Diamond Transition Research Articles

Structural transition and migration of incoherent twin boundary in diamond.

Grain boundaries (GBs), with their diversity in both structure and structural transitions, play an essential role in tailoring the properties of polycrystalline materials1-5. As a unique GB subset, {112} incoherent twin boundaries (ITBs) are ubiquitous in nanotwinned, face-centred cubic materials6-9. Although multiple ITB configurations and transitions have been reported7,10, their transition mechanisms and impacts on mechanical properties remain largely unexplored, especially in regard to covalent materials. Here we report atomic observations of six ITB configurations and structural transitions in diamond at room temperature, showing a dislocation-mediated mechanism different from metallic systems11,12. The dominant ITBs are asymmetric and less mobile, contributing strongly to continuous hardening in nanotwinned diamond13. The potential driving forces of ITB activities are discussed. Our findings shed new light on GB behaviour in diamond and covalent materials, pointing to a new strategy for development of high-performance, nanotwinned materials.

Network Phases with Multiple-Junction Geometries at the Gyroid-Diamond Transition.

A novel phase sequence for the transition from the double diamond to the double gyroid cubic phases via two non-cubic intermediate phases, an orthorhombic Fmmm (O69) phase and a hexagonal P63/m (H176) phase, is reported for specifically designed bolapolyphiles composed of a linear rod-like bistolane core with sticky glycerol ends and two branched central and two linear peripheral side chains. These liquid crystalline (LC) phases represent members of a new class of unicontinuous network phases, formed by longitudinal rod bundles with polar spheres acting as junctions and the alkyl chains forming the continuum around them. In contrast to previously known bicontinuous cubic networks, they combine different junctions with different angles in a common structure, and one of them even represents a triple network instead of the usually found double networks. This provides new perspectives for the design of soft network phases with enhanced structural complexity, inspiring the search for new supramolecular networks, nano-particle arrays, and photonic band-gap materials.

Transitions in polycrystalline diamond probed by steady state, modulated and transient surface photovoltage spectroscopy

Numerous electronic transitions in polycrystalline chemical vapor deposition diamond with characteristic nucleation and growth sides were obtained by highly sensitive surface photovoltage (SPV) spectroscopy in dc (Kelvin probe), ac (modulated) and ac (transient) regimes from near infrared to deep ultraviolet. In the dc regime, defect transitions D1 to D8 were detected at 0.8–0.9, 1, 1.37, 1.78, 2.27, 3.15, 4.2 and 5.42 eV, respectively. Hints for more transitions were found for measurements in the ac (modulated) regime in the range near D3. SPV measurements in the ac (transient) regime showed the importance of disorder for relaxation of SPV signals excited at different photon energies. Phonon assisted transitions were observed at E g− E x− hν LA,LO, E g− E x + hν LA,LO and E g− E x + hν TO. The developed SPV techniques are suitable for applications in research and quality control not only for diamond but also for any other semiconductor with ultra-wide bandgap.

Brittle-ductile transition and nano-surface generation in diamond turning of single-crystal germanium

Single-crystal germanium (Ge) has been widely used in infrared optics and ultra-precision components, however, the appearance of alternating brittle and ductile fan-shaped patterns on machined surfaces seriously affects the machined surface quality. The present research focused on the brittle-ductile transition and its effect on the nano-surface generation in single-point diamond turning (SPDT) of (100), (110) and (111) crystal planes of Ge. Face cutting and plunge cutting experiments were performed, the surface anisotropy and brittle-ductile transition regime during cutting were analyzed, and the evolution of crystalline structure was discussed with molecular dynamics (MD) simulation. Theoretical and experimental results show: (1) The anisotropy of the machined surface is closely related to the atomic arrangement order and lattice structure. The anisotropy of the (111) crystal plane is stable while the (110) is unstable. (2) The critical length and depth of both the elastic-ductile and brittle-ductile transition are anisotropic; the depth and length of brittle regions are shallower and shorter than those of ductile regions and proportional to the tool edge radius. (3) The critical depth and thrust force of the elastic-ductile transition obtained in MD simulation can be employed to predict the changing trend of the brittle-ductile transition. (4) The thickness of amorphous layer produced on the machined surface is proportional to the ratio of cut depth to tool edge radius. The 1¯10 crystal orientation of the (111) crystal plane shows the best cutting performance. This paper provides deep insight into the brittle-ductile transition and nano-surface generation in SPDT of Ge.

