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The properties of van der Waals (vdW) materials often vary dramatically with the atomic stacking order between layers, but this order can be difficult to control. Trilayer graphene (TLG) stacks in either a semimetallic ABA or a semiconducting ABC configuration with a gate-tunable band gap, but the latter has only been produced by exfoliation. Here we present a chemical vapor deposition approach to TLG growth that yields greatly enhanced fraction and size of ABC domains. The key insight is that substrate curvature can stabilize ABC domains. Controllable ABC yields ~59% were achieved by tailoring substrate curvature levels. ABC fractions remained high after transfer to device substrates, as confirmed by transport measurements revealing the expected tunable ABC band gap. Substrate topography engineering provides a path to large-scale synthesis of epitaxial ABC-TLG and other vdW materials., We thank Prof. Paul Heiney for useful discussions and Mr. Sheng Wang for help in creating Fig. 4a. This work was supported by the NSF EFRI 2-DARE 1542879. The Synchrotron SOLEIL is supported by the Centre National de la Recherche Scientifique (CNRS) and the Commissariat `a l’Energie Atomique et aux Energies Alternatives (CEA), France. J.B. and D.J.S. thank Jian Han for useful discussions and were supported as part of the Center for the Computational Design of Functional Layered Materials, an Energy Frontier Research Center funded by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES) under Award DE-SC0012575. Part of J.B.ʼs contribution was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. W.P. and M.D. acknowledge support from the NSF EFRI 2-DARE 1542707 and NSF EAGER 1838456. Z.L. appreciates support by the NSFC-RGC Joint Research Scheme (N_HKUST607/17) and the Guangzhou Science and Technology Project (201704030134). J.G. thanks the German Research Council (DFG) for support via grants GE2827/1-1 and GE2827/2-1. J.M.K. thanks the support from NSF MRSEC DMR-1720530. The infrared near-field nanoscopy measurements is supported by the Director, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division of the U.S. Department of Energy under Contract No. DE-AC02-05-CH11231 (Sub-wavelength Metamaterial program). A.M.R. acknowledges support of the National Science Foundation, under Grant DMR-1719353.

Ultralow friction can be achieved with 2D materials, particularly graphene and MoS2. The nanotribological properties of these different 2D materials have been measured in previous atomic force microscope (AFM) experiments sequentially, precluding immediate and direct comparison of their frictional behavior. Here, friction is characterized at the nanoscale using AFM experiments with the same tip sliding over graphene, MoS2, and a graphene/MoS2 heterostructure in a single measurement, repeated hundreds of times, and also measured with a slowly varying normal force. The same material systems are simulated using molecular dynamics (MD) and analyzed using density functional theory (DFT) calculations. In both experiments and MD simulations, graphene consistently exhibits lower friction than the MoS2 monolayer and the heterostructure. In some cases, friction on the heterostructure is lower than that on the MoS2 monolayer. Quasi-static MD simulations and DFT calculations show that the origin of the friction contrast is the difference in energy barriers for a tip sliding across each of the three surfaces.

We developed a high-yield synthesis of highly crystalline bilayer graphene (BLG) with two preferential stacking modes using a Ni-Cu gradient alloy growth substrate. Previously reported approaches for BLG growth include flat growth substrates of Cu or Ni-Cu uniform alloys and "copper pocket" structures. Use of flat substrates has the advantage of being scalable, but the growth mechanism is either "surface limited" (for Cu) or carbon precipitation (for uniform Ni-Cu), which results in multicrystalline BLG grains. For copper pockets, growth proceeds through a carbon back-diffusion mechanism, which leads to the formation of highly crystalline BLG, but scaling of the copper pocket structure is expected to be difficult. Here we demonstrate a Ni-Cu gradient alloy that combines the advantages of these earlier methods: the substrate is flat, so easy to scale, while growth proceeds by a carbon back-diffusion mechanism leading to high-yield growth of BLG with high crystallinity. The BLG layer stacking was almost exclusively Bernal or twisted with an angle of 30°, consistent with first-principles calculations we conducted. Furthermore, we demonstrated scalable production of transistor arrays based crystalline Bernal-stacked BLG with a band gap that was tunable at room temperature.

