Thick Flakes Research Articles

One‐Step Creation of Quantum Emitter Arrays in Hexagonal Boron Nitride by Local Stress Application

AbstractHexagonal boron nitride (hBN) has recently emerged as a promising platform for hosting quantum emitters (QEs) in atomically thin solid‐state materials, as it operates at room temperature, possess exceptional brightness, and the two‐dimensional nature of hBN enables versatile integration with various photonic elements compared to bulk hosts. However, high‐yield fabrication of site‐specific QEs at designated locations still requires complicated pre‐ or post‐processes. In this work, QEs are created in defect‐free exfoliated hBN through localized stress application in series. Indentation is performed at target positions within the hBN flakes of various thickness without any subsequent post‐treatment, resulting in quantum emission from the majority of the indented regions. This technique yields 73% of QE generation per predefined indented site at optimized conditions, highly exceeding the 30–40% yield of previously reported top‐down approaches. In addition, the zero phonon line wavelengths of stress‐induced QEs are focused at 595 ± 18.9 nm, which implies that the emitters originate from the same family of defects. The facile one‐step, on‐demand, and high‐yield fabrication of QE arrays opens the possibility to realize scalable quantum light sources integrated on chip‐scale quantum photonic circuits.

Photoelectrochemical behavior of GaTe nanoflakes prepared by exfoliation

Gallium telluride (GaTe), a layered semiconductor, was investigated in the photoelectrochemical (PEC) hydrogen evolution reaction (HER). GaTe nanoflakes were prepared by both mechanical and liquid phase exfoliations, and were subsequently used to obtain micro-, and macroelectrodes. When studying them as photocathodes, the highest photocurrent density of ∼6 mA cm−2 was achieved with a 808±50 nm thick flake. Importantly, the GaTe microelectrodes presented a thickness dependent PEC activity. Macroscopic (1 cm2) GaTe electrodes were also prepared from the liquid phase exfoliated nanoflakes, by immobilizing them on glassy carbon electrodes. GaTe photocathodes showed loading and solar power flux dependent PEC activity and reaching a maximum photocurrent of ∼4 mA cm–2 with a good photostability. Overall, these results add another candidate to the pool of photoelectrodes applicable in different PEC processes.

Electron Beam Restructuring of Quantum Emitters in Hexagonal Boron Nitride

AbstractHexagonal boron nitride (hBN) holds promise as a solid state, van der Waals host of single photon emitters for on‐chip quantum photonics. The B‐center defect emitting at 436 nm is particularly compelling as it can be generated by electron beam irradiation. However, the emitter generation mechanism is unknown, the robustness of the method is variable, and it has only been applied successfully to thick flakes of hBN (≫ 10 nm). Here, it is used in situ time‐resolved cathodoluminescence (CL) spectroscopy to investigate the kinetics of B‐center generation. It is shown that the generation of B‐centers is accompanied by quenching of a carbon‐related emission at ≈305 nm and that both processes are rate‐limited by electromigration of defects in the hBN lattice. It identifies problems that limit the efficacy and reproducibility of the emitter generation method and solve them using a combination of optimized electron beam parameters and hBN pre‐and postprocessing treatments. It is achieved B‐center quantum emitters in hBN flakes as thin as 8 nm, elucidate the mechanisms responsible for electron beam restructuring of quantum emitters in hBN, and gain insights toward the identification of the atomic structure of the B‐center quantum emitter.

The effects of processing technique on formation temperature of calcium aluminate magnesium (CaO.2MgO.8Al2O3)

