Large-area Single-layer Graphene Research Articles

High-order harmonic spectroscopy of polycrystalline graphene

Present mass production of large-area single-layer graphene relies fundamentally on chemical vapor deposition methods. The generation of grain boundaries, which divides the sample into a set of crystalline domains, is inherent to these fabrication methods. Recent studies have demonstrated a strong anisotropy in the ultrafast non-linear response of single-layer graphene when subjected to non-perturbative, intense laser fields below the damage threshold. We propose to exploit this anisotropy to characterize the size distribution of graphene domains in polycrystals via high-order harmonic polarimetry. Our simulation results demonstrate the sensitivity of the harmonic polarization state to details of the polycrystal grain distribution. In particular, we show that the rotation in the polarization tilt of the highest-order harmonics holds information about the grain distribution in the polycrystal. As a proof-of-concept, we propose a method to determine the standard deviation of the grain size distribution from the values of the most frequent grain size and the standard deviation of the harmonic tilt rotation from a set of hypothetical measurements on different polycrystal realizations. Our work reveals the capability of high-order harmonic polarimetry to characterize polycrystalline two-dimensional materials.

Growth of free-standing SnO nanostructures on single layer graphene

The physical properties of nano-sized metal oxide semiconductors are influenced by the nanostructures as well as the crystallinity, thus the growth of crystalline tin oxide (SnO) with controlled morphologies would be important to its applications. To date, the various morphologies of SnO have been successfully synthesized such as nanodisks, nanobelts, nanoplatelets, nanoslabs, nanoflowers, and nanobranches. Here, the vertical free-standing SnO nanostructures were densely grown on single layer graphene (SLG) by vapor transport method. Compared with the tilted SnO nanostructures on SiO2/Si substrate, surface area ratio of the free-standing SnO nanostructures exhibit over of 4.4 times. According to analysis results such as Raman spectroscopy, FE-SEM, XRD, and FE-TEM, uniform and high quality free-standing SnO nanostructures were grown on large area SLG. We observed that the synthetic shapes of SnO nanostructures are different depending on the presence or absence of graphene film on substrate.

Origin of residual particles on transferred graphene grown by CVD

Large-area single-layer graphene can be grown on Cu foil by CVD, but for device applications, the layer must to be transferred onto an insulating substrate. As residual particles are often observed on transferred graphene, we investigated their origin using scanning electron microscopy and energy-dispersive X-ray spectrometry (EDX). The results show that these residual particles are composed either of silicon or an alloy of a few metals, and hence, likely originate from the quartz tube of the CVD furnace and the impurities contained in the Cu foil.

Rivet Graphene.

Large-area graphene has emerged as a promising material for use in flexible and transparent electronics due to its flexibility and optical and electronic properties. The anchoring of transition metal nanoparticles on large-area single-layer graphene is still a challenge. Here, we report an in situ preparation of carbon nano-onion-encapsulated Fe nanoparticles on rebar graphene, which we term rivet graphene. The hybrid film, which allows for polymer-free transfer and is strong enough to float on water with no added supports, exhibits high optical transparency, excellent electric conductivity, and good hole/electron mobility under certain tensile/compressive strains. The results of contact resistance and transfer length indicate that the current in the rivet graphene transistor does not just flow at the contact edge. Carbon nano-onions encapsulating Fe nanoparticles on the surface enhance the injection of charge between rivet graphene and the metal electrode. The anchoring of Fe nanoparticles encapsulated by carbon nano-onions on rebar graphene will provide additional avenues for applications of nanocarbon-based films in transparent and flexible electronics.

In-situ measurement of the heat transport in defect- engineered free-standing single-layer graphene.

Utilizing nanomachining technologies, it is possible to manipulate the heat transport in graphene by introducing different defects. However, due to the difficulty in suspending large-area single-layer graphene (SLG) and limited temperature sensitivity of the present probing methods, the correlation between the defects and thermal conductivity of SLG is still unclear. In this work, we developed a new method for fabricating micro-sized suspended SLG. Subsequently, a focused ion beam (FIB) was used to create nanohole defects in SLG and tune the heat transport. The thermal conductivity of the same SLG before and after FIB radiation was measured using a novel T-type sensor method on site in a dual-beam system. The nanohole defects decreased the thermal conductivity by about 42%. It was found that the smaller width and edge scrolling also had significant restriction on the thermal conductivity of SLG. Based on the calculation results through a lattice dynamics theory, the increase of edge roughness and stronger scattering on long-wavelength acoustic phonons are the main reasons for the reduction in thermal conductivity. This work provides reliable data for understanding the heat transport in a defective SLG membrane, which could help on the future design of graphene-based electrothermal devices.