Dynamical quantum phase transition in diamond: Applications in quantum metrology

Nonequilibrium dynamics is a paramount scenario for studying quantum systems. The emergence of new features with no equilibrium counterpart, such as dynamical quantum phase transition (DQPT), has attracted wide attention. In this paper, we depart from the well-known Ising model and showcase an experimentally accessible configuration of a negatively charged nitrogen-vacancy center that interacts with nearby carbon-13 nuclear spins. We provide insights into this system in the context of DQPT. We show that nuclear spins undergo DQPT by appropriately choosing the relation between the transverse and longitudinal components of an external magnetic field. Furthermore, we can steer the DQPT via a time-dependent longitudinal magnetic field and apply this control to enhance the estimation of the coupling strength between the nuclear spins. Moreover, we propose a quenched dynamics that originates from the rotation of the central electron spin, which controls the DQPT relying on the anisotropy of the hyperfine coupling.

Transient surface photovoltage spectroscopy of diamond

Contactless and highly sensitive probing of electronic transitions in diamond over a wide spectral range from near infrared to deep ultraviolet is still challenging. Surface photovoltage (SPV) signals depend on electronic transitions and transport phenomena leading to charge separation in space and allow for a contactless study of electronic transitions. Here, transient SPV spectroscopy in an arrangement with a charge amplifier and a laser tunable over a wide range was applied to study an undoped diamond single crystal between 0.8 and 5.9 eV at room temperature in ambient air. SPV transients were measured without and with weak visible bias light, which allowed for suppression of possible parasitic contributions in SPV signals not related to diamond and distinction of processes of charge separation that were independent of band bending. Transitions at 1.0 and 3.1 eV led to preferential separation of photogenerated holes toward the surface. In contrast, a transition at 1.8 eV caused preferential separation of photogenerated electrons toward the surface. Transitions near the indirect bandgap of diamond were observed at 5.27, 5.32, 5.48, and 5.53 eV and could be assigned to absorption assisted (i) by an indirect exciton and absorption of longitudinal optical and acoustic phonons, (ii) by absorption of transverse acoustic phonons, (iii) by emission of transverse acoustic phonons, and (iv) by emission of longitudinal optical and acoustic phonons, respectively. Charge separation under excitation at 5.27 eV was caused by directed charge transfer at/near the diamond surface after exciton diffusion followed by exciton dissociation.

Intermediate carbon phase. New experimental data and atomic model

We investigated phase transitions in diamond under cyclic loading near the graphite-diamond equilibrium line at temperatures sufficiently below the Debye temperature (maximum stresses and temperature in the diamond do not exceed 6 GPa and 420 K accordingly). Using Raman and transmission electron microscopy, we reveal the following stages of the transition of diamond to graphite-type phase (onions). The first stage is the plastic deformation by the mechanical twinning along with martensitic phase transition of diamond to intermediate carbon phase (ICP). The next stage is the transition of ICP to onions with a radius below ~5 nm. The proposed theoretical ICP model based on the crosslinking of graphene layers by Frenkel defects displays not only good agreement with the available structural experimental data but also specific properties like enhanced out-of-plane stiffness and metallicity.

