Crystalline Flakes Research Articles

Wettability Engineering for Studying Ion Transport in 2D Layered Materials

AbstractLayered materials are widely used for their capacity to incorporate and store foreign ionic species. It has become possible to exploit this process at the level of single crystals consisting of individual atomic layers: 2D materials and their van der Waals heterostructures. Due to the small size of available high‐quality specimen, however, it remains challenging to probe ion transport and storage in these systems. To promote future advances in this field, wettability engineering is introduced as a strategy for the on‐chip integration of electrolytes with micrometer‐sized samples of 2D materials. Contact angles are systematically measured for a variety of electrolyte–surface couples to identify a rational device design, and engineer lateral contrasts in surface chemistry to control the rims of drop cast electrolytes with micrometer precision. This allows covering single crystalline flakes of 2D materials only partially, leaving most of the sample surface uncovered and imposing directionality on ion transport. Wettability engineering is used to fabricate few‐layer graphene electrochemical micro two‐compartment cells that display chemical diffusion of lithium on the order of 10−8 cm2 s−1. While the approach is transferrable to other 2D materials and ionic species, it can also benefit device design for the study of nonlocal electrolyte gating effects.

Enhanced nitrogen and phosphorus adsorption performance and stabilization by novel panda manure biochar modified by CMC stabilized nZVZ composite in aqueous solution: Mechanisms and application potential

Direct discharge of water containing excessive nitrogen and phosphorus concentrations into the aquatic environment can lead to loss of resources and eutrophication. Biochar prepared by thermochemical treatment of feedstocks, particularly the metal modified biochar, which can be used as a new adsorbent for aquatic environmental treatment. Herein, a novel panda manure biochar (PMBC) modified by carboxymethyl cellulose (CMC) stabilized nano zero-valent zinc (nZVZ-CMC-PMBC) was developed and applied for efficiently removal of NH4+and PO43−. Batch sorption experiments verified the kinetics and equilibrium isotherms of NH4+ and PO43− adsorption, the results showed that PMBC had significantly better adsorption performance on nitrogen and phosphorus compared to bamboo biochar. Moreover nZVZ-CMC-PMBC demonstrated maximum theoretical adsorption capacities of 40.31 mg•g−1 NH4+-N and 154.30 mg•g−1 total phosphorus (TP). Characterization result of SEM-EDS, BET, FTIR, XRD and XPS reveal that PMBC had larger specific surface area and pore diameter than bamboo biochar which confirmed that PMBC contained more adsorption sites. As for nZVZ-CMC-PMBC, crystalline nZVZ flakes are dispersed and impregnated into the frame of CMC-nZVZ- PMBC composite, characterization analysis unveiled the formed complexes on its surface. Therefore, the adsorption mechanism of nZVZ-CMC-PMBC for NH4+-N and TP mainly involved Zn2+ precipitates, surface electrostatic attraction and surface complexation.

Single crystal flake parameters of MoS2 and MoSe2 exfoliated using anodic bonding technique and its potential in rapid prototyping

Rapid prototyping of devices using exfoliated Molybednum di-Sulphide (MoS2) and Molybdenum di-Selenide (MoSe2) requires an experimental protocol for maximizing the probability of realizing flakes with desired physical dimension and properties. In this work, we analyzed the size and thickness distribution of MoS2 and MoSe2 single crystalline flakes exfoliated using anodic bonding technique and established a correlation between physical dimension of the flakes and the bonding parameters. Anodic bonding was carried out by applying a fixed voltage of 200 V with a set temperature of 150 °C for four different bonding time intervals. On analyzing the flake parameters from the four anodic bonded substrates using the optical and atomic force microscopy, it is found that the probability of getting flakes with large lateral size (>200 μm) increases as the bonding time interval is increased. Most of these large sized flakes have thickness of more than one hundred mono-layers and a tiny fraction of them have thickness of the order of few monolayers. A similar trend was also observed for MoSe2 single crystals. To demonstrate the feasibility of this technique in rapid prototyping, ultra thin MoS2 flakes was directly bridged between two ITO electrodes and their transport properties was investigated. Micro-Raman and photoluminescence studies were taken on selected regions of the thicker and thinner exfoliated flakes and their physical properties are compared.