Large-area single layer exfoliation of graphene oxide via Couette-Taylor flow reactor

L graphene sheets can reduce the contact resistance caused by the number of graphene-graphene contacts and enhance the electrical conductivity. Furthermore, larger graphene sheets can be more effective in load transfer while graphene is used as reinforcement filler in composites. In previous studies, graphite oxide synthesized by the conventional Hummers’ method was exfoliated through sonication process to obtain single layer graphene oxide. However, its lateral size was less than 10μm and additionally, omnidirectional mechanical energy input, such as sonication, reduces the lateral size of graphene oxide sheets. This lateral size reduction of single layer graphene oxide attributes to the further weakening of graphene sheets resulting from the hydroxyl and epoxy sites functionalized edge and plane of graphene sheets. Here, we describe a facile method to prepare large-area single layer graphene oxide, which uses the exfoliation of graphite oxide in a Couette-Taylor flow reactor. We found that the formation of Taylor vortex flow with shearing stress in the reactor is effective for exfoliation of graphite oxide, which allows for the production of a more than 100μm in lateral size single or few-layer graphene oxide platelets at a high yield of 90% within 60 min of reaction time. The properties of fabricated graphene oxide were examined by field emission scanning electron microscope, Raman spectra, atomic force microscope, X-ray photoelectron spectroscope and X-ray diffraction.

Synthesis and applications of large-area single-layer graphene

The progresses in syntheses of large-area single-layer graphene and applications in membrane separation are summarized in this review.

Morphological modulation of graphene-mediated hybridization in plasmonic systems.

We investigated the plasmonic response of a 2-dimensional ordered array of closely spaced (few-nm apart) Au nanoparticles covered by a large-area single-layer graphene sheet. The array consisted of coherently aligned nanoparticle chains, endowed with a characteristic uniaxial anisotropy. The joint effect of such a morphology and of the very small particle size and spacing led to a corresponding uniaxial wrinkling of graphene in the absence of detectable strain. The deposition of graphene redshifted the Au plasmon-resonance, strongly increased the optical absorption of the array and, most importantly, induced a marked optical anisotropy in the plasmonic response, absent in the pristine nanoparticle array. The experimental observations are accounted for by invoking a graphene-mediated resistive coupling between the Au nanoparticles, where the optical anisotropy arises from the wrinkling-induced anisotropic electron mobility in graphene at optical frequencies.

Angle-selective perfect absorption with two-dimensional materials.

Two-dimensional (2D) materials have great potential in photonic and optoelectronic devices. However, the relatively weak light absorption in 2D materials hinders their application in practical devices. Here, we propose a general approach to achieve angle-selective perfect light absorption in 2D materials. As a demonstration of the concept, we experimentally show giant light absorption by placing large-area single-layer graphene on a structure consisting of a chalcogenide layer atop a mirror and achieving a total absorption of 77.6% in the mid-infrared wavelength range (~13 μm), where the graphene contributes a record-high 47.2% absorptivity of mid-infrared light. Construction of such an angle-selective thin optical element is important for solar and thermal energy harvesting, photo-detection and sensing applications. Our study points to a new opportunity to combine 2D materials with photonic structures to enable novel device applications.

Photonic and Plasmonic Guided Modes in Graphene–Silicon Photonic Crystals

We report the results of systematic studies of plasmonic and photonic guided modes in large-area single-layer graphene integrated into a nanostructured silicon substrate. The interaction of light with graphene and substrate photonic crystals can be classified in distinct regimes of plasmonic and photonic modes.

Deducing the apparent flat-band position Vafb and the doping level of large area single layer graphene MOS capacitors

A capacitance–voltage (CV) study on large area CVD single layer graphene MOS capacitors has been carried out. The CV features are carefully examined to reveal the electronic origins of the observed frequency and bias dependence. In this study we investigate the frequency dispersion, propose the definition and extraction of the apparent flat-band Vafb, and finally perform the low temperature CV measurement to deduce the doping level of the graphene MOSCAPs.

