High Electrical Mobility Research Articles

Two-Dimensional Carbon: A Review of Synthesis Methods, and Electronic, Optical, and Vibrational Properties of Single-Layer Graphene

Graphite has been widely used by humans for a large part of their history. Nevertheless, it has only recently been possible to isolate its basic unit: carbon atoms arranged in a honeycomb structure on a single plane, namely graphene. Since its discovery, many techniques have been developed and improved to properly synthesize graphene and its derivatives which are part of the novel class of two-dimensional materials. These advanced materials have imposed themselves in nanotechnology thanks to some outstanding physical properties due to their reduced dimensions. In the case of graphene, its reduced dimension gives rise to a high electrical mobility, a large thermal conductivity, a high mechanical resistance, and a large optical transparency. Therefore, such aspect is of great scientific interest for both basic and applied research, ranging from theoretical physics to surface chemistry and applied solid state physics. The connection between all these fields is guaranteed by spectroscopy and especially by Raman spectroscopy which provides a lot of information about structural and electronic features of graphene. In this review, the authors present a systematized collection of the most important physical insights on the fundamental electronic and vibrational properties of graphene, their connection with basic optical and Raman spectroscopy, and a brief overview of main synthesis methods.

Influence of seeding promoters on the properties of CVD grown monolayer molybdenum disulfide

Chemical vapor deposition (CVD) is the most efficient method to grow large-area two dimensional (2D) transition metal dichiacogenides (TMDCs) in high quality. Monolayer molybdenum disulfide (MoS2) and seed-assistant are the mostly selected 2D TMDC and growth strategy for such CVD processes, respectively. Though the advantages of seed catalysts in facilitating the nucleation, achieving higher yield and better repeatability, as well as their effects on the morphologies of as-grown MoS2 have been studied, the influence of seeding promoters on both optical and electrical properties of as-grown monolayer MoS2 is not known comprehensively, which is indeed critical for understanding fundamental physics and developing practical application of such emerging 2D semiconductors. In this report, we systematically investigated the effect of different seeding promoters on the properties of CVD-grown monolayer MoS2. It is found that different seed molecules lead to different impacts on the optical and electrical properties of as-grown monolayer MoS2. Among three different seed catalysts (perylene-3,4,9,10-tetracarboxylic acid tetrapotassium salt (PTAS), copper phthalocyanine (CuPc), and crystal violet (CV)), PTAS performs better in obtaining large area monolayer MoS2 with good optical quality and high electrical mobility than the other two. Our work gives a guide for modifying the properties of as-grown monolayer MoS2 and other 2D transition metal dichalcogenides in seeding promoters-assisted synthesis process.

Fabrication of high quality and large area graphite thin films by pyrolysis and graphitization of polyimides

We have developed large-area, high-quality graphite thin films that are 0.5–3 μm thick. To prepare the graphite films, we used pyrolysis and graphitization of polyimide up to temperatures in the range 2900 °C-3300 °C, which is applicable to industrial-production processing. Given that the graphite layers of the films are highly oriented in the surface plane, they have good physical properties. The resulting electrical conductivity of graphite film of thickness 1.4 μm and area 10 × 10 cm2 that was treated at 3200 °C is 24,800 S/cm, and the electrical conductivity showed metallic temperature dependence. The carrier mobility of the film is 11,700 V/cm2, and the carrier concentrations of electrons (Ne) and holes (Nh) are 6.98 × 1018 cm−3 and 5.90 × 1018 cm−3, respectively. We consider the high electrical conductivity and mobility to have been derived from the homogeneous, highly oriented graphite layers. We found that graphitization of films less than 3 μm thick progresses uniformly throughout the film, and the resulting graphite films have structures with uniform, highly oriented layers. The produced graphite thin films have excellent physical properties and can be easily handled, so we believe that many industrial applications are possible.

Methodology of Thermoelectric Power Factor Enhancement by Controlling Nanowire Interface.