This study aims at the synthesis of a ternary phase crystal CaO.2MgO.8Al2O3 (CM2A8) in the Al-rich part of the ternary system CaO-Al2O3-MgO through two techniques: (1) sol–gel citrate and (2) solid-state reaction. Based on the measurement of particle size, density, phase analysis, nature of bounds, and microscopic observation, the effect of the processing technique on the crystal purity, crystallite size, and synthesis temperature of the as-prepared CM2A8 powder was investigated. Phases analysis (XRD patterns) showed in the sol–gel citrate combustion method almost a single-phase material (calcium aluminate magnesium phases) at relatively lower temperatures while in the solid-state method as-prepared CM2A8 powder with a small amount of spinel (MgAl2O4 = MA) and calcium hexaaluminate (CaAl12O19 = CA6) secondary phases. The generated heat through the combustion of the reaction of ammonium nitrate and citrate-based complexes decreases the synthesis temperature of nanocrystalline CM2A8, and the CM2A8 phase is formed at a temperature of 1400 °C. While growth mechanism in the solid-state reaction method adheres to the two-dimensional nucleation theory and a thick hexagonal flake known as CM2A8 is produced at a temperature of 1700 °C. In comparison with the crystals synthesized by the solid-state reaction method, the powder density of the nanocrystals synthesized by a sol–gel combustion method exhibits higher values (3.54 g.cm−3 and 3.65 g.cm−3, respectively). Peak intensities in the 400–1000 cm−1 range, according to FTIR measurements, correspond to metal–oxygen–metal (M–O–M) bonds (vibrations of Al–O stretching in Mg–O–Al and Ca–O–Al), and they grow as the temperature rises from 200 °C to 1400 °C, indicating that CM2A8 was the crystal that was formed. The morphological analysis (FESEM) revealed that most of the agglomerate particles synthesized by the sol–gel citrate combustion remained within the range of 50–83 nm, while the particles synthesized by the had particles as 1 μm. The specific surface area synthesized CM2A8 via the sol–gel method is in the range of 10–24 m2/g. The main advantages of the sol–gel citrate process are the high purity of the product, the narrow particle size distribution, and the achievement of uniform nanostructure at low temperatures.

The Robust Ferroelectric and Electrical Response in 2D Bi2O2Se Semiconductor

AbstractThe amelioration of atomically thin ferroelectric materials is imperative for next‐generation outperformed two‐dimensional (2D) electronics, which is elusive by their bulk counterparts. These remarkable materials’ ferroelectric and piezoelectric features are the fundamental urges in optoelectronics, electronics, and energy harvesting. In this work, 2D ferroelectric Bi2O2Se flakes have been synthesized using a modified chemical vapor deposition technique. The 6 nm thick Bi2O2Se flake provides a robust ferroelectric switching under an applied voltage of ±10 V by piezoresponse force microscopy, further confirmed by first principles. Leveraging the successful growth, the high‐quality Bi2O2Se flakes permit the fabrication of a field‐effect transistor (FET) with state‐of‐the‐art performance. The FET device rewards a high current on–off ratio of 108 and field effect mobility of almost 131 cm2 V−1 s−1, owing to the small carrier effective mass of 0.2 m0. Combined, the electric field‐induced local polarization of ferroelectric switching and unprecedented device performance of Bi2O2Se semiconductors are certified for their utilization in advanced nanoelectronics and miniaturization of multifunctional devices with multifunctionalities.

Synthesis of multi-phases MoO3-MoS2-Mo2S3 nanostructure catalyst for degradation of methylene blue, rhodamine B, and crystal violet dyes

In this work, we present the synthesis of multi-phase MoO3–MoSx (Mo–O–S) nanostructure as an outstanding photocatalyst through the straightforward hydrothermal method. The as-synthesized Mo–O–S nanostructure exhibited high purity and well-defined crystallite phases, featuring rods with average diameters ranging from 100–200 nm and thick flakes of 10–25 nm. The optical characterization of the as-prepared Mo–O–S nanostructure reveals four distinct emission peaks within the 520–680 nm wavelength range. The photocatalytic activity of the Mo–O–S nanostructure was evaluated through the degradation of rhodamine B (RhB), methylene blue (MB), and crystal violet (CV) dyes. The results unveil impressive degradation efficiencies, achieving 65%, 82%, and 89% after 180 min of exposure to UV irradiation for RhB, MB, and CV dyes, respectively. This pioneer investigation underscores the potential of the MoO3–MoSx nanostructure as a promising catalyst for the effective degradation of multiple dyes.

Selective Isolation of Mono- to Quadlayered 2D Materials via Sonication-Assisted Micromechanical Exfoliation.

