Browsing by Author "Najmaei, Sina"
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Item 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, MadanItem Defect-mediated transport and electronic irradiation effect in individual domains of CVD-grown monolayer MoS2(AIP Publishing, 2015) Durand, Corentin; Zhang, Xiaoguang; Fowlkes, Jason; Najmaei, Sina; Lou, Jun; Li, An-PingThe authors study the electrical transport properties of atomically thin individual crystalline grains of MoS2ﾠwith four-probeﾠscanning tunneling microscopy.ﾠTheﾠmonolayerﾠMoS2ﾠdomains are synthesized byﾠchemical vapor depositionﾠon SiO2/Si substrate. Temperature dependent measurements on conductance andﾠmobilityﾠshow that transport is dominated by an electron charge trapping and thermal release process with very lowﾠcarrier densityﾠandﾠmobility.ﾠThe effects of electronicﾠirradiationﾠare examined by exposing the film toﾠelectron beamﾠin theﾠscanning electron microscopeﾠin an ultrahigh vacuum environment. Theﾠirradiationﾠprocess is found to significantly affect theﾠmobilityﾠand theﾠcarrier densityﾠof the material, with the conductance showing a peculiar time-dependent relaxation behavior. It is suggested that the presence of defects in active MoS2ﾠlayer and dielectric layer create charge trapping sites, and a multiple trapping and thermal release process dictates the transport andﾠmobilityﾠcharacteristics. Theﾠelectron beamﾠirradiationﾠpromotes the formation of defects and impact the electrical properties of MoS2. Our study reveals the important roles of defects and theﾠelectron beamﾠirradiationﾠeffects in the electronic properties of atomic layers of MoS2.Item 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, MadanMolybdenum 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.Item 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, MadanThe 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.Item Enhancing the photocurrent and photoluminescence of single crystal monolayer MoS2ﾠwith resonant plasmonic nanoshells(AIP Publishing LLC., 2014) Sobhani, Ali; Lauchner, Adam; Najmaei, Sina; Ayala-Orozco, Ciceron; Wen, Fangfang; Lou, Jun; Halas, Naomi J.Monolayer molybdenum disulfide (MoS2) produced by controlled vapor-phase synthesis is a commercially promising new two-dimensional material for optoelectronics because of its direct bandgap and broad absorption in the visible and ultraviolet regimes. By tuning plasmonic core-shell nanoparticles to the direct bandgap of monolayer MoS2 and depositing them sparsely (<1% coverage) onto the material's surface, we observe a threefold increase in photocurrent and a doubling of photoluminescence signal for both excitonic transitions, amplifying but not altering the intrinsic spectral response.Item Growth methods for controlled large-area fabrication of high-quality graphene analogs(2017-02-28) Najmaei, Sina; Liu, Zheng; Ajayan, Pulickel M.; Lou, Jun; Rice University; United States Patent and Trademark OfficeIn some embodiments, the present disclosure pertains to methods of growing chalcogen-linked metallic films on a surface in a chamber. In some embodiments, the method comprises placing a metal source and a chalcogen source in the chamber, and gradually heating the chamber, where the heating leads to the chemical vapor deposition of the chalcogen source and the metal source onto the surface, and facilitates the growth of the chalcogen-linked metallic film from the chalcogen source and the metal source on the surface. In some embodiments, the chalcogen source comprises sulfur, and the metal source comprises molybdenum trioxide. In some embodiments, the growth of the chalcogen-linked metallic film occurs by formation of nucleation sites on the surface, where the nucleation sites merge to form the chalcogen-linked metallic film. In some embodiments, the formed chalcogen-linked metallic film includes MoS2.Item 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, MadanWe 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.Item Metallic 1T phase source/drain electrodes for field effect transistors from chemical vapor deposited MoS2(AIP, 2014) Kappera, Rajesh; Voiry, Damien; Yalcin, Sibel Ebru; Jen, Wesley; Acerce, Muharrem; Torrel, Sol; Branch, Brittany; Lei, Sidong; Chen, Weibing; Najmaei, Sina; Lou, Jun; Ajayan, Pulickel M.; Gupta, Gautam; Mohite, Aditya D.; Chhowalla, ManishTwo dimensional transitionmetal dichalcogenides (2D TMDs) offer promise as optoelectronic materials due to their direct band gap and reasonably good mobility values. However, most metals form high resistance contacts on semiconducting TMDs such as MoS2. The large contact resistance limits the performance of devices. Unlike bulk materials, low contact resistance cannot be stably achieved in 2D materials by doping. Here we build on our previous work in which we demonstrated that it is possible to achieve low contact resistance