Large-area Synthesis Research Articles

Characterization of graphene-on-insulator films formed using plasma based surface chemistry

This research explores the surface chemistry of halogen based plasmas on silicon carbide and is aimed at the synthesis of large area graphene-on-insulator films. In these studies, 6H–SiC (0001) substrates were etched using either CF4 and Cl2 based plasmas and then thermally annealed. The resulting surfaces were analyzed using x-ray photoelectron spectroscopy, reflection high energy electron diffraction, atomic force microscopy, and Raman spectroscopy. The analyses showed that the etching process selectively etched silicon to produce carbon rich surface layers on the silicon carbide substrate, and when annealed, these carbon rich layers formed graphene films with halogen- and oxygen-based defects. The thickness of the graphene was controlled by the plasma etch parameters. Two point current–voltage measurements were used to characterize the electrical properties of the films. The current–voltage plots exhibited back-to-back Schottky behavior which suggests that the defects open a band gap in these films. Current-voltage data were used to determine the Schottky barrier height, carrier density, and the upper limit for the film resistivity.

Large-area synthesis of high-quality and uniform monolayer WS2 on reusable Au foils.

Large-area monolayer WS2 is a desirable material for applications in next-generation electronics and optoelectronics. However, the chemical vapour deposition (CVD) with rigid and inert substrates for large-area sample growth suffers from a non-uniform number of layers, small domain size and many defects, and is not compatible with the fabrication process of flexible devices. Here we report the self-limited catalytic surface growth of uniform monolayer WS2 single crystals of millimetre size and large-area films by ambient-pressure CVD on Au. The weak interaction between the WS2 and Au enables the intact transfer of the monolayers to arbitrary substrates using the electrochemical bubbling method without sacrificing Au. The WS2 shows high crystal quality and optical and electrical properties comparable or superior to mechanically exfoliated samples. We also demonstrate the roll-to-roll/bubbling production of large-area flexible films of uniform monolayer, double-layer WS2 and WS2/graphene heterostructures, and batch fabrication of large-area flexible monolayer WS2 film transistor arrays.

Synthesis and Application of Monolayer Semiconductors (June 2015)

Recently, semiconducting monolayers, such as MoS2 and WSe2, have been highlighted for their spin-valley coupling, diverse band structures, bendability, and excellent optoelectronic performances. With a subnanometer thickness of atomic layers, the transition metal dichalcogenides (TMDc) atomic layers demonstrate a significant photoresponse, considerable absorption to incident sunlight and favorable transport performances, leading to applications in the electronic circuit requiring low stand-by power, diverse optoelectronic devices, and next-generation nanoelectronics. Therefore, the class of monolayer TMDc offers a burgeoning field in materials science, fundamental physics, and optoelectronics. A feasible synthetic process to realize controlled synthesis of large area and high quality of TMDc monolayers is in demands. In this review, we will introduce the progress on synthesis and applications of the TMDc atomic layers.

Synthesis of Large-Area Highly Crystalline Monolayer Molybdenum Disulfide with Tunable Grain Size in a H2 Atmosphere.

Large-area and highly crystalline monolayer molybdenum disulfide (MoS2) with a tunable grain size was synthesized in a H2 atmosphere. The influence of introduced H2 on MoS2 growth and grain size, as well as the corresponding mechanism, was tentatively explored by controlling the H2 flow rate. The as-grown monolayer MoS2 displays excellent uniformity and high crystallinity evidenced by Raman and high-resolution transmission electron microscopy. The Raman results also give an indication that the quality of the monolayer MoS2 synthesized in a H2 atmosphere is comparable to that synthesized by using seed or mechanical exfoliation. In addition, the electronic properties and dielectric inhomogeneity of MoS2 monolayers were also detected in situ via scanning microwave microscopy, with measurements on impedance and differential capacitance (dC/dV). Back-gated field-effect transistors based on highly crystalline monolayer MoS2 shows a field-effect mobility of ∼13.07 cm2 V(-1) s(-1) and an Ion/Ioff ratio of ∼1.1×10(7), indicating that the synthesis of large-area and high-quality monolayer MoS2 with H2 is a viable method for electronic and optoelectronic applications.

Large-Area Synthesis of High-Quality Uniform Few-Layer MoTe2

The controlled synthesis of large-area, atomically thin molybdenum ditelluride (MoTe2) crystals is crucial for its various applications based on the attractive properties of this emerging material. In this work, we developed a chemical vapor deposition synthesis to produce large-area, uniform, and highly crystalline few-layer 2H and 1T' MoTe2 films. It was found that these two different phases of MoTe2 can be grown depending on the choice of Mo precursor. Because of the highly crystalline structure, the as-grown few-layer 2H MoTe2 films display electronic properties that are comparable to those of mechanically exfoliated MoTe2 flakes. Our growth method paves the way for the large-scale application of MoTe2 in high-performance nanoelectronics and optoelectronics.