Brittle-Ductile Transition and Nano-Surface Generation in Single-Point Diamond Turning of Single-Crystal Germanium

Single-crystal germanium (Ge) has been widely used in infrared optics and ultra-precision components, however, the appearance of alternating brittle and ductile fan-shaped patterns on machined surfaces seriously affects the machined surface quality. The present research focused on the brittle-ductile transition and its effect on the nanosurface generation in single-point diamond turning (SPDT) of (100), (110) and (111) crystal planes of Ge. Face cutting and plunge cutting experiments were performed, the surface anisotropy and brittle-ductile transition regime during cutting were analyzed, and the evolution of crystalline structure were discussed with molecular dynamics (MD) simulation. Theoretical and experimental results show: (1) The anisotropy of the machined surface is closely related to the atomic arrangement order and lattice structure. The anisotropy of the (111) crystal plane is stable while the (110) is unstable. (2) The critical length and depth of both the elastic-ductile and brittle-ductile transition are anisotropic; the depth and length of brittle regions are shallower and shorter than those of ductile regions and proportional to the tool edge radius. (3) The critical depth and thrust force of the elastic-ductile transition obtained in MD simulation can be employed to predict the changing trend of the brittle-ductile transition. (4) The thickness of amorphous layer produced on the machined surface is proportional to the ratio of cut depth to tool edge radius. The crystal orientation of the (111) crystal plane shows the best cutting performance. This paper provides deep insight into the brittle-ductile transition and nanosurface generation in SPDT of Ge.

Intracenter dipole transitions of a hydrogen-like boron acceptor in diamond: Oscillator strengths and line broadening

Substitutional boron in diamond acts as an acceptor center with the ionization energy of about 372 meV. Unlike its analogues in elemental semiconductors (silicon, germanium), boron bound states are poorly described by the electronic mass approximation due to the large ionization energy and the small spin-orbit splitting of the valence band in diamond. Thereby experimental investigations appear to be a main way of study of discrete electronic states of boron in diamond.In this paper, we report infrared (IR) absorption spectroscopy results obtained for HPHT-grown single crystal diamonds doped with natural boron (80% 11B – 20% 10B), and with isotopically enriched boron (99% 11B – 1% 10B). A moderate dopant concentration (1016–1017 cm−3), low concentration of lattice defects and vanishing thermal impact at cryogenic temperatures provided significantly reduced line broadening so that more than 60 boron transitions could be spectrally resolved. Mathematical approximation of the spectral lines revealed the main empiric features of zero-phonon dipole intracenter transitions: their energy, linewidth and integrated absorption. This allowed us to calculate oscillator strength of the boron acceptor transitions in diamond. The Lorentzian shape of the spectral lines indicates dominating uniform broadening of the transitions. Together with the relatively broad linewidths (0.1–0.5 meV) it reveals strong electron-phonon interaction in boron-doped diamond.

Formation of diamond nanostructures from graphite using 10 W fibre laser

The high activation energy required for graphite–diamond transition limits its applicability in novel areas. To exploit fully the multifunctional properties of diamond in diverse fields, there is a necessity to explore more efficient ways for its synthesis. In this study, we have demonstrated a new approach for nanodiamonds formation by employing a commercially available low power 10 W continuous-wave fibre laser. The laser system is modulated to generate the high-pressure high temperature environment necessary for the phase conversion of graphite to diamond. The microsecond pulse duration combined with liquid confinement effect on plasma provide scope for a lower rate of supercooling, which restricts the epitaxial growth of the crystals. The sample is characterized by X-ray powder diffraction, transmission electron microscope and Raman spectroscopy, confirming the presence of different types of nanodiamonds including newly discovered n-diamond. The process offers many important advantages like scalable process, non-catalyst-based eco-friendly and cost-effective synthesis of metastable nanodiamonds. The results demonstrate the effectuality of inexpensive commercial lasers towards attaining the localized extreme environment necessary for direct phase conversion of diamond materials.

Dissolution susceptibility of glass-like carbon versus crystalline graphite in high-pressure aqueous fluids and implications for the behavior of organic matter in subduction zones