electrodes by phase transformation. We show that similar to the previously demonstrated mechanically exfoliated samples, it is possible to decrease the contact resistance and enhance the FET performance by locally inducing and patterning the metallic 1T phase of MoS2 on chemically vapor deposited material. The device properties are substantially improved with 1T phase source/drain electrodes.Item Nanoantenna-Enhanced Light-Matter Interaction in Atomically Thin WS2(American Chemical Society, 2015) Kern, Johannes; Trügler, Andreas; Niehues, Iris; Ewering, Johannes; Schmidt, Robert; Schneider, Robert; Najmaei, Sina; George, Antony; Zhang, Jing; Lou, Jun; Hohenester, Ulrich; de Vasconcellos, Steffen Michaelis; Bratschitsch, RudolfAtomically thin transition metal dichalcogenides (TMDCs) are an emerging class of two-dimensional semiconductors. Recently, the first optoelectronic devices featuring photodetection as well as electroluminescence have been demonstrated using monolayer TMDCs as active material. However, the lightﾖmatter coupling for atomically thin TMDCs is limited by their small absorption length and low photoluminescence quantum yield. Here, we significantly increase the lightﾖmatter interaction in monolayer tungsten disulfide (WS2) by coupling the atomically thin semiconductor to a plasmonic nanoantenna. Due to the plasmon resonance of the nanoantenna, strongly enhanced optical near-fields are generated within the WS2ﾠmonolayer. We observe an increase in photoluminescence intensity by more than 1 order of magnitude, resulting from a combined absorption and emission enhancement of the exciton in the WS2monolayer. The polarization characteristics of the coupled system are governed by the nanoantenna. The robust nanoantennaﾖmonolayer hybrid paves the way for efficient photodetectors, solar cells, and light-emitting devices based on two-dimensional materials.Item Second harmonic microscopy of monolayer MoS2(American Physical Society, 2013) Kumar, Nardeep; Najmaei, Sina; Cui, Qiannan; Ceballos, Frank; Ajayan, Pulickel M.; Lou, Jun; Zhao, HuiWe show that the lack of inversion symmetry in monolayer MoS2 allows strong optical second harmonic generation. The second harmonic of an 810-nm pulse is generated in a mechanically exfoliated monolayer, with a nonlinear susceptibility on the order of 10−7 m/V. The susceptibility reduces by a factor of seven in trilayers, and by about two orders ofmagnitude in even layers. A proof-of-principle second harmonicmicroscopymeasurement is performed on samples grown by chemical vapor deposition, which illustrates potential applications of this effect in the fast and noninvasive detection of crystalline orientation, thickness uniformity, layer stacking, and single-crystal domain size of atomically thin films of MoS2 and similar materials.Item Surface enhanced resonant Raman scattering in hybrid MoSe2@Au nanostructures(Optical Society of America, 2018) Abid, Inès; Chen, Weibing; Yuan, Jiangtan; Najmaei, Sina; Peñafiel, Emil C.; Péchou, Renaud; Large, Nicolas; Lou, Jun; Mlayah, AdnenWe report on the surface enhanced resonant Raman scattering (SERRS) in hybrid MoSe2@Au plasmonic-excitonic nanostructures, focusing on the situation where the localized surface plasmon resonance of Au nanodisks is finely tuned to the exciton absorption of monolayer MoSe2. Using a resonant excitation, we investigate the SERRS in MoSe2@Au and the resonant Raman scattering (RRS) in a MoSe2@SiO2 reference. Both optical responses are compared to the non-resonant Raman scattering signal, thus providing an estimate of the relative contributions from the localized surface plasmons and the confined excitons to the Raman scattering enhancement. We determine a SERRS/RRS enhancement factor exceeding one order of magnitude. Furthermore, using numerical simulations, we explore the optical near-field properties of the hybrid MoSe2@Au nanostructure and investigate the SERRS efficiency dependence on the nanodisk surface morphology and on the excitation wavelength. We demonstrate that a photothermal effect, due to the resonant plasmonic pumping of electron-hole pairs into the MoSe2 layer, and the surface roughness of the metallic nanostructures are the main limiting factors of the SERRS efficiency.Item Synthesis and Defect Engineering in Molybdenum Disulfide (MoS2) Atomic layers(2014-11-04) Najmaei, Sina; Lou, Jun; Ajayan, Pulickel; Kono, JunichiroThe unique physical properties of two-dimensional (2D) molybdenum disulfide (MoS2) and its promising applications in future optoelectronics have motivated an extensive study of its physical properties. However, a major limiting factor in investigation of 2D MoS2 is its large area and high quality preparation. The existence of various types of defects in 2D MoS2 makes the characterization of defect types and understanding their roles in the physical properties of this material