In Situ Synthesis of Porous Carbons by Using Room-Temperature, Atmospheric-Pressure Dielectric Barrier Discharge Plasma as High-Performance Adsorbents for Solid-Phase Microextraction.

A one-step, template-free method is described to synthesize porous carbons (PCs) in situ on a metal surface by using a room-temperature, atmospheric-pressure dielectric barrier discharge (DBD) plasma. This method not only features high efficiency, environmentally friendliness, and low cost and simple equipment, but also can conveniently realize large-area synthesis of PCs by only changing the design of the DBD reactor. The synthesized PCs have a regulated nestlike morphology, and thus, provide a high specific surface area and high pore volume, which result in excellent adsorption properties. Its applicability was demonstrated by using a PC-coated stainless-steel fiber as a solid-phase microextraction (SPME) fiber to preconcentrate polycyclic aromatic hydrocarbons (PAHs) prior to analysis by gas chromatography with flame ionization detection (GC-FID). The results showed that the fiber exhibited excellent enrichment factors (4.1×10(4) to 3.1×10(5)) toward all tested PAHs. Thus, the PC-based SPME-GC-FID provides low limits of detection (2 to 20 ng L(-1)), good precision (<7.8%), and good recoveries (80-115%) for ultra-sensitive determination of PAHs in real water samples. In addition, the PC-coated fiber could be stable enough for more than 500 replicate extraction cycles.

Rapid epitaxy-free graphene synthesis on silicidated polycrystalline platinum.

Large-area synthesis of high-quality graphene by chemical vapour deposition on metallic substrates requires polishing or substrate grain enlargement followed by a lengthy growth period. Here we demonstrate a novel substrate processing method for facile synthesis of mm-sized, single-crystal graphene by coating polycrystalline platinum foils with a silicon-containing film. The film reacts with platinum on heating, resulting in the formation of a liquid platinum silicide layer that screens the platinum lattice and fills topographic defects. This reduces the dependence on the surface properties of the catalytic substrate, improving the crystallinity, uniformity and size of graphene domains. At elevated temperatures growth rates of more than an order of magnitude higher (120 μm min−1) than typically reported are achieved, allowing savings in costs for consumable materials, energy and time. This generic technique paves the way for using a whole new range of eutectic substrates for the large-area synthesis of 2D materials.

Bridging the Gap - Rediscovering Black Phosphorus As an Anisotropic Layered Material for Electronics and Optoelectronics

In this talk, I will discuss our recent work in introducing black phosphorus (BP) to the layered-material family as a novel anisotropic 2D material for electronic and optoelectronic applications [1] [2]. Narrow gap BP thin film (0.3 eV in bulk) serendipitously fill the energy space between zero-gap graphene and large-gap TMDCs, making it an promising material for near and mid-infrared optoelectronics [3]. BP thin films show high mobility above 650 cm2/V.s at room temperature along the light effective mass (x) direction, implying its promising potential for high frequency, thin-film electronics. Furthermore, its anisotropic nature within the plane of the layers may allow for the realization of conceptually new electronic and photonic devices impossible in other 2D materials. In the talk, I will also present our work in demonstrating 20 GHz black phosphorus radio-frequency transistors [4]. Our recent progress in characterizing the optical properties of monolayer BP [5] and in achieving large-area synthesis of black phosphorus thin film will also be discussed. I will conclude with remarks on promising future directions of black phosphorus research and how this new material is expected to benefit the next-generation electronics and photonics technologies.

Synthesis of Transition Metal Dichalcogenide WSe2thin Films By Atomic Layer Deposition

