2:30 PM - *EP10.3.01
Flat Photonics on Graphene: Widely Tunable Plasmonics Metasurfaces for High Performance Detectors and Modulators
Federico Capasso 1,Yu Yao 2
1 Harvard Univ Cambridge United States,2 School of Electrical, Computer amp; Energy Engineering Ira A. Fulton School of Engineering Tempe United StatesShow Abstract
By combining metal and graphene in hybrid plasmonic structure, we have greatly enhance graphene-light interaction leading to greatly improved optoelectronic devices. We first demonstrated that graphene can be integrated into the nano-gaps of coupled plasmonic antennas to achieve broad tuning of plasmonic antennas. By incorporating suitable MIM structures in optical antenna designs, we implemented an electrically tunable coupled antenna array on graphene with a large tuning range (1100 nm, i.e. 250 cm-1, nearly 20% of the resonance frequency) of the antenna resonance at mid-infrared (MIR) wavelengths (6-7 µm), almost twice the bandwidth than previously reported using gratings of linear coupled Au optical antennas on graphene.1
Applications to graphene detectors so far are still limited by the weak optical absorption (only 2.3% in the monolayer graphene sheet) and short photo-carrier lifetime (< 1 ps). In the second part of this work we will report on an investigation of closely-coupled metallic antenna structures on graphene and show that they can be utilized to simultaneously improve both the light absorption and photo-carrier collection2. The coupled antennas concentrate free space light into the nano-scale antenna gaps (~λ0/100), where the graphene light interaction is greatly enhanced as a result of the ultra-high electric field intensity inside the gap. In addition, the metallic antennas can be designed as electrodes to collect the generated photo-carriers very efficiently due to the short carrier transit time (sub-picosecond). We have demonstrated room temperature mid-infrared (mid-IR) antenna-assisted graphene detectors with more than 200 times enhancement of responsivity (~0.4 V/W) compared to devices without antennas (<2 mV/W).
Widely tunable metasurface composed of optical antennas on graphene can be incorporated into a subwavelength-thick optical cavity to create an electrically tunable perfect absorber.3 By switching the absorber in and out of the critical coupling condition via the gate voltage applied on graphene, a modulation depth of up to 100% can be achieved. In particular, we demonstrated ultrathin (thickness
3:00 PM - EP10.3.02
Gate Tunable Mid-Infrared Optical Response of (Bi1-xSbx)2Te3 Topological Insulators
William Whitney 4,Victor Brar 1,Yunbo Ou 2,Ke He 3,Qi-Kun Xue 3,Harry Atwater 1
1 Thomas J. Watson Laboratory of Applied Physics California Institute of Technology Pasadena United States,4 Department of Physics California Institute of Technology Pasadena United States,1 Thomas J. Watson Laboratory of Applied Physics California Institute of Technology Pasadena United States2 Beijing National Laboratory for Condensed Matter Physics Institute of Physics, Chinese Academy of Sciences Beijing China3 State Key Laboratory of Low-Dimensional Quantum Physics, Department of Physics Tsinghua University Beijing ChinaShow Abstract
The electronic properties of topological insulators – narrow band-gap semiconductors that exhibit insulating bulk and semimetallic Dirac surface states – have been the subject of intense study over the past several years. The optical and optoelectronic behavior of these materials, however, remain widely uncharacterized. We report here experiments that demonstrate electronic control of the optical properties of 5-20 nm thick (Bi1-xSbx)2Te3 films grown by Van der Waals epitaxy, both as-grown on strontium titanate substrates, and after epitaxial lift off and transfer to silicon dioxide on silicon.
It has previously been shown that electrostatic gating can be used to tune the Fermi level in the Dirac semimetal graphene, modifying interband transitions and free carrier absorption. We find strong optical absorption modulation for (Bi1-xSbx)2Te3 films by electrostatic gating, and argue that this modulation is due to bulk states, not topological surface states. In particular, we see 2 and 5 relative percent modulation of transmittance and reflectance, respectively, in the 3-10 micron wavelength range for films transferred to silicon dioxide on silicon substrates. We see similar behavior in as-grown films via ellipsometry. The spectral locations of the modulation features, combined with electrical measurements, indicate that we are modifying bulk carrier interband transitions and free carrier absorption by gate-induced modulation of the Fermi level in p-type (Bi1-xSbx)2Te3 films. We also report a novel epitaxial lift off process for topological insulator films grown on strontium titanate – which is a preferred growth substrate for TI materials, but which is non-ideal for infrared optical characterization – to transfer films to other substrates. These results demonstrate the gate tuning of semiconductor interband transitions, and provide a characterization of the bulk optical response of topological insulators; transport properties of these films will also be discussed.
