show extremely long-time constants for establishing the redox equilibrium and very low exchange current densities making the identification and characterization of the redox process a demanding experimental task.

Nucleophilic-alkylation-reoxidation: a functionalization sequence for single-wall carbon nanotubes.

A new reaction sequence for the chemical functionalization of single-wall carbon nanotubes (SWNTs) consisting of the nucleophilic addition of t-BuLi to the sidewalls of the tubes and the subsequent reoxidation of the intermediates t-Bu(n)SWNT(n-) leading to t-Bu(n)SWNT was developed. During the formation of the t-Bu(n)SWNT(n-), a homogeneous dispersion in benzene was formed due to the electrostatic repulsion of the negatively charged intermediates causing debundling. The entire reaction sequence can be repeated, and the degree of functionalization of the products (t-Bu(n))(m)SWNT (m = 1-3) increases with increasing m. Degrees of functionalization expressed as the carbon-to-addend ratio of up to 31 were reached. The reaction was studied in detail by photoelectron spectroscopy, Raman spectroscopy, and scanning tunneling microscopy (STM). The C 1s core level spectra reveal that the nucleophilic attack of the t-BuLi leads to negatively charged SWNTs. Upon oxidation, this negative charge is removed. The valence band spectra of the functionalized samples exhibit a significant reduction in the pi-derived density of states. In STM, the covalently bonded t-butyl groups attached to the sidewalls have been visualized. Raman spectroscopy reveals that addition of the nucleophile to metallic tubes is preferred over the addition to semiconducting tubes.

Effect of SOCl2 treatment on electrical and mechanical properties of single-wall carbon nanotube networks.

Chemical modification by SOCl2 of an entangled network of purified single-wall carbon nanotubes, also known as 'bucky paper', is reported to profoundly change the electrical and mechanical properties of this system. Four-probe measurements indicate a conductivity increase by up to a factor of 5 at room temperature and an even more pronounced increase at lower temperatures. This chemical modification also improves the mechanical properties of SWNT networks. Whereas the pristine sample shows an overall semiconducting character, the modified material behaves as a metal. The effect of SOCl2 is studied in terms of chemical doping of the nanotube network. We identified the microscopic origin of these changes using SEM, XPS, NEXAFS, EDX, and Raman spectroscopy measurements and ab initio calculations. We interpret the SOCl2-induced conductivity increase by p-type doping of the pristine material. This conclusion is reached by electronic structure calculations, which indicate a Fermi level shift into the valence band, and is consistent with the temperature dependence of the thermopower.

Functionalization of single-walled carbon nanotubes with (R-)oxycarbonyl nitrenes.

Sidewall functionalization of single-walled carbon nanotubes (SWCNTs) via the addition of (R-)oxycarbonyl nitrenes allows for the covalent binding of a variety of different groups such as alkyl chains, aromatic groups, dendrimers, crown ethers, and oligoethylene glycol units. Such additions lead to a considerable increase in the solubility in organic solvents such as 1,1,2,2-tetrachloroethane (TCE), dimethyl sulfoxide (DMSO), and 1,2-dichlorobenzene (ODCB). The highest solubilities of 1.2 mg/mL were found for SWCNT adducts with nitrenes containing crown ether of oligoethylene glycol moieties in DMSO and TCE, respectively. The presence of chelating donor groups within the addends allowed for the complexation of Cu(2+) and Cd(2+). Atomic force microscopy (AFM) and transmission electron microscopy (TEM) revealed that the functionalized tubes form thin bundles with typical diameters of 10 nm. The presence of thin bundles in solution is supported by (1)H NMR spectroscopy. The elemental composition of the functionalized SWCNT was determined by X-ray photoelectron spectroscopy (XPS). The use of Raman and electron absorption spectroscopy (UV/Vis-nIR) showed that the electronic properties of the SWCNTs are mostly retained after functionalization, indicating a low degree of addition within this series of SWCNT derivatives.