Two-dimensional layered transition metal dichalcogenides, such as MoS2, WSe2 and MoSe2 have received a lot of attention recently due to their promising unique thermal, mechanical, electrical, optical properties. Tungsten Diselenide (WSe2), one of the transition metal dichalcogenides of interest for 2-D electronic systems, exhibits not only good thermal stability and high melting point, but also excellent optical and electrical properties. WSe2 is a semiconductor with a bandgap which could be used for the fabrication of solar cells and LEDs of various colors [1, 2]. Many techniques have been developed for synthesis of WSe2 films on different substrates in recent years, such as sold state reaction, elctrodepositon, RF-magnetron sputtering, electrodepostion, pulsed laser depostion, chemical vapor depositon. However, there are very few references that report atomic layer deposition (ALD) results of WSe2 films. This technique exhibits self-limiting surface reactions, which means accidental overdosing of precursors does not result in increased film deposition on the substrate surface. This self-terminating character is able to accurately control thickness and composition of ultra thin films and achieves uniformity, conformality and even monolayers of transition metal dichalcogenides. Here we report on a large-area synthesis of WSe2 films on native oxide covered and hydroxyl terminated Si substrates by using the Savannah 100 ALD system from Ultratech/Cambridge Nanotech. Tungsten hexacarbonyl (W(CO)6) and (trimethylsilyl) selenide ((Me3Si)2Se) are employed as the chemical ALD precursors for Tungsten and selenium, respectively. Generally 20 sccm N2 was used as a carrier gas for the precursors. The growth temperature was set at 275, 300 and 325oC. The crystal structure and chemical composition of WSe2 films were identified by using X-ray diffraction (XRD) and energy dispersive X-ray spectroscopy (EDS). The film morphology was characterized by field emission scanning electron microscopy (FESEM) and atomic force microscopy (AFM). The optical property was analyzed by Raman spectroscopy and photoluminescence (PL) spectroscopy. The strong Raman scattering of in-plane E1 2g and out-of-plane A1g vibration modes, and the dominant PL of direct transition from Kc to Kv and indirect transition from Λc to Γv with absence of Kc to Γv are typical optical spectra of layered WSe2. The reduction of out-of-plane vibration mode and the absence of PL from indirect transition provide the evidence of fewer layer WSe2.

Synthesis and structure of two-dimensional transition-metal dichalcogenides

Abstract

Ultraclean and large-area monolayer hexagonal boron nitride on Cu foil using chemical vapor deposition

Atomically thin hexagonal boron nitride (h-BN) has been demonstrated to be an excellent dielectric layer as well as an ideal van der Waals epitaxial substrate for fabrication of two-dimensional (2D) atomic layers and their vertical heterostructures. Although many groups have obtained large-scale monolayer h-BN through low pressure chemical vapor deposition (LPCVD), it is still a challenge to grow clean monolayers without the reduction of domain size. Here we report the synthesis of large-area (4 × 2 cm2) high quality monolayer h-BN with an ultraclean and unbroken surface on copper foil by using LPCVD. A detailed investigation of the key factors affecting growth and transfer of the monolayer was carried out in order to eliminate the adverse effects of impurity particles. Furthermore, an optimized transfer approach allowed the nondestructive and clean transfer of the monolayer from copper foil onto an arbitrary substrate, including a flexible substrate, under mild conditions. Atomic force microscopy indicated that the root-mean-square (RMS) roughness of the monolayer h-BN on SiO2 was less than 0.269 nm for areas with fewer wrinkles. Selective area electron diffraction analysis of the h-BN revealed a pattern of hexagonal diffraction spots, which unambiguously demonstrated its highly crystalline character. Our work paves the way toward the use of ultraclean and large-area monolayer h-BN as the dielectric layer in the fabrication of high performance electronic and optoelectronic devices for novel 2D atomic layer materials.

Monitoring defects on monolayer graphene using nematic liquid crystals.

Defects in graphene governs electrical and optical properties. Although grain boundaries in graphene inevitably formed during large area synthesis process, which act as scattering centers for charge carriers to degrade mobility, have been studied extensively, point defects have been rarely investigated mainly due to the absence of facile observation tools. Here, we report polarized optical microscopy to observe defect distributions in monolayer graphene. This was realized by aligning liquid crystal s (LC) on graphene where the defect population was modulated by irradiating ultraviolet (UV) light directly on graphene surface under moisture condition. Aromatic rings in LC molecules are oriented with hexagonal rings in graphene to have preferred orientation, providing a way to identify relative orientations of graphene domains and point defects. Our studies show that point defects generated by prolonged UV irradiation time give rise to irregular LC alignment with disclination lines on the graphene surface and a large-size LC domain associated with graphene single domain eventually disappeared. This indicates that defects associated with oxygen-containing functional groups cause to reduce the strong stacking interaction between graphene and LC molecules.

