Conduction Band Of Diamond Research Articles

Enhancement of spin-to-charge conversion of diamond NV centers at ambient conditions using surface electrodes

The nitrogen-vacancy (NV) center in diamond is a heavily studied defect due to its potential applications to quantum metrology and computation, particularly in ambient conditions. The key mechanism to using the NV in any application lies in the ability to read out the spin state of the defect which is typically done optically. The optical contrast is then the key metric for electron spin readout fidelity and one of the key limiting factors in the NV's overall performance. We present a mechanism for high contrast readout using the spin-to-charge conversion mechanism in conjunction with an electrode to improve the spin contrast by altering the NV energy levels relative to the diamond conduction band. Theoretical modeling predicts an optical spin contrast at 42% which would be the highest optical contrast for the NV at room temperature and the technique opens up a range of alternative research pathways for the NV which are discussed.

Spin-to-Charge Conversion with Electrode Confinement in Diamond

The nitrogen-vacancy ($\mathrm{N}$-$V$) center in diamond has a wide range of potential applications in quantum metrology, communications, and computation. The key to its use lies in how large the optical spin contrast is and the associated fidelity of the spin-state readout. In this paper, we propose a mechanism for improving contrast with a spin-to-charge protocol that relies on the use of an external electrode and cryogenic temperatures to discretize the diamond conduction band for spin-selective resonant photoionization. We use effective-mass theory to calculate the discrete eigenenergies in this system and use them to formulate a spin-to-charge protocol that involves resonant photoionization out of the $\mathrm{N}$-$V$ ground state into the diamond conduction band. The major sources of broadening, which guide the design of the experiment, are also addressed. With this mechanism, we calculate an optical spin contrast that and an associated spin-readout fidelity of 85%. This significant improvement can be applied to a number of cryogenic quantum technologies.

Detection of Aqueous Solvated Electrons Produced by Photoemission from Solids Using Transient Absorption Measurements.

Solvated electrons in water have long been of interest to chemists. While readily produced using intense multiphoton excitation of water and/or irradiation with high-energy particles, the possible role of solvated electrons in electrochemical and photoelectrochemical reactions at electrodes has been controversial. Recent studies showed that excitation of electrons to the conduction band of diamond leads to barrier-free emission of electrons into water. While these electrons can be inferred from the reactions they induce, direct detection by transient absorption measurements provides more direct evidence. Here, we present studies demonstrating direct detection of solvated electrons produced at diamond electrode surfaces and the influence of electrochemical potential and solution-phase electron scavengers. We further present a more detailed analysis of experimental conditions needed to detect solvated electrons emitted from diamond and other solid materials through transient optical absorption measurements.

Enhanced Photocatalytic Activity of Diamond Thin Films Using Embedded Ag Nanoparticles.

Silver nanoparticles embedded into the diamond thin films enhance the optical absorption and the photocatalytic activity toward the solvated electron-initiated reduction of N2 to NH3 in water. Here, we demonstrate the formation of diamond films with embedded Ag nanoparticles <100 nm in diameter. Cross-sectional scanning electron microscopy (SEM), energy-dependent SEM, and energy-dispersive X-ray analysis demonstrate the formation of encapsulated nanoparticles. Optical absorption measurements in the visible and ultraviolet region show that the resulting films exhibit plasmonic resonances in the visible and near-ultraviolet region. Measurements of photocatalytic activity using supraband gap (λ < 225 nm) and sub-band gap (λ > 225 nm) excitation show significantly enhanced ability to convert N2 to NH3. Incorporation of Ag nanoparticles induces a nearly 5-fold increase in activity using a sub-band gap excitation with λ > 225 nm. Our results suggest that internal photoemission, in which electrons are excited from Ag into diamond's conduction band, is an important process that extends the wavelength region beyond diamond's band gap. Other factors, including Ag-induced optical scattering and formation of graphitic impurities are also discussed.

