Browsing by Author "Vajtai, R."

Now showing 1 - 4 of 4
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    Effects of etchants in the transfer of chemical vapor deposited graphene
    (AIP, 2018) Wang, M.; Yang, E.H.; Vajtai, R.; Kono, J.; Ajayan, P.M.
    The quality of graphene can be strongly modified during the transfer process following chemical vapor deposition (CVD) growth. Here, we transferred CVD-grown graphene from a copper foil to a SiO2/Si substrate using wet etching with four different etchants: HNO3, FeCl3, (NH4)2S2O8, and a commercial copper etchant. We then compared the quality of graphene after the transfer process in terms of surface modifications, pollutions (residues and contaminations), and electrical properties (mobility and density). Our tests and analyses showed that the commercial copper etchant provides the best structural integrity, the least amount of residues, and the smallest doping carrier concentration.
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    Negative Differential Conductance & Hot-Carrier Avalanching in Monolayer WS2 FETs
    (Springer Nature, 2017) He, G.; Nathawat, J.; Kwan, C.-P.; Ramamoorthy, H.; Somphonsane, R.; Zhao, M.; Ghosh, K.; Singisetti, U.; Perea-López, N.; Zhou, C.; Elías, A.L.; Terrones, M.; Gong, Y.; Zhang, X.; Vajtai, R.; Ajayan, P.M.; Ferry, D.K.; Bird, J.P.
    The high field phenomena of inter-valley transfer and avalanching breakdown have long been exploited in devices based on conventional semiconductors. In this Article, we demonstrate the manifestation of these effects in atomically-thin WS2 field-effect transistors. The negative differential conductance exhibits all of the features familiar from discussions of this phenomenon in bulk semiconductors, including hysteresis in the transistor characteristics and increased noise that is indicative of travelling high-field domains. It is also found to be sensitive to thermal annealing, a result that we attribute to the influence of strain on the energy separation of the different valleys involved in hot-electron transfer. This idea is supported by the results of ensemble Monte Carlo simulations, which highlight the sensitivity of the negative differential conductance to the equilibrium populations of the different valleys. At high drain currents (>10 μA/μm) avalanching breakdown is also observed, and is attributed to trap-assisted inverse Auger scattering. This mechanism is not normally relevant in conventional semiconductors, but is possible in WS2 due to the narrow width of its energy bands. The various results presented here suggest that WS2 exhibits strong potential for use in hot-electron devices, including compact high-frequency sources and photonic detectors.
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    On-chip integrated vertically aligned carbon nanotube based super- and pseudocapacitors
    (Springer Nature, 2017) Pitkänen, O.; Järvinen, T.; Cheng, H.; Lorite, G.S.; Dombovari, A.; Rieppo, L.; Talapatra, S.; Duong, H.M.; Tóth, G.; Juhász, K.L.; Kónya, Z.; Kukovecz, A.; Ajayan, P.M.; Vajtai, R.; Kordás, K.
    On-chip energy storage and management will have transformative impacts in developing advanced electronic platforms with built-in energy needs for operation of integrated circuits driving a microprocessor. Though success in growing stand-alone energy storage elements such as electrochemical capacitors (super and pseusocapacitors) on a variety of substrates is a promising step towards this direction. In this work, on-chip energy storage is demonstrated using architectures of highly aligned vertical carbon nanotubes (CNTs) acting as supercapacitors, capable of providing large device capacitances. The efficiency of these structures is further increased by incorporating electrochemically active nanoparticles such as MnOx to form pseudocapacitive architectures thus enhancing device capacitance areal specific capacitance of 37 mF/cm2. The demonstrated on-chip integration is up and down-scalable, compatible with standard CMOS processes, and offers lightweight energy storage what is vital for portable and autonomous device operation with numerous advantages as compared to electronics built from discrete components.
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    Universal ac conduction in large area atomic layers of CVD-grown MoS2
    (American Physical Society, 2014) Ghosh, S.; Najmaei, S.; Kar, S.; Vajtai, R.; Lou, J.; Pradhan, N.R.; Balicas, L.; Ajayan, P.M.; Talapatra, S.
    Here, we report on the ac conductivity [σ’(ω); 10 mHz < ω < 0.1 MHz] measurements performed on atomically thin, two-dimensional layers of MoS2 grown by chemical vapor deposition (CVD). Σ’(ω) is observed to display a “universal” power law, i.e., σ’(ω) ∼ ωs measured within a broad range of temperatures, 10 K< T <340 K. The temperature dependence of ‘‘s” indicates that the dominant ac transport conduction mechanism in CVD-grown MoS2 is due to electron hopping through a quantum mechanical tunneling process. The ac conductivity also displays scaling behavior, which leads to the collapse of the ac conductivity curves obtained at various temperatures into a single master curve. These findings establish a basis for our understanding of the transport mechanism in atomically thin, CVD-grown MoS2 layers.