3:15 PM - EP10.3.03
Thermal Camouflage with Graphene
Shahnaz Aas 1,Coskun Kocabas 2,Osman Balci 1
1 Physics Bilkent University Ankara Turkey,2 Physics Bilkent University Ankara TurkeyShow Abstract
The control of thermal radiation from a hot object has wide range of applications including camouflage systems. Kirchhoff’s radiation law connects the thermal emission from a surface with its optical absorption. One can engineer the thermal radiation by coating the surface with photonic crystals or plasmonic structures. However, the electrical control of thermal radiation without changing the temperature has been an outstanding challenge due to the lack of an active material in infrared frequencies. Here, we demonstrate a new class of active surfaces capable of efficient real-time electrical-control of their thermal emission over full infrared (IR) spectrum. Our approach relies on electro-modulation of IR absorption and emissivity of multilayer graphene via reversible intercalation of ions. The demonstrated devices are light and ultra-flexible which can conformably coat their environment. Using these active thermal surfaces, we fabricated an adaptive thermal camouflage system which can reconfigure its thermal appearance and blend itself with the thermal background in a few seconds. We anticipate that, the electrical control of thermal radiation would enable variety of new technologies ranging from thermal camouflage to heat management for outer space application.
3:30 PM - EP10.3.04
Ultra-Broadband and High Responsivity Graphene Infrared Photodetector and Imaging Array
Che-Hung Liu 1,You-Chia Chang 1,Fuxi Cai 1,Wei Lu 1,Theodore Norris 1,Zhaohui Zhong 1
1 Univ of Michigan-Ann Arbor Ann Arbor United States,Show Abstract
The ability to efficiently detect light over a broad spectral range is central to various technology applications such as imaging, sensing, spectroscopy and communication etc. Graphene is among one of the most promising candidate materials for ultra-broadband photodetectors, as its absorption spectrum covers the entire ultraviolet to far-infrared range due to its unique gapless linear band structure. However, the photoresponsivity of graphene-based photodetectors has so far been limited due to the small optical absorption coming from its monolayer structure nature. Here, we report an ultra-broadband photodetector design based on graphene heterostructures. The detector is a phototransistor utilizing the tunneling barrier formed by the heterojunction interface. Under optical illumination, photo-excited hot carriers generated in the absorption layer tunnel into channel layer, leading to a charge build-up in absorption layer and a strong photo-gating effect on the channel conductance. The single pixel detector demonstrates room temperature photodetection from visible to the near-infrared range, with near-infrared responsivity higher than 1 A/W. In addition, infrared imaging using a 1000-pixel detector array will also be discussed. These results address the key challenges for broadband infrared imaging array and showcase the promise of graphene-based optoelectronic applications.
3:45 PM - EP10.3.05
Hybrid, Gate-Tunable, van der Waals p-n Heterojunctions from Pentacene and MoS2#xD;
Deep Jariwala 1,Sarah Howell 1,Kan-Sheng Chen 1,Junmo Kang 1,Vinod Sangwan 1,Stephen Filippone 1,Riccardo Turrisi 1,Tobin Marks 1,Lincoln Lauhon 1,Mark Hersam 1
1 Northwestern Univ Evanston United States,Show Abstract
The emergence of a wide variety of two-dimensional (2D) materials has created new opportunities for device designs and applications. In particular, the availability of semiconducting transition metal dichalcogenides,1 in addition to semi-metallic graphene and insulating boron nitride, has enabled the fabrication of ‘all 2D’ van der Waals heterostructure devices. However, the concept of van der Waals heterostructures1, 2 has the potential to be significantly broadened beyond layered solids. This principle has been exploited to fabricate heterojunctions between conventional silicon and 2D materials3 as well as carbon nanotubes with 2D materials4 and amorphous oxide semiconductors.5 Similarly, molecular and polymeric organic solids,6 whose surface atoms possess saturated bonds, have the potential to interact via van der Waals forces and thus offer an alternative for scalable integration with 2D materials. Despite this potential, the integration of organic semiconductors with 2D materials for optoelectronic devices has thus far been limited. Here, we demonstrate the integration of an organic small molecule p-type semiconductor, pentacene, with a 2D n-type semiconductor, MoS2. The resulting p-n heterojunction diode is gate-tunable and shows asymmetric control over the anti-ambipolar transfer characteristic. The operating principles and band profiles of this system are characterized by direct charge transport measurements, scanning photocurrent microscopy, electrostatic force microscopy, and finite element modeling. This comprehensive experimental and computational study reveals that pentacene forms a type-II heterojunction with MoS2 exhibiting a photovoltaic effect, thereby demonstrating n-type MoS2 as a candidate non-fullerene acceptor in organic photovoltaics.