Atomically Asymmetric Inversion Scales up to Mesoscopic Single-Crystal Monolayer Flakes.

Symmetry is highly relevant with various quantities and phenomena in physics. While the translational symmetry breaks at the edges of two-dimensional hexagonal crystalline flakes, it is usually associated with the breaking of central inversion symmetry that is yet to be observed in terms of physical properties. Here, we report an experiment-theory joint study on in-plane compressed single-crystal monolayer WS2 flakes. Although the flakes show a hexagonal appearance with a C6 symmetry, our density functional theory calculations predict that their in-plane strain, geometric structure, work-function, energy bandgap, and mechanical modulus are nonequivalent among the triangular regions with different edge terminations at the atomic scale, and the flakes exhibit self-patterns with a C3 symmetry. Such nonequivalence of physical properties and concomitant self-patterns persist even in a 50 μm-sized monolayer WS2, observed using atomic force microscopy. This indicates that the symmetry arising from the atomic geometry could preserve up to tens of microns for both geometric and properties of the flake, regardless of its mesoscopic geometry, i.e., C6 here. Such a detectable mesoscopic scale and symmetric nano- to mesoscale patterns provide promising building blocks for 2D materials and devices and also allow edge terminations of 2D flakes to be directly distinguished.

Chemical purification processes of the natural crystalline flake graphite for Li-ion Battery anodes

We investigated the effect of the acid species, the liquid-to-solid ratio, the reaction time, and temperature on the carbon content (CC) of natural crystalline flake graphite in Ethiopia. Specifically, after physical purification through a froth flotation, CC was observed to be 94.71 %. However, when the acid-alkali-acid, H2SO4/H2O2-NaOH−HCl, chemical purification process was employed in this study, CC was significantly improved to 99.68 % (or 99.72 % after additional HF leaching), based on both a loss-on-ignition test and thermogravimetric analysis. Here, the purified graphite powder exhibited a conductivity of 21.9 S/cm, placing it in the range of semiconductor materials. The hexagonal structure of graphite, when analyzed by X-ray diffraction, was independent of the purification process. However, according to the scanning electron microscopy image, the impurities in the graphite sample were removed effectively through the purification process. Finally, based on the overall material properties of this purified graphite, this natural flake graphite could be used for high tech applications such as Li-ion battery anodes.

Activated coal-based graphene with hierarchical porous structures for ultra-high energy density supercapacitors

Coal is a low-cost and abundant natural resource for energy and chemicals generation, as well as production of various carbon materials. Herein, interconnected porous graphene with 3D structures has been fabricated from graphitized coal through the combination of liquid intercalation/oxidation, thermal exfoliation and chemical activation. Compared with conventional graphene prepared from crystalline flake graphite, the as-obtained graphene from graphitized coal possesses unique 3D interconnected structures, which can provide high ion-accessible surface area (2428.6 m2 g−1) and large pore volume (1.82 cm3 g−1). The supercapacitor from coal-based graphene shows high specific capacitance of 225 F g−1 at 0.5 A g−1 and high capacitance retention of 63% at 100 A g−1 in 6 M KOH aqueous electrolyte. The energy density of coal-based graphene is evaluated to be 79.4 Wh kg−1 in EMIMBF4 ionic liquid electrolyte. Our strategy may open up a new avenue for massive preparation of hierarchical porous graphene for high-performance supercapacitors.

