Browsing by Author "Dubey, Madan"

Now showing 1 - 5 of 5
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    Blueshift of the A-exciton peak in folded monolayer 1H-MoS2
    (American Physical Society, 2013) Crowne, Frank J.; Amani, Matin; Birdwell, A. Glen; Chin, Matthew L.; O'Regan, Terrance P.; Najmaei, Sina; Liu, Zheng; Ajayan, Pulickel M.; Lou, Jun; Dubey, Madan
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    Electrical performance of monolayer MoS2 field-effect transistors prepared by chemical vapor deposition
    (American Institute of Physics, 2013) Amani, Matin; Chin, Matthew L.; Birdwell, A. Glen; O'Regan, Terrance P.; Najmaei, Sina; Liu, Zheng; Ajayan, Pulickel M.; Lou, Jun; Dubey, Madan
    Molybdenum disulfide (MoS2) field effect transistors (FET) were fabricated on atomically smooth large-area single layers grown by chemical vapor deposition. The layer qualities and physical properties were characterized using high-resolution Raman and photoluminescence spectroscopy, scanning electron microscopy, and atomic force microscopy. Electronic performance of the FET devices was measured using field effect mobility measurements as a function of temperature. The back-gated devices had mobilities of 6.0 cm2/V s at 300K without a high-j dielectric overcoat and increased to 16.1 cm2/V s with a high-j dielectric overcoat. In addition the devices show on/off ratios ranging from 105 to 109.
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    Enabling Ultrasensitive Photo-detection Through Control of Interface Properties in Molybdenum Disulfide Atomic Layers
    (Springer Nature, 2016) Najmaei, Sina; Lei, Sidong; Burke, Robert A.; Nichols, Barbara M.; George, Antony; Ajayan, Pulickel M.; Franklin, Aaron D.; Lou, Jun; Dubey, Madan
    The interfaces in devices made of two-dimensional materials such as MoS2 can effectively control their optoelectronic performance. However, the extent and nature of these deterministic interactions are not fully understood. Here, we investigate the role of substrate interfaces on the photodetector properties of MoS2 devices by studying its photocurrent properties on both SiO2 and self-assembled monolayer-modified substrates. Results indicate that while the photoresponsivity of the devices can be enhanced through control of device interfaces, response times are moderately compromised. We attribute this trade-off to the changes in the electrical contact resistance at the device metal-semiconductor interface. We demonstrate that the formation of charge carrier traps at the interface can dominate the device photoresponse properties. The capture and emission rates of deeply trapped charge carriers in the substrate-semiconductor-metal regions are strongly influenced by exposure to light and can dynamically dope the contact regions and thus perturb the photodetector properties. As a result, interface-modified photodetectors have significantly lower dark-currents and higher on-currents. Through appropriate interfacial design, a record high device responsivity of 4.5 × 103 A/W at 7 V is achieved, indicative of the large signal gain in the devices and exemplifying an important design strategy that enables highly responsive two-dimensional photodetectors.
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    Growth-substrate induced performance degradation in chemically synthesized monolayer MoS2ﾠfield effect transistors
    (AIP Publishing LLC., 2014) Amani, Matin; Chin, Matthew L.; Mazzoni, Alexander L.; Burke, Robert A.; Najmaei, Sina; Ajayan, Pulickel M.; Lou, Jun; Dubey, Madan
    We report on the electronic transport properties of single-layer thick chemical vapor deposition (CVD) grown molybdenum disulfide (MoS2) field-effect transistors (FETs) on Si/SiO2 substrates. MoS2 has been extensively investigated for the past two years as a potential semiconductor analogue to graphene. To date, MoS2 samples prepared via mechanical exfoliation have demonstrated field-effect mobility values which are significantly higher than that of CVD-grown MoS2. In this study, we will show that the intrinsic electronic performance of CVD-grown MoS2 is equal or superior to that of exfoliated material and has been possibly masked by a combination of interfacial contamination on the growth substrate and residual tensile strain resulting from the high-temperature growth process. We are able to quantify this strain in the as-grown material using pre- and post-transfer metrology and microscopy of the same crystals. Moreover, temperature-dependent electrical measurements made on as-grown and transferred MoS2 devices following an identical fabrication process demonstrate the improvement in field-effect mobility.
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    Lithium storage mechanisms in purpurin based organic lithium ion battery electrodes
    (Nature Publishing Group, 2012) Reddy, Arava Leela Mohana; Nagarajan, Subbiah; Chumyim, Porramate; Gowda, Sanketh R.; Pradhan, Padmanava; Jadhav, Swapnil R.; Dubey, Madan; John, George; Ajayan, Pulickel M.
    Current lithium batteries operate on inorganic insertion compounds to power a diverse range of applications, but recently there is a surging demand to develop environmentally friendly green electrode materials. To develop sustainable and eco-friendly lithium ion batteries, we report reversible lithium ion storage properties of a naturally occurring and abundant organic compound purpurin, which is non-toxic and derived from the plant madder. The carbonyl/hydroxyl groups present in purpurin molecules act as redox centers and reacts electrochemically with Li-ions during the charge/discharge process. The mechanism of lithiation of purpurin is fully elucidated using NMR, UV and FTIR spectral studies. The formation of the most favored six membered binding core of lithium ion with carbonyl groups of purpurin and hydroxyl groups at C-1 and C-4 positions respectively facilitated lithiation process, whereas hydroxyl group at C-2 position remains unaltered.