Single Flakes Research Articles

Resistive memory based on single-crystalline black phosphorus flake/HfOx structure

Two-dimensional materials are gaining more and more attention in the field of electronic devices because of their unique advantages, such as high crystalline quality and clean and flat contact planes; compared to traditional materials, the use of two-dimensional materials as the working layer of a resistive random-access memory (RRAM) has the potential to further reduce the device size and enhance its performance. Herein, a black phosphorus (BP) single crystal flake passivated by hafnium oxide is used as the working layer for an RRAM. The devices show a switching on/off ratio of 102 in more than 100 cycles, and others can even be as high as 106. We speculated the working mechanism of the BP RRAM based on the results of serial experiments and transport analysis.

Direct oxidation of methane to methanol on Co embedded N-doped graphene: Comparing the role of N2O and O2 as oxidants

In this work, the effects of N-doping into the Co-doped single vacancy (Co-SV-G) and di-vacancy graphene flake (Co-dV-G) are investigated and compared toward direct oxidation of methane to methanol (DOMM) employing two different oxidants (N2O and O2) using density functional theory (DFT) calculation. We found that DOMM on CoN3-G utilizing the N2O molecule as oxygen-donor proceeds via a two-step reaction with low activation energies. In addition, we found that although CoN3-G might be a good catalyst for methane conversion, it can also catalyze the oxidation of methanol to CO2 and H2O due to the required low activation barriers. Moreover, the adsorption behaviors of CHx (x = 0–4) species and dehydrogenation of CHx (x = 1–4) species on CoN3-G are investigated. We concluded that CoN3-G can be used as an efficient catalyst for DOMM and N2O reduction at ambient conditions which may serve as a guide for fabricating effective C/N catalysts in energy-related devices.

Direct tuning of graphene work function via chemical vapor deposition control

Besides its unprecedented physical and chemical characteristics, graphene is also well known for its formidable potential of being a next-generation device material. Work function (WF) of graphene is a crucial factor in the fabrication of graphene-based electronic devices because it determines the energy band alignment and whether the contact in the interface is Ohmic or Schottky. Tuning of graphene WF, therefore, is strongly demanded in many types of electronic and optoelectronic devices. Whereas study on work function tuning induced by doping or chemical functionalization has been widely conducted, attempt to tune the WF of graphene by controlling chemical vapor deposition (CVD) condition is not sufficient in spite of its simplicity. Here we report the successful WF tuning method for graphene grown on a Cu foil with a novel CVD growth recipe, in which the CH4/H2 gas ratio is changed. Kelvin probe force microscopy (KPFM) verifies that the WF-tuned regions, where the WF increases by the order of ~250 meV, coexist with the regions of intrinsic WF within a single graphene flake. By combining KPFM with lateral force microscopy (LFM), it is demonstrated that the WF-tuned area can be manipulated by pressing it with an atomic force microscopy (AFM) tip and the tuned WF returns to the intrinsic WF of graphene. A highly plausible mechanism for the WF tuning is suggested, in which the increased graphene-substrate distance by excess H2 gases may cause the WF increase within a single graphene flake. This novel WF tuning method via a simple CVD growth control provides a new direction to manipulate the WF of various 2-dimensional nanosheets as well as graphene.

Infrared Permittivity of the Biaxial van der Waals Semiconductor α-MoO3 from Near- and Far-Field Correlative Studies.

The biaxial van der Waals semiconductor α-phase molybdenum trioxide (α-MoO3 ) has recently received significant attention due to its ability to support highly anisotropic phonon polaritons (PhPs)-infrared (IR) light coupled to lattice vibrations-offering an unprecedented platform for controlling the flow of energy at the nanoscale. However, to fully exploit the extraordinary IR response of this material, an accurate dielectric function is required. Here, the accurate IR dielectric function of α-MoO3 is reported by modeling far-field polarized IR reflectance spectra acquired on a single thick flake of this material. Unique to this work, the far-field model is refined by contrasting the experimental dispersion and damping of PhPs, revealed by polariton interferometry using scattering-type scanning near-field optical microscopy (s-SNOM) on thin flakes of α-MoO3 , with analytical and transfer-matrix calculations, as well as full-wave simulations. Through these correlative efforts, exceptional quantitative agreement is attained to both far- and near-field properties for multiple flakes, thus providing strong verification of the accuracy of this model, while offering a novel approach to extracting dielectric functions of nanomaterials. In addition, by employing density functional theory (DFT), insights into the various vibrational states dictating the dielectric function model and the intriguing optical properties of α-MoO3 are provided.