Mechanical exfoliation methods of two-dimensional materials have been an essential process for advanced devices and fundamental sciences. However, the exfoliation method usually generates various thick flakes, and a bunch of thick bulk flakes usually covers an entire substrate. Here, we developed a method to selectively isolate mono- to quadlayers of transition metal dichalcogenides (TMDCs) by sonication in organic solvents. The analysis reveals the importance of low interface energies between solvents and TMDCs, leading to the effective removal of bulk flakes under sonication. Importantly, a monolayer adjacent to bulk flakes shows cleavage at the interface, and the monolayer can be selectively isolated on the substrate. This approach can extend to preparing a monolayer device with crowded 17 electrode fingers surrounding the monolayer and for the measurement of electrostatic device performance.

Hydrolysis of NaBH4 using carbonized melamine foam-supported cobalt borate composite catalyst for H2 production

Catalysts are crucial for the hydrolysis of NaBH4, and noble metal catalysts cannot be used in large-scale NaBH4 hydrolysis because of their high cost. Therefore, developing low-cost, high-activity, non-precious metal catalysts is the focus of current research. In this study, a composite catalyst with a coral rod-like structure was synthesized by loading CoB particles on the surface of a carbonized melamine foam (CMF)-CoO composite carrier. For preparing CMF-CoO, thick flakes of Co-MOF precursors were grown on CMF and calcined in an inert atmosphere. Then, CoB particles were loaded using a chemical reduction method to obtain the CMF-CoO-CoB catalyst. The morphology and microstructure of the CMF-CoO-CoB catalyst were characterized through scanning electron microscopy and X-ray photoelectron spectroscopy. The catalyst efficiently catalyzes the hydrolysis of NaBH4, providing a hydrogen release rate of 6974 mL∙min−1∙g−1. The activation energy is 35.05 kJ∙mol−1 at 30 °C. In addition, 80.4% of the catalytic activity of CMF-CoO-CoB was retained after five catalytic cycles. The CMF-CoO-CoB catalyst has a significantly higher activity and better cycling stability than the catalyst using a single carrier. Thus, the CMF-CoO-CoB composite catalyst with excellent performance has high potential for application in the hydrolysis of NaBH4.

Reentrant Proximity-Induced Superconductivity for GeTe Semimetal

We experimentally investigate charge transport in In–GeTe and In–GeTe–In proximity devices, which are formed as junctions between superconducting indium leads and thick single crystal flakes of α-GeTe topological semimetal. We observe nonmonotonic effects of the applied external magnetic field, including reentrant superconductivity in In–GeTe–In Josephson junctions: supercurrent reappears at some finite magnetic field. For a single In–GeTe Andreev junction, the superconducting gap is partially suppressed in zero magnetic field, while the gap is increased nearly to the bulk value for some finite field before its full suppression. We discuss possible reasons for the results obtained, taking into account spin polarization of Fermi arc surface states in topological semimetal \\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\alpha $$\\end{document}-GeTe with a strong spin–orbit coupling. In particular, the zero-field surface state spin polarization partially suppresses the superconductivity, while it is recovered due to the modified spin-split surface state configuration in finite fields. As an alternative possible scenario, the transition into the Fulde–Ferrell–Larkin–Ovchinnikov state is discussed. However, the role of strong spin–orbit coupling in forming the nonmonotonic behavior has not been analyzed for heterostructures in the Fulde–Ferrell–Larkin–Ovchinnikov state, which is crucial for junctions involving GeTe topological semimetal.

Crossover from gapped-to-gapless Dirac surface states in magnetic topological insulator MnBi2Te4

Intrinsic magnetic topological insulators (MTIs) host exotic topological phases such as quantized anomalous Hall insulating phase, arising due to the large magnetic exchange gap. However, the interplay of magnetism and topology in these systems in different temperature regimes remains elusive. In this work, we present the logarithmic temperature-dependence of conductivity for sub-100 nm thick exfoliated flakes of MTI MnBi2Te4 in the presence of out-of-plane magnetic fields and extracted the linear slope, κ. We observed a characteristic change, in the low-temperature regime, indicating the gapped Dirac surface state according to Lu–Shen theory. We also report the recovery of topological properties in the system via the weak-antilocalization effect in the vicinity of antiferromagnetic to paramagnetic transition and in the paramagnetic regime. Hikami–Larkin–Nagaoka analysis suggested the presence of topological surface states. Therefore, our study helps in understanding how intrinsic magnetism masks topological properties in an MTI as long as magnetic ordering persists.

