Browsing by Author "Lei, Sidong"
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Item 2D Optoelectronics: Challenges and Opportunities(2015-12-17) Lei, Sidong; Ajayan, Pulickel M; Lou, Jun; Kono, JunichiroIndium Selenide (InSe) is one of atomically layered 2D materials attracting broad interests recently, because of its good optoelectronic properties. Based on the challenges of 2D optoelectronics, several topics will be covered in this defense, such as trap states and low absorption rate. InSe is selected as a platform to study these topics. The localized states and trap states in InSe system was characterized through low temperature photocurrent measurement to reveal the evolution of band structure and origin of the localized states in few layered InSe. It is found the surface electron orbitals contribute to the localized states. By modifying the surface electron via metallic ions, the Fermi level can be tuned significantly and the inertia surface of the pristine 2D surface can be sensitized for functionalization. Via this method, the InSe photodetector can be improved by organic photosensitive molecules. On the other hand, local gating can induce trap states in 2D materials, helping to improve the photoresponse, but slowing down the response speed. By utilizing this effect, 2D charge coupled device can be fabricated to serve as flexible image sensor which can help correct the optical aberration. The discussion is based on InSe, however, the principle is very universal that can be easily apply to other 2D system. The research can help to promote the research and device development in 2D optoelectronics.Item Charge coupled device based on atomically layered van der waals solid state film for opto-electronic memory and image capture(2016-08-30) Lei, Sidong; Ge, Liehui; George, Antony; Li, Bo; Vajtai, Robert; Ajayan, Pulickel M.; Rice University; United States Patent and Trademark OfficeAn opto-electronic sensor may provide one or more layers of atomically layered photo-sensitive materials. The sensor may include a gate electrode layer, a dielectric layer in contact with the gate electrode layer, and a working media layer that is photo-sensitive deposited on the dielectric layer. The working media layer may provide one or more layers of one or more materials where each of the one or more layers is an atomic layer. The sensor may also include side electrodes in contact with the working media layer.Item Electric Double Layer Field-Effect Transistors Using Two-Dimensional (2D) Layers of Copper Indium Selenide (CuIn7Se11)(MDPI, 2019) Patil, Prasanna D.; Ghosh, Sujoy; Wasala, Milinda; Lei, Sidong; Vajtai, Robert; Ajayan, Pulickel M.; Talapatra, SaikatInnovations in the design of field-effect transistor (FET) devices will be the key to future application development related to ultrathin and low-power device technologies. In order to boost the current semiconductor device industry, new device architectures based on novel materials and system need to be envisioned. Here we report the fabrication of electric double layer field-effect transistors (EDL-FET) with two-dimensional (2D) layers of copper indium selenide (CuIn7Se11) as the channel material and an ionic liquid electrolyte (1-Butyl-3-methylimidazolium hexafluorophosphate (BMIM-PF6)) as the gate terminal. We found one order of magnitude improvement in the on-off ratio, a five- to six-times increase in the field-effect mobility, and two orders of magnitude in the improvement in the subthreshold swing for ionic liquid gated devices as compared to silicon dioxide (SiO2) back gates. We also show that the performance of EDL-FETs can be enhanced by operating them under dual (top and back) gate conditions. Our investigations suggest that the performance of CuIn7Se11 FETs can be significantly improved when BMIM-PF6 is used as a top gate material (in both single and dual gate geometry) instead of the conventional dielectric layer of the SiO2 gate. These investigations show the potential of 2D material-based EDL-FETs in developing active components of future electronics needed for low-power applications.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 High photoresponse of individual WS2 nanowire-nanoflake hybrid materials(AIP Publishing LLC, 2018) Asres, Georgies Alene; Järvinen, Topias; Lorite, Gabriela S.; Mohl, Melinda; Pitkänen, Olli; Dombovari, Aron; Tóth, Geza; Spetz, Anita Lloyd; Vajtai, Robert; Ajayan, Pulickel M.; Lei, Sidong; Talapatra, Saikat; Kordas, Krisztianvan der Waals solids have been recognized as highly photosensitive materials that compete conventional Si and compound semiconductor based devices. While 2-dimensional nanosheets of single and multiple layers and 1-dimensional nanowires of molybdenum and tungsten chalcogenides have been studied, their nanostructured derivatives with complex morphologies are not explored yet. Here, we report on the electrical and photosensitive properties of WS2 nanowire-nanoflake hybrid materials we developed lately. We probe individual hybrid nanostructured particles along the structure using focused ion beam deposited Pt contacts. Further, we use conductive atomic force microscopy to analyze electrical behavior across the nanostructure in the transverse direction. The electrical measurements are complemented by in situ laser beam illumination to explore the photoresponse of the nanohybrids in the visible optical spectrum. Photodetectors with responsivity up to ∼0.4 AW−1 are demonstrated outperforming graphene as well as most of the other transition metal dichalcogenide based devices.