Graphene plasmonic-assisted enhancement of linear and nonlinear optical properties of conic-shaped InAs/GaAs quantum dots with wetting layer

Graphene plasmonic (GP) in terahertz spectral range has been used to enhance the linear and nonlinear optical properties of conic-shaped InAs/GaAs quantum dots (QDs) coupled to wetting layer. Single layer graphene flakes with rectangular geometry in three configurations were located beneath the wetting layer (WL) and by exciting the plasmonic edge modes the transition dipole moments (TDMs) of S-to-P transition were calculated. Optimization on distance and thickness has been done to reach highest TDM. The results showed that the real and imaginary parts of linear susceptibility were enhanced ten and eighteen times, respectively. For third-order susceptibility, seven hundred and twelve thousand times enhancements have been observed for real and imaginary parts, respectively. • Graphene plasmonic has been used in quantum dots. • Plasmonic edge modes of graphene were excited in emission frequency of quantum dot. • Huge enhancements in optical properties were observed in linear and third-order susceptibilities.

Tiled Monolayer Films of 2D Molybdenum Disulfide Nanoflakes Assembled at Liquid/Liquid Interfaces.

Thin films of MoS2 bilayer nanoflakes, which are predominantly a single flake thick and with flakes in edge-to-edge contact, have been produced via self-assembled tiling at the planar interface between two immiscible liquids. Films of several square centimeters extent can be produced with a total covered area approaching 90% and over 70% of the film covered by single flakes without overlap. Films produced through liquid/liquid assembly are shown to produce a lower uncovered area fraction and more uniform thickness when compared with films of similar areal coverage produced by the “top-down” techniques of spin coating and spray coating. Statistical analysis of flake coverage data, measured by atomic force microscopy (AFM), shows that liquid/liquid assembly produces a distinctly different variation in film thickness than conventional top-down deposition. This supports the hypothesis that the two-dimensional (2D) confinement of liquid/liquid assembly produces more uniform films. Demonstrator field-effect transistors (FETs) manufactured from the films exhibit mobility and on/off current ratios of 0.73 cm2 V–1 s–1 and 105, respectively, comparable to FETs of similar layout and chemical vapor deposition (CVD)-grown or mechanically cleaved single-crystal MoS2 channel material. This work demonstrates the use of liquid/liquid interfaces as a useful tool for the self-assembly of high-performance thin-film devices made from dispersions of 2D materials.

A modified Mori–Tanaka approach incorporating filler-matrix interface failure to model graphene/polymer nanocomposites

A novel modified Mori-Tanaka (M-T) approach to modeling the stress-strain response of graphene nanocomposites subjected to large deformation is described. It is hypothesized that slip at the filler-matrix interface occurs, and this limits the stress which can be transferred to the filler through the interface. Although interfacial slip has a significant influence on the mechanical response of polymer nanocomposites, the M-T method, commonly employed to model nanocomposites, does not accommodate this effect. A pseudo-filler phase is defined to represent the filler when interfacial slip occurs, and this is modeled using finite elements to simulate a single filler flake encapsulated by the interface, interacting with the matrix. The stiffness of the pseudo-filler is determined by examination of the variation of stress in the filler when interfacial slip occurs; this interfacial slip model is incorporated into the M-T method. Quasi-static tensile loading and unloading tests were conducted on polyvinyl alcohol (PVA)-graphene oxide (GO) nanocomposite samples, and significant enhancement of the stress-strain response of the nanocomposite was observed by the addition of GO; e.g. a 5 wt% GO addition increases the initial elastic modulus of PVA from 330 MPa to 1.29 GPa. The modified M-T method was used to simulate the stress-strain response of the nanocomposites, and the theoretical results compare well with experimental stress-strain data for both loading and unloading.

Ambipolar Charge Transport in Two-Dimensional WS2 Metal-Insulator-Semiconductor and Metal-Insulator-Semiconductor Field-Effect Transistors.

