Single Crystal Diamond Surfaces Research Articles

Diamond surface functionalization via visible light–driven C–H activation for nanoscale quantum sensing

Nitrogen-vacancy (NV) centers in diamond are a promising platform for nanoscale NMR sensing. Despite significant progress toward using NV centers to detect and localize nuclear spins down to the single spin level, NV-based spectroscopy of individual, intact, arbitrary target molecules remains elusive. Such sensing requires that target molecules are immobilized within nanometers of NV centers with long spin coherence. The inert nature of diamond typically requires harsh functionalization techniques such as thermal annealing or plasma processing, limiting the scope of functional groups that can be attached to the surface. Solution-phase chemical methods can be readily generalized to install diverse functional groups, but they have not been widely explored for single-crystal diamond surfaces. Moreover, realizing shallow NV centers with long spin coherence times requires highly ordered single-crystal surfaces, and solution-phase functionalization has not yet been shown with such demanding conditions. In this work, we report a versatile strategy to directly functionalize C-H bonds on single-crystal diamond surfaces under ambient conditions using visible light, forming C-F, C-Cl, C-S, and C-N bonds at the surface. This method is compatible with NV centers within 10 nm of the surface with spin coherence times comparable to the state of the art. As a proof-of-principle demonstration, we use shallow ensembles of NV centers to detect nuclear spins from surface-bound functional groups. Our approach to surface functionalization opens the door to deploying NV centers as a tool for chemical sensing and single-molecule spectroscopy.

Microwave hydrogen plasma etching on the laser-cut surface of single crystal diamond: Phases, etching morphology, stress evolution and the effect of etching on the subsequent homoepitaxial growth

In order to improve the pretreating efficiency of single crystal diamond (SCD) before its homoepitaxial growth, a strategy coupling laser cutting and microwave hydrogen plasma etching was proposed to replace the traditional pretreatment steps, which underwent laser cutting, mechanical polishing, acid cleaning, and microwave plasma etching in order. The feasibility of this idea has been studied by growing SCD after directly etching its laser-cut surface using microwave hydrogen plasma. The effect of the etching time on the SCD including surface phase, etching morphology, etching pit shape, residual stress, growth morphology, and quality of SCD are investigated and analyzed. The results show that microwave hydrogen plasma etching can quickly (within 10 min) remove the residual graphite carbon on the SCD surface caused by laser cutting. The extension of the etching time mainly affects the surface morphology and the internal stress of SCD. Etching pits with different shapes (square, trapezoid, inverted pyramid) and sizes can be found on the hydrogen plasma etched surface under different etching times. Fortunately, these etching pits do not introduce polycrystals on the SCD during subsequent homoepitaxial growth. All the hydrogen plasma etched laser cut SCD samples exhibit typical step-flow growth morphology after 20 h of homoepitaxial growth. Moreover, the step-flows are orderly and parallel, which is better than the disorderly step-flows on the polished SCD. Besides, a higher growth quality and a lower residual stress are obtained for the etched SCD. The above results indicate that replacing the traditional complex pretreatment steps with laser cutting and etching pretreatment is feasible. The new approach can effectively improve the pretreatment efficiency and result in superior quality of SCD after homoepitaxial growth.

Study on the mechanism of chemical mechanical polishing on high-quality surface of single crystal diamond

Due to the ultra-high hardness and excellent chemical inertness of single crystal diamond (SCD), achieving a high-quality surface of SCD by chemical mechanical polishing (CMP) is challenging. To obtain a high-quality surface of SCD, we developed a novel slurry containing a mixed oxidant of hydrogen peroxide (H2O2) and potassium ferrate (K2FeO4). The surface micro-morphology, profile, roughness, and damage layer characteristics of SCD were characterized by a 3D surface optical profiler, atomic force microscopy (AFM), scanning electron microscopy (SEM), and transmission electron microscope (TEM). Simultaneously, the reasons for the formation of high-quality surface of SCD were investigated. The results demonstrated that the polished SCD surface was smooth and scratch-free with an average Ra of 0.681 ± 0.023 nm over six areas of 868 × 868 μm2 and a damage layer thickness of 0.6 nm. Furthermore, the state of the chemical bonds of elements on the SCD surface was surveyed by X-ray photoelectron spectroscopy (XPS), and the polishing mechanism was deduced as follows: hydroxyl radicals were adsorbed on the SCD surface to oxidize and formed COH and CO bonds, while the damage due to lattice distortion resulted in the breakage of the CC bond to achieve carbon atom removal.

