Single Flakes Research Articles

Nickel diffusion into MoS2 and the effect of annealing on contact resistance

Metal-semiconductor interfaces are crucial for the performance of many devices and understanding interactions between material interfaces after processing is therefore of interest. Here, field-effect transistors with Ni contacts are studied to determine the effects of annealing on contact resistance. Contact resistance and total resistance of as-deposited and annealed field-effect transistors with Ni/Ti/Au contacts increase upon annealing, with higher annealing temperatures yielding a greater increase in contact resistance. It appears that Ni diffusion into MoS2 correlates with the observed higher contact resistance, and transmission electron microscopy, energy dispersive spectroscopy, and electron energy loss spectroscopy reveal the presence of Ni in single crystal MoS2 flakes after annealing Ni/MoS2 in ultra-high purity Ar gas at 400 °C. Metal diffusion into 2D semiconductors like MoS2 may help or hurt device performance, and this work highlights the importance of considering the effects metal diffusion may have on contact resistance.

Ultrasensitive broadband photodetector using electrostatically conjugated MoS2-upconversion nanoparticle nanocomposite

Hybrid or composite nanomaterials have emerged that demonstrates superior optoelectronic performance over pure nanomaterials that lacks broadband usage, or responsivity, or both, mainly because of the limitation of the collection of photogenerated carriers. We have addressed this problem by using a composite of MoS2 and a multi-photon absorbing lanthanide doped upconversion nanoparticles (UCNPs), that emits in the visible, to make a photodetector (PD) device with ultrahigh broadband responsivity. Single flake MoS2 electrostatically conjugated with UCNPs were used to fabricate the PD device with platinum, and gold contacts. The device was irradiated with UV-to-NIR illumination, at different power density, to study its broadband photosensitivity. Photoresponsivities in excess of 100 A W-1 is easily obtained; a highest responsivity of 1254 A W-1 is reported for 980 nm at 1.0 V bias. An unprecedented normalized gain of 7.12 × 10−4 cm2 V−1, and Detectivity of 1.05 × 1015 Jones (@980 nm, 1 V) was obtained which is, to the best of our knowledge, the highest reported till date for this device class. Under vacuum conditions even higher values of these device parameters were obtained, while losing on the response speeds. The photoresponsivity in the nanocomposite followed the trend of the convoluted optical absorption of the individual components. Real application of the PD device was demonstrated using non-laser domestic appliances such as sodium vapour lamp, mobile phone flash light, and air-condition remote controller.

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.

Thermal properties of thin films made from MoS2 nanoflakes and probed via statistical optothermal Raman method

A deep understanding of the thermal properties of 2D materials is crucial to their implementation in electronic and optoelectronic devices. In this study, we investigated the macroscopic in-plane thermal conductivity (κ) and thermal interface conductance (g) of large-area (mm2) thin film made from MoS2 nanoflakes via liquid exfoliation and deposited on Si/SiO2 substrate. We found κ and g to be 1.5 W/mK and 0.23 MW/m2K, respectively. These values are much lower than those of single flakes. This difference shows the effects of interconnections between individual flakes on macroscopic thin film parameters. The properties of a Gaussian laser beam and statistical optothermal Raman mapping were used to obtain sample parameters and significantly improve measurement accuracy. This work demonstrates how to address crucial stability issues in light-sensitive materials and can be used to understand heat management in MoS2 and other 2D flake-based thin films.

Highly Stable Two‐Dimensional Tin(II) Iodide Hybrid Organic–Inorganic Perovskite Based on Stilbene Derivative

AbstractHybrid organic–inorganic perovskites have recently emerged as potential disruptive photovoltaic technology. However, the toxicity of lead used in state‐of‐the‐art hybrid perovskites solar cell prevents large‐scale commercialization, which calls for lead‐free alternatives. Sn‐based perovskites have been considered as alternatives but they are limited by rapid oxidation and decomposition in ambient air. Here, an Sn‐based two‐dimensional hybrid organic–inorganic perovskites [A2B(n‐1)SnnI(3n+1)] (n = 1 and 2) are reported with improved air stability, using bulky stilbene derivatives as the organic cations (2‐(4‐(3‐fluoro)stilbenyl)ethanammonium iodide (FSAI)). The moisture stability of the [(FSA)2SnI4] perovskites is attributed to the hydrophobic properties of fluorine‐functionalized organic chains (FSA), as well as the strong cohesive bonding in the organic chains provided by H bonds, CH···X type H bonds, weak interlayer F···F interaction, and weak face‐to‐face type π‐π interactions. The photodetector device fabricated on exfoliated single crystal flake of [(FSA)2SnI4] exhibits fast and stable photoconductor response.