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 Observing the interplay between surface and bulk optical nonlinearities in thin van der Waals crystals(Macmillan Publishers Limited, 2016) Deckoff-Jones, Skylar; Zhang, Jingjing; Petoukhoff, Christopher E.; Man, Michael K.L.; Lei, Sidong; Vajtai, Robert; Ajayan, Pulickel M.; Talbayev, Diyar; Madéo, Julien; Dani, Keshav M.Van der Waals materials, existing in a range of thicknesses from monolayer to bulk, allow for interplay between surface and bulk nonlinearities, which otherwise dominate only at atomically-thin or bulk extremes, respectively. Here, we observe an unexpected peak in intensity of the generated second harmonic signal versus the thickness of Indium Selenide crystals, in contrast to the quadratic increase expected from thin crystals. We explain this by interference effects between surface and bulk nonlinearities, which offer a new handle on engineering the nonlinear optical response of 2D materials and their heterostructures.Item Synthesis and photo-response study on GaSe and InSe atomic layers(2013-12-02) Lei, Sidong; Ajayan, Pulickel M.; Kono, Junichiro; Lou, JunThe two dimensional atomically layered materials are drawing intense attention in recent years, because of their special physical properties. Graphene, as a layered material with large charge carrier mobility, has been studied for years. Besides, graphene, large amounts of materials exhibit unique electronic and optical properties. GaSe (Gallium Selenide) and InSe (Indium Selenide) are van der Waals type layered crystal and widely applied in the area of photo-sensing, photo-voltage, non-linear optics, etc. The atomically layered GaSe and InSe may exhibit unique optical and electrical properties. In this study, GaSe single atomically layered crystal was grown by vapor phase transport method, InSe atomically layered flakes were prepared by mechanical exfoliation method. TEM and SEM were applied to characterize the quality of the crystals. Raman studies revealed the changes of vibration modes as the number of layers varied. Photo-conductivity measurements were conducted to reveal the band-structure changes. As the number of atomically layers become less and less, the band gaps of both of GaSe and InSe increase. From our study, it is observed that the layered atomically layered GaSe and InSe shows larger photo-current on/off ratio, indicating less non-photo-generated charge carrier in layered GaSe and InSe. Meanwhile, InSe atomically layers have stronger visible-light response than GaSe, which makes InSe more suitable for atomicallyally layered photo-voltage device, and GaSe more suitable for UV detector. Besides, the home-made opto-electronic measurement probe-station is also discussed in detail.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 Vertical and in-plane heterostructures from WS2/MoS2 monolayers(Nature Publishing Group, 2014) Gong, Yongji; Lin, Junhao; Wang, Xingli; Shi, Gang; Lei, Sidong; Lin, Zhong; Zou, Xiaolong; Ye, Gonglan; Vajtai, Robert; Yakobson, Boris I.; Terrones, Humberto; Terrones, Mauricio; Tay, Beng Kang; Lou, Jun; Pantelides, Sokrates T.; Liu, Zheng; Zhou, Wu; Ajayan, Pulickel M.Layer-by-layer stacking or lateral interfacing of atomic monolayers has opened up unprecedented opportunities to engineer two-dimensional heteromaterials. Fabrication of such artificial heterostructures with atomically clean and sharp interfaces, however, is challenging. Here, we report a one-step growth strategy for the creation of high-quality vertically stacked as well as in-plane interconnected heterostructures of WS2/MoS2 via control of the growth temperature. Vertically stacked bilayers with WS2 epitaxially grown on top of the MoS2 monolayer are formed with preferred stacking order at high temperature. A strong interlayer excitonic transition is observed due to the type II band alignment and to the clean interface of these bilayers. Vapour growth at low temperature, on the other hand, leads to lateral epitaxy of WS2 on MoS2 edges, creating seamless and atomically sharp in-plane heterostructures that generate strong localized photoluminescence enhancement and intrinsic p–n junctions. The fabrication of heterostructures from monolayers, using simple and scalable growth, paves the way for the creation of unprecedented two-dimensional materials with exciting properties.Item Wafer-scale monodomain films of spontaneously aligned single-walled carbon nanotubes(Springer Nature, 2016) He, Xiaowei; Gao, Weilu; Xie, Lijuan; Li, Bo; Zhang, Qi; Lei, Sidong; Robinson, John M.; Hároz, Erik H.; Doorn, Stephen K.; Wang, Weipeng; Vajtai, Robert; Ajayan, Pulickel M.; Adams, W. Wade; Hauge, Robert H.; Kono, JunichiroThe one-dimensional character of electrons, phonons and excitons in individual single-walled carbon nanotubes leads to extremely anisotropic electronic, thermal and optical properties. However, despite significant efforts to develop ways to produce large-scale architectures of aligned nanotubes, macroscopic manifestations of such properties remain limited. Here, we show that large (>cm2) monodomain films of aligned single-walled carbon nanotubes can be prepared using slow vacuum filtration. The produced films are globally aligned within ±1.5° (a nematic order parameter of ∼1) and are highly packed, containing 1 × 106 nanotubes in a cross-sectional area of 1 μm2. The method works for nanotubes synthesized by various methods, and film thickness is controllable from a few nanometres to ∼100 nm. We use the approach to create ideal polarizers in the terahertz frequency range and, by combining the method with recently developed sorting techniques, highly aligned and chirality-enriched nanotube thin-film devices. Semiconductor-enriched devices exhibit polarized light emission and polarization-dependent photocurrent, as well as anisotropic conductivities and transistor action with high on/off ratios.