The simultaneous realization of low thermal conductivity and high thermoelectric power factor in materials has long been the goal for the social use of high-performance thermoelectric modules. Nanostructuring approaches have drawn considerable attention because of the success in reducing thermal conductivity. On the contrary, enhancement of the thermoelectric power factor, namely, the simultaneous increase of the Seebeck coefficient and electrical conductivity, has been difficult. We propose a method for the power factor enhancement by introducing coherent homoepitaxial interfaces with controlled dopant concentration, which enables the quasiballistic transmission of high-energy carriers. The wavenumber of the high-energy carriers is nearly conserved through the interfaces, resulting in simultaneous realization of a high Seebeck coefficient and relatively high electrical mobility. Here, we experimentally demonstrate the dopant-controlled epitaxial interface effect for the thermoelectric power factor enhancement using our "embedded-ZnO nanowire structure" having high-quality nanowire interfaces. This presents the methodology for substantial power factor enhancement by interface carrier scattering.

High Electrical Conductivity 2D MXene Serves as Additive of Perovskite for Efficient Solar Cells.

MXenes, a newly intriguing family of 2D materials, have recently attracted considerable attention owing to their excellent properties such as high electrical conductivity and mobility, tunable structure, and termination groups. Here, the Ti3 C2 Tx MXene is incorporated into the perovskite absorber layer for the first time, which aims for efficiency enhancement. Results show that the termination groups of Ti3 C2 Tx can retard the crystallization rate, thereby increasing the crystal size of CH3 NH3 PbI3 . It is found that the high electrical conductivity and mobility of MXene can accelerate the charge transfer. After optimizing the key parameters, 12% enhancement in device performance is achieved by 0.03 wt% amount of MXene additive. This work unlocks opportunities for the use of MXene as potential materials in perovskite solar cell applications.

High electrical conductivity and carrier mobility in oCVD PEDOT thin films by engineered crystallization and acid treatment.

Air-stable, lightweight, and electrically conductive polymers are highly desired as the electrodes for next-generation electronic devices. However, the low electrical conductivity and low carrier mobility of polymers are the key bottlenecks that limit their adoption. We demonstrate that the key to addressing these limitations is to molecularly engineer the crystallization and morphology of polymers. We use oxidative chemical vapor deposition (oCVD) and hydrobromic acid treatment as an effective tool to achieve such engineering for conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT). We demonstrate PEDOT thin films with a record-high electrical conductivity of 6259 S/cm and a remarkably high carrier mobility of 18.45 cm2 V-1 s-1 by inducing a crystallite-configuration transition using oCVD. Subsequent theoretical modeling reveals a metallic nature and an effective reduction of the carrier transport energy barrier between crystallized domains in these thin films. To validate this metallic nature, we successfully fabricate PEDOT-Si Schottky diode arrays operating at 13.56 MHz for radio frequency identification (RFID) readers, demonstrating wafer-scale fabrication compatible with conventional complementary metal-oxide semiconductor (CMOS) technology. The oCVD PEDOT thin films with ultrahigh electrical conductivity and high carrier mobility show great promise for novel high-speed organic electronics with low energy consumption and better charge carrier transport.

Graphene a promising electrode material for supercapacitors-A review

The global demand for high performance and environmentally friendly energy storage systems leads to intensive research on new and advanced electrode materials that are able to satisfy the fast-growing global market in various applications. The 2D graphene material is one of the most promising candidates for next-generation energy storage applications, particularly supercapacitor devices due to its exceptional intrinsic properties such as highest theoretical specific surface area (2600 m2/g), high electrical charges mobility (230 000 cm2/V·s), thermal conductivity (3000 W/mK), and highest strength (130 GPa). This comprehensive review summarizes the most recent progress made on the graphene material in its different structural forms of foams (3D), thin films (2D), nano-fibers (1D), and nano-dotes (0D) for supercapacitor electrodes. It initiates with a brief historical introduction on graphene discovery and its current production techniques that retain its intrinsic properties ranging from mechanical exfoliation of graphene in high quality to its epitaxial growth by chemical vapor deposition on metal substrates and its derivation by chemical reduction of graphene oxide. In addition to highlighting its main characterization techniques such as Raman spectroscopy, atomic force microscopy, and transmission electron microscopy, as well as, its critical properties including electrical, optical, mechanical, and thermal properties. Its potential applications are also illustrated with emphasizing on its usage as an electrode material in supercapacitors. Finally, its main challenges and future prospects are considerably pointed out.