Quantum Hall effect of Dirac surface states of as-grown single crystal flakes in Sn0.02-Bi1.08Sb0.9Te2S without gate control

The quantum spin Hall effect (QSHE) in three-dimensional topological insulators (3D-TIs) is an important hallmark of realization of ultralow dissipative electrical transport originating from nontrivial topological surface Dirac states (TSDSs). The observation of QSHE, however, had been limited at low temperatures for thin film crystals prepared by epitaxial growth or ultrathin (nanometer-level) flakes, and the finite Fermi energy tuning was required using field effect transistor (FET) construction. Here, we show that a clear observation of QSHE with a quantized level of e2/h can be possible for high quality single crystal flakes of Sn-(Bi,Sb)2(Te,S)3 (Sn-BSTS) prepared by an exfoliation method without any gate control under FET construction. The QSHE is observed in an extremely large sample area of millimeter square at the largest thickness of micrometer and up to the highest temperature of 20 K, which is greatly superior to those reported for other 3D-TIs so far. The accurate band picture of both top and bottom topological surface states of Sn-BSTS is determined from both the QSHE and the Shubnikov-de Hass (SdH) oscillations. High quality Sn-BSTS of 3D-TIs will effectively be used for experiments targeting on science as well as many technological applications expected in the future.

Temperature-Dependent Opacity of the Gate Field Inside MoS2 Field-Effect Transistors.

The transport behaviors of MoS2 field-effect transistors (FETs) with various channel thicknesses are studied. In a 12 nm thick MoS2 FET, a typical switching behavior is observed with an Ion/Ioff ratio of 106. However, in 70 nm thick MoS2 FETs, the gating effect weakens with a large off-current, resulting from the screening of the gate field by the carriers formed through the ionization of S vacancies at 300 K. Hence, when the latter is dual-gated, two independent conductions develop with different threshold voltage (VTH) and field-effect mobility (μFE) values. When the temperature is lowered for the latter, both the ionization of S vacancies and the gate-field screening reduce, which revives the strong Ion/Ioff ratio and merges the two separate channels into one. Thus, only one each of VTH and μFE are seen from the thick MoS2 FET when the temperature is less than 80 K. The change of the number of conduction channels is attributed to the ionization of S vacancies, which leads to a temperature-dependent intra- and interlayer conductance and the attenuation of the electrostatic gate field. The defect-related transport behavior of thick MoS2 enables us to propose a new device structure that can be further developed to a vertical inverter inside a single MoS2 flake.

In-situ visualization of hydrogen evolution sites on helium ion treated molybdenum dichalcogenides under reaction conditions

Nanostructured 2D transition metal dichalcogenides play an increasingly important role in heterogeneous catalysis. These materials are abundant (co-)catalysts with tunable properties to catalyze a number of key reactions related to energy provision, for instance the hydrogen evolution reaction (HER). It is vital to understand which surface sites are active in order to maximize their number and to improve the overall (photo-)catalytic behavior of those materials. Here, we visualize these active sites under HER conditions at the surface of molybdenum dichalcogenides (MoX2, X = Se, S) with lateral resolution on the nanometer scale by means of electrochemical scanning tunneling microscopy. The edges of single MoX2 flakes show high catalytic activity, whereas their terraces are inactive. We demonstrate how the inert basal planes of these materials can be activated towards the HER with the help of a focused beam of a He-ion microscope. Our findings demonstrate that the He-ion induced defects contribute at lower overpotentials to the HER, while the activity of the edges exceeds the activity of the basal defects for sufficiently high overpotentials. Given the lithographic resolution of the helium ion microscope, our results show the possibility to generate active sites in transition metal dichalcogenides with a spatial resolution below a few nanometers.