Organic matter, showing variable degrees of crystallinity and thus of graphitization, is an important source of carbon in subducted sediments, as demonstrated by the isotopic signatures of deep and ultra-deep diamonds and volcanic emissions in arc settings. In this experimental study, we investigated the dissolution of sp2 hybridized carbon in aqueous fluids at 1 and 3GPa, and 800 °C, taking as end-members (i) crystalline synthetic graphite and (ii) X-ray amorphous glass-like carbon. We chose glass-like carbon as an analogue of natural “disordered” graphitic carbon derived from organic matter, because unlike other forms of poorly ordered carbon, it does not undergo any structural modification at the investigated experimental conditions, allowing approach to thermodynamic equilibrium. Textural observations, Raman spectroscopy, synchrotron X-ray diffraction and dissolution susceptibility of char produced by thermal decomposition of glucose (representative of non-transformed organic matter) at the same experimental conditions support this assumption. The redox state of the experiments was buffered at ΔFMQ ≈ –0.5 using double capsules and either fayalite-magnetite-quartz (FMQ) or nickel-nickel oxide (NNO) buffers. At the investigated P–T–fO2 conditions, the dominant aqueous dissolution product is carbon dioxide, formed by oxidation of solid carbon. At 1GPa and 800 °C, oxidative dissolution of glass-like carbon produces 16–19 mol% more carbon dioxide than crystalline graphite. In contrast, fluids interacting with glass-like carbon at the higher pressure of 3GPa show only a limited increase in CO2 (fH2NNO) or even a lower CO2 content (fH2FMQ) with respect to fluids interacting with crystalline graphite. The measured fluid compositions allowed retrieval of the difference in Gibbs free energy (ΔG) between glass-like carbon and graphite, which is +1.7(1) kJ/mol at 1GPa–800 °C and +0.51(1) kJ/mol (fH2NNO) at 3GPa–800 °C. Thermodynamic modeling suggests that the decline in dissolution susceptibility at high pressure is related to the higher compressibility of glass-like carbon with respect to crystalline graphite, resulting in G–P curves crossing at about 3.4GPa at 800 °C, close to the graphite–diamond transition. The new experimental data suggest that, in the presence of aqueous fluids that flush subducted sediments, the removal of poorly crystalline “disordered” graphitic carbon is more efficient than that of crystalline graphite. This occurs especially at shallow levels of subduction zones, where the difference in free energy is higher and the availability of poorly organized metastable carbonaceous matter and of aqueous fluids produced by devolatilization of the downgoing slab is maximized. At depths greater than 110 km, the small differences in ΔG imply that there is minimal energetic drive for transforming “disordered” graphitic carbon to ordered graphite; “disordered” graphitic carbon could even be energetically slightly favored in a narrow P interval.

Femtosecond Laser-Induced Electron Emission from Nanodiamond-Coated Tungsten Needle Tips.

We present femtosecond laser-induced electron emission from nanodiamond-coated tungsten tips. Based on the shortness of the femtosecond laser pulses, electrons can be photoexcited for wavelengths from the infrared (1932nm) to the ultraviolet (235nm) because multiphoton excitation becomes efficient over the entire spectral range. Depending on the laser wavelength, we find different dominant emission channels identified by the number of photons needed to emit electrons. Based on the band alignment between tungsten and nanodiamond, the relevant emission channels can be identified as specific transitions in diamond and its graphitic boundaries. It is the combination of the character of initial and final states (i.e., bulk or surface-near, direct or indirect excitation in the diamond band structure), the number of photons providing the excitation energy, and the peak intensity of the laser pulses that determines the dominant excitation channel for photoemission. A specific feature of the hydrogen-terminated nanodiamond coating is its negative electron affinity that significantly lowers the work function and enables efficient emission from the conduction band minimum into vacuum without an energy barrier. Emission is stable for bunch charges of up to 400electrons per laser pulse. We infer a normalized emittance of <0.20 nm rad and a normalized peak brightness of >1.2×10^{12} A m^{-2} sr^{-1}. The properties of these tips are encouraging for their use as laser-triggered electron sources in applications such as ultrafast electron microscopy as well as diffraction and novel photonics-based laser accelerators.