Investigation of DC-RF and breakdown behaviour in Lg = 20 nm novel asymmetric GaAs MHEMTs for future submillimetre wave applications

In this work, we systematically investigated the DC-RF and breakdown voltage characteristics of Lg = 20 nm novel asymmetric GaAs metamorphic high electron mobility transistor (MHEMT) using Sentaurus-TCAD tool. The highlights of the novel asymmetric GaAs MHEMT are the cavity in the asymmetric Γ-gate region, n+-type In0.52Al0.48As/In0.53Ga0.47As/In0.75Ga0.25As cap layers and n+-type In0.52Ga0.48As vertical source/drain (S/D) regions. The influence of asymmetric gate recess width on DC-RF and ON and OFF-state breakdown voltage characteristics of the novel asymmetric GaAs MHEMT has been systematically investigated using hydrodynamic (HD) charge transport model. The physical models such as impact ionization model, high electric field electron mobility model, Shockley-Read-Hall and recombination models and density gradient model are also included during room temperature Sentaurus-TCAD simulation in order to accurately obtain the DC-RF and breakdown performance parameters. The 20 nm gate length novel asymmetric GaAs MHEMT obtained a maximum transconductance (gm_max) and drain current IDS_max of 3350 mS/mm and 1230 mA/mm respectively. The transit frequiencies fT and fmax obtained by the proposed 20 nm gate length GaAs MHEMT are 643 GHz and 1230 GHz respectively. The 20 nm gate length novel asymmetric GaAs MHEMT also exhibited an ON-state (BVON) and OFF-state (BVOFF) breakdown voltages of 3.1 V and 7.2 V respectively. To the best knowledge of authors, this is the record combination of DC–RF and breakdown performance parameter values obtained for GaAs MHEMT which makes them highly suitable for next generation high power high speed applications.

Naphthalene diimide self-assembled ribbons with high electrical conductivity and mobility without doping

The thermally activated electrical conductivity and charge transfer properties of self-assembled naphthalene diimide molecule have been studied with N,N’di(hexadecyl)naphthalene tetracarboxylic diimide (HD-NDI) derivative. The self-assembly of HD-NDI has been resulted in one-dimensional large micron-size ribbons. The aggregate formation and self-assembling property have been extensively studied by absorption, fluorescence and X-ray diffraction analysis. Absorption and fluorescence measurements were taken in variety of solvents in fresh and aged samples, and the study suggested J-type aggregation for self-assembly formation. The electrical conductivity of self-assembled HD-NDI material was measured as a function of temperature where the conductivity increased with temperature and the highest conductivity (1 × 10−5 S/cm) was obtained at 250 °C. SEM images clearly show the formation of ribbons of micron size. Further the electron transport property of HD-NDI was also evaluated by SCLC method at room temperature by fabricating electron-only devices with annealing the film at ~ 200 °C. HD-NDI shows excellent electron mobility (1.8 × 10−3 cm2 V−1 s−1) because of effective channel formation due to self-assembly of HD-NDI molecules, and this material may find potential application in variety of electronic devices.

A Kinetic Pathway toward High-Density Ordered N Doping of Epitaxial Graphene on Cu(111) Using C5NCl5 Precursors.

Pristine graphene possesses high electrical mobility, but its low charge carrier density severely limits its technological significance. Past efforts to increase graphene's carrier density via chemical doping have shown limited successes, accompanied by substantial reductions in the mobility caused by disordered dopants. Here, based on first-principles calculations, we propose to grow graphene on Cu(111) via self-assembly of C5NCl5 molecular precursors to achieve high-density (1/6) and highly ordered nitrogen doping. Such a process relies on the elegant concerted roles played by the London dispersion, chemical, and screened Coulomb repulsive forces in enhancing molecular adsorption, facilitating easy dechlorination, and dictating the overall orientation of the C5N radicals, respectively. Further growth from the orientationally correlated graphene islands is accompanied by significantly minimized density of grain boundaries as the grains coalesce to form larger N-doped graphene sheets, which are further shown to possess superb electronic properties for future device applications. Initial kinetic processes involved in N-doped graphene growth using C5NH5 precursors are also investigated and contrasted with that of C5NCl5.