Visualization of the chemical structures of two-dimensional (2D) materials and their interfaces at the virtually atomic scale is an imperative step toward devising highly efficient ultrathin optoelectronic devices. Herein, we demonstrate a universal method featuring time-of-flight secondary ion mass spectrometry (ToF-SIMS), coupled with the structure simplicity of 2D materials, as a versatile tool to reveal the vertical atomic layers of various two-dimensional (2D) materials including graphene, hexagonal boron nitride (h-BN), and transition metal dichalcogenides (TMDs). We demonstrated that the vertical atomic layers of those 2D materials can be unveiled layer-by-layer using a strategy of ToF-SIMS three-dimensional (3D) analysis developed in this work. Moreover, we found that the extreme surface sensitivity and chemical specificity of ToF-SIMS also enables the examination of the lateral uniformity of 2D materials. During this process, we first removed interference of adsorbed organic contamination by anne...

Heterostructures of transition metal dichalcogenides (TMDs) offer the attractive prospect of combining distinct physical properties derived from different TMD structures. Here, we report direct chemical vapor deposition of in-plane monolayer heterostructures based on 1H-MoS2 and 1T′-MoTe2. The large lattice mismatch between these materials led to intriguing phenomena at their interface. Atomic force microscopy indicated buckling in the 1H region. Tip-enhanced Raman spectroscopy showed mode structure consistent with Te substitution in the 1H region during 1T′-MoTe2 growth. This was confirmed by atomic resolution transmission electron microscopy, which also revealed an atomically stitched, dislocation-free 1H/1T′ interface. Theoretical modeling revealed that both the buckling and absence of interfacial misfit dislocations were explained by lateral gradients in Te substitution levels within the 1H region and elastic coupling between 1H and 1T′ domains. Phase field simulations predicted 1T′ morphologies with ...

Bilayer two-dimensional (2D) van der Waals (vdW) materials are attracting increasing attention due to their predicted high quality electronic and optical properties. Here we demonstrate dense, selective growth of WSe(2) bilayer flakes by chemical vapor deposition with the use of a 1:10 molar mixture of sodium cholate and sodium chloride as the growth promoter to control the local diffusion of W-containing species. A large fraction of the bilayer WSe(2) flakes showed a 0 (AB) and 60° (AA’) twist between the two layers, while moiré 15 and 30°-twist angles were also observed. Well-defined monolayer-bilayer junctions were formed in the as-grown bilayer WSe(2) flakes, and these interfaces exhibited p-n diode rectification and an ambipolar transport characteristic. This work provides an efficient method for the layer-controlled growth of 2D materials, in particular, 2D transition metal dichalcogenides and promotes their applications in next-generation electronic and optoelectronic devices.

The adsorption of chemical species on the surface of graphene alters the concentration of charge carries by either increasing or decreasing it depending on the nature of the adsorbed chemical species and inducing noticeable changes in the material's electronic properties. This remarkable feature enables graphene-based sensors to detect a wide range of biomolecules, chemicals, and gas/vapors. A lot of progress has been made in this field and technologies based on reduced graphene oxide flakes have been well reviewed. Graphene grown by chemical vapor deposition has proven to be the most prominent route to large-scale production of devices. However, until now, a comprehensive review on scalable, macro-sized, chemical vapor-deposited, graphene-based field-effect transistor sensors was missing. Here we review the present status, current challenges, and future prospects for the bio/chemical sensing applications of scalable, chemical vapor-deposited, graphene field-effect transistors. We explain and examine the fabrication techniques, sensing mechanisms, and various assay strategies. We also discuss the sensing performance of graphene based on quantum mechanics simulations and theoretical calculations.

In the past few years, considerable progress has been made on the controlled synthesis of bilayer van der Waals (vdW) materials such as graphene and transition metal dichalcogenides (TMDs), which are of interest due to their attractive optical and electronic properties. A variety of methods have been developed to synthesize bilayer vdW materials. This review is devoted to recent advances in the properties and synthesis of bilayer graphene (BLG) and TMDs. We emphasize the intriguing properties of BLG and TMDs, depending on their composition, stacking configurations, and twisting angles. The large-scale chemical vapor deposition (CVD) growth of BLG and TMDs with large domain size, high quality, and strong interlayer coupling is highlighted. We then expand the discussion to the current understanding of the growth mechanisms of BLG by CVD and synthetic methods of bilayer TMDs. Finally, the crucial challenges of BLG synthesis and our perspective of the future of bilayer TMDs will be laid out for potential applications of vdW materials.