Few-Layer to Multilayer Germanium(II) Sulfide: Synthesis, Structure, Stability, and Optoelectronics.

Among 2D/layered semiconductors, group IV monochalcogenides such as SnS(e) and GeS(e) have attracted attention as phosphorene/black phosphorus analogues with anisotropic structures and predicted unusual properties. In contrast to SnS, for which bottom-up synthesis has been reported, few-layer GeS has been realized primarily via exfoliation from bulk crystals. Here, we report the synthesis of large (up to >20 μm), faceted single crystalline GeS flakes with anisotropic properties using a vapor transport process. In situ electron microscopy is used to identify the thermal stability and sublimation pathways, and demonstrates that the GeS flakes are self-encapsulated in a thin, sulfur-rich amorphous GeSx shell during growth. The shell provides exceptional chemical stability to the layered GeS core. In contrast to exfoliated GeS, which rapidly degrades during exposure to air, the synthesized GeS-GeSx core-shell structures show no signs of chemical attack and remain unchanged in air for extended time periods. Measurements of the optoelectronic properties by photoluminescence spectroscopy show a tunable bandgap due to out-of-plane quantum confinement in flakes with thickness below 100 nm. Cathodoluminescence (CL) spectroscopy with nanoscale excitation provides evidence for interfacial charge transfer due to a type II heterojunction between the crystalline core and amorphous shell. Measurements by locally excited CL yield a minority carrier (electron) diffusion length in the p-type GeS core = 0.27 μm, on par with diffusion lengths in the highest-quality layered chalcogenide semiconductors.

Thin-Film Deposition of Surface Passivated Black Phosphorus.

A single-step, direct silicon-substrate growth of black phosphorus (BP) crystals is achieved in a self-contained short-way transport technique under low-pressure conditions (<1.5 MPa). A 115 nm-thick BP hero single crystal is formed with lateral dimensions of 10 × 85 μm. The synthesis proceeds with Sn, SnI4, and red phosphorus and has a well-defined phosphorus phase dependency on the SnI4 concentration. Furthermore, in situ Sn passivation of BP occurs. This allows long-term stability with no sign of any degradation after 4 months of exposure to ambient conditions. Single-crystal BP flakes and multigrain flakes with high- and low-angle grain boundaries are achieved. Electron backscatter diffraction determined crystal growth to be independent of the substrate, which is further supported by successful growth on various substrates, including sapphire, silicon nitride, silicon, and silicon oxide. Cross-sectional transmission electron microscopy of a single crystal flake provides valuable insight into the growth mechanism. Elemental Sn encapsulates BP crystals, and crystalline SnI x inclusions are found to be scattered throughout the BP crystal. It is suggested that SnI x inclusions may provide the dominant mechanism for seeding vertical growth. IR absorption measurements for thin and bulk BP recipes show an equal response below Eg dominated by free carrier absorption. FET devices fabricated from thin-film and bulk BP recipes show improved device performance compared to unpassivated BP films of equal thickness with an on/off current ratio >102.

Low temperature growth of clean single layer hexagonal boron nitride flakes and film for graphene-based field-effect transistors

二维六方氮化硼是一种理想的石墨烯电学器件介电层材料, 然而, 制备低成本和高质量的氮化硼材料仍是一个挑战. 本文使用等离子体化学气相沉积法, 500°C下在铜箔衬底上制备了三角形的BN晶畴及其薄膜. 通过使用锡箔纸包裹原料的方法避免了残留原料在BN表面的沉积. 当BN作为石墨烯场效应晶体管的介电层时, 基于石墨烯的场效应器件空穴与电子的迁移率分别为10500和4750 cm2 V‒1 s‒1, 明显优于在高温条件下制备的BN作为介电层的石墨烯器件, 间接表明了该方法可得到高质量的BN. 另外, 本工作也揭示了氮化硼的质量对石墨烯场效应器件的性能至关重要.