A Ternary Alloy Substrate to Synthesize Monolayer Graphene with Liquid Carbon Precursor.

Here we demonstrate a ternary Cu2NiZn alloy substrate for controllably synthesizing monolayer graphene using a liquid carbon precursor cyclohexane via a facile CVD route. In contrast with elemental metal or bimetal substrates, the alloy-induced synergistic effects that provide an ideal metallic platform for much easier dehydrogenation of hydrocarbon molecules, more reasonable strength of adsorption energy of carbon monomer on surface and lower formation energies of carbon chains, largely renders the success growth of monolayer graphene with higher electrical mobility and lower defects. The growth mechanism is systemically investigated by our DFT calculations. This study provides a selective route for realizing high-quality graphene monolayer via a scalable synthetic method by using economic liquid carbon supplies and multialloy metal substrates.

Electrons on the surface of 2D materials: from layered electrides to 2D electrenes

We review layered and ultrathin electrides with exciting properties like high electrical mobility, high carrier concentrations, and low work functions.

Interface electronic structure and the Schottky barrier at Al-diamond interface: hybrid density functional theory HSE06 investigation

Diamond is regarded as one of the most promising semiconductor materials used for high power devices because of its superior physical and electrical properties, such as wide bandgap, high breakdown electric field, high mobility, and high thermal conductivity. Highpower diamond devices are now receiving much attention. In particular, Schottky diode based on a metal/diamond junction has promising applications, and high breakdown voltage has been achieved, though unfortunately its forward resistance is high. In this paper, the first principles calculations are performed to study the electronic structure of interface and the Schottky barrier height of Al-diamond interface. The projection of the density of states on the atomic orbitals of the interface atoms reveals that the typical Al-induced gap states are associated with a smooth density of states in the bulk diamond band gap region, and these gap states are found to be localized within three atom layers. At the same time, electronic charge transfer makes the Fermi level upgrade on the side of diamond. Besides, the typical Al-induced gap state model gives a simple picture about what determines Schottky barrier height at Al-diamond interface, by assuming an ideal, defect-free and laterally homogeneous Schottky interface in which the only interaction comes from the decay of the electron wave function from the metal into the semiconductor, which in turn induces electronic charges to be rearranged in the region close to the interface. As for the electronic charge transfer, this potential shift can be extracted by subtracting the superimposed planar or macroscopically averaged electrostatic potentials of the Al and diamond surfaces (at frozen atomic positions), from the planar or macroscopically averaged potential of the relaxed Al-diamond interface. The electronic charge transfer suggests that the formation of an interface should be associated with the formation of new chemical bonds and substantial rearrangements of the electron charge density. Especially, we obtain the Schottky barrier height of 1.03 by the first principle, which is in good agreement with the results from phenomenological model and experiment. The research results in this paper can provide a theoretical basis for the research of the metal diamond Schottky junction diode, and can also give a theoretical reference for the research of the metal-semiconductor highpower device based on diamond material.

High-temperature carrier density and mobility enhancements in AlGaN/GaN HEMT using AlN spacer layer

In this paper, we experimentally studied the effect of AlN spacer layer on optical and electrical properties of AlGaN/GaN high electric mobility transistors (HEMTs) grown by metal organic chemical vapor deposition method. For AlGaN layer in HEMT structure, the Al composition of the sample was determined using x-ray diffraction and photoluminescence. Electrolyte electro-reflectance (EER) measurement not only confirmed the aluminum composition of AlGaN layer, but also determined the electric field strength on the AlGaN layer through the Franz-Keldysh oscillation phenomenon. This result indicated that the electric field on the AlGaN layer could be improved from 430 to 621kV/cm when AlN spacer layer was inserted in HEMT structure, which increased the concentration of two dimensional electron gas (2DEG) and improve the mobility. The temperature dependent Hall results show that both the mobility and the carrier concentration of 2DEG would decrease abruptly causing HEMT loss of function due to phonon scattering and carrier thermal escape when temperature increases above a specific value. Meanwhile, our study also demonstrates using AlN spacer layer could be beneficial to allow the mobility and carrier density of 2DEG sustaining at high temperature region.