Quantum confinement effects on optical transitions in nanodiamonds containing nitrogen vacancies

Colored nitrogen-vacancy (NV) centers in nanosize diamonds ($d\ensuremath{\sim}5$ nm) are promising probe materials because their optical transitions are sensitive to mechanical, vibrational, and spin changes in the surroundings. Here, a linear response time-dependent density functional theory approach is used to describe the optical transitions in several NV-doped diamond quantum dots (QDs) in order to investigate size effects on the absorption spectra. By computing the full optical spectrum up to band-to-band transitions, we analyze both the localized ``pinned'' midgap and the charge-transfer excitations for an isolated reduced NV center. Subband charge-transfer excitations are shown to be size dependent, involving the excitation of the dopant $s{p}^{3}$ electrons to the diamond conduction band. Additionally, the NV-doped systems exhibit characteristic $s{p}^{3}\ensuremath{-}s{p}^{3}$ excitations whose experimental energies are reproduced well and do not depend on QD size. However, the NV position and global cluster symmetry can affect the amount of the energy splitting of the vertical excitation energies of the midgap transitions.

Photoelectric detection of electron spin resonance of nitrogen-vacancy centres in diamond.

The readout of negatively charged nitrogen-vacancy centre electron spins is essential for applications in quantum computation, metrology and sensing. Conventional readout protocols are based on the detection of photons emitted from nitrogen-vacancy centres, a process limited by the efficiency of photon collection. We report on an alternative principle for detecting the magnetic resonance of nitrogen-vacancy centres, allowing the direct photoelectric readout of nitrogen-vacancy centres spin state in an all-diamond device. The photocurrent detection of magnetic resonance scheme is based on the detection of charge carriers promoted to the conduction band of diamond by two-photon ionization of nitrogen-vacancy centres. The optical and photoelectric detection of magnetic resonance are compared, by performing both types of measurements simultaneously. The minima detected in the measured photocurrent at resonant microwave frequencies are attributed to the spin-dependent ionization dynamics of nitrogen-vacancy, originating from spin-selective non-radiative transitions to the metastable singlet state.

Diamond electron emission

Abstract

Carbon electrode interfaces for synthesis, sensing and electrocatalysis: general discussion.

Robert Hamers replied: One can think of water as being like an amorphous semiconductor, in that there are strong interactions between adjacent atoms but there is no long-range order. J.V. Coe published several important papers in which he describes the water conduction band. Terminology is important, as one can refer to an optical conduction band and an adiabatic conduction band. The optical conduction band is determined by nding the position of the valence band relative to vacuum, and then using the optical bandgap (onset of optical absorption) to dene the position of the conduction band. For our purposes, the more important denition is that of the adiabatic conduction band, which is essentially determined by the electron affinity level of the water. That is, it refers to a state in which there is an excess electron, but the water molecules have not yet polarized around it to form a solvated electron. In this limit, the electron is essentially a plane wave and water behaves like a continuum dielectric, with the electron stabilized with respect to vacuum by ~0.1 V due to the long-range polarization forces that the electron exerts. A key aspect of the ‘adiabatic conduction band’ is that it has been used to explain how electrons created in water (usually created by high-energy ionizing radiation) are able to travel signicant distances in water before the water nally polarizes around them to form solvated electrons, which then have much lower mobility. In our studies, the conduction band may be important in establishing how far the solvated electrons are located away from the electrode surface. If they enter the water with ~1 eV of kinetic energy (roughly the energy difference between the conduction band of diamond and the conduction band of water), then they may be able to travel some

Freestanding Ultrananocrystalline Diamond Films with Homojunction Insulating Layer on Conducting Layer and Their High Electron Field Emission Properties

Freestanding ultrananocrystalline diamond (UNCD) films with homojunction insulating layer in situ grown on a conducting layer showed superior electron field emission (EFE) properties. The insulating layer of the films contains large dendrite type grains (400-600 nm in size), whereas the conducting layer contains nanosize equi-axed grains (5-20 nm in size) separated by grain boundaries of about 0.5-1 nm in width. The conducting layer possesses n-type (or semimetallic) conductivity of about 5.6 × 10(-3) (Ω cm)(-1), with sheet carrier concentration of about 1.4 × 10(12) cm(-2), which is ascribed to in situ doping of Li-species from LiNbO(3) substrates during growth of the films. The conducting layer intimately contacts the bottom electrodes (Cu-foil) by without forming the Schottky barrier, form homojunction with the insulating layer that facilitates injection of electrons into conduction band of diamond, and readily field emitted at low applied field. The EFE of freestanding UNCD films could be turned on at a low field of E(0) = 10.0 V/μm, attaining EFE current density of 0.2 mA/cm(2) at an applied field of 18.0 V/μm, which is superior to the EFE properties of UNCD films grown on Si substrates with the same chemical vapor deposition (CVD) process. Such an observation reveals the importance in the formation of homojunction on enhancing the EFE properties of materials. The large grain granular structure of the freestanding UNCD films is more robust against harsh environment and shows high potential toward diamond based electronic applications.