Lithographically patterned metallic conduction in single-layer MoS2 via plasma processing

Tailoring the electrical transport properties of two-dimensional transition metal dichalcogenides can enable the formation of atomically thin circuits. In this work, cyclic hydrogen and oxygen plasma exposures are utilized to introduce defects and oxidize MoS2 in a controlled manner. This results in the formation of sub-stochiometric MoO3−x, which transforms the semiconducting behavior to metallic conduction. To demonstrate functionality, single flakes of MoS2 were lithographically oxidized using electron beam lithography and subsequent plasma exposures. This enabled the formation of atomically thin inverters from a single flake of MoS2, which represents an advancement toward atomically thin circuitry.

Facile synthesis of nanowire and rectangular flakes of Co3O4 onto Ni foam for high-performance asymmetric supercapacitors

In this study, Co3O4 with different morphologies (nanowires of single phase, nanowires, and rectangular flakes of both phases) for supercapacitor electrode were synthesized through a hydrothermal approach in the presence of Triton X-100 followed by annealing at 350 °C for 1 h. The different morphologies of the samples were obtained with the assistance of surfactant by adjusting the quantity of reactants. Relative to Co3O4 nanowire (Co3O4 NW) samples, the Co3O4 nanowire-flake (Co3O4 NW-F) samples exhibit excellent electrochemical performance with higher capacity of 1230.65 F g−1 at a current density of 1 A g−1, and with regard to cycling stability, they perform well in a three-electrode system during the larger specific surface area. Moreover, an asymmetric supercapacitor was assembled with Co3O4 NW-F as positive electrode and activated carbon as negative electrode; the device shows a high energy density of 61.60 W h kg−1 at a power density of 1440 W kg−1.

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$_{0.03}$.

The impact of flake tool attributes and butcher experience on carcass processing time and efficiency during experimental butchery trials

AbstractButchery involves using tools to process animal carcasses during consumption of tissue packages, and this technological adaptation has a deep connection with evolving human dietary ecology. Studying butchery informs archaeological inferences about lithic artefact function and site formation and describes skeletal traces of carcass consumption, which may corroborate the butchery function of certain tools. Diverse methodologies including subjective personal observations and well‐controlled experiments that investigate abstracted slicing mechanics or realistic butchery scenarios provide mixed conclusions about which kinds of tools are more efficient for butchery and whether their archaeological traces can be discriminated. Much less analytical attention is devoted to investigating whether a butcher's experience using stone tools to process carcasses impacts their performance, which may confound experimental assessment of how tool attributes are related to butchery efficiency. These experiments examine butchery performance in two novices over 40 goat forelimb and hindlimb trials where a single flake was used to deflesh and disarticulate a limb. By measuring butchery performance as defleshing efficiency (the amount of meat removed per second) and the time necessary to disarticulate limb elements, we demonstrate that in general, both butchers performed similarly when butchering forelimbs and hindlimbs and did not exhibit a strong learning curve of performance improvement. As well, flake weight, size, and cutting edge length were positively related to defleshing efficiency and negatively related to disarticulation time, suggesting that larger flakes are better butchery tools. Our results suggest that anatomical differences between forelimbs and hindlimbs did not impact butchery timing or efficiency and butchery performance is similar across novices. To maximize comparability of experimental butchery results, we encourage future research to examine butchery performance with respect to animal or tissue package size and investigate how butcher experience impacts performance.

Integration of atomically thin layers of transition metal dichalcogenides into high-Q, monolithic Bragg-cavities: an experimental platform for the enhancement of the optical interaction in 2D-materials

We demonstrate a new approach to integrate single layer MoSe2 and WSe2 flakes into monolithic all-dielectric planar high-quality micro-cavities. These distributed-Bragg-reflector (DBR) cavities may, e.g., be tuned to match the exciton resonance of the 2D-materials. They are highly robust and compatible with cryogenic and room-temperature operation. The integration is achieved by a customized ion-assisted physical vapor deposition technique, which does not degrade the optical properties of the 2D-materials. The monolithic 2D-resonator is shown to have a high Q-factor in excess of 4500. We use photoluminescence (PL) experiments to demonstrate that the coating procedure with a SiO2 coating on a prepared surface does not significantly alter the electrooptical properties of the 2D-materials. Moreover, we observe a resonance induced modification of the PL-spectrum for the DBR embedded flake. Our system thus represents a versatile platform to resonantly enhance and tailor light-matter-interaction in 2D-materials. The gentle processing conditions would also allow the integration of other sensitive materials into these highly resonant structures.