Neuronal growth on high-aspect-ratio diamond nanopillar arrays for biosensing applications

Monitoring neuronal activity with simultaneously high spatial and temporal resolution in living cell cultures is crucial to advance understanding of the development and functioning of our brain, and to gain further insights in the origin of brain disorders. While it has been demonstrated that the quantum sensing capabilities of nitrogen-vacancy (NV) centers in diamond allow real time detection of action potentials from large neurons in marine invertebrates, quantum monitoring of mammalian neurons (presenting much smaller dimensions and thus producing much lower signal and requiring higher spatial resolution) has hitherto remained elusive. In this context, diamond nanostructuring can offer the opportunity to boost the diamond platform sensitivity to the required level. However, a comprehensive analysis of the impact of a nanostructured diamond surface on the neuronal viability and growth was lacking. Here, we pattern a single crystal diamond surface with large-scale nanopillar arrays and we successfully demonstrate growth of a network of living and functional primary mouse hippocampal neurons on it. Our study on geometrical parameters reveals preferential growth along the nanopillar grid axes with excellent physical contact between cell membrane and nanopillar apex. Our results suggest that neuron growth can be tailored on diamond nanopillars to realize a nanophotonic quantum sensing platform for wide-field and label-free neuronal activity recording with sub-cellular resolution.

Experimental studies of electron affinity and work function from titanium on oxidised diamond (100) surfaces

Sub-monolayers of titanium were deposited onto oxidised (100) single-crystal diamond surfaces and annealed in vacuo at temperatures up to 1000 °C to find a temperature-stable termination procedure that produces a surface with Negative Electron Affinity (NEA). The samples were analysed by X-ray Photoelectron Spectroscopy, Ultraviolet Photoelectron Spectroscopy and Energy-Filtered Photoemission Electron Microscopy to determine their electron affinity and work function values. NEA values were observed on samples following annealing above 400 °C, with the largest NEA value being –0.9 eV for a sample coated with a half-monolayer of Ti annealed at 400 °C. Work function values were ∼4.5 eV for all samples annealed at temperatures between 400 and 600 °C, then rose at higher temperatures due to the loss of substantial amounts of O from the surface. Work-function maps indicated that the surface was uniform over areas 5700 μm2, suggesting that the deposition and annealing steps used are reliable methods to produce films with homogeneous surface properties.

Characterization of Fenton reaction-based material removal on single crystal diamond surface

Single Crystal Diamond (SCD), a new generation semiconductor material, is used in biochip substrates, sensors, and solid-state quantum emitters for quantum communication due to its excellent optoelectronic properties. However, its low processing efficiency arises from its extremely high hardness and chemical inertness. For efficient processing of SCD, an intensive analysis of its material removal mechanism is required. The material removal characteristics of the SCD surface under different processing conditions were studied using the method of wear scratch variation on SCD surface. The results show that the Fenton reaction could induce oxidation on the SCD surface and improve its material removal, which was more suitable for SCD processing than other reaction solution environments. The addition of diamond abrasive to the Fenton reaction solution resulted in a maximum SCD surface material removal amount of 586.43 μm2, which was higher than the Fenton reaction condition alone by a factor of 1.56, higher than the effect of diamond abrasive alone by a factor of 0.81, and higher than the combination of the two by a factor of 0.06. Among those factors that influence the Fenton reaction assisted SCD material removal, the mechanical removals of abrasive and tool rod accounted for 47.2 % and 8 %, respectively, while the synergy of chemical reaction with mechanical action by the abrasive and tool rod accounted for 44.8 %. The synergistic effect of chemical reaction and mechanical removal can improve the processing efficiency of SCD.

A novel visible-light catalyzed assisted single crystal diamond chemical mechanical polishing slurry and polishing mechanism