Two-dimensional (2D) materials with ambipolar transport characteristics have attracted considerable attention as post-complementary metal-oxide semiconductor (CMOS) materials. These materials allow for electron- or hole-dominant conduction to be achieved in a single channel of the field-effect transistors (FETs) without an extrinsic doping. In this study, all-2D metal-insulator-semiconductor (MIS)-based devices, which were composed of all-2D graphene, hexagonal boron nitride, and WS2, exhibited ambipolar and symmetrical transport characteristics with a low surface state density (Dit,min ≈ 7 × 1011 cm-2·eV-1). Hole- or electron-dominant inversion under the influence of electrostatic doping was obtained in a WS2-based 2D capacitor up to a frequency range of 1 MHz. n- and p-channel conductions with enhancement-mode operations were selectively realized in a single MISFET, which presented a current on/off ratio of >106 and high field-effect mobility (μe = 58-67 cm2/V·s and μh = 19-30 cm2/V·s). Furthermore, a monolithic CMOS-like logic inverter, which employed a single WS2 flake, exhibited a high gain of 78. These results can be used to reduce the footprints of the device architectures and simplify the device fabrication processes of next-generation CMOS integrated circuits.

Grain Boundary Induced Ultralow Threshold Random Laser in a Single GaTe Flake.

Random lasing is a lasing phenomenon realized in random media, and it has attracted a great deal of attention in recent years. An essential requirement for strong random lasing is to achieve strong and recurrent scattering among grain boundaries of a disordered structure. Herein, we report a random laser (RL) based on individual polycrystalline GaTe microflakes (MFs) with a lasing threshold of 4.15 kW cm-2, about 1-2 orders of magnitude lower than that of the reported single GaN microwire random laser. The strongly enhanced light scattering and trapping benefit from the reduced grain size in the polycrystalline GaTe MF, resulting in a ultralow threshold. We also investigate the dependence of spatially localized cavities' dimension on the pumping intensity profile and temperature. The findings provide a feasible route to realize RL with a low threshold and small size, opening up a new avenue in fulfilling many potential optoelectronic applications of RL.

Scalable Manufacturing of Free-Standing, Strong Ti3 C2 Tx MXene Films with Outstanding Conductivity.

Free-standing films that display high strength and high electrical conductivity are critical for flexible electronics, such as electromagnetic interference (EMI) shielding coatings and current collectors for batteries and supercapacitors. 2D Ti3 C2 Tx flakes are ideal candidates for making conductive films due to their high strength and metallic conductivity. It is, however, challenging to transfer those outstanding properties of single MXene flakes to macroscale films as a result of the small flake size and relatively poor flake alignment that occurs during solution-based processing. Here, a scalable method is shown for the fabrication of strong and highly conducting pure MXene films containing highly aligned large MXene flakes. These films demonstrate record tensile strength up to ≈570 MPa for a 940 nm thick film and electrical conductivity of ≈15 100 S cm-1 for a 214 nm thick film, which are both the highest values compared to previously reported pure Ti3 C2 Tx films. These films also exhibit outstanding EMI shielding performance (≈50 dB for a 940 nm thick film) that exceeds other synthetic materials with comparable thickness. MXene films with aligned flakes provide an effective route for producing large-area, high-strength, and high-electrical-conductivity MXene-based films for future electronic applications.

Directional Anisotropy of the Vibrational Modes in 2D-Layered Perovskites.

The vibrational modes in organic/inorganic layered perovskites are of fundamental importance for their optoelectronic properties. The hierarchical architecture of the Ruddlesden–Popper phase of these materials allows for distinct directionality of the vibrational modes with respect to the main axes of the pseudocubic lattice in the octahedral plane. Here, we study the directionality of the fundamental phonon modes in single exfoliated Ruddlesden–Popper perovskite flakes with polarized Raman spectroscopy at ultralow frequencies. A wealth of Raman bands is distinguished in the range from 15 to 150 cm–1 (2–15 meV), whose features depend on the organic cation species, on temperature, and on the direction of the linear polarization of the incident light. By controlling the angle of the linear polarization of the excitation laser with respect to the in-plane axes of the octahedral layer, we gain detailed information on the symmetry of the vibrational modes. The choice of two different organic moieties, phenethylammonium (PEA) and butylammonium (BA), allows us to discern the influence of the linker molecules, evidencing strong anisotropy of the vibrations for the (PEA)2PbBr4 samples. Temperature-dependent Raman measurements reveal that the broad phonon bands observed at room temperature consist of a series of sharp modes and that such mode splitting strongly differs for the different organic moieties and vibrational bands. Softer molecules such as BA result in lower vibrational frequencies and splitting into fewer modes, while more rigid molecules such as PEA lead to higher frequency oscillations and larger number of Raman peaks at low temperature. Interestingly, in distinct bands the number of peaks in the Raman bands is doubled for the rigid PEA compared to the soft BA linkers. Our work shows that the coupling to specific vibrational modes can be controlled by the incident light polarization and choice of the organic moiety, which could be exploited for tailoring exciton–phonon interaction, and for optical switching of the optoelectronic properties of such 2D layered materials.