A simple technique of doping graphene by manipulating adsorbed impurity charges is presented. Using a field effect transistor configuration, controlled polarization of a ferroelectric polymer gate is used to compensate and neutralize charges of one type. The uncompensated charges of the opposite type then dope graphene. Both n- and p-type doping are possible by this method, which is non-destructive and reversible. We observe a change in n-type dopant concentration of 8 × 1012 cm−2 and a change in electron mobility of 650%. The electron and hole mobilities are inversely proportional to the impurity concentration, as predicted by theory. Selective doping of graphene can be achieved using this method by patterning gate electrodes at strategic locations and programming them independently. Such charge control without introducing hard junctions, therefore, permits seamless integration of multiple devices on a continuous graphene film.A simple technique of doping graphene by manipulating adsorbed impurity charges is presented. Using a field effect transistor configuration, controlled polarization of a ferroelectric polymer gate is used to compensate and neutralize charges of one type. The uncompensated charges of the opposite type then dope graphene. Both n- and p-type doping are possible by this method, which is non-destructive and reversible. We observe a change in n-type dopant concentration of 8 × 1012 cm−2 and a change in electron mobility of 650%. The electron and hole mobilities are inversely proportional to the impurity concentration, as predicted by theory. Selective doping of graphene can be achieved using this method by patterning gate electrodes at strategic locations and programming them independently. Such charge control without introducing hard junctions, therefore, permits seamless integration of multiple devices on a continuous graphene film.

We present a novel method for fabricating a high-density carbon nanotube microelectrode array (MEA) chip. Vertically aligned carbon nanotubes (VACNTs) were synthesized by microwave plasma-enhanced chemical vapor deposition and thermal chemical vapor deposition. The device was characterized using electrochemical experiments such as cyclic voltammetry, impedance spectroscopy and potential transient measurements. Through-silicon vias (TSVs) were fabricated and partially filled with polycrystalline silicon to allow electrical connection from the high-density electrodes to a stimulator microchip. In response to the demand for higher resolution implants, we have developed a unique process to obtain a high-density electrode array by making the microelectrodes smaller in size and designing new ways of routing the electrodes to current sources. Keywords: Microelectrode array, Neural implant, Carbon nanotubes, Through-silicon via interconnects, Microfabrication

Atomic diffusion-induced catalyst evolution affects the iron catalyst lifetime and the collective termination of VACNT growth on a metal substrate. We demonstrated that controlled catalyst diffusion in the pretreatment stage is capable of extending the iron catalyst lifetime on an aluminum thin-film substrate to allow enhanced vertically aligned carbon nanotube (VACNT) growth by a conventional low-pressure thermal chemical vapor deposition. In adopting a fast-heating pretreatment to control the catalyst loss from diffusion, catalyst lifetime was extended, leading to the VACNT growth with heights up to 65% taller than those without the diffusion control strategy. Atomic diffusion of iron catalyst along the grain boundaries of aluminum thin films was found to play a critical role in the catalyst loss mechanism. This work provides the basic understanding of catalyst diffusion control to facilitate the growth of VACNT arrays on metallic substrates for various applications such as through silicon via interconn...

We have uncovered some unusual thermal interface properties of a three-dimensional, flexible and interconnected graphene foam (GF). The thermal interfacial resistance of GF at Si–Al interface is as low as 0.04 cm2K W−1, which is one order of magnitude lower than conventional thermal grease and thermal paste-based thermal interfacial material (TIM). The thermal contact resistance was found to dominate the overall interfacial resistance of GF-based TIM, in as much as the bulk thermal conductivity of GF is rather high. The contact pressure-dependent thermal interfacial resistance of GF exhibits an asymptotic behavior, which converges into a plateau value at an ultralow contact pressure (∼0.1 MPa). Significantly, the GF-based TIM has shown a superior performance to vertically aligned carbon nanotubes currently held as the gold standard (at least ∼75% improvement in thermal interfacial resistance at Si–Al interface), thus providing a strong candidate for the next generation of high-performance carbon-based TIM.

Graphene of different layer numbers was fabricated using thermal chemical vapor deposition (TCVD), and it was demonstrated as a heat spreader in electronic packaging. Platinum thermal evaluation chips were used to evaluate the thermal performance of the graphene heat spreaders. The temperature of a hot spot driven at a heat flux of up to 430 W cm(-2) was decreased from 121 degrees C to 108 degrees C (Delta T approximate to 13 degrees C) with the insertion of the monolayer graphene heat spreader, compared with the multilayer (n = 6-10) ones' temperature drop of similar to 8 degrees C. Various parameters affecting the thermal performance of graphene heat spreaders were discussed, e.g. layer numbers of graphene, phonon scattering, thermal boundary resistance. We demonstrate the potentials of using a complementary metal oxide semiconductor compatible TCVD process to utilize graphene as a heat spreader for heat dissipation purposes.