Study of ammonia molecule adsorbing on diamond (1 0 0) surface

The diamond (1 0 0) surface with amino terminations is investigated based on density function theory within the generalized gradient approximation. Our calculated negative electron affinity of diamond (1 0 0) surface with hydrogen termination provides a necessary condition for initiating radical reaction. The results display that the ammonia molecule can form stable C–N covalent bonds on the diamond surface. In addition, due to the lower adsorption energy of one amino group binding on diamond surface, single amino group (SAG) model is easy to be realized in experiment with the comparison of double amino group (DAG) model. The adsorbed ammonia molecule will induce acceptor-like gap states with little change of the valence and conduction band of diamond in SAG model. The adsorption mechanism in the formation of ammonia monolayer on H-terminated diamond (1 0 0) surface, and two possible adsorption structures (SAG and DAG) were especially studied.

Electron emission from conduction band of diamond with negative electron affinity

Experimental evidence explaining the extremely low-threshold electron emission from diamond reported in 1996 has been obtained for the first time. Direct observation using combined ultraviolet photoelectron spectroscopy/field emission spectroscopy (UPS/FES) proved that the origin of field-induced electron emission from heavily nitrogen (N)-doped chemical vapour deposited (CVD) diamond was at conduction band minimum (CBM) utilising negative electron affinity (NEA). The significance of the result is that not only does it prove the utilisation of NEA as the dominant factor for the extremely low-threshold electron emission from heavily N-doped CVD diamond, but also strongly implies that such low-threshold emission is possible from other types of diamond, and even other materials having NEA surface. The low-threshold voltage, along with the stable intensity and remarkably narrow energy width, suggests that this type of electron emission can be applied to develop a next generation vacuum nano-electronic devices with long lifetime and high energy resolution.

The Origin of Field-induced Electron Emission from N-doped CVD Diamond Characterized by Combined XPS/UPS/FES System

AbstractElectric field of less than 5 V/um is enough to extract electrons from CVD diamond, whereas field of one to two orders of magnitude higher is needed to extract electrons from metal emitter tips. Diamond has various advantages as an electron emitter in addition to the low-threshold voltage, negative electron affinity (NEA), high thermal conductivity, and high chemical stability. The difficulty in clarification of electron emission mechanism is the factor preventing diamond from being used in practical Quite a few numbers of possible mechanisms have been proposed based on conventional emission current-anode voltage (I-V) characteristics, however, difficulty remained in the determination of origin for emitting electrons. In our previous study, we have succeeded in determining the origin for lightly nitrogen (N)-doped CVD diamond by using combined x-ray photoemission spectroscopy / ultraviolet photoemission spectroscopy / field emission spectroscopy (XPS/UPS/FES) system. The origin at 1.47 eV above valence band maximum (VBM) was consistent with donor level for aggregated N. The origin was at VBM for natural IIb diamond measured as a reference, indicating the emitted electrons strongly depend on the dopant In this study, the origin of emitting electrons for heavily N-doped CVD diamond was characterized by the means of combined XPS/UPS/FES. Extremely low-threshold electron emission at 0.5 V/um is still one of the lowest threshold voltages. Such a low-threshold emission may be possible if electrons are somehow injected to conduction band of diamond and emitted into vacuum through NEA surface. Hence, we strongly believe the determination of the origin for heavily N-doped CVD diamond leads to the clarification of mechanism for low-threshold electron emission from diamond. The system in which we have developed for this study consists of ultraviolet source built in an X-ray photoelectron spectrometer. The diamond sample could be negatively biased up to 4kV relative to the mesh grid for FES. Individual spectroscopy of XPS was performed prior to the combined spectroscopy to determine any possible contaminants on CVD diamond. Peaks referred to C1s and O1s of diamond were observed in addition to Gold, used in a mesh grid set above the sample. Combined spectroscopy of UPS/FES was then, conducted on the CVD diamond to identify the origin of field emitted electrons. The diamond was first illuminated by He I excitation for photoemission. NEA with the typical spectra shape for diamond was observed. After confirming stable operation of individual UPS, negative voltage was applied simultaneously for field emission. Exceeding threshold voltage, a sharp peak, which can be referred to field-emitted electrons appeared in addition to typical UPS spectra. The origin of emitting electrons as well as its dependence on the applied voltage will be discussed.