Single crystal diamond has significant chemical and mechanical stability, which makes it challenging to efficiently obtain high quality single crystal diamond surface. In this study, the effect of four different sets of chemical mechanical polishing (CMP) slurries on the surface roughness Ra and the material removal rate MRR was investigated. The optimum slurry consisted of hydrogen peroxide, diamond micro-powder, synthesis product of nanodiamond and CuFe-layered double hydroxide (ND/LDH), oxalic acid and deionized water. After CMP by the optimum slurry, the average Ra value of three scanning ranges of 868 µm× 868 µm was 0.112 ± 0.003 nm and the average MRR value of three measurements was 416 ± 5 nm/h. High efficiency to obtain high quality single crystal diamond surface was achieved. The variation of hydroxyl radicals (·OH) concentration in the optimum slurry with time was measured. The optimum slurry CMP mechanism was elucidated by X-ray diffractometry (XRD) and X-ray photoelectron spectrometer (XPS). Visible-light photoelectrons and the rapid oxidation of C2O4·- radicals were accelerated the redox cycle of Fe3+/Fe2+ and Cu2+/Cu+ to produce·OH. The·OH oxidised the C atoms on the single crystal diamond surface to form -C-O-C-, -C-O-H and -CO. Finally, they were removed by diamond micro-powder abrasive, obtaining an ultra-smooth surface.

Diamond mirrors for high-power continuous-wave lasers

High-power continuous-wave (CW) lasers are used in a variety of areas including industry, medicine, communications, and defense. Yet, conventional optics, which are based on multi-layer coatings, are damaged when illuminated by high-power CW laser light, primarily due to thermal loading. This hampers the effectiveness, restricts the scope and utility, and raises the cost and complexity of high-power CW laser applications. Here we demonstrate monolithic and highly reflective mirrors that operate under high-power CW laser irradiation without damage. In contrast to conventional mirrors, ours are realized by etching nanostructures into the surface of single-crystal diamond, a material with exceptional optical and thermal properties. We measure reflectivities of greater than 98% and demonstrate damage-free operation using 10 kW of CW laser light at 1070 nm, focused to a spot of 750 μm diameter. In contrast, we observe damage to a conventional dielectric mirror when illuminated by the same beam. Our results initiate a new category of optics that operate under extreme conditions, which has potential to improve or create new applications of high-power lasers.

Comparison of surface and subsurface damage of mosaic single-crystal diamond substrate processed by mechanical and plasma-assisted polishing

The surface and subsurface damage of a mosaic single-crystal diamond (SCD) (100) substrate processed by mechanical polishing (MP) and plasma-assisted polishing (PAP) were studied. Scanning white-light interferometer (SWLI) (84-μm square) observation for different local positions showed that the surface damage such as scratches and deep pits were observed on the mosaic SCD substrate after MP, while such damage was completely removed after PAP. Scanning cathodoluminescence (CL) measurement of band-A emission for different local positions showed that scratches with high luminescence intensity attributed to subsurface damage were observed on the mosaic SCD substrate after MP, while such specific luminescence was not observed after PAP. Micro-Raman spectroscopy measurement showed that peak position shift of diamond Raman lines attributed to residual stress and full width at half maximum (FWHM) broadening attributed to destruction of the crystalline structure were observed on the mosaic SCD substrate after MP, while such a peak position shift and FWHM broadening were not observed after PAP.

Ablation characteristics and material removal mechanisms of a single-crystal diamond processed by nanosecond or picosecond lasers.

The microstructures on a diamond surface have attracted extensive attention in microelectronics, ultra-precision machining tools, and optical elements, etc. In this work, microgrooves were fabricated on a single-crystal diamond surface using ultraviolet nanosecond or infrared picosecond laser pulses. The surface and internal morphologies of the microgrooves were characterized. The chemical composition and phase transition of the diamond after laser irradiation were analyzed. Furthermore, the ablation threshold, ablation rate, and material removal rate of the diamond processed by nanosecond or picosecond lasers were also calculated. In addition, the temperature distributions of the diamond ablated by nanosecond or picosecond lasers were simulated. Finally, the material removal mechanisms of a single-crystal diamond processed by nanosecond or picosecond lasers were revealed. This work is expected helpful to provide a guidance for the laser fabrication of microstructures on diamond.

Smoothing of single crystal diamond by high-speed three-dimensional dynamic friction polishing: Optimization and surface bonds evolution mechanism