The modeling and synthesis of nanodiamonds by laser ablation of graphite and diamond-like carbon in liquid-confined ambient

Nanodiamonds have attracted considerable interest for their potential applications in quantum computation, sensing, and bioimaging. However, synthesis of nanodiamonds typically requires high pressures and temperatures, and is still a challenge. Here, we demonstrate production of nanodiamonds by pulsed laser ablation of graphite and diamond-like carbon in water. Importantly, this technique enables production of nanocrystalline diamonds at room temperature and standard pressure conditions. Moreover, we propose a method for the purification of nanodiamonds from graphitic and amorphous carbon phases that do not require strong acids and harsh chemical conditions. Finally, we present a thermodynamic model that describes the formation of nanodiamonds during pulsed laser ablation. We show that synthesis of the crystalline phase is driven by a graphite–liquid–diamond transition process that occurs at the extreme thermodynamic conditions reached inside the ablation plume.

Simulation of the graphite–diamond transition in an isentropic process

An equation of state for graphite and diamond has been derived in wide density and temperature ranges. A set of equations for the graphite–diamond phase transition has been presented. Hugoniots for graphite and diamond have been calculated. Numerical simulation data for the graphite–diamond transition in the isentropic compression process using a metallic z-pinch with diamond saving have been reported.

Coherent control of a strongly driven silicon vacancy optical transition in diamond

The ability to prepare, optically read out and coherently control single quantum states is a key requirement for quantum information processing. Optically active solid-state emitters have emerged as promising candidates with their prospects for on-chip integration as quantum nodes and sources of coherent photons connecting these nodes. Under a strongly driving resonant laser field, such quantum emitters can exhibit quantum behaviour such as Autler–Townes splitting and the Mollow triplet spectrum. Here we demonstrate coherent control of a strongly driven optical transition in silicon vacancy centre in diamond. Rapid optical detection of photons enabled the observation of time-resolved coherent Rabi oscillations and the Mollow triplet spectrum. Detection with a probing transition further confirmed Autler–Townes splitting generated by a strong laser field. The coherence time of the emitted photons is comparable to its lifetime and robust under a very strong driving field, which is promising for the generation of indistinguishable photons.

Experimental subsolidus phase relations in the system CaCO3–CaMg(CO3)2 up to 6.5 GPa and implications for subducted marbles

The dissociation of dolomite to aragonite + magnesite and the reaction of dolomite + aragonite to Mg-calcite have been experimentally determined by piston cylinder experiments in the range of 650–1000 °C and 2–6.5 GPa. Friction decay within the salt sleeves used as pressure medium was monitored by continuously logging the travel distance of the piston. Piston movement ceased after the first 24 h of the experiment, and all experiments were kept at the steady-state conditions for an additional 24–196 h. The reaction of dolomite to aragonite + magnesite has a positive slope and has been bracketed at 650 °C between 5.1 and 5.8 GPa and at 750 °C between 5.7 and 6.5 GPa. The reaction is sluggish with a 0.6-GPa interval where all three phases coexist. The reaction dolomite + aragonite to Mg-calcite has been determined in 0.5-GPa intervals from 660 °C at 2 GPa to 940 °C at 6.5 GPa. The composition of Mg-calcite at the reaction curve is buffered and changes systematically from X(Ca) of 0.90 at 2 GPa to 0.57 at 6.5 GPa. The solvus between dolomite and Mg-calcite shrinks slightly with increasing pressure, providing evidence for a small negative slope of the critical curve dolomite = Mg-calcite. The degree of disorder in dolomite has been determined in some of the retrieved samples by Rietveld refinement based on X-ray diffraction patterns. The order parameter S decreases gradually from 1 to 0.7 as a function of temperature as the critical curve is approached and then drops dramatically after crossing it. No ordering on quench has been observed. The experimental results permit the construction of a petrogenetic grid for the system dolomite, aragonite, Mg-calcite, and magnesite, which has an invariant point at ~7.5 GPa and 980 °C, representing the maximum pressure stability of dolomite. X(Ca) isopleths of Mg-calcite coexisting with dolomite serve mainly as a thermometer, whereas in the presence of aragonite they are a useful barometer. The experimental results have been applied to diamond-bearing marbles of the Kokchetav Massif, where Mg-calcite inclusions in garnet and zircon with X(Ca) of 0.76 most likely formed during prograde metamorphism close to the graphite–diamond transition.