The origin of shallow n-type conductivity in boron-doped diamond with H or S co-doping: Density functional theory study

The ionization potentials and the electron affinities of doped diamond were calculated using B3LYP hybrid density functional theory and nanocrystalline cluster models, while taking into account the quantum confinement of the charge carriers. In many cases donor and acceptor levels were created in the middle of the gap between the conduction and valence bands. A possible explanation for the n-type behavior created by co-doping diamond films with boron and sulfur is given in terms of thermally activated electron donation from an SVS (V is vacancy) donor to a BB acceptor band. Both lie deep in the band gap. It is proposed that electrons in the BB acceptor band are mobile charge carriers. It is also proposed that the conversion of boron-doped diamond from p-type conductivity, with hole charge carriers in the top of the valence band, to n-type conductivity, following treatment in a deuterium plasma, may arise from formation of interstitial hydrogen donor levels and B n H m acceptor levels that create an acceptor band in which electrons are mobile. Again, both are deep in the band gap of pure diamond. In a prior attempt to explain this n-type behavior, BH n defects with unrelaxed structures were proposed to be shallow donors to the diamond conduction band. This paper shows that these defects become deep donors when their structures are optimized. Finally, defects created from vacancies with 1 to 4 H in them are shown to be deep donors to the diamond conduction band.

Monte Carlo simulations of electron transport properties of diamond in high electric fields using full band structure

Electron transport properties in diamond under high electric fields (⩽5×106 V/cm) have been investigated by means of Monte Carlo simulations which include a full band structure, a wave-vector- and frequency-dependent dielectric function, phonon scattering rates with phonon dispersion relations, and impact ionization rates. The full band structure of diamond was calculated using an empirical pseudopotential method with an expansion of 113 plane waves, and was utilized to evaluate the dielectric function using the Lindhard method while suitable deformation potential coefficients were chosen in an adiabatic bond-charge model. Calculated results such as transition energies at the main points of symmetry and lines in the Brillouin zone as well as phonon dispersions were in good agreement with corresponding experimental data previously reported. The impact ionization rates of electrons in diamond were then evaluated from Fermi’s golden rule using the full band structure and dielectric function. The electric field (F) dependence of the electron drift velocity obtained reproduced experimental results previously reported on hot electron effects well and was fitted well by an analytical equation. It is found that the impact ionization probability rapidly increases with F⩾1×106 V/cm so that an electron in the diamond conduction band can yield electron-hole pair production for 1 μm travel on average at ≈1.5×106 V/cm. The F dependence of the impact ionization probabilities obtained is discussed in relation to energy (E) distributions of hot electrons created in high F on the order of 106 V/cm which are represented well by a Gaussian function, exp[−(E–Egap)2/wF2], for E≈7 eV, where Egap is the band gap energy and wF is a F-dependent constant corresponding to energy spread of ≈4 eV.

Single spin states in a defect center resolved by optical spectroscopy

Individual paramagnetic defect centers in diamond nanocrystals have been investigated by low-temperature high-resolution optical spectroscopy. Narrow fluorescence excitation spectral lines have been found, indicating transitions between individual spin sublevels. Spectral diffusion is explained by cross relaxation among spin sublevels and by the presence of excited electrons in the conduction band of diamond. The relaxation times are in the millisecond range. The system may be useful for quantum information processing with individual electron spins.