A novel Ni-S-Mn electrode with hierarchical morphology fabricated by gradient electrodeposition for hydrogen evolution reaction

A novel Ni-S-Mn electrode with hierarchical porous morphology of multi-channel wind-erosion-gully was fabricated through gradient electrodeposition. XRD and XPS were adopted to investigate chemical composition and elemental valence states; SEM, TEM and EDS were employed to characterize morphology and microstructure; LSV, CV and EIS were used to study electrocatalytic properties and stability of Ni-S-Mn electrode. The characterization of microstructure combined with the evaluation of electrocatalytic performances proved that the Ni-S-Mn electrode surface composition are of hierarchical porous morphologies, including the crystalline Ni3S2 and MnS flake structure as framework and the amorphous Ni as intergranular fillings; electron transfer efficiency can be effectively accelerated by crystalline sulfide; the layered structure of electrode surface facilitates the mass transfer of hydrogen molecules and electrolyte solution during hydrogen evolution reaction (HER). Particularly, the as-fabricated Ni-S-Mn electrode has a remarkable electrochemical surface area (ECSA) of 43180 cm2, that is extremely higher among all other Ni-based electrode fabricated by electrodeposition. In addition to its excellent electrochemical stability confirmed by chronoamperometry (CA) at −100 mV vs. RHE for 15 h, the current density of Ni-S-Mn deposits on this novel electrode can reach as high as 713.8 mA cm−2 at an overpotential of 0.4 V vs. RHE.

The Methods to Unlock Molybdenum Disulfide

Author(s): Rodriguez, Anthony | Abstract: In today’s world, we’ve invented many new technologies that have allowed us to map out the earth’s surface but just as we can map out surfaces in a large scale, we can also map out surfaces in a very small scale such as in nanometers. Thus, giving us opportunities to study a discover various materials such as molybdenum disulfide (MoS2) crystalline flakes. MoS2 being a material that has a similar yet different structure to graphene, low friction when compared to SiO2 (a low friction material as is), and provides many potential applications that can be implemented into aerospace as a solid lubricant. But like any fellow researcher studying a material, each has their own methods/techniques that they use to obtain results. In our proposal, we will cover the research that has taken place to provide a consensus of MoS2’s potential application as a solid lubricant in aerospace to its low frictional properties in the nanometer scale. And the technologies we’ll be looking into to access for our methods/techniques for researching will be Atomic Force Microscopy (AFM), Scanning Electron Microscopy (SEM), X-Ray Photoelectric Spectroscopy (XPS), X-Ray Diffraction (XRD), and Tribometers at the University of California, Merced (UC Merced).

A review on thermophysical properties of flexible graphite

Flexible graphite (FG) is the last stage of compression of expanded graphite. Natural and highly crystalline graphite flakes are expanded and rolled together without any binder in order to shape foils of different thicknesses and densities. The result is an anisotropic but highly conductive material, with low strength and stiffness compared to common graphite, but gathering a collection of different peculiar properties. About mechanical field, FG is mostly used in sealing and gasketing applications due to its resilience and capability of dissipate energy during vibration. It also shows high resistance against chemical agents and can retain its structural integrity at temperature up to 2500℃ in inert atmosphere. All the mechanical, electrical and thermal properties are strongly affected by the inherent anisotropy that make it exploitable in thermal cooling, interface insulation, electromagnetic shielding and in electrical conduction components such as fuel cell electrodes. In this work the manufacturing process is summarized together with the effect of density on strength and modulus and the experimental results found in literature among anisotropy, stiffness, thermal and electrical conductivity, specific heat and thermal expansion are reviewed together with some modeling approaches.

Large and Robust Charge-to-Spin Conversion in Sputtered Disordered WTe <sub>x</sub>

Topological insulators have recently shown great promise for ultralow-power spin-orbit torque (SOT) devices thanks to their large charge-to-spin conversion efficiency originating from the spin-momentum-locked surface states. Recently, Weyl semimetals emerge as an alternative SOT source with spin-polarized surface as well as bulk states, robustness against magnetic and structural disorder, and higher electrical conductivity for integration in metallic magnetic tunnel junctions. Here, we report that long-range disordered sputtered WTex thin films exhibit local chemical and structural order as those of Weyl semimetal WTe2 and conduction behavior that is consistent with semi-metallic Weyl fermion. We find large charge-to-spin conversion properties in thermally annealed sputtered WTex films that are comparable with those in crystalline WTe2 flakes. Besides, the strength of unidirectional spin Hall magnetoresistance in annealed WTex/Mo/CoFeB heterostructure is 5 to 20 times larger than typical SOT/ferromagnet bilayers reported at room temperature. We further demonstrate room temperature field-free magnetization switching at a current density of 1 – 2 MA/cm2. These large charge-to-spin conversion properties that are robust in the presence of long-range disorder and thermal annealing pave the way for industrial integration of such topological materials. Our results open a new class of sputtered semimetals for memory and computing based on magnetic tunnel junctions as well as broader planar heterostructures consisting of SOT layer/ferromagnet interfaces.