The high-speed three-dimensional movement dynamic friction polishing (3DM-DFP) has been recognized as an efficient approach for ultra-smoothing single crystal diamond (SCD) surface. Continuing from the previous works focusing on the subsurface cleavage of diamond after 3DM-DFP, process optimization and surface reaction evolution mechanism as a fundamental building block is investigated, for the first time, for comprehensively understanding this fast-smoothing manner. By systematically adjusting the controlling factor, stronger load (0.3 MPa) and appropriate duration (0.5 h) as well as moderate sliding speed (in the range of 30 to 45 m s−1) is found to be able to obtain the smooth surface of SCD without uncontacted traces or break-surface cleavage. Subtle residual clues on SCD surface as a function of progressive DFP procedure indicate that Fe catalytic oxidation mainly produce Fe2O3 and partial intermediate oxides Fe1-yO. Meanwhile, the activated oxygen inserts sp3 CC bonds could form CO or CO and C-O-V (vacancy) at existing reactive surface sites. The (100) favorable CO bonds can be rebuilt if (100) surface is reformed, although the CO bonds associated with non-(100) rough surface would replace them during DFP procedure. The formed COC and concomitant C-O-V as well as the oxidized graphite give rise to the increase of CO proportion, and finally the covered defective graphitic phase has an approximate CO/CO ratio of 1.25. All these are endowed potential value for future upgrading of DFP technique for diamond surface smoothing.

Laser-Induced Phosphorus-Doped Conductive Layer Formation on Single-Crystal Diamond Surfaces.

A laser-induced doping method was employed to incorporate phosphorus into an insulating monocrystalline diamond at ambient temperature and pressure conditions. Pulsed laser beams with nanosecond duration (20 ns) were irradiated on the diamond substrate immersed in a phosphoric acid liquid, in turns, and a thin conductive layer was formed on its surface. Phosphorus incorporation in the depth range of 40-50 nm below the irradiated surface was confirmed by secondary ion mass spectroscopy (SIMS). Electrically, the irradiated areas exhibited ohmic contacts even with tungsten prober heads at room temperature, where the electrical resistivity of irradiated areas was greatly decreased compared to the original surface. The temperature dependence of the electrical conductivity implies that the surface layer is semiconducting with activation energies ranging between 0.2 eV and 54 meV depending on irradiation conditions. Since after laser treatment no carbon or graphitic phases other than diamond is found (the D and G Raman peaks are barely observed), the incorporation of phosphorus is the main origin of the enhanced conductivity. It was demonstrated that the proposed technique is applicable to diamond as a new ex situ doping method for introducing impurities into a solid in a precise and well-controlled manner, especially with electronic technology targeting of smaller devices and shallower junctions.

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.

Influence of diamond crystal orientation on the interaction with biological matter

Diamond has been a popular material for a variety of biological applications due to its favorable chemical, optical, mechanical and biocompatible properties. While the lattice orientation of crystalline material is known to alter the interaction between solids and biological materials, the effect of diamond’s crystal orientation on biological applications is completely unknown. Here, we experimentally evaluate the influence of the crystal orientation by investigating the interaction between the <100>, <110> and <111> surfaces of the single crystal diamond with biomolecules, cell culture medium, mammalian cells and bacteria. We show that the crystal orientation significantly alters these biological interactions. Most surprising is the two orders of magnitude difference in the number of bacteria adhering on <111> surface compared to <100> surface when both the surfaces were maintained under the same condition. We also observe differences in how small biomolecules attach to the surfaces. Neurons or HeLa cells on the other hand do not have clear preferences for either of the surfaces. To explain the observed differences, we theoretically estimated the surface charge for these three low index diamond surfaces and followed by the surface composition analysis using x-ray photoelectron spectroscopy (XPS). We conclude that the differences in negative surface charge, atomic composition and functional groups of the different surface orientations lead to significant variations in how the single crystal diamond surface interacts with the studied biological entities.

Ohmic Contact of Pt/Au on Hydrogen-Terminated Single Crystal Diamond

In this study, contact properties of platinum/gold (Pt/Au) on the surface of hydrogen-terminated single crystal diamond (H-SCD) were studied with several treatment conditions. The electrodes of Pt (20 nm)/Au (100 nm) were deposited on H-SCD surface by electron beam evaporation technique. Then, the specific contact resistance (ρc) of the as-fabricated sample was measured by the circular transmission line model, which showed good ohmic properties with the value of 5.65 × 10−4 Ω·cm2. To identify the thermal stability of Pt/Au/H-SCD, the sample was annealed in hydrogen ambient from 200 to 700 °C for 20 min at each temperature. As the temperature increased, ρc demonstrated better thermal stability. In addition, the barrier height was evaluated to be −0.67 ± 0.12 eV by X-ray photoelectron spectroscopy (XPS) technique.