Three dimensional graphdiyne polymers with tunable band gaps

Four three-dimensional graphdiyne polymers, namely Tri-C18, Hex-C36, Tri-C54, and Orth-C72, are predicted with first principles calculations. These carbon allotropes are constituted with interlinked sp3-hybridized carbon pillars and sp2-hybridized graphene-like nanoribbons. Graphdiyne polymers are energetically more stable than C60 and graphdiyne, and show lower phase transition energy barriers from AA-graphdiyne than that of graphite–diamond transition. Except for the semimetallic Tri-C18, these polymers show narrow direct (or quasi-direct) band gap of 1.55–1.74eV. The porous structure and the high in-plane Young’s modulus of graphdiyne sheet are inherited by these polymers. Meanwhile, a greatly enhanced Young’s modulus as high as ca. 1TPa along the direction perpendicular to graphdiyne sheet and high hardness are developed in these systems.

Strong coupling betweenP1diamond impurity centers and a three-dimensional lumped photonic microwave cavity

We report strong coupling between an ensemble of N impurity (P1) centres in diamond and microwave photons using a unique double post re-entrant cavity. The cavity is designed so that the magnetic component of the cavity field is spatially separated from the electric component and focused into the small volume in which the diamond sample is mounted. The novelty of the structure simultaneously allows high magnetic filling factor (38.4\%) and low frequencies necessary to interact, at low magnetic field, with transitions in diamond such as those in NV$^{-}$ and P1 centres. Coupling strength (or normal-mode splitting) of 51.42 MHz, was achieved with P1 centres at 6.18 GHz and 220 mT in a centimetre-scale cavity, with a corresponding cooperativity factor of 4.7. This technique offers an alternative way, with some significant advantages, to couple 3D cavities to transitions in diamond and achieve the strong coupling necessary for applications to quantum information processing.

Modeling of Nonthermal Solid‐to‐Solid Phase Transition in Diamond Irradiated with Femtosecond x‐ray FEL Pulse

AbstractIn this contribution we review in detail our recently developed hybrid model able to trace simultaneously nonequilibrium electron kinetics, evolution of an electronic structure, and eventually nonthermal phase transition in solids irradiated with femtosecond free‐electron laser pulses. Diamond irradiated with an ultrashort intense x‐ray pulse serves as an example to show how an irradiated material undergoes an ultrafast phase transition on sub‐picosecond timescales. The transition of diamond into graphite is induced by an excitation of electrons from the valence band into the conduction band, which, in turn, induces a rapid change of the interatomic potential. Our theoretical model incorporates: a Monte‐Carlo method for tracing high‐energy electrons and K‐shell holes in diamond; a temperature equation for the valence‐band and low‐energy conduction‐band electrons; a tight binding method for calculation of the evolving electronic structure of the material and potential energy surfaces; and molecular dynamics propagating atomic trajectories. This unified approach predicts the damage threshold of diamond in a good agreement with experimentally measured values. It reveals a multi‐step nature of nonthermal phase transition being an interplay between electronic excitation, changes of the band structure, and atomic reordering. An effect of pulse parameters, such as photon energy and temporal pulse shape, on the phase transition is discussed in detail. (© 2015 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Study of percolation and modeling of the order–disorder transition for zincblende–diamond structures: Percolation and the existence of a unique band of events

In this paper, the dependence of the crystallographic direction in the critical concentration of IV-type atoms for the zincblende–diamond (order–disorder) transition in (III–V)–IV semiconductor alloys were found for the (001), (110), and (111) growth directions by using Monte Carlo simulations. Intervals for the concentration values were determined by simulation, generating a band that is consistent with experimental data. Selecting the average value of this band produces good agreement with previous experimental results. It was also found that the relationship between order parameter and concentration does not have unique values and that there exists a band of events. The existence of this band can explain the scatter in the experimental data reported in x-ray, Raman, and optical anisotropy. It was found that the site percolation threshold is independent of the crystallographic direction of growth and converges to a single concentration value x ≈ 0.437.