An Interdigital Capacitive Humidity Sensor With Layered Black Phosphorus Flakes as a Sensing Material

Black phosphorus (BP) atomic layers have received significant attention for chemical sensing applications due to its exceptional electrical, mechanical, and surface properties. However, they are known to undergo chemical degradation in ambient conditions. In this paper, an interdigital capacitive humidity sensor with layered BP flakes as a sensing material is reported. The BP flakes were prepared from crystal powders by the mechanical exfoliation and then deposited on the interdigital electrode (IDE) structures by a spin-coating method. The deposited BP flakes were characterized by the atomic force microscope (AFM) and Raman spectroscopy, which demonstrated the uniformity of highly crystalline BP flakes. The interdigital capacitive humidity sensors were measured under different relative humidity (RH) levels. The results obtained show that the capacitance increases gradually as the RH levels with the sensitivity of about 0.2pF/%RH at the low RH levels (<45%RH) while the capacitance increases rapidly as the RH levels with the high sensitivity of about 10pF%RH. The maximum hysteresis was measured to be about 3%RH. Stability measurement showed that the capacitance of the sensors increased in the first 15 hours and then tended to be stable. Good repeatability was observed in the measurement, suggesting that degradation was prevented. Oxide layers were formed on the surface instead and the inside layers of BP kept intact. The estimated response and recovery time of the capacitive sensors was about 17s and 4.7s, respectively. The equivalent RC networks model for the capacitive sensors is proposed to explain the mechanisms. The fast response and excellent sensitivity of the capacitive sensors at the high RH levels showed potential applications in high RH monitoring areas.

Highly active oxygen evolution integrated with efficient CO2 to CO electroreduction

Electrochemical reduction of CO2 to useful chemicals has been actively pursued for closing the carbon cycle and preventing further deterioration of the environment/climate. Since CO2 reduction reaction (CO2RR) at a cathode is always paired with the oxygen evolution reaction (OER) at an anode, the overall efficiency of electrical energy to chemical fuel conversion must consider the large energy barrier and sluggish kinetics of OER, especially in widely used electrolytes, such as the pH-neutral CO2-saturated 0.5 M KHCO3 OER in such electrolytes mostly relies on noble metal (Ir- and Ru-based) electrocatalysts in the anode. Here, we discover that by anodizing a metallic Ni-Fe composite foam under a harsh condition (in a low-concentration 0.1 M KHCO3 solution at 85 °C under a high-current ∼250 mA/cm2), OER on the NiFe foam is accompanied by anodic etching, and the surface layer evolves into a nickel-iron hydroxide carbonate (NiFe-HC) material composed of porous, poorly crystalline flakes of flower-like NiFe layer-double hydroxide (LDH) intercalated with carbonate anions. The resulting NiFe-HC electrode in CO2-saturated 0.5 M KHCO3 exhibited OER activity superior to IrO2, with an overpotential of 450 and 590 mV to reach 10 and 250 mA/cm2, respectively, and high stability for >120 h without decay. We paired NiFe-HC with a CO2RR catalyst of cobalt phthalocyanine/carbon nanotube (CoPc/CNT) in a CO2 electrolyzer, achieving selective cathodic conversion of CO2 to CO with >97% Faradaic efficiency and simultaneous anodic water oxidation to O2 The device showed a low cell voltage of 2.13 V and high electricity-to-chemical fuel efficiency of 59% at a current density of 10 mA/cm2.