Direct Observation of Propagating Spin Waves in the 2D van der Waals Ferromagnet Fe5GeTe2.

Magnetism in reduced dimensionalities is of great fundamental interest while also providing perspectives for applications of materials with novel functionalities. In particular, spin dynamics in two dimensions (2D) have become a focus of recent research. Here, we report the observation of coherent propagating spin-wave dynamics in a ∼30 nm thick flake of 2D van der Waals ferromagnet Fe5GeTe2 using X-ray microscopy. Both phase and amplitude information were obtained by direct imaging below TC for frequencies from 2.77 to 3.84 GHz, and the corresponding spin-wave wavelengths were measured to be between 1.5 and 0.5 μm. Thus, parts of the magnonic dispersion relation were determined despite a relatively high magnetic damping of the material. Numerically solving an analytic multilayer model allowed us to corroborate the experimental dispersion relation and predict the influence of changes in the saturation magnetization or interlayer coupling, which could be exploited in future applications by temperature control or stacking of 2D-heterostructures.

Quaternary, layered, 2D chalcogenide, Mo1−x W x SSe: thickness dependent transport properties

Highly oriented, single crystalline, quaternary alloy chalcogenide crystal, Mo x W1−x S2y Se2(1−y), is synthesized using a high temperature chemical vapor transport technique and its transport properties studied over a wide temperature range. Field effect transistors (FET) with bottom gated configuration are fabricated using Mo0.5W0.5SSe flakes of different thicknesses, from a single layer to bulk. The FET characteristics are thickness tunable, with thin flakes (1–4 layers) exhibiting n-type transport behaviour while ambipolar transfer characteristics are observed for thicker flakes (>90 layers). Ambipolar behavior with the dominance of n-type over p-type transport is noted for devices fabricated with layers between 9 and 90. The devices with flake thickness ∼9 layers exhibit a maximum electron mobility 63 ± 4 cm2 V−1s−1 and an I ON/I OFF ratio >108. A maximum hole mobility 10.3 ± 0.4 cm2 V−1s−1 is observed for the devices with flake thickness ∼94 layers with I ON/I OFF ratio >102–103 observed for the hole conduction. A maximum I ON/I OFF for hole conduction, 104 is obtained for the devices fabricated with flakes of thickness ∼7–19 layers. The electron Schottky barrier height values are determined to be ∼23.3 meV and ∼74 meV for 2 layer and 94 layers flakes respectively, as measured using low temperature measurements. This indicates that an increase in hole current with thickness is likely to be due to lowering of the band gap as a function of thickness. Furthermore, the contact resistance (R ct) is evaluated using transmission line model (TLM) and is found to be 14 kohm.μm. These results suggest that quaternary alloys of Mo0.5W0.5SSe are potential candidates for various electronic/optoelectronic devices where properties and performance can be tuned within a single composition.

Diverse Morphologies of Nb2O5 Nanomaterials: A Comparative Study for the Growth Optimization of Elongated Spiky Nb2O5 and Carbon Nanosphere Composite

AbstractControlled synthesis and design of nanomaterials with intricate morphologies and active phases offer new prospects in harnessing their unique chemical and physical properties for various applications. Herein, a facile and efficient hydrothermal approach is reported for obtaining various complex Nb2O5 nanostructures, including thin sheets, thick flakes, spiky and elongated spiky sea urchin morphologies using urotropin as a growth‐directing and hydrolyzing agent in various mixed and pure solvents. The detailed structural and chemical composition, surface morphology and crystallinity of as‐synthesized Nb2O5 nanostructures are presented. The urotropin concentration, reaction time, and water‐ethanol solvent mixture play a critical role for obtaining the elongated spiky sea urchin morphologies. The spiky Nb2O5 structures show a pseudohexagonal phase with less urotropin content, while thin sheets are obtained with a higher urotropin concentration and are primarily amorphous. These structures undergo transformation in their crystal phase and morphologies during calcination at higher temperatures revealing the active role of urotropin in stabilizing them. A composite of spiky sea urchin Nb2O5‐carbon nanospheres (suNb2O5‐CNS) is achieved by in‐situ growth of Nb2O5 in the presence of CNS without compromising on morphology, phase, and crystallinity. suNb2O5‐CNS composite is shown to possess higher charge storage capacity compared to its constituents for supercapacitor applications.