We have developed a scalable fabrication process for the production of DNA biosensors based on gold nanoparticle-decorated graphene field effect transistors (AuNP-Gr-FETs), where monodisperse AuNPs are created through physical vapor deposition followed by thermal annealing. The FETs are created in a four-probe configuration, using an optimized bilayer photolithography process that yields chemically clean devices, as confirmed by XPS and AFM, with high carrier mobility (3590 ± 710 cm2/V·s) and low unintended doping (Dirac voltages of 9.4 ± 2.7 V). The AuNP-Gr-FETs were readily functionalized with thiolated probe DNA to yield DNA biosensors with a detection limit of 1 nM and high specificity against noncomplementary DNA. Our work provides a pathway toward the scalable fabrication of high-performance AuNP-Gr-FET devices for label-free nucleic acid testing in a realistic clinical setting.

Large-area growth of monolayer films of the transition metal dichalcogenides is of the utmost importance in this rapidly advancing research area. The mechanical exfoliation method offers high quality monolayer material but it is a problematic approach when applied to materials that are not air stable. One important example is 1T'-WTe2, which in multilayer form is reported to possess a large non saturating magnetoresistance, pressure induced superconductivity, and a weak antilocalization effect, but electrical data for the monolayer is yet to be reported due to its rapid degradation in air. Here we report a reliable and reproducible large-area growth process for obtaining many monolayer 1T'-WTe2 flakes. We confirmed the composition and structure of monolayer 1T'-WTe2 flakes using x-ray photoelectron spectroscopy, energy-dispersive x-ray spectroscopy, atomic force microscopy, Raman spectroscopy and aberration corrected transmission electron microscopy. We studied the time dependent degradation of monolayer 1T'-WTe2 under ambient conditions, and we used first-principles calculations to identify reaction with oxygen as the degradation mechanism. Finally we investigated the electrical properties of monolayer 1T'-WTe2 and found metallic conduction at low temperature along with a weak antilocalization effect that is evidence for strong spin-orbit coupling.

Monolayer graphene was fabricated using thermal chemical vapor deposition (TCVD) as heat spreaders in electronic packaging. Platinum (Pt) thermal evaluation chips were utilized to evaluate the thermal performance of the graphene heat spreaders. Temperature of hot spot driven at a heat flux of up to 430W·cm-2 was decreased by about 13 °C with the attaching of the graphene heat spreader. We demonstrate the potentials of using CMOS compatible TCVD process to make graphene as heat spreader for power dissipation needs.

A fast-heating scheme is utilized for long VACTN-TIM fabrication directly on metal substrate. VACNT arrays with lengths of more than 100 μm were achieved on Al thin film by a conventional thermal CVD at a low temperature of 600 °C. Our methodology allows patterned VACNT array fabrication using a standard lift-off process for various applications, such as thermal interface materials, super capacitor field emission devices and electronic interconnects.

Biosensors based on graphene field effect transistors (GFETs) decorated with antibody-functionalized platinum nanoparticles (PtNPs) are developed for the quantitative detection of breast cancer biomarker HER3. High-quality chemical vapor deposited graphene is prepared and transferred over gold electrodes microfabricated on an SiO2/Si wafer to yield an array of 52 GFET devices. The GFETs are modified with PtNPs to obtain a hybrid nanostructure suitable for attachment of HER3-specific, genetically engineered thiol-containing single-chain variable fragment antibodies (scFv) to realize a biosensor for HER3. Physical and electrical characterization of Bio-GFET devices is carried out by electron microscopy, atomic force microscopy, Raman spectroscopy, and current–gate voltage measurements. A concentration-dependent response of the biosensor to HER3 antigen is found in the range 300 fg mL−1 to 300 ng mL−1 and is in quantitative agreement with a model based on the Hill–Langmuir equation of equilibrium thermodynamics. Based on the dose–response data, the dissociation constant is estimated to be 800 pg mL−1, indicating that the high affinity of the scFv antibody is maintained after immobilization. The limit of detection is 300 fg mL−1, showing the potential for PtNP/G-FETs to be used in label-free biological sensors.