Evolution of Phosphorene Sheets through Direct Crystallization of Thin‐Film Red Phosphorus

A novel hydrogen plasma treatment to convert an amorphous phosphorus film deposited on silicon substrates into a thin crystalline layer is successfully developed. The amorphous phosphorus layer is deposited on desired substrates as silicon using vacuum evaporation in a direct‐current plasma reactor in a controllable fashion. The formation of 2D phosphorene layers is based on the phase transition of a previously deposited amorphous film into the crystalline black phosphorus flakes. Direct transformation from red phosphorus to black can be achieved at a temperature of 300 °C on desired substrates as silicon, although mica and glass can also be used. This allotrope transformation can be achieved without any high pressure or high temperature. Apart from hydrogen plasma treatment, ultraviolet (UV) exposure to see any possible improvement in the 2D layer formation is also used herein. Various characterization techniques including scanning electron microscopy, transmission electron microscopy, atomic force microscopy, and Raman spectroscopy are used to study the crystallinity and morphology of the samples. In addition, the plasma‐treated flakes are used to realize photodetectors which show excellent response to illuminating light. While the hydrogen plasma treatment leads to crystalline phosphorene sheets, the UV treatment results in granular and partially crystalline nanostructures.

Graphene oxide (GO)/Copper doped Hematite (α-Fe2O3) nanoparticles for organic pollutants degradation applications at room temperature and neutral pH

Pure, copper doped and GO/copper doped α-Fe2O3 nanoparticles with variable dopant have been synthesized by sol-gel method using ferric nitrate nonahydrate (Fe(NO3)3.9H2O), cupric acetate monohydrate as precursors of iron and copper respectively. Gelatin, polymerizing agent, provides long term stability to nanoparticles restricting agglomeration. Crystallographic and morphological characteristics, of the synthesized nanomaterials, have been studied by powder x-ray diffraction (XRD) and transmission electron microscope (TEM), respectively. Microscopic studies have shown formation nano flakes with heterogeneous size distribution with single crystalline nano flakes. Methylene blue (MB) dye, pollutant in aqueous media, has been degraded up to 70% using Cu-Fe2O3 catalyst within 120 min under the simulated visible irradiation. To further enhance degradation capability, GO/ Cu-Fe2O3 has been synthesized and showed close to complete degradation, i.e. ∼100%, of dye within same time period of 120 min. Present method shows quick and full degradation capability of aqueous pollutants that has potential application in the field of cleaning organic aqueous pollutants, a major problem these days.

Origin of the butterfly magnetoresistance in ZrSiS

ZrSiS has been identified as a topological material made from non-toxic and earth-abundant elements. Together with its extremely large and uniquely angle-dependent magnetoresistance this makes it an interesting material for applications. We study the origin of the so-called butterfly magnetoresistance by performing magnetotransport measurements on four different devices made from exfoliated crystalline flakes. We identify near-perfect electron-hole compensation, tuned by the Zeeman effect, as the source of the butterfly magnetoresistance. Furthermore, the observed Shubnikov-de Haas oscillations are carefully analyzed using the Lifshitz-Kosevich equation to determine their Berry phase and thus their topological properties. Although the link between the butterfly magnetoresistance and the Berry phase remains uncertain, the topological nature of ZrSiS is confirmed.

A Novel Laser‐Stimulated Technique for Direct Formation of Few‐Layer Phosphorene on Silicon Substrates

Direct formation of few‐layered phosphorene flakes through a laser stimulated crystallization on silicon substrates is reported. The formation of such layers is initiated by depositing red phosphorus (RP) on silicon followed by laser‐induced crystallization. After deposition, a glass plate is used to encapsulate the amorphous layer on the substrate. Being capable of a programmable scanning, a 1064‐nm laser is used as the source to impart energy onto the RP layer and to achieve the desired allotrope transformation. Exposure to laser beam at the presence of temperature and local pressure exerted by argon ambient leads to the conversion of amorphous material into crystalline flakes. Furthermore, interesting core‐shell, flower‐like and vertical structures are achieved. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), Raman spectroscopy and energy dispersive X‐ray spectroscopy (EDS) are used to examine the conversion of red phosphorus into few‐layer phosphorene flakes. These flakes are used to fabricate a photodetector with a reasonable response to green laser and a field‐effect transistor with a mobility of 12 cm2 V−1 s−1, further indicating the evolution of well‐crystalline layers. The fabricated transistors demonstrate triode and saturation regimes in their electrical response, evidencing their high quality.