(Invited) Synthesis, Characterization, and Applications of Single- and Double-Layer Graphene Grown on Epitaxial Metal Films

To develop graphene’s excellent and unique properties for future electronic devices, the synthesis of large area, high-quality graphene sheets with a controlled number of layers and well-defined structure is important. Here, we present our work to grow high-quality graphene with controlled orientation. Our method is based on heteroepitaxial CVD using highly crystalline metal catalyst deposited on single crystalline substrates, such as sapphire and MgO [1-6]. We investigated the CVD growth of single-layer graphene on Cu(111) and Cu(100) and found that the orientation of the graphene is strongly influenced by the underlying Cu lattice [2-4]. Large hexagonal graphene domains with a lateral size of 50-100 μm are aligned on the Cu(111) surface [5]. On the other hand, the orientation of graphene on Cu(100) has two main domain orientations, reflecting the symmetry mismatch between graphene and Cu(100) lattice. The carrier transport across the neighboring domains merged with the same orientation was investigated. We found a higher carrier mobility for devices inside one domain (mean value is 7,200 cm2/Vs at 280 K) than those with a domain interface in the channel (2,000 cm2/Vs), suggesting that the domain interface acts as a scattering site even when they merged with the same orientation [6]. Moreover, we have realized the selective growth of double-layer graphene on the heteroepitaxial metal films, which is expected to be applicable to semiconductor devices. The growth mechanism and the stacking order of the double-layer graphene will be discussed. Finally, our recent progress on the synthesis of two-dimensional heterostructures, such as graphene-MoS2 and graphene-NbS2hybrid structures, will be also presented [7,8]. [1] H. Ago et al., ACS Nano, 4, 7407 (2010). [2] B. Hu et al., Carbon 50, 57 (2012). [3] C. M. Orofeo et al., Carbon, 50, 2189 (2012). [4] Y. Ogawa et al., J. Phys. Chem. Lett., 3, 219 (2012). [5] H. Ago et al., Appl. Phys. Exp., 6, 75101 (2013). [6] Y. Ogawa et al., Nanoscale, 6, 7288 (2014). [7] W. Ge et al., Nanoscale, 5, 5773 (2013). [8] H. Ago et al., submitted.

(Industrial Electrochemistry & Electrochemical Engineering Division H. H. Dow Memorial Student Achievement Award) An Investigation of the Growth Mechanism of Coal Derived Graphene Films

Graphene is a unique material with remarkable properties suitable for a wide-array of applications.1, 2 Chemical vapor deposition (CVD) is demonstrated to be a simple technique for large area and high quality synthesis of graphene films.3 Traditionally, hydrocarbon gases are used for the graphene synthesis.4 Unconventional solid carbon sources, such as various polymers5, and amorphous carbon thin film6 have shown great potential for synthesis of graphene films. Coal is one such carbon enriched and abundantly available unconventional source. We have successfully utilized coal as carbon source to synthesize large area, transparent and high quality amorphous carbon films and graphene films7, 8. Utilizing coal to produce high quality graphene films makes it an inexpensive process. The process of producing coal-derived graphene films incorporates vacuum-CVD technique. Coal pyrolysis produces the required hydrocarbon gases that are precursors for formation of carbon species. These carbon species nucleate, and grow on copper foil to form graphene grains. The graphene grains merge together to form graphene films. These coal-derived graphene films have also been utilized as electrodes for various applications.9, 10 A detailed study of the growth mechanism of graphene films using coal as the carbon source has been investigated by using various spectroscopy, crystallography and chromatography techniques, which will be presented at the conference. Raman spectra of coal-derived carbon films are shown in the figure. Wide D (~1350cm-1) and G (~1595cm-1) bands are observed for carbon films synthesized at low temperatures demonstrating the amorphous nature. As the synthesis temperature is increased the bands get narrower and an additional peak 2D (~2700cm-1) is observed at 1055ºC synthesis. This peak is boundary phonon mode in second order Raman spectra associated with graphene films, thus demonstrating the formation of graphene at 1055ºC. Electro-oxidation of coal at intermediate temperatures to produce hydrogen has been utilized as an alternative to high temperature coal gasification11. The coal after the electrolysis is designated as waste (coal-char) from the coal electrolysis process. This coal-char is rich in organo-metalic compounds12. Utilizing the coal-char for producing high-value product such as graphene could further be economically favorable for scale up12. An overview of this process will also be discussed in the conference.

Towards Wafer-Scale Monocrystalline Graphene Growth and Characterization.

Since its discovery in 2004, graphene has boosted numerous fundamental sciences and technological applications due to its massless Dirac particle-like linear band dispersion, that causes unprecedented physical properties. Among the various methods for synthesizing graphene, chemical vapor deposition is the most suitable approach for scalable production on a wafer scale, which is a critical step for practical applications. Graphene grain boundaries (GGBs), consisting of nonhexagonal carbon rings and therefore modulating the properties of graphene films, are inevitably formed via the merging of adjacent graphene domains with different orientations. Large-area monocrystalline graphene synthesis without forming GGBs has been challenging, let alone observing such boundaries. Here, an up-to-date review is presented of how to grow wafer-scale monocrystalline graphene without GGBs. One approach is to make single domain sizes as large as possible by reducing or passivating the number of nucleation sites. Another approach is to align graphene domains in identical orientations, and then merge them atomically. The recently developed methods for observing graphene orientation and GGBs both at the atomic and macro-scales are also presented. Finally, perspectives for future research in graphene growth are discussed.