Nano-manipulation and laser treatment as alternative routes for strain engineering in Graphene

In this work we present two alternative routes for strain engineering in graphene. The first technique involves a laser treatment a low power that was found to create micro-bubbles on single layer graphene flakes. The effect of strain and doping of the graphene along these micro-bubbles is mapped and analysed using Raman Spectroscopy and it is found that a pronounced split in the G-band phonon mode can be attributed to large strain induction. The second technique discussed involves the use of nano-manipulating probes to manoeuvre and deform multilayer graphene flakes. Devices fabricated from such deformed flakes exhibit transport deviating from the expected 2D/quasi-2D electron gas observed in single layer graphene and graphite. We observe signatures of quantum linear magnetoresistance up to large fields in such devices.

Tunable structure and optical properties of single crystal SnS2 flakes

Large-scale hexagonal SnS2 flakes with high quality and tunable structures have been fabricated by an improved chemical vapor deposition method. The flakes follow a spiral growth mechanism and exhibit a two-dimensional structure with a large width-to-height ratio. Raman oscillation induced by laser interference has been observed on the flakes with a pyramid-shaped top. Based on the analysis on oscillatory period and height profile, the index of refraction for the grown SnS2 can be determined. SnS2 flakes with optimized structures have been further designed and prepared successfully, on which enhanced Raman emission can be obtained across the whole flake.

Broadband and high-sensitivity permeability measurements on a single magnetic particle by transformer coupled permeameter

We have developed a system to measure permeability over a wide frequency range with high sensitivity, which we call the transformer coupled permeameter. In this system, the magnetic sample is sandwiched by two short-terminated coplanar waveguides that form a loosely coupled transformer. In this way, the stimulus signal acting as a background becomes smaller as the frequency decreases, thus significantly suppressing the deterioration of the signal-to-noise ratio (SNR) at low frequencies that most conventional permeameters suffer from. It has been demonstrated that this system can provide a sensitivity high enough to measure the permeability of a single ferromagnetic flake particle comprising a noise suppression sheet over 10 MHz–20 GHz with a high SNR.

Multi-level flash memory device based on stacked anisotropic ReS2-boron nitride-graphene heterostructures.

Charge-trapping memory devices based on two-dimensional (2D) material heterostructures possess an atomically thin structure and excellent charge transport capability, making them promising candidates for next-generation flash memories to achieve miniaturized size, high storage capacity, fast switch speed, and low power consumption. Here, we report a nonvolatile floating-gate memory device based on an ReS2/boron nitride/graphene heterostructure. The implemented ReS2 memory device displays a large memory window exceeding 100 V, leading to an ultrahigh current ratio over 108 between programming and erasing states. The ReS2 memory device also exhibits an ultrafast switch speed of 1 μs. In addition, the device can endure hundreds of switching cycles and shows stable retention characteristics with ∼40% charge remaining after 10 years. More importantly, taking advantage of its anisotropic electrical properties, a single ReS2 flake can achieve direction-sensitive multi-level data storage to enhance the data storage density. On the basis of these characteristics, the proposed ReS2 memory device is potentially able to serve the entire memory device hierarchy, meeting the need for scalability, capacity, speed, retention, and endurance at each level.