Synthesis of carbon spheres by atmospheric pressure chemical vapor deposition from a serial of aromatic hydrocarbon precursors

Uniform shaped carbon spheres, a material with several potential applications, were synthesized from a serial of aromatic hydrocarbon pyrolysis precursors (benzene, naphthalene, anthracene and pyrene), by chemical vapor deposition at atmospheric pressure, and at moderately low temperatures with stainless steel catalyst. A high carbon sphere yield, without secondary products, was achieved at temperatures above 850 °C. Sphere diameters decrease, in some cases, according to the temperature increments. Field emission scanning electron microscopy images show a completely solid core. Raman and Fourier transform infrared spectra suggest a larger amount of CHx bonds on the sphere surface with major curvature. Spheres are formed by small crystalline graphene flakes, observed by high resolution transmission electron microscopy and confirmed by the G band intensity in Raman spectra and X-ray diffraction signals.

Nonlocal signatures of the chiral magnetic effect in the Dirac semimetal Bi0.97Sb0.03

The field of topological materials science has recently been focussing on three-dimensional Dirac semimetals, which exhibit robust Dirac phases in the bulk. However, the absence of characteristic surface states in accidental Dirac semimetals (DSM) makes it difficult to experimentally verify claims about the topological nature using commonly used surface-sensitive techniques. The chiral magnetic effect (CME), which originates from the Weyl nodes, causes an $\textbf{E}\cdot\textbf{B}$-dependent chiral charge polarization, which manifests itself as negative magnetoresistance. We exploit the extended lifetime of the chirally polarized charge and study the CME through both local and non-local measurements in Hall bar structures fabricated from single crystalline flakes of the DSM Bi$_{0.97}$Sb$_{0.03}$. From the non-local measurement results we find a chiral charge relaxation time which is over one order of magnitude larger than the Drude transport lifetime, underlining the topological nature of Bi$_{0.97}$Sb$_{0.03}$.

Restoring Superconductivity in the Quantum Metal Phase of NbSe2 Using Dissipative Coupling.

Localization arguments forbid the appearance of a metallic ground state in two dimensions. Yet, a large variety of disordered superconductors are known to manifest an anomalous metal phase in the zero temperature limit. While previous observations were confined to noncrystalline "dirty" superconductors, the recent observation of the so-called Bose metal phase in crystalline thin flakes of NbSe2 has sparked off intense debate. While the exact nature of this phase remains unknown, it is thought that quantum fluctuations play a decisive role in Bose metal physics. In this work, we study the response of the anomalous metal phase in thin flakes of NbSe2 to dissipative coupling. We evince a dramatic quenching of the Bose metal phase when dissipative coupling is strong, fully restoring a zero resistance superconducting state in the entire region of the magnetic field (H)-temperature (T) phase diagram where the Bose metal phase is otherwise observed. The suppression of the Bose metal phase by dissipative coupling is possible only in a quantum system where dissipation can directly affect system thermodynamics. Our observation of a dissipative phase transition in two-dimensional NbSe2 firmly establishes the quantum nature of the anomalous metal phase in this class of "clean" superconductors.

Dominant s -wave superconducting gap in PdTe2 observed by tunneling spectroscopy on side junctions

We have fabricated superconductor-normal metal side-junctions with different barrier transparencies out of PdTe$_2$ crystalline flakes and measured the differential conductance spectra. Modeling our measurements using a modified Blonder Tinkham Klapwijk (BTK) formalism confirms that the superconductivity is mostly comprised of the conventional $s$-wave symmetry. We have found that for junctions with very low barrier transparencies, the junctions can enter a thermal regime, where the critical current becomes important. Adding this to the BTK-model allows us to accurately fit the experimental data, from which we conclude that the superconductivity in the $a$-$b$ plane of PdTe$_2$ is dominated by conventional $s$-wave pairing.