Enhanced photocatalytic property of BiFeO3/N-doped graphene composites and mechanism insight

A series of BiFeO3/(N-doped) graphene composites are prepared by a facile hydrothermal method. BiFeO3/N-doped graphene shows photocatalytic performance superior to that of BiFeO3/graphene and pristine BiFeO3. The enhanced photo-degradation performance of BiFeO3/N-doped graphene are mainly attributable to the improved light absorbance of the composite, abundant active adsorption sites and high electrical charge mobility of N-doped graphene, and the downward band bending of BiFeO3 at the composite interface. In particular, X-ray photoelectron spectroscopy analyses reveal that the electron energy band of BiFeO3 is downward bent by 1.0eV at the interface of BiFeO3/N-doped graphene, because of different work functions of both materials. This downward band bending facilitates the transfer of photogenerated electrons from BiFeO3 to N-doped graphene and prompts the separation of photo-generated electron-hole pairs, leading eventually to the enhanced photocatalytic performance.

Breakthrough performance for 2-D material

A new design has enabled researchers to make the best-performing transistor yet from two-dimensional molybdenum disulfide. They say the results show the material has the right stuff to compete with silicon in the future—though many technological challenges still stand in its way (Nano Lett. 2016, DOI: 10.1021/acs.nanolett.6b02713). Atomically thin 2-D materials such as MoS2 and graphene have unique electronic properties that make them promising as potential replacements for silicon. But researchers have to show that the performance of devices made from ultrathin MoS2 can approach or surpass that of silicon. MoS2 has high electrical mobility and is a semiconductor with a large band gap, which means it can be made into transistors that switch between conducting and insulating states, providing the 1s and 0s of the digital logic that’s at the heart of computing. But so far, MoS2’s promise has not been realized. “It’s clear what should be possible, but

Direct observation of the layer-dependent electronic structure in phosphorene

Phosphorene, a single atomic layer of black phosphorus, has recently emerged as a new two-dimensional (2D) material that holds promise for electronic and photonic technologies. Here we experimentally demonstrate that the electronic structure of few-layer phosphorene varies significantly with the number of layers, in good agreement with theoretical predictions. The interband optical transitions cover a wide, technologically important spectral range from the visible to the mid-infrared. In addition, we observe strong photoluminescence in few-layer phosphorene at energies that closely match the absorption edge, indicating that they are direct bandgap semiconductors. The strongly layer-dependent electronic structure of phosphorene, in combination with its high electrical mobility, gives it distinct advantages over other 2D materials in electronic and opto-electronic applications.

High-Rate Homoepitaxial Growth of Heavily Boron-Doped Diamond (100) Films By Microwave Plasma-Enhanced Chemical Vapor Deposition Using High Microwave Power Density