Non-contact polishing of single crystal diamond by ion beam etching

We propose a non-contact surface finishing method for brittle substrates by ion beam etching and we experimentally demonstrate polishing of (100) single crystal diamond surface. We model and simulate the polishing process, and verify the results experimentally by monitoring individual defects during the surface treatment. Rapid flattening of scratches and digs, as typically present on brittle substrates after mechanical polishing, is observed: trench depth is typically removed by 95% in less than 30 min. The polishing method relies on physical bombardment of the substrate surface with accelerated inert gas ions, rendering it highly versatile and applicable to a wide variety of materials.

Bonding, structural properties and thermal stability of low damage RF (N2) plasma treated diamond (100) surfaces studied by XPS, LEED, and TPD

In this work, we investigate the interaction of N2 radio frequency (RF) plasma with single crystal diamond (100) surface. This study shows that a low-level damaged nitrogen terminated diamond (100) surface may be produced using a bent plasma source and a grounded mesh electrode placed at the source exit. Optical emission spectroscopy (OES) was used to characterize the species present in the plasma. The chemical, structural and thermal stability of the N-terminated diamond (100) surface were investigated in-situ by X-ray photoelectron spectroscopy (XPS), temperature programmed desorption (TPD) and low energy electron diffraction (LEED). OES showed that increasing the flow rate and operating pressure could decrease the ratio of charged-to-neutral nitrogen particles. From XPS results it is determined that the adsorbed nitrogen to the diamond surface may occupy a number of bonding configurations and that the damage to the surface upon nitridation, as reflected in the C(1s) peak shape analysis, can be minimized. From annealing experiments, it was found that some of the incorporated nitrogen onto the diamond surface is stable upon annealing to 1000 °C. LEED results show that upon a short RF (N2) plasma exposure time a (1 × 1) pattern was observed which was preserved upon annealing to 1000 °C suggesting that a thermally stable and ordered nitrogen terminated surface was formed. The TPD spectrum shows two low-temperature N2 desorption peaks that are associated to weakly bonded nitrogen chemisorbed species to the diamond surface at 210 and 560 °C.

Limits of single crystal diamond surface mechanical polishing

We demonstrate the process of mechanical polishing of a single crystal diamond substrate to a roughness of no >0.5 nm. We used the anisotropy of the polishing intensity depending on the crystallographic orientation of the diamond substrate relative to the rotation direction of the cast iron grinding wheel (scaife). The surface distortions and its roughness were measured by atomic force microscopy (AFM) and X-ray reflectometry (XRR). The proposed technique optimization allows preparation of ultra-smooth surfaces in a relatively short time of <10 min.

Fabrication of monolithic diamond photodetector with microlenses.

A monolithic diamond photodetector with microlenses is fabricated by etching microlens arrays (MLAs) on single crystal diamond surface and patterning tungsten electrode strips on the edge of these arrays. Firstly, compact MLAs are etched on half of diamond sample surface by thermal reflow method. Secondly, via magnetron sputtering technique, two sets of interdigitated tungsten electrodes are patterned on the sample surface, one set is on the edge of MLAs, the other set is on the planar area. The optoelectronic performances of photodetectors have been investigated and indicated that the photocurrent of microlens photodetector increases by 74.8 percent at 10 V under 220 nm UV light illumination by comparing with that in planar case. Simulations of photodetectors' electrical and optical properties have been carried out, illustrating an improvement of charge collection ability and light absorption efficiency in microlens case. Furthermore, the present device structure can be extended to other semiconductor photodetectors.

Etching Kinetics of (100) Single Crystal Diamond Surfaces in a Hydrogen Microwave Plasma, Studied with In Situ Low‐Coherence Interferometry

A low‐coherence interferometry (LCI) was used to measure in situ the etch rate (ER) of synthetic single crystal (SC) diamonds in H2 microwave plasma, at substrate temperatures in the broad range of 800–1370 °C. The method allows the collection of the kinetic data on a single sample without switching off the plasma. (100)‐orientated SC plates of CVD and IIa type HPHT diamond were systematically etched in pure hydrogen at pressure p = 130 Torr and microwave power density of ≈300 W cm−3. The activation energies Ea of 42 ± 5 and 32 ± 4 kCal mol−1 have been determined for the CVD and HPHT substrates. An enhanced etching rate of a subsurface defected layer with thickness of ∼1 μm or less, formed upon polishing of the samples, is revealed. Surface morphology, roughness, and the shape of etch pits produced by a selective etching of defects, were characterized with optical profilometry. CH and dimer C2 radicals were detected in the H2 plasma with optical emission spectroscopy, as a result of the diamond etching, and the emission intensity of these species was linked to the substrate etch rate.