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The growth of single-layer graphene on copper foil by chemical vapor deposition (CVD) method has been investigated extensively by several groups, however, achieving the same for the bilayer graphene, using a fast and reproducible process, is proven to be difficult and most of the efforts in this direction so far have been on controlling the nucleation phase during active growth regime. In this article we show that by regulating the gases introduced during the cooling phase, uniform and continuous growth of both the single and bilayer graphene can be obtained on copper foils with growth phase duration reduced to 3 min (i.e., 5–60 times faster than previous methods). We demonstrated growth of bilayer graphene on 30 × 30 cm copper foils. We show that the use of vacuum cooling enhanced the growth of single-layer graphene while the introduction of hydrogen gas during the cooling phase promoted the growth of bilayer graphene. We explain observed results elucidating a crucial role of hydrogen leading to a growth of bilayer graphene. The characterization of single and bilayer graphene have been supported by extensive statistical analysis of Raman spectroscopy, selected area electron diffraction measurements as well as fabrication of graphene field effect transistors.

In this article, we report only 10 atomic layer thick, high mobility, transparent thin film transistors (TFTs) with ambipolar device characteristics fabricated on both a conventional silicon platform as well as on a flexible substrate. Monolayer graphene was used as metal electrodes, 3-4 atomic layers of h-BN were used as the gate dielectric, and finally bilayers of WSe2 were used as the semiconducting channel material for the TFTs. The field effect carrier mobility was extracted to be 45 cm(2)/(V s), which exceeds the mobility values of state of the art amorphous silicon based TFTs by ∼100 times. The active device stack of WSe2-hBN-graphene was found to be more than 88% transparent over the entire visible spectrum and the device characteristics were unaltered for in-plane mechanical strain of up to 2%. The device demonstrated remarkable temperature stability over 77-400 K. Low contact resistance value of 1.4 kΩ-μm, subthreshold slope of 90 mv/decade, current ON-OFF ratio of 10(7), and presence of both electron and hole conduction were observed in our all two-dimensional (2D) TFTs, which are extremely desirable but rarely reported characteristics of most of the organic and inorganic TFTs. To the best of our knowledge, this is the first report of all 2D transparent TFT fabricated on flexible substrate along with the highest mobility and current ON-OFF ratio.

Carboxylic functionalization (-COOH groups) of carbon nanotubes is known to improve their dispersion properties and increase the electrical conductivity of carbon-nanotube-polymer nanocomposites. We have studied experimentally the effects of this type of functionalization on the thermal conductivity of the nanocomposites. It was found that while even small quantities of carbon nanotubes (~1 wt %) can increase the electrical conductivity, a larger loading fraction (~3 wt %) is required to enhance the thermal conductivity of nanocomposites. Functionalized multi-wall carbon nanotubes performed the best as filler material leading to a simultaneous improvement of the electrical and thermal properties of the composites. Functionalization of the single-wall carbon nanotubes reduced the thermal conductivity enhancement. The observed trends were explained by the fact that while surface functionalization increases the coupling between carbon nanotube and polymer matrix, it also leads to formation of defects, which impede the acoustic phonon transport in the single-wall carbon nanotubes. The obtained results are important for applications of carbon nanotubes and graphene flakes as fillers for improving thermal, electrical and mechanical properties of composites.

Continuous scaling of Si CMOS devices and circuits, increased speed and integration densities resulted in problems with thermal management of nanoscale device and computer chips [1]. Further progress in information, communication and energy storage technologies requires more efficient heat removal methods and stimulates the search for thermal interface material (TIMs) with enhanced thermal conductivity. The commonly used TIMs are filled with the particles such as silver or silica. The conventional TIMs require high volume fractions of the filler (∼70%) to achieve thermal conductivity of ∼1–5 W/mK. Recently, some of us discovered that graphene has extremely high intrinsic thermal conductivity, which exceeds that of carbon nanotubes [2–4]. To use this property for thermal management of nanoscale electronic devices, we utilized the inexpensive liquid-phase exfoliated graphene and multi-layer graphene (MLG) as filler materials in TIMs (Figure 1). The thermal properties of the obtained graphene-epoxy composites were measured using the “laser flash” technique (Figure 2). It was found that the thermal conductivity enhancement factor exceeded a factor of 23 at 10% of the graphene volume loading fraction [5]. This enhancement is larger than anything that has been achieved using other fillers. We have also tested graphene flakes in the electrically-conductive hybrid graphene-metal particle TIMs. The thermal conductivity of resulting composites was increased by a factor of ∼5 in a temperature range from 300 K to 400 K at a small graphene loading fraction of 5-vol.-% [6]. The unusually strong enhancement of thermal properties was attributed to the high thermal conductivity of graphene, strong graphene coupling to matrix materials and the large range of the length-scale - from nanometers to micrometers - of the graphene and silver particle fillers. Graphene-based TIMs have a number of other advantages related to their viscosity and adhesion, which meet the industry requirements. Our results suggest that graphene can become excellent filler materials in the next generation of TIMs for the electronic, optoelectronic and photovoltaic solar cell applications.

We have fabricated dedicated MEMS tribotesters made from boron doped ultrananocrystalline diamond (B-UNCD) as the structural material, and carried out comprehensive nano-tribological measurements when two B-UNCD sidewall surfaces underwent sliding interaction in a micro-electromechanical systems (MEMS) in a humid and dry atmosphere. We have investigated the evolution of tribological contacts during sliding interactions and corresponding surface modification under repeated cyclic sliding conditions, while measuring displacement and lateral force with 4 nm and 64 nN resolution, respectively. We have observed the formation of carbon-based ultra-thin protective layer at the sliding interface as characterized by Raman spectroscopy and scanning electron microscopy. Interestingly, the formation of this protective layer occurs in both a dry and wet atmosphere, albeit at different rates when the energy dissipated due to friction reaches a plateau, starting from 200 000 and 400 000 cycles, respectively. Once this layer is formed, we do not observe any measurable wear indicating stable operation for an extended time period. Our results demonstrate that B-UNCD is a very promising material to overcome the wear-related reliability problems in MEMS.