Metal-insulator-vacuum type electron emission from N-containing chemical vapor deposited diamond

This letter presents a clear explanation of the electron emission mechanism of the high-resistivity N-doped diamond cathode. Due to the very low barrier to emission of electrons from the N-doped diamond conduction band into vacuum, electrons in the conduction band of diamond can establish an appreciable leakage current at very low anode voltage. When such a current starts to flow, there is a field which is developed across the diamond bulk. This field is observed as an increase in the electric field at the back contact, causing the injected tunneling current increases exponentially. This process leads to the low threshold emission from the high resistivity N-doped diamond cathode.

Electron Transmission Through Modified Schottky Barriers

AbstractThe effects of a Coulomb-like potential in the Schottky barrier existing between a material-diamond interface is analyzed. The inclusion is intended to mimic the effects of an ionized trap within the barrier, and therefore to account for charge injection into the conduction band of diamond via a Poole-Frenkel transport mechanism. The present treatment is to provide a qualitative account of the increase in current density near the inclusion, which can be substantial. The model is first reduced to an analytically tractable one-dimensional tunneling problem addressable by an Airy Function approach in order to investigate the nature of the effect. A more comprehensive numerical approach is then applied. Finally, statistical arguments are used to estimate emission site densities using the results of the aforementioned analysis.

Calculation of the field emission current density from n-SI through injection in N-doped diamond

A model for electron field injection from the n-silicon conduction band (CB) into the nitrogen– diamond CB is presented. The model takes into account the electric field penetration into the silicon base and the disturbance of the electron distribution therein. Allowing for a nonlinear anode voltage dependence of the field in the diamond film (and thus in vacuum too) a good fit to experimental current density–voltage (J–V) characteristics was obtained. The model can also predict the temperature dependence of the emission current in accordance with some existing experimental data.

The use of internal field emission to inject electronic charge carriers into the conduction band of diamond films: a review

Several thin film composite metal(semiconductor) diamond cold cathode sources have recently been fabricated exhibiting high current–low power characteristics. We have modeled the field emission in these thin film diamond electron sources as a three-step process (electron injection, transport and vacuum emission). Critical to the operation of these devices is a mechanism for populating the conduction band (CB) of diamond with charge carriers. Internal field emission has been proposed for the injection of electrons by tunneling from metal (semiconductor) substrates into the diamond CB. A thin (Schottky) tunneling barrier is created at the substrate–diamond interface by heavily doping the diamond with nitrogen and roughening the metal (semiconductor) interface to enhance the internal field. In this paper we review model calculations of the internal field emission process for both metal and semiconductor substrates. The results show good agreement with experiment, implying the usefulness of the internal field emission mechanism to provide electronic charge carriers in the CB of diamond films.

Model calculations of internal field emission and J–V characteristics of a composite n-Si and N–diamond cold cathode source

A model to describe internal field emission through the interface between highly n-doped Si and nitrogen (N)-doped diamond is presented. We describe the roughness on the Si surface as a collection of sharp, spherically pointed Si asperities embedded in the diamond film. These “tips” provide enhancement of the applied electric field, which, in conjunction with the high N doping of diamond, results in the formation of a Schottky barrier which allows for tunneling or internal field emission from the Si into the conduction band of diamond. This enhanced electric field is also sufficient to induce valence band tunneling from the Si into the diamond conduction band. In our model limitations on the field mediated transport of holes from the n-doped Si/diamond interface to the cathode base leads to charging of the Si asperities. This charge accumulation results in band bending in Si and a significant reduction in the valence band current. The calculated J–V characteristics for the internal field emission lead to nonlinear behavior when plotted in Fowler–Nordheim coordinates. This is a consequence of the limitation of the conduction band current due to density of states effects at high fields in addition to the suppression of the valence band current. The calculated results are in qualitative agreement with recent field emission studies of Okano et al. [K. Okano, S. Koizumi, S. Ravi, P. Silva, and G. A. J. Amaratunga, Nature 381, 140 (1996)] for a composite n-Si and N–diamond cold cathode source. A plausible geometric argument suggests that there is also reasonable quantitative agreement.