Single crystal diamond membranes for nanoelectronics

Reimagining Carbon Nanomaterial Analysis: Empowering Transfer Learning and Machine Vision in Scanning Electron Microscopy for High-Fidelity Identification

In this report, we propose a novel technique for identifying and analyzing diverse nanoscale carbon allotropes using scanning electron micrographs. By precisely controlling the quenching rates of undercooled molten carbon through laser irradiation, we achieved the formation of microdiamonds, nanodiamonds, and Q-carbon films. However, standard laser irradiation without proper undercooling control leads to the formation of sparsely located diverse carbon polymorphs, hindering their discovery and classification through manual analyses. To address this challenge, we applied transfer-learning approaches using convolutional neural networks and computer vision techniques to achieve allotrope discovery even with sparse spatial presence. Our method achieved high accuracy rates of 92% for Q-carbon identification and 94% for distinguishing it from nanodiamonds. By leveraging scanning electron micrographs and precise undercooling control, our technique enables the efficient identification and characterization of nanoscale carbon structures. This research significantly contributes to the advancement of the field, providing automated tools for Q-materials and carbon polymorph identification. It opens up new opportunities for the further exploration of these materials in various applications.

Highly Stable Electrochemical Supercapacitor Performance of Self-Assembled Ferromagnetic Q-Carbon

Novel phase Q-carbon thin films exhibit some intriguing features and have been explored for various potential applications. Herein, we report the growth of different Q-carbon structures (i.e., filaments, clusters, and microdots) by varying the laser energy density from 0.5 to 1.0 J/cm² during pulsed laser annealing of amorphous diamond-like carbon films with different sp³-sp² carbon compositions. These unique nano- and microstructures of Q-carbon demonstrate exceptionally stable electrochemical performance by cyclic voltammetry, galvanostatic charging-discharging, and electrochemical impedance spectroscopy for energy applications. The temperature-dependent magnetic studies (magnetization vs magnetic field and temperature) reveal the ferromagnetic nature of the Q-carbon microdots. The saturation magnetization and coercive field values decrease from 132 to 14 emu/cc and 155 to 92 Oe by increasing the temperature from 2 to 300 K, respectively. The electrochemical performances of Q-carbon filament, cluster, and microdot thin-film supercapacitors were investigated by two-electrode configurations, and the highest areal specific capacitance of ∼156 mF/cm² was observed at a current density of 0.15 mA/cm² in the Q-carbon microdot thin film. The Q-carbon microdot electrodes demonstrate an exceptional capacitance retention performance of ∼97.2% and Coulombic efficiency of ∼96.5% after 3000 cycles due to their expectational reversibility in the charging-discharging process. The kinetic feature of the ion diffusion associated with the charge storage property is also investigated, and small changes in equivalent series resistance of ∼9.5% and contact resistance of ∼9.1% confirm outstanding stability with active charge kinetics during the stability test. A high areal power density of ∼5.84 W/cm² was obtained at an areal energy density of ∼0.058 W h/cm² for the Q-carbon microdot structure. The theoretical quantum capacitance was obtained at ∼400 mF/cm² by density functional theory calculation, which gives an idea about the overall capacitance value. The obtained areal specific capacitance, power density, and impressive long-term cyclic stability of Q-carbon thin-film microdot electrodes endorse substantial promise in high-performance supercapacitor applications.

Fabrication of Q-Carbon Nanostructures, Diamond and Their Composites with Wafer-Scale Integration

We report the formation of Q-carbon nanolayers, Q-carbon nanoballs, nanodiamonds, microdiamonds, and their composites by controlling laser and substrate variables. The choice of these parameters is guided by the SLIM (simulation of laser interactions with materials) computer modeling. For a constant film thickness and initial sp3 content, we obtain different microstructures with increasing pulse energy density as a result of different quenching rate and undercooling. This is related to decreasing undercooling with increasing pulse energy density. The structure of thin film Q-carbon evolves into Q-carbon nanoballs with the increase in laser annealing energy density. These Q-carbon nanoballs interestingly self-organize in the form of rings with embedded nanodiamonds to form Q-carbon nanoballs/diamond composites. We form high quality, epitaxial nano, and micro diamond films at a higher energy density and discuss a model showing undercooling and quenching rate generating a pressure pulse, which may play a critical role in a direct conversion of amorphous carbon into Q-carbon or diamond or their composites. This ability to selectively tune between diamond or Q-carbon or their composites on a single substrate is highly desirable for a variety of applications ranging from protective coatings to nanosensing and field emission to targeted drug delivery. Furthermore, Q-carbon nanoballs and nanodiamonds are utilized as seeds to grow microdiamond films by HFCVD. It is observed that the Q-carbon nanoballs contain diamond nuclei of critical size, which provide available nucleation sites for diamond growth, leading to stress-free, adherent, and denser films, which are needed for a variety of coating applications.

Fabrication and Characterization of Single-Crystal Diamond Membranes for Quantum Photonics with Tunable Microcavities

The development of quantum technologies is one of the big challenges in modern research. A crucial component for many applications is an efficient, coherent spin-photon interface, and coupling single-color centers in thin diamond membranes to a microcavity is a promising approach. To structure such micrometer thin single-crystal diamond (SCD) membranes with a good quality, it is important to minimize defects originating from polishing or etching procedures. Here, we report on the fabrication of SCD membranes, with various diameters, exhibiting a low surface roughness down to 0.4 nm on a small area scale, by etching through a diamond bulk mask with angled holes. A significant reduction in pits induced by micromasking and polishing damages was accomplished by the application of alternating Ar/Cl₂ + O₂ dry etching steps. By a variation of etching parameters regarding the Ar/Cl₂ step, an enhanced planarization of the surface was obtained, in particular, for surfaces with a higher initial surface roughness of several nanometers. Furthermore, we present the successful bonding of an SCD membrane via van der Waals forces on a cavity mirror and perform finesse measurements which yielded values between 500 and 5000, depending on the position and hence on the membrane thickness. Our results are promising for, e.g., an efficient spin-photon interface.

Thin Diamond Film on Silicon Substrates for Pressure Sensor Fabrication

Thin polycrystalline diamond films chemically vapor deposited on thinned silicon substrates were used as membranes for pressure sensor fabrication by means of selective chemical etching of silicon. The sensing element is based on a simple low-finesse Fabry–Pérot (FP) interferometer. The FP cavity is defined by the end-face of a single mode fiber and the diamond diaphragm surface. Hence, pressure is evaluated by measuring the cavity length by an optoelectronic system coupled to the single mode fiber. Exploiting the excellent properties of Chemical Vapor Deposition (CVD) diamond, in terms of high hardness, low thermal expansion, and ultra-high thermal conductivity, the realized sensors have been characterized up to 16.5 MPa at room temperature. Preliminary characterizations demonstrate the feasibility of such diamond-on-Si membrane structure for pressure transduction. The proposed sensing system represents a valid alternative to conventional solutions, overcoming the drawback related to electromagnetic interference on the acquired weak signals generated by standard piezoelectric sensors.

Nonequilibrium Structural Evolution of Q-Carbon and Interfaces

Q-carbon is a densely packed metastable phase of carbon formed by ultrafast quenching of carbon melt in a super-undercooled state. After quenching, diamond tetrahedra are randomly packed with >80% packing efficiency. This discovery has opened a pathway to fabricate various interesting heterostructures following the highly nonequilibrium route of nanosecond pulsed laser annealing. In the present work, we demonstrate the evolution of Q-carbon/α-carbon and Q-carbon/diamond heterostructures with atomically sharp interfaces, controlled via varying solidification rates of the undercooled C melt. This structure consists of ultrahard Q-carbon (∼80% sp³ and rest sp²) with an overlayer of soft α-carbon (∼40% sp³) on the inert c-Al₂O₃ substrate. Using high-resolution scanning transmission electron microscopy and Raman spectroscopy analysis, we present the formation of the highly dense Q-carbon/α-carbon bilayer structure with distinctly different atomic and electronic structures. The laser-solid interaction simulations coupled with atomistic ab initio modeling further confirm the conversion of C melt into Q-carbon by achieving maximum undercooling near the substrate and further into α-carbon with a decrease in regrowth velocity (<6 m/s) away from the substrate. We present details of the evolution of heterointerfaces formed from carbon melt for designing heterostructures far from equilibrium for various functional applications by using pulsed laser processing.

Quantum nanophotonics with group IV defects in diamond

Diamond photonics is an ever-growing field of research driven by the prospects of harnessing diamond and its colour centres as suitable hardware for solid-state quantum applications. The last two decades have seen the field shaped by the nitrogen-vacancy (NV) centre with both breakthrough fundamental physics demonstrations and practical realizations. Recently however, an entire suite of other diamond defects has emerged-group IV colour centres-namely the Si-, Ge-, Sn- and Pb-vacancies. In this perspective, we highlight the leading techniques for engineering and characterizing these diamond defects, discuss the current state-of-the-art group IV-based devices and provide an outlook of the future directions the field is taking towards the realisation of solid-state quantum photonics with diamond.