Ultrafine Pt nanoparticle induced doping/strain of single layer graphene: experimental corroboration between conduction and Raman characteristics

The doping/strain induced by ultrafine sputtered Pt nanoparticles (NPs) of different sizes on single layer graphene is studied through conduction channel modification of graphene-based field-effect transistors and subsequent Raman characterization. For sub-nm (0.5 nm) sized Pt NPs, a substantial Dirac point shift is observed in the I–V characteristics, suggestive of n-type doping of the large area single layer graphene through the process of charge transfer and chemical interaction. Conversely, for larger (1.1 nm) Pt NPs, minimal Dirac point shift is observed, indicating lack of the charge transfer induced doping effect. The representative Raman signatures corroborate with the electrical characterization results and indicate while charge transfer dominates Raman peak shift for the 0.5 nm Pt NP decorated graphene, strain effect dominates in case of the larger 1.1 nm Pt NP.

Thermally reduced kaolin-graphene oxide nanocomposites for gas sensing.

Highly sensitive graphene-based gas sensors can be made using large-area single layer graphene, but the cost of large-area pure graphene is high, making the simpler reduced graphene oxide (rGO) an attractive alternative. To use rGO for gas sensing, however, require a high active surface area and slightly different approach is needed. Here, we report on a simple method to produce kaolin-graphene oxide (GO) nanocomposites and an application of this nanocomposite as a gas sensor. The nanocomposite was made by binding the GO flakes to kaolin with the help of 3-Aminopropyltriethoxysilane (APTES). The GO flakes in the nanocomposite were contacting neighboring GO flakes as observed by electron microscopy. After thermal annealing, the nanocomposite become conductive as showed by sheet resistance measurements. Based on the conductance changes of the nanocomposite films, electrical gas sensing devices were made for detecting NH3 and HNO3. These devices had a higher sensitivity than thermally annealed multilayer GO films. This kaolin-GO nanocomposite might be useful in applications that require a low-cost material with large conductive surface area including the demonstrated gas sensors.

Metal-assisted exfoliation (MAE): green, roll-to-roll compatible method for transferring graphene to flexible substrates

Graphene is expected to play a significant role in future technologies that span a range from consumer electronics, to devices for the conversion and storage of energy, to conformable biomedical devices for healthcare. To realize these applications, however, a low-cost method of synthesizing large areas of high-quality graphene is required. Currently, the only method to generate large-area single-layer graphene that is compatible with roll-to-roll manufacturing destroys approximately 300 kg of copper foil (thickness = 25 μm) for every 1 g of graphene produced. This paper describes a new environmentally benign and scalable process of transferring graphene to flexible substrates. The process is based on the preferential adhesion of certain thin metallic films to graphene; separation of the graphene from the catalytic copper foil is followed by lamination to a flexible target substrate in a process that is compatible with roll-to-roll manufacturing. The copper substrate is indefinitely reusable and the method is substantially greener than the current process that uses relatively large amounts of corrosive etchants to remove the copper. The sheet resistance of the graphene produced by this new process is unoptimized but should be comparable in principle to that produced by the standard method, given the defects observable by Raman spectroscopy and the presence of process-induced cracks. With further improvements, this green, inexpensive synthesis of single-layer graphene could enable applications in flexible, stretchable, and disposable electronics, low-profile and lightweight barrier materials, and in large-area displays and photovoltaic modules.

Effect of Sub 1-Nm Pt Nanoparticle on the Conduction Properties of Graphene Based Field Effect Transistor

We have developed a low-power tilted-target sputtering process for achieving sub-1 nm metal nanoparticles (NPs) with controlled sizes and number densities and narrow size distributions on graphene. In this report, the unique properties of Pt NPs and their applications in graphene-based field-effect transistor by modifying the conduction channel with sub 1-nm Pt NPs has been studied. By using ultra-small size Pt NPs with quantized energy levels, we influence the electrical properties of large-area single-layer graphene by both electron transfer, carrier scattering and strain, resulting in substantial Dirac point shifting in the I-V characteristics and change in conductivity, respectively, which could be helpful to better understand the metal NP-graphene interaction. We also show that the plasma damage created on the graphene surface from the sputter deposition of metal NPs can be removed by 250 °C annealing in hydrogen.