Experimental setup for anisotropic thermoelectric transport measurements using MPMS

At reduced dimensions the electrical transport properties of materials often depend on the measurement direction. Here we report on a measurement setup designed on the manual insertion utility probe (MIUP) of the Quantum Design’s magnetic property measurement system (MPMS) to measure anisotropic transport properties in the temperature range of 10–400 K and magnetic fields up to 5 Tesla. The setup is capable of measuring the Seebeck coefficient and electrical resistivity both along and perpendicular to the applied magnetic field. The Seebeck measurement is based on the differential measurement technique; the four-probe electrical resistivity measurement can easily be performed on the opposite side of the setup. The setup consists of a small copper cube (5 × 5 × 5 mm3) fitted with diagonally cut electrically insulated square strips. The strips are fitted with copper-constantan thermocouple in differential arrangement for the temperature-gradient (ΔT) measurement and symmetrically arranged surface mount (SM) resistors to create the ΔT across the sample. The cube can be rotated to alter the direction of the applied magnetic field on the sample. We demonstrate the directional dependence of the measured transport properties for a normal platinum wire and for single crystal flakes of CuCr2Se4.

Electrochemical Visualization of Defect-induced Density of States at Single Graphene Flake

The density of states (DOS) at the Fermi level plays crucial role in the property and application of graphene-based material. We introduce an electrochemical approach that allows straightforwardly probing the defect-induced DOS at single flake of chemically reduced graphene oxide (r-GO). The idea is based on the earlier belief that the non-adiabatic electron transfer (ET) reaction rate at electrode/electrolyte interface increases with the Fermi-level DOS of electrode. By investigating the voltammetric responses of defect-rich and defect-lean r-GO nanoflakes using a variety of redox probes, we observe strong correlation between the ET kinetics and the defect densities on r-GO flakes, which are tuned by varying the flake sizes and preparation procedures. Our results thus validate a few general beliefs, namely, the defects can boost the Fermi-level DOS in graphene, the ET reaction on graphene-based material is non-adiabatic, and the non-adiabatic electrochemical ET kinetics indeed increases with the DOS of electrode.

Black phosphorus inverter devices enabled by in-situ aluminum surface modification

Two-dimensional black phosphorus (BP) generally exhibits a hole-dominated transport characteristic when configured as field-effect transistor devices. The effective control of charge carrier type and concentration is very crucial for the application of BP in complementary electronics. Herein, we report a facile and effective electron doping methodology on BP, through in situ surface modification with aluminum (Al). The electron mobility of few-layer BP is found to be largely enhanced to ∼ 10.6 cm2·V–1·s–1 by over 6 times after aluminum modification. In situ photoelectron spectroscopy characterization reveals the formation of Al–P covalent bond at the interface, which can also serve as local gate to tune the transport properties in BP layers. Finally, a spatially-controlled aluminum doping technique is employed to establish a p–n homojunction on a single BP flake, and hence to realize the complementary inverter devices, where the highest gain value of ∼ 33 is obtained.

Anisotropic Third-Order Nonlinearity in Pristine and Lithium Hydride Intercalated Black Phosphorus

Two-dimensional (2D) black phosphorus (BP) displays unique anisotropic properties in its linear optical responses, but its third-order nonlinearity (Kerr effect) has remained relatively unexplored. Here we measured the third-order nonlinear optical responses in exfoliated BP and in lithium hydride intercalated BP (LiH-BP) flakes using polarization-resolved microscopic femtosecond Z-scan techniques. Strong optical Kerr nonlinearities were measured for single flakes of BP and LiH-BP, with third-order nonlinear susceptibilities that are larger than those for many 2D materials. The nonlinear coefficients of LiH-BP are higher than those of BP, indicating that lithium hydride intercalation enhances not only the ambient stability of BP but also its nonlinear optical response. Highly anisotropic polarization-dependent Kerr nonlinearities of both BP and LiH-BP flakes were observed. The strong nonlinear and anisotropic optical responses of BP and LiH-BP indicate the great potential of these materials in nonlinear p...