Diamond is expected as a next-generation semiconductor material for high-power devices because of its high breakdown electric field, mobility, and thermal conductivity. For the power devices with high current operation, vertical structures are effective because the large contact area is easily obtained by trench structures in the same way as silicon and silicon carbide power devices. The low resistivity diamond single-crystal substrates are required to fabricate the diamond vertical power devices and are realized by heavy boron doping. The highest growth rate of heavily boron-doped ([B]film > 1020 atoms/cm3) diamond films obtained by microwave plasma-enhanced chemical vapor deposition (MPECVD) is 6 μm/h [1]. The higher growth rate is required for manufacturing cost reduction of the low resistivity diamond substrates. However, the higher the boron concentration is, the lower the growth rate of the diamond films becomes [1]. It is known that there are mainly two methods for increasing the growth rate of the diamond films. One is a nitrogen addition to the plasma. However, incorporated nitrogen into diamond acts as a donor, which is compensator for boron-doped diamond [2]. The other is high density of the microwave power input into the plasma. Therefore, we tried to change the shape of a diamond substrate holder for MPECVD to improve the growth rate by increasing the power density near the surface of substrate. Figure 1 shows the plasma states of (a) the conventional substrate holder and (b) the improved substrate holder. The improved substrate holder has a shallower bottom than the conventional one. In the case of the improved substrate holder, the top of the diamond substrate is higher than that of the holder, as shown in Fig.1(b). As a result, the diamond growth rate and incorporation of boron atoms into the diamond film are enhanced at the same time because the electric field is concentrated at the top surface of the diamond substrate and the radical density on the diamond surface increases. High-pressure, high-temperature (HPHT) synthetic Ib(100) single-crystal diamonds were used as substrates for the homoepitaxial diamond growth by MPECVD with a spherical resonator structure (Arios Inc. in Japan). Before the CVD growth, the diamond substrates were treated with HF diluted with H2O for 15 min. Next, the substrates were heated in 2:1 mixture of H2O2 and H2SO4 at 400°C for 15 min. The growth was carried out using CH4 and B(CH3)3 diluted with H2. The microwave power, the total pressure and the methane concentration were maintained at 900 W, 50 kPa and 2.4 %, respectively. The boron/carbon (B/C) ratios in the gas phase were fixed at 14,000 ppm for the conventional holder and 20,000 ppm for the improved substrate holder, respectively. The growth of the heavily boron-doped diamond films was conducted for two hours in the above conditions. After the growth, the surface morphologies of the heavily boron-doped diamond films were observed by optical microscopy (OM). The resistivities of the films were measured by a van der Pauw contact configuration and the growth rates were estimated by the micrometer. Figure 2 shows the OM images of typical surface morphologies of the heavily boron-doped diamond films grown on diamond (100) substrates by (a) the conventional substrate holder and (b) the improved substrate holder. Although the diamond film grown by the conventional substrate holder had some non-epitaxial crystallites with density of 1.6 × 104 cm-2, which degrade the diamond device performances, the diamond film grown by the improved substrate holder had no non-epitaxial crystallites. The resistivities of the films were 3.3 × 10-2 Ωcm for the improved substrate holder and 2.3 × 10-2 Ωcm for the conventional substrate holder, respectively. The boron concentrations in these films were 1~3 × 1020 cm-3, which is estimated from the growth condition. The growth rates were 29.5 μm/h for the improved substrate holder and 13.0 μm/h for the conventional substrate holder, respectively. The growth rate of 29.5 μm/h is the highest growth rate for heavily boron-doped ([B]film> 1020 cm-3) diamond films. Acknowledgements This work was partially supported by Adaptable and Seamless Technology Transfer Program through Target-driven R&D, JST, and Kanazawa University SAKIGAKE Project.

Design and application of carbon nanomaterials for photoactive and charge transport layers in organic solar cells.

Commercialization of organic solar cell (OSC) has faltered due to their low power conversion efficiency (PCE) compared to inorganic solar cell. Low electrical conductivity, low charge mobility, and short-range light absorption of most organic materials limit the PCE of OSCs. Carbon nanomaterials, especially carbon nanotubes (CNTs) and graphenes, are of great interest for use in OSC applications due to their high electrical conductivity, mobility, and unique optical properties for enhancing the performance of OSCs. In this review, recent progress toward the integration of carbon nanomaterials into OSCs is described. The role of carbon nanomaterials and strategies for their integration into various layers of OSCs, including the photoactive layer and charge transport layer, are discussed. Based on these, we also discuss the prospects of carbon nanomaterials for specific OSC layers to maximize the PCE.

Electrophoretic deposition of aramid nanofibers on carbon fibers for highly enhanced interfacial adhesion at low content

Here, an anodic electrophoretic deposition was adopted to facilitate the large-scale uniform coating of nano-fillers onto carbon fibers to enhance the interfacial properties between carbon fibers and epoxy matrix. As interface–reinforcing materials, aramid nanofibers were introduced because of their superior mechanical properties and epoxy matrix-friendly functional groups. Furthermore, aramid nanofibers can be readily coated on carbon fibers via electrophoretic deposition because they are negatively-charged in solution with high electrical mobility. Finally, aramid nanofiber-coated carbon fibers showed significantly improved interfacial properties such as higher surface free energy and interfacial shear strengths (39.7% and 34.9% increases, respectively) than those of a pristine carbon fiber despite a very small amount of embedding (0.025wt% of aramid nanofibers in a carbon fiber), and the short beam strength of the laminated composite prepared with the aramid nanofiber-coated carbon fibers was also improved by 17.0% compared to a non-modified composite.