Large area synthesis of a nanoporous two-dimensional polymer at the air/water interface.

We present the synthesis of a two-dimensional polymer at the air/water interface and its nm-resolution imaging. Trigonal star, amphiphilic monomers bearing three anthraceno groups on a central triptycene core are confined at the air/water interface. Compression followed by photopolymerization on the interface provides the two-dimensional polymer. Analysis by scanning tunneling microscopy suggests that the polymer is periodic with ultrahigh pore density.

Aerosol-assisted chemical vapour deposition synthesis of multi-wall carbon nanotubes: III. Towards upscaling

We report the optimisation of the experimental variables for the synthesis of large-area (90cm2), mm-thick carpets of vertically aligned multi-wall carbon nanotubes (MWCNTs) using the aerosol-assisted chemical vapour deposition technique. The effect of the reactor size, temperature, time and the flow rate of the carrier gas on the catalytic conversion rate of the precursor, composition of the atmosphere of the reactor and various properties of MWCNTs such as the mass, length, outer diameter, defect density, oxidation resistance and residual catalyst concentration was investigated. By increasing the volume of the reactor from around 0.2 to 8.5L, the mass of synthesised MWCNTs and the efficiency of the precursor were increased fivefold (up to 14g/h.) and twice (up to 88%), respectively. Free-standing and on-substrate carpets of MWCNTs could withstand normal handling without falling apart, making them suitable for characterisations and applications from nano to macro scales. The synthesis recipe also improved the production rate of nitrogen-containing MWCNTs by an order of magnitude.

Highly scalable, atomically thin WSe2 grown via metal-organic chemical vapor deposition.

Tungsten diselenide (WSe2) is a two-dimensional material that is of interest for next-generation electronic and optoelectronic devices due to its direct bandgap of 1.65 eV in the monolayer form and excellent transport properties. However, technologies based on this 2D material cannot be realized without a scalable synthesis process. Here, we demonstrate the first scalable synthesis of large-area, mono and few-layer WSe2 via metal-organic chemical vapor deposition using tungsten hexacarbonyl (W(CO)6) and dimethylselenium ((CH3)2Se). In addition to being intrinsically scalable, this technique allows for the precise control of the vapor-phase chemistry, which is unobtainable using more traditional oxide vaporization routes. We show that temperature, pressure, Se:W ratio, and substrate choice have a strong impact on the ensuing atomic layer structure, with optimized conditions yielding >8 μm size domains. Raman spectroscopy, atomic force microscopy (AFM), and cross-sectional transmission electron microscopy (TEM) confirm crystalline monoto-multilayer WSe2 is achievable. Finally, TEM and vertical current/voltage transport provide evidence that a pristine van der Waals gap exists in WSe2/graphene heterostructures.

Large-area synthesis of WSe2 from WO3 by selenium–oxygen ion exchange

Few-layer tungsten diselenide (WSe2) is attractive as a next-generation electronic material as it exhibits modest carrier mobilities and energy band gap in the visible spectra, making it appealing for photovoltaic and low-powered electronic applications. Here we demonstrate the scalable synthesis of large-area, few-layer WSe2 via replacement of oxygen in hexagonally stabilized tungsten oxide films using dimethyl selenium. Cross-sectional transmission electron microscopy reveals successful control of the final WSe2 film thickness through control of initial tungsten oxide thickness, as well as development of layered films with grain sizes up to several hundred nanometers. Raman spectroscopy and atomic force microscopy confirms high crystal uniformity of the converted WSe2, and time domain thermo-reflectance provide evidence that near record low thermal conductivity is achievable in ultra-thin WSe2 using this method.

A review of large-area bilayer graphene synthesis by chemical vapor deposition.

Bilayer graphene has attracted considerable attention due to its potential as a tunable band gap in AB-stacked bilayers. Recently, great advancements have been made in the synthesis of chemical-vapor-deposited bilayer graphene. This featured article provides a detailed and up-to-date review of the synthesis of bilayer graphene by chemical vapor deposition (CVD). We will discuss various approaches to synthesize bilayer graphene and the corresponding growth dynamics. Methods for identifying the growth mechanism of bilayer graphene on Cu enclosures are highlighted for a deeper understanding of better control over uniformity and thickness.