Spectral Visualization of Near-Infrared Enhancement in 2D Layered WS2

Field enhancement of semiconductors is highly dependent on the wavelength of light. The wavelength dependence is particularly promising for light–matter interaction applications, such as sensors, photocatalysts, and solar cells, as well as to achieve an understanding of plasmon-free field enhancement. However, there are few reports on the field enhancement of two-dimensional (2D) semiconductors and no reports on the wavelength dependence to date. Here, we present the field enhancement of tungsten disulfide (WS2) for a 2D layered semiconductor. The enhancement factor (EF) of a single WS2 flake was evaluated by using confocal fluorescence microspectroscopy. The wavelength dependence was investigated both by experiment and with theoretical calculations for the range from the visible to the near-infrared (NIR) region (400–2000 nm). As a result, the EFs for a single WS2 flake were visualized as maps upon six different excitation wavelengths. The maps were compared with maps of electric fields from finite-difference time-domain calculations, based on the x–y–z position of a single WS2 flake at the same position measured by using atomic force microscopy. The EF in the NIR region showed the largest enhancement, and the maximum EF was demonstrated at a specific number of layers (150 layers) as EF = 100.

Preparation of Hybrid Molybdenum Disulfide/Single Wall Carbon Nanotube–n-Type Silicon Solar Cells

Carbon nanotube/silicon (CNT/Si) heterojunction solar cells represent one new architecture for photovoltaic devices. The addition of MoS2 to the devices is shown to increase the efficiency of the devices. Two structures are explored. In one case, the single wall carbon nanotubes (SWCNTs) and MoS2 flakes are mixed to make a hybrid, which is then used to make a film, while in the other case, a two layer system is used with the MoS2 deposited first followed by the SWCNTs. In all cases, the solar cell efficiency is improved largely due to significant increases in the fill factor. The rise in fill factor is due to the semiconducting nature of the MoS2, which helps with the separation of charge carriers.

Revealing the structure of composite nanodiamond–graphene oxide aqueous dispersions by small-angle scattering

The work presents the results of studying the structure of binary liquid nanocarbon systems obtained by mixing hydrosol of detonation nanodiamond and aqueous dispersions of single layer graphene oxide flakes. We studied size and space distribution of nanocarbon clusters formed upon interaction of the components in aqueous media by mutually complement methods of small-angle X-ray and neutron scattering. The formation of small secondary agglomerates of nanodiamond particles on the surface of graphene oxide flakes was concluded and supported by the data of transmission electron microscopy from dried samples. The observed effect can significantly modify the structure of nanocarbon composites formed of nanodiamond and graphene oxide. The structural features of binary dispersions detonation nanodiamond–graphene oxide should be taken into account at the preparation of the conductive composites of reduced graphene oxide for energy storage systems.

Review—Two-Dimensional Titanium Carbide MXenes and Their Emerging Applications as Electrochemical Sensors

MXenes, new two-dimensional transition metal carbides and nitrides, are attracting considerable attention as multifunctional materials in various research fields due to their high surface area, hydrophilicity, good mechanical properties, metallic-like conduction and adsorption characteristics, combined with the ability to tailor the surface properties with functional groups. In this review, the applications of MXenes as electrochemical sensors are summarised and discussed. The review is focused on Ti3C2Tx as these are the only MXenes that have so far been considered as electrochemical sensors, primarily due to the fact that they have metallic-like conduction and can be easily formed as single and multi-layered flakes through chemical etching of the parent MAX phase. The fabrication of the Ti3C2Tx-based sensors is first described and then followed by an account of their applications as electrochemical sensors, for the detection of small molecules, environmental contaminants and biosensors for biomedical sensing applications. Finally, their overall stability when exposed to aqueous media, which will have implications in the future development of MXene-based sensors, is discussed together with their future prospects as electrochemical-based sensors.

Few-Layer PbI2 Nanoparticle: A 2D Semiconductor with Lateral Quantum Confinement.

Inspired by the superior optoelectronic performances of various 2D semiconductors, their new compositions and structures are being actively pursued in order to foster novel fundamental physics and device applications. As a layered semiconductor with a direct bandgap, few-layer PbI2 should have drawn much research attention due to their capability of emitting photons at short wavelengths of the visible spectrum. Here we chemically synthesize few-layer PbI2 flakes and nanoparticles, which demonstrate unique exciton properties that have rare counterparts in other 2D semiconductors. For three layers and more, the single PbI2 flakes can be utilized to show how the bandgap energy of a 2D semiconductor evolves with the changing layer thickness. The single PbI2 nanoparticles are associated with an ultranarrow photoluminescence line width of ∼1 meV, thus reflecting the influence of lateral quantum confinement on the energy-level structures of a 2D semiconductor. The above findings mark the emergence of a potent 2D platform that is more than complementary to well-studied transition-metal dichalcogenide monolayers.