1. Jariwala D., Sangwan V.K., Lauhon L.J., Marks T.J., Hersam M.C. Emerging device applications for semiconducting two-dimensional transition metal dichalcogenides. ACS Nano 8, 1102–1120 (2014).
2. Grigorieva I.V., Geim A.K. Van der Waals heterostructures. Nature 499, 419-425 (2013).
3. Yang H., Heo J., Park S., Song H.J., Seo D.H., Byun K.-E., et al. Graphene Barristor, A Triode Device with a Gate-Controlled Schottky Barrier. Science 336, 1140-1143 (2012).
4. Jariwala D., Sangwan V.K., Wu C.-C., Prabhumirashi P.L., Geier M.L., Marks T.J., et al. Gate-Tunable Carbon Nanotube–MoS2 Heterojunction p-n Diode. Proc. Nat. Acad. Sci. USA 110, 18076–18080 (2013).
5. Jariwala D., Sangwan V.K., Seo J.-W.T., Xu W., Smith J., Kim C.H., et al. Large-Area, Low-Voltage, Antiambipolar Heterojunctions from Solution-Processed Semiconductors. Nano Lett. 15, 416-421 (2015).
6. Marks T.J. Materials for organic and hybrid inorganic/organic electronics. MRS Bull. 35, 1018-1027 (2010).
4:30 PM - *EP10.3.06
Ultra-Fast and Nanoscale Opto-Electronic Phenomena in 2D Material Heterostructures
Frank Koppens 1
1 ICFO - The Institute of Photonic Sciences Barcelona Spain,Show Abstract
The optoelectronic response of two-dimensional (2D) crystals, such as graphene and transition metal dichalcogenides (TMDs), is currently subject to intensive investigations. Owing to its broadband absorption, gapless character and ultrafast carrier dynamics, graphene is a promising material for nano-photonics and high-speed photodetectors , whereas TMDs have emerged as potential candidates for sensitive photodetection thanks to their enhanced photon absorption. Vertically assembling these crystals in so-called van der Waal heterostructures allows the creation of novel and versatile optoelectronic devices that combine the complementary properties of their constituent materials.
Here we present a various new device capabilities, varying from nano-photonic devices to ultra-fast and broadband electrical detectors [1-5]. We applied femtosecond time-resolved photocurrent measurements on 2d material heterostructures, which probes the transit of photoexcited charges across the photoactive TMD layer – and thus current generation – directly in the time domain. Remerkably fast photoresponse of 5 ps is observed.
In adition, we show that graphene-WSe2-graphene heterostructure devices offer this possibility through the photo-thermionic (PTI) effect: the absorbed photon energy in graphene is efficiently transferred to the electron bath, leading to a thermalized ‘hot’ carrier distribution. Carriers with energy higher than the Schottky barrier between graphene and WSe2 can be emitted over the barrier, thus creating photocurrent. We experimentally demonstrate that the PTI effect enables detection of sub-bandgap photons, while being size- scalable, electrically tunable, broadband and ultrafast.
 Photodetectors based on graphene, other two-dimensional materials and hybrid systems
F. H. L. Koppens, T. Mueller, Ph. Avouris, A. C. Ferrari, M. S. Vitiello, M. Polini
Nature Nanotechnol. 9, 780-793 (2014)
 High-Responsivity Graphene–Boron Nitride Photodetector and Autocorrelator in a Silicon Photonic Integrated Circuit.
Ren-Jye Shiue, Yuanda Gao, Yifei Wang, Cheng Peng, Alexander D Robertson, Dmitri K Efetov, Solomon Assefa, Frank HL Koppens, James Hone, Dirk Englund
Nano Letters, 15 (11), 2015.
 Picosecond photoresponse in van der Waals heterostructures
M. Massicotte, P. Schmidt, F. Vialla, K. G. Schädler, A. Reserbat-Plantey, K. Watanabe, T. Taniguchi, K. J. Tielrooij and F. H. L. Koppens
Nature Nanotechnology 11.1 (2016)
 Hot-carrier photocurrent effects at graphene–metal interfaces
K. J. Tielrooij, M. Massicotte, L. Piatkowski, A. Woessner, Q. Ma, P. Jarillo-Herrero, N. F. van Hulst, F. H. L. Koppens
J. Phys.: Condens. Matter 27, 164207 (2015)
 Generation of photovoltage in graphene on a femtosecond timescale through efficient carrier heating
K. J. Tielrooij, L. Piatkowski, M. Massicotte, A. Woessner, Q. Ma, Y. Lee, K. S. Myhro, C. N. Lau, P. Jarillo-Herrero, N. F. van Hulst, and F. H. L. Koppens
Nature Nanotechnology 10, 437-443 (2015)
5:00 PM - EP10.3.07
Electrically Gated Graphene Plasmon Modulated Erbium Emission
Jeremy Brouillet 1,Victor Brar 1,Adriana Scarangella 2,Evan Miyazono 1,Michelle Sherrott 1,Wei Hsiang Lin 1,Andrei Faraon 1,Maria Miritello 2,Francesco Priolo 2,Harry Atwater 1
1 Applied Physics California Institute of Technology Pasadena United States,2 University of Catania and CNR-IMM-MATIS Catania ItalyShow Abstract
The enhancement of spontaneous emission rates is an important goal of the nanophotonics community for applications such as lasers, single photon sources, ultrabright LEDs, and ultrafast signal processing. Plasmonic structures are promising candidates for accomplishing this goal because they strongly confine light, thus increasing the local density of optical states, LDOS, leading to enhanced spontaneous emission. We demonstrate the first reported increase in spontaneous emission rate by means of plasmonic coupling of graphene nanoribbons to spontaneous emitters, using Er as an active infrared luminescent center. By electrostatically tuning the plasmon resonance to that of the erbium 2.94µm 4I11/2 → 4I13/2 transition, we show an enhancement in luminescence for the 1550nm 4I13/2 → 4I15/2 transition. We realize this experimentally using graphene nanoresonators patterned with widths between 15nm and 25nm on top of 20nm to 200nm thick layers of erbium-dopedY2O3. We add an ion gel to aid in doping of the graphene by charge accumulation. The sample is optically pumped at a wavelength of 532nm and its emission is measured near 1550nm as a function of voltage applied to the ion gel. We improve the luminescence enhancement of adding a plasmonically-resonant 25nm graphene nanoribbon by 75% compared to undoped nanoribbons. As the plasmon only interacts with the top few tens of nanometers of material and the effect we observe is an indirect measurement of the transition, this increase in luminescence implies a strong increase in the spontaneous transition rate of the 2.94µm transition. We therefore show an electrically-tunable Purcell-enhanced plasmonic emitter that can increase luminescence at 1550nm.
Previous work in metallic nanoparticles has demonstrated spontaneous emission rate enhancements exceeding 1,0001. We also discuss the outlook for upper limits to graphene plasmon-enhanced Er spontaneous emission. Prior work has shown that graphene nanoresonators support highly confined plasmonic modes in the mid-infrared with a LDOS of 106 times greater than that of free space2. It has been predicted that this extremely large LDOS can proportionately enhance the emission of rare-earth emitters, via emission to a plasmonic mode of gated graphene nanoresonators, thus allowing widely tunable control of the Purcell enhancement3.
 T. Hoang, G. Akselrod, C. Argyropoulos, J. Huang, D. Smith, and M. Mikkelsen, Ultrafast spontaneous emission source using plasmonic nanoantennas. Nature Communications. 2015, 6 (7788).
 V. Brar, M. Jang, M. Sherrott, J. Lopez, and H. Atwater. Highly Confined Tunable Mid-Infrared Plasmonics in Graphene Nanoresonators. Nano Letters. 2013, 13 (6), 2541-2547.
. K. Tielrooij, et al. Electrical control of optical emitter relaxation pathways enabled by graphene. Nature Physics. 2015, 11, 281-287.
5:15 PM - EP10.3.08
Gate Tunable Coherent Terahertz Absorption in Graphene
Nurbek Kakenov 1,Osman Balci 1,Hakan Altan 2,Coskun Kocabas 1
1 Bilkent University Ankara Turkey,2 Middle East Technical University Ankara TurkeyShow Abstract
In this work, we demonstrate coherent perfect absorption of terahertz radiation in electrolyte gated graphene. We developed a new electrically tunable terahertz cavity using graphene and gold electrodes, filled with a room-temperature ionic liquid electrolyte. The conductivity of the graphene electrode, thus the terahertz reflectivity is controlled by a voltage applied between the electrodes. We were able to modulate the resonant reflectivity by 99% with a bias voltage of 2V. We anticipate that the gate-tunable coherent perfect absorption would lead to efficient active terahertz devices such as modulators and detectors.
5:30 PM - EP10.3.09
Engineering the Charge Transfer in Two-Dimensional Heterostructures for Photodetector Application
Adrien Robin 2,Emmanuel Lhuillier 2,Abdelkarim Ouerghi 3,Benoit Dubertret 1
1 LPEM ESPCI ParisTech Paris France,2 Nexdot Paris France,4 INSP UPMC Paris France,2 Nexdot Paris France3 LPN Marcoussis France1 LPEM ESPCI ParisTech Paris FranceShow Abstract
Two-Dimensional layered heterostructures open up great prospects in photodetector applications1. It is a promising route to decouple the charge photogeneration from their transport by mean of photogating, as in the case of hybrid graphene-2D semiconductors2,3. In this context, the charge transfer control between the light absorber layer and the transport layer is of crucial importance. In order to evidence and overcome the restricting parameters for charge transfer, we build a model system by hybridizing colloidal 2D CdSe nanoplatelets4 with epitaxial graphene5. This strategy bypasses the limited mobility of nanocrystals films originating from the hopping transport6.
The charge transfer is investigated by building an electrolyte gated photo-transistor. We show that the large semiconductor exciton binding energy originating from the 2D character of the material prevents an efficient charge transfer to the graphene channel. This bottleneck can be overcome by growing a heterostructure at the nanocrystal level, allowing us to control the magnitude and the direction - electrons or holes - of the charge transfer to graphene. We eventually show that functionalizing graphene by nanocrystals only leads to a limited change in the magnitude of the 1/f noise.
(1) Buscema, M.; Island, J. O.; Groenendijk, D. J.; Blanter, S. I.; Steele, G. A.; van der Zant, H. S. J.; Castellanos-Gomez, A. Chem. Soc. Rev. 2015, 44 (11), 3691–3718.
(2) Konstantatos, G.; Badioli, M.; Gaudreau, L.; Osmond, J.; Bernechea, M.; de Arquer, F. P. G.; Gatti, F.; Koppens, F. H. L. Nat. Nanotechnol. 2012, 7 (6), 363–368.
(3) Sun, Z.; Liu, Z.; Li, J.; Tai, G.; Lau, S.-P.; Yan, F. Adv. Mater. 2012, 24 (43), 5878–5883.
(4) Ithurria, S.; Tessier, M. D.; Mahler, B.; Lobo, R. P. S. M.; Dubertret, B.; Efros, A. L. Nat. Mater. 2011, 10 (12), 936–941.
(5) Robin, A.; Lhuillier, E.; Xu, X. Z.; Ithurria, S.; Aubin, H.; Ouerghi, A.; Dubertret, B. Submitted.
(6) Lhuillier, E.; Robin, A.; Ithurria, S.; Aubin, H.; Dubertret, B. Nano Lett. 2014, 14 (5), 2715–2719.
5:45 PM - EP10.3.10
Electric and Photovoltaic Behavior of Novel Few Layer α-MoTe2 / MoS2 Dichalcogenide Heterojunction
Atiye Pezeshki 1,Seongil Im 1
1 Inst of Physics amp; Applied Physics Yonsei University Seoul Korea (the Republic of),Show Abstract
Transition-metal dichalcogenides (TMDs) are two dimensional (2D) nanomaterial with the common formula MX2, where M is a transition metal element from group IV-VII (M = Mo, W, Nb, Re, and so on) while X is a chalcogene element (X = S, Se, Te). In general, M atoms are sandwiched between X atoms to form a single layer, and each layer can be stacked together via van der Waals forces, which make 2D TMDs easily cleaved by scotch tape or other similar techniques. Among TMD families with ultra-thin layers, molybdenum disulfide (MoS2) and tungsten diselenide (WSe2) are well-known semiconductors with their bandgaps of more than 1 eV; their band gap increases to 1.6~1.8 eV for monolayer and band properties changes from indirect to direct type. As one of quite recent 2D materials, molybdenum ditelluride (α-MoTe2) has also been attracting attention due to its optical and electrical properties. Monolayer a-MoTe2 exhibits a direct optical bandgap of 1.10 eV, while its bulk form becomes an indirect semiconductor with the band gap of 0.88~1.0 eV. Interestingly, it is reported that MoTe2 shows structural and electronic phase transition. The structural phase transition from hexagonal (2H) phase to monoclinic (distorted octahedral or 1T ) phase is reversible at a high temperature.
According to a literature, few-layered a-MoTe2 field effect transistors (FETs) showed ambipolar type conduction with somewhat low mobilities of 0.2 ~ 3.7 cm2V-1s-1. But another report displayed a p-channel MoTe2 nanoflake FET of quite enhanced mobility of 10~30 cm2V-1s-1 using thermal annealing for contact. MoTe2-based homo- or hetero-junction p-n diode has not been reported yet although p-n diode is one of the basic building blocks for electronics and optoelectronics. In fact, forming two different types of conduction in the same nanoflake might not be easy. So, heterojunction p-n diode studies have always been preferred but limited to n-MoS2/p-Si bulk, n-MoS2/p- WSe2 and n-MoS2/p-BP (black phosphorous) systems, to the best of our knowledge.
In the present study, we attempted van der Waals heterojunction p-n diode fabrication by direct imprinting technique transferring p-type α-MoTe2 onto n-type MoS2 nanoflake, since in this way we would achieve Mo-based dichalcogene 2D p-n diode which appears novel in view of using the same transition metal for both p- and n-type. Pt electrode appeared very good for ohmic contact with p-type α-MoTe2 even without thermal annealing. Our few-layered a-MoTe2/MoS2 p-n diode demonstrates low voltage operation at 5 V and high reasonable ON/OFF current ratio of 103~4×103 (~4×103 on SiO2/Si and ~103 on glass substrate) along with good ideality factors of 1.06~1.34. Kilohertz fast dynamic rectification was achieved in the dark while dynamic photo-voltaic switching was also exhibited under red, green, blue LEDs, and 800 nm infrared laser. We regard that our new 2D p-n diode is quite promising in the prospects of nano-electronics and optoelectronics.
EP10.4: Poster Session: Synthesis and Optical Characterization of 2D Materials
Thursday AM, March 31, 2016
Sheraton, Third Level, Phoenix Ballroom
9:00 PM - EP10.4.01
Using Water Soluble Conjugated Polyelectrolyte/Reduced Graphene Oxide as Hole Injection Layer
Afsoon Fallahi 3,Ezeddin Mohajerani 4,Masoud Alahbakhshi 4,Faramarz Afshar Taromi 2
1 Harvard-MIT Division of Health Science and Technology Cambridge United States,2 Polymer Eng. Dept. Amirkabir University of Technology Tehran Iran (the Islamic Republic of),3 INST, Sharif University Tehran Iran (the Islamic Republic of),4 Laser and Plasma Research Institute (LAPRI), Shahid Beheshti University Tehran Iran (the Islamic Republic of)2 Polymer Eng. Dept. Amirkabir University of Technology Tehran Iran (the Islamic Republic of)Show Abstract
This study presents using a nanocomposite comprising of a cationic conjugated polyelectrolyte (CPE), Poly[(2,5-bis(2-(N,N-diethylammonium bromide)ethoxy)-1,4- phenylene)-alt-1,4-phenylene] or (PPPNEt2.HBr), with graphene oxide (GO) as a new hole
injection layer (HIL) for organic light emitting diodes. It is demonstrated that using the designed ionically functionalized water soluble conjugated polymers instead of polyethylene dioxythiophene: polystyrene sulfonate (PEDOT:PSS) is a promising approach to overcome
strong acidic nature of PEDOT:PSS besides excluding its non-conductive PSS part. As the other aspiration of this work, we introduce a good partner for dissolving and spin-casting of GO as a simple and economic technique to use the hole conductive and electron blocking nature of GO in hole injection portion of assembled devices. Using this new binary blend showed enhanced charge carrier mobility, good electroluminescent and J-V characteristics in comparison with the conventional devices. Such improvement is interpreted with induced ion space charge of HIL at the interface and resulting electric field screening effect due to ion migration.
9:00 PM - EP10.4.02
Amino-Functionalized Reduced Graphene Oxide as a High-Efficiency Counter Electrode in Dye-Sensitized Solar Cells
Ce Hao 1,Yiyi Jia 1
1 State Key Laboratory of Fine Chemicals, School of Chemistry, Dalian University of Technology Dalian China,Show Abstract
Researching and developing of high efficiency and inexpensive alternatives to precious metals Pt counter electrode material is critical for promoting the dye-sensitized solar cell (DSSCs) of large-scale commercialization. First by studying the mechanism of iodine reduction reaction (IRR) occurring on the counter electrode, concluded that elementary reactions (III) is rate-determining steps to the overall electro-catalytic activities. So IRR rate mainly depends on the ionization energy of the electro-catalyst. And by screening the volcanic figures, amino-functionalized reduced graphene oxide (EFG) is preferably to be used as counter electrode material owing to the lower ionization energy and optimal iodine binding energy. Then under mild experimental conditions we synthesized EFG by ethylene diamine nucleophilic epoxy functional groups surface in graphene oxide and assembled into DSSCs as a counter electrode. Benefiting from lower ionization energy and the unique structure, EFG has a high power conversion efficiency (PCE) close to that based traditional Pt counter electrode, which will transfer electrons rapidly from external circuit to the I3−, thus reducing the charge transfer resistance (Rct) and improving the photovoltaic performance. The present work has a guiding significance for theoretical design of high-efficiency carbon electrodes in energy conversion/storage devices.
9:00 PM - EP10.4.03
Synthesis of Single Crystal Monolayer Graphene Islands on Germanium Substrate by Hot Filament Chemical Vapor Deposition
Tej Limbu 2,Frank Mendoza 1,Brad Weiner 3,Gerardo Morell 2
1 Institute for Functional Nanomaterials San Juan United States,2 Physics University of Puerto Rico, Rio Piedras San Juan United States,1 Institute for Functional Nanomaterials San Juan United States1 Institute for Functional Nanomaterials San Juan United States,3 Chemistry University of Puerto Rico, Rio Piedras San Juan United StatesShow Abstract
Hot filament chemical vapor deposition (HFCVD) reactor shows a great potential of being implemented in the high quality synthesis of carbon nanomaterials. Herein, we report the synthesis of large area single crystal monolayer graphene islands of area up to 100 µm on hydrogen terminated germanium (110) surface by using methane gas as a carbon precursor in HFCVD reactor. As synthesized graphene on germanium substrate was primarily characterized by Raman spectroscopy which shows narrow G peak at 1588 cm-1 with a full wave at half maximum (FWHM) of about 13 cm-1 and 2D peak at 2682 cm-1 with a FWHM of about 34 cm-1. Field emission scanning electron microscopy image shows the growth of multiple single crystal islands on the germanium surface. By optimizing the values of gas flow rates and deposition time, we were able to control the size of the graphene islands on the germanium substrate. The anisotropic twofold symmetry of the germanium (110) surface allows unidirectional alignment of multiple graphene islands so we believe that such islands merge to produce a single crystal large area graphene under an optimized condition.
9:00 PM - EP10.4.04
Effect of Silicon