Carbon nanotube (CNT) growth termination mechanism has been studied using Molecular Dynamic (MD) simulation. Due to catalyst diffusion and interaction between the catalyst and the CNT growth substrates, the catalytic activity needed for CNT nucleation is depleted. Diffusion coefficient of the Fe catalyst on different substrates has been evaluated using averaged mean square displacement (MSD) of the Fe atoms in the simulation. At the CNT growth temperature, Fe on Cu substrate shows the highest diffusion coefficient. The calculated low diffusion coefficient of Fe atoms in Al 2 O 3 substrate confirms the use of Al 2 O 3 as the choice material for the diffusion buffer layer. This is consistent with observations in CNT growth experiments using thermal chemical vapor deposition (CVD). The MD simulation provides a clear demonstration of Al 2 O 3 as an effective diffusion buffer material.

With more and more high power high brightness LEDs adopted in general lighting applications, the state-of-the-art packages cannot meet the requirement of thermal management. Over-heating of an LED will cause premature failure because the efficiency, the spectrum, the reliability and the life of solid state lighting devices strongly depend on successful thermal management. Therefore, the requirement for improving the performance and the reliability poses a significant challenge for novel cooling solutions including applications of heat sinks with active cooling methods and high thermal performance materials. Conventional mechanical fans are not preferred in LED devices due to its high energy consumption, short life and high noise. In this study, cooling fixtures with piezoelectric units are designed to be integrated with high power LED modules. The cooling effect of high performance TIM and piezoelectric units are investigated using numerical modeling method.

An application using vertically aligned carbon nanotube (VACNT) as thermal interface material (TIM) in high-brightness LEDs is demonstrated. VACNT arrays have been synthesized on the top and the bottom surfaces of the aluminum heat slugs and integrated into the LED modules using the designed packaging process. The thermal resistance of the LEDs is characterized using the thermal transient test system (MicRed, T3Ster). Compared with silver epoxy, VACNT-TIM shows a comparable thermal performance. It is found that a proper packaging process plays an important role in the final thermal performance of the LEDs with VACNT-TIM.

With the increasing application of high power LEDs in general lighting, more effective cooling solutions should be considered to maintain a better performance and reliability with lower LED junction temperature. In this paper, it is discussed firstly in detail how to effectively take advantage of high thermal performance materials, such as CNTs, to improve the heat conduction in LED packages. Secondly, the air flow velocity field generated by piezoelectric fans is simulated using 3D fluid structure interaction method (FSI) and verified with experimental data. A cooler with a piezoelectric fan inside is designed as a preliminary study on how to apply piezoelectric fans in LED active cooling.

High quality vertically aligned carbon nanotube (VACNT) arrays have been synthesized on bulk Al alloy (Al6063) substrates with an Fe catalyst using low pressure chemical vapor deposition (LPCVD). The pretreatment process of the catalyst was shown to play a critical role, and it was studied comprehensively and optimized to grow high quality VACNT arrays within a wide range thicknesses of catalyst layer (2nm – 11 nm) and acetylene (C 2 H 2 ) flow rates (100sccm – 300sccm). The thermal performance of the resulting VACNT arrays is evaluated. The minimum thermal interfacial resistance of the Al-CNT-Si interfaces achieved so far is only 4 mm2·K/W, and the average value is 14.6 mm2·K/W.

Herein, we reported on the study of percolation dynamics in thermoset-based die attach (DA) materials and its effect on percolation conductivity. Two types of percolation mechanism in thermoset based DA were discovered, i.e., the curing reaction-induced percolation and the physical aging-induced percolation. The former features in a fast percolation network growth rate, which is one order of magnitude higher than the latter. It is demonstrated that the percolation kinetics largely affects the apparent percolation conductivity under the traditional packaging conditions; and reaction-induced percolation allows ultrahigh efficiency in reaching the volume fraction-limiting percolation conductance, resulting in enhanced thermal performance of DA.

High quality vertically aligned carbon nanotube (VACNT) arrays have been synthesized on bulk Al alloy (Al6063) substrates with an electron-beam (E-beam) evaporated Fe catalyst using low pressure chemical vapor deposition (LPCVD). The pretreatment process of the catalyst was shown to play a critical role. This was studied comprehensively and optimized to repeatedly grow high quality VACNT arrays within a wide range of thicknesses of catalyst layer (2-11 nm) and acetylene (C(2)H(2)) flow rates (100-300 sccm). The thermal performance of the resulting VACNT arrays was evaluated. The minimum interfacial thermal resistance of the Si/VACNT/Al interfaces achieved so far is only 4 mm(2) K W(-1), and the average value is 14.6 mm(2) K W(-1).