Enhancing the electrical properties of a flexible transparent graphene-based field-effect transistor using electropolished copper foil for graphene growth.

Flexible transparent graphene-based field-effect transistors (Gr-FETs) were fabricated using large-area single-layer graphene synthesized through low-pressure chemical vapor deposition on a pretreated copper (Cu) foil, followed by transfer of the graphene from the Cu foil to a poly(ethylene terephthalate) (PET) substrate. The electropolishing method was adopted to smooth the surface of the Cu foil, which is a crucial factor because it affects the defect density of graphene films on the PET substrate after transfer and the electronic transport property of the graphene-based devices. The influence of the electropolishing process on the graphene properties was examined using a Raman spectroscope, a scanning electron microscope, and an optical microscope. When the electropolishing process was adopted to improve the graphene quality, the carrier mobility of the flexible transparent Gr-FETs was enhanced from 90 to 340 cm(2)/(V s). Furthermore, variation of the carrier mobility was lower than 10% when the bending radius of the flexible device was decreased from 6.0 to 1.0 cm.

Induction heating-assisted repeated growth and electrochemical transfer of graphene on millimeter-thick metal substrates

Chemical vapor deposition in a hot wall reactor is the most common technique for the production of large area single layer graphene. However, growth in this type of reactors is time consuming and the results are limited by the surface quality of the widely used catalytic metal foils as growth substrates. In this work we demonstrate the use of millimeter-thick Cu and Pt substrates for graphene growth via inductive magnetic heating, which allows for fast temperature ramps during heat up and cooling. Based on a detailed growth study, a two-step growth process resulting in continuous monolayer graphene films of high crystal quality with grain size of larger than 90μm is established. An electrochemical transfer process is used to separate the graphene film from the metallic substrate, yielding excellent results in terms of defect density, doping and residual contamination. Back-gated graphene field-effect transistors fabricated on Si/SiO2 structures exhibit a high reproducibility with a peak mobility higher than 4000cm2/Vs. The combination of the highly time efficient graphene growth and electrochemical transfer together with the reusability of the growth substrates and the possibility of applying novel surface pretreatments pave the way for the use of high quality substrates in industrial applications.

Effect of Sub 2-Nm Pt Nanoparticle on the Conduction Properties of Graphene Based Field Effect Transistor

Graphene-based transistors are attractive platforms due to graphene’s promising electrical properties. Currently, depositing and controlling the size distribution of metal nanoparticles (NPs) on a support surface, such as graphene, without damaging the support is an issue faced in applications involving metal (NPs) in new age electronics. To overcome this barrier, we have developed a low power tilted target sputtering process for achieving sub-2 nm metal (NPs) on graphene with controlled sizes, number densities, and narrow size distributions. In this report, we study the unique properties of platinum (Pt) NPs and their applications in graphene-based FETs by modifying the conduction channel with sub-nm Pt NPs. By using ultra-small size Pt NPs with quantized energy level, we influence the electrical properties of large area single layer graphene through both electron transfer and carrier scattering, resulting in large Dirac point shifting (0.15MV/m for 300nm of SiO2 as blocking oxide) in the I-V characteristics and change in conductivity, respectively; all of which could be helpful to better understand the metal NP-Graphene interaction. We also show that the plasma damage created on the graphene surface from the sputter deposition of metal nanoparticles can be removed by 250 oC annealing in hydrogen.

Single-layer graphene oxide films on a silicon surface

A method is proposed to produce large-area single-layer graphene oxide films on the surface of semiconductor silicon wafers by precipitation from aqueous suspensions. Graphene oxide is synthesized from natural crystalline graphite during chemical oxidation and represents a wide-gap insulator. Single-layer graphene with a homogeneous-fragment size up to 50 μm can be formed by the reduction of graphene oxide films, and this size is significantly larger than those achieved to date.

Dense Arrays of Highly Aligned Graphene Nanoribbons Produced by Substrate‐Controlled Metal‐Assisted Etching of Graphene

Dense arrays of aligned graphene nanoribbons (GNRs) are fabricated by substrate-controlled etching of large-area single-layer graphene. An adequate choice of etching substrate and catalyst deposition method allows densities up to 25 nanoribbons μm(-1) to be obtained with average widths of 19 nm. The efficacy of the method is evidenced by the high on/off ratios of back-gated transistors made with these GNRs, which can go up to 5000.