Optically Active Spin Defects in Few-Layer Thick Hexagonal Boron Nitride.

Optically active spin defects in hexagonal boron nitride (hBN) are promising quantum systems for the design of two-dimensional quantum sensing units offering optimal proximity to the sample being probed. In this Letter, we first demonstrate that the electron spin resonance frequencies of boron vacancy centers (V_{B}^{-}) can be detected optically in the limit of few-atomic-layer thick hBN flakes despite the nanoscale proximity of the crystal surface that often leads to a degradation of the stability of solid-state spin defects. We then analyze the variations of the electronic spin properties of V_{B}^{-} centers with the hBN thickness with a focus on (i)the zero-field splitting parameters, (ii)the optically induced spin polarization rate and (iii)the longitudinal spin relaxation time. This Letter provides important insights into the properties of V_{B}^{-} centers embedded in ultrathin hBN flakes, which are valuable for future developments of foil-based quantum sensing technologies.

Thickness-Controlled Synthesis of Uniform Hexagonal Boron Nitride Films on Metal Alloys by Chemical Vapor Deposition

Two-dimensional (2D) multilayer hexagonal boron nitride (hBN) is a superior substrate for other 2D materials due to its insulating property, atomic-scale smoothness, and absence of dangling bonds, enhancing electrical or optical properties in high-performance 2D material-based devices. In most fabrication processes for 2D heterostructure devices, thick hBN flakes exfoliated from bulk have been used because of their high quality and simplicity in the process. However, exfoliated hBN flakes are limited in size and thickness control, impeding scale-up production for industrialization. Therefore, new synthesis strategies are required to overcome the drawback of the exfoliation method. In this work, we investigated the synthesis of large-area, multilayer hBN via atmospheric pressure chemical vapor deposition using single crystalline metal and single crystal alloy substrates. The synthesized results are characterized by optical microscopy, scanning electron microscopy, Raman spectroscopy, atomic force microscopy, etc. These studies provide a more comprehensive understanding of the growth mechanism of hexagonal boron nitride.

Electrical detection of spin pumping in van der Waals ferromagnetic Cr2Ge2Te6 with low magnetic damping

The discovery of magnetic order in atomically-thin van der Waals materials has strengthened the alliance between spintronics and two-dimensional materials. An important use of magnetic two-dimensional materials in spintronic devices, which has not yet been demonstrated, would be for coherent spin injection via the spin-pumping effect. Here, we report spin pumping from Cr2Ge2Te6 into Pt or W and detection of the spin current by inverse spin Hall effect. The magnetization dynamics of the hybrid Cr2Ge2Te6/Pt system are measured, and a magnetic damping constant of ~ 4–10 × 10−4 is obtained for thick Cr2Ge2Te6 flakes, a record low for ferromagnetic van der Waals materials. Moreover, a high interface spin transmission efficiency (a spin mixing conductance of 2.4 × 1019/m2) is directly extracted, which is instrumental in delivering spin-related quantities such as spin angular momentum and spin-orbit torque across an interface of the van der Waals system. The low magnetic damping that promotes efficient spin current generation together with high interfacial spin transmission efficiency suggests promising applications for integrating Cr2Ge2Te6 into low-temperature two-dimensional spintronic devices as the source of coherent spin or magnon current.

Gate-Dependent Nonlinear Hall Effect at Room Temperature in Topological Semimetal GeTe

We experimentally investigate nonlinear Hall effect as zero-frequency and second-harmonic transverse voltage responses to ac electric current for topological semimetal GeTe. A thick single-crystal GeTe flake is placed on the Si/SiO2 substrate, where the p-doped Si layer serves as a gate electrode. We confirm that electron concentration is not gate-sensitive in thick GeTe flakes due to the gate field screening by bulk carriers. In contrast, by transverse voltage measurements, we demonstrate that the nonlinear Hall effect shows pronounced dependence on the gate electric field at room temperature. Since the nonlinear Hall effect is a direct consequence of a Berry curvature dipole in topological media, our observations indicate that Berry curvature can be controlled by the gate electric field. This experimental observation can be understood as a result of the known dependence of giant Rashba splitting on the external electric field in GeTe. For possible applications, the zero-frequency gate-controlled nonlinear Hall effect can be used for the efficient broad-band rectification.