critically important. In this dissertation, I will examine synthetic approaches for preparation of 2D MoS2 and the understanding of defect types and role in its electronic and optical properties. First I will examine the research efforts in understanding exfoliation, direct sulfurization, and chemical vapour deposition (CVD) of MoS2 monolayers as main approaches for preparation of such atomic layers. Recognizing that a natural consequence of the synthetic approaches is the addition of sources of defects, I will initially focus on identifying these imperfections with intrinsic and extrinsic origins. I will reveal the predominant types of point and grain boundary defects in the crystal structure of polycrystalline MoS2 using transmission electron microscopy (TEM), and understand how they modify the electronic band structure of this material using first principles-calculations. The observations and calculations reveal the main types of vacancy-defects, substitutional-defects, and dislocation cores at the grain boundaries of MoS2. Since the sources of defects in two-dimensional atomic layers can, in principle, be controlled and studied with more precision as compared to its bulk counterparts, understanding their roles in the physical properties of this material may provide opportunities for their property modulation. Therefore, I next examined the general electronic properties of single crystalline 2D MoS2 and study the role of grain boundaries in the electrical transport and photoluminescence properties of its polycrystalline counterparts. These results reveal the important role played by point defects and grain boundaries in affecting charge carrier mobility and excitonic properties of these atomic layers. In addition to the intrinsic defects, growth process induced substrate impurities and strain induced band structure perturbations are revealed as major sources of disorders in CVD grown 2D MoS2. I further explore substrate defects for modification and control of electronic and optical properties of 2D MoS2 through interface engineering. Self-assembled monolayer based interface modification, as a versatile technique adaptable to different conventional and flexible substrates, is used to promote significant tunability in the key MoS2 field-effect device parameters. This approach provides a powerful tool for modification of native substrate defect characteristics and allows for a wide range of property modulations. The results signify the role of intrinsic and extrinsic defects in the physical properties of MoS2 and unveil strategies that can utilize these characteristics.Item Ternary CuIn7Se11: Towards Ultra-Thin Layered Photodetectors and Photovoltaic Devices(Wiley, 2014) Lei, Sidong; Sobhani, Ali; Wen, Fangfang; George, Antony; Wang, Qizhong; Huang, Yihan; Dong, Pei; Li, Bo; Najmaei, Sina; Bellah, James; Gupta, Gautam; Mohite, Aditya D.; Ge, Liehui; Lou, Jun; Halas, Naomi J.; Vajtai, Robert; Ajayan, Pulickel2D materials have been widely studied over the past decade for their potential applications in electronics and optoelectronics. In these materials, elemental composition plays a critical role in defining their physical properties. Here we report the first successful synthesis of individual high quality CuIn7Se11 (CIS) ternary 2D layers and demonstrate their potential use in photodetection applications. Photoconductivity measurements show an indirect bandgap of 1.1 eV for few-layered CIS, an external quantum efficiency of 88.0 % with 2 V bias across 2 μm channel with and a signal-to-noise ratio larger than 95 dB. By judicious choice of electrode materials, we demonstrate the possibility of layered CIS-based 2D photovoltaic devices. This study examines this ternary 2D layered system for the first time, demonstrating the clear potential for layered CIS in 2D material-based optoelectronic device applications.Item Ultrafast Optical Microscopy of Single Monolayer Molybdenum Disulfide Flakes(Macmillan Publishers Limited, 2016) Seo, Minah; Yamaguchi, Hisato; Mohite, Aditya D.; Boubanga-Tombet, Stephane; Blancon, Jean-Christophe; Najmaei, Sina; Ajayan, Pulickel M.; Lou, Jun; Taylor, Antoinette J.; Prasankumar, Rohit P.We have performed ultrafast optical microscopy on single flakes of atomically thin CVD-grown molybdenum disulfide, using non-degenerate femtosecond pump-probe spectroscopy to excite and probe carriers above and below the indirect and direct band gaps. These measurements reveal the influence of layer thickness on carrier dynamics when probing near the band gap. Furthermore, fluence-dependent measurements indicate that carrier relaxation is primarily influenced by surface-related defect and trap states after above-bandgap photoexcitation. The ability to probe femtosecond carrier dynamics in individual flakes can thus give much insight into light-matter interactions in these two-dimensional nanosystems.