Thickness modulation of incline-grown Bi2O2Se for the construction of high-performance phototransistors

Newly emerging two-dimensional (2D) Bi2O2Se has received intense research interest due to its unique band structure and ultrafast optical properties. However, the device performance of Bi2O2Se-based photodetectors is far from the expectation because of the undesirable contact issues of the contaminates from the fabrication process or the high Schottky barrier caused by the large work function mismatch. In this work, highly efficient photodetection based on an “all-Bi2O2Se” device geometry has been demonstrated. By controlling the growth conditions, Bi2O2Se flakes with thicknesses of 8–15 nm (thin) and >40 nm (thick) are obtained. The thin one is a typical n-type semiconductor, while the thick one shows the degenerated n-type behavior with a higher Fermi level. Two thick flakes are adopted as 2D contacts for the absorption layer of thin flake, leading to the upward movement of the thin flake band structures. By tailoring the Schottky barrier frame at the interface junction, the high barriers are eliminated, which boost the transport and collection of photo-generated electrons. The photodetector demonstrates strong photoresponse to visible and near-infrared light. High photoresponsivity and specific detectivity of 3.34 × 104 A/W and 6.61 × 1013 Jones, respectively, are achieved under the 640 nm light illumination.

Investigation and field effect tuning of thermoelectric properties of SnSe2 flakes

The family of Van der Waals dichalcogenides (VdWDs) includes a large number of compositions and phases, exhibiting varied properties and functionalities. They have opened up a novel electronics of two-dimensional materials, characterized by higher integration and interfaces which are atomically sharper and cleaner than conventional electronics. Among these functionalities, some VdWDs possess remarkable thermoelectric properties. SnSe2 has been identified as a promising thermoelectric material on the basis of its estimated electronic and transport properties. In this work we carry out experimental meas-urements of the electric and thermoelectric properties of SnSe2 flakes. For a 30 micron thick SnSe2 flake at room temperature, we measure electron mobility of 40 cm^2 V^-1 s^-1, a carrier density of 4 x 10^18 cm^-3, a Seebeck coefficient S around -400 microV/K and thermoelectric power factor around 0.35 mW m^-1 K^-2. The comparison of experimental results with theoretical calculations shows fair agreement and indicates that the dominant carrier scattering mechanisms are polar optical phonons at room temperature and ionized im-purities below 50 K. In order to explore possible improvement of the thermoelectric properties, we carry out reversible electrostatic doping on a thinner flake, in a field effect setup. On this 75 nm thick SnSe2 flake, we measure a field effect variation of the Seebeck coefficient of up to 290 % at low temperature, and a corresponding variation of the thermoelectric power factor of up to 1050 %. We find that the power factor increases with the depletion of n-type charge carriers. Field effect control of thermoelectric transport opens perspectives for boosting energy harvesting and novel switching technologies based on two-dimensional materials.

Identification of the monolayer thickness difference in a mechanically exfoliated thick flake of hexagonal boron nitride and graphite for van der Waals heterostructures

Exfoliated flakes of layered materials, such as hexagonal boron nitride (hBN) and graphite with a thickness of several tens of nanometers, are used to construct van der Waals heterostructures. A flake with a desirable thickness, size, and shape is often selected from many exfoliated flakes placed randomly on a substrate using an optical microscope. This study examined the visualization of thick hBN and graphite flakes on SiO2/Si substrates through calculations and experiments. In particular, the study analyzed areas with different atomic layer thicknesses in a flake. For visualization, the SiO2 thickness was optimized based on the calculation. As an experimental result, the area with different thicknesses in a hBN flake showed different brightness in the image obtained using an optical microscope with a narrow band-pass filter. The maximum contrast was 12% with respect to the difference of monolayer thickness. In addition, hBN and graphite flakes were observed by differential interference contrast (DIC) microscopy. In the observation, the area with different thicknesses exhibited different brightnesses and colors. Adjusting the DIC bias had a similar effect to selecting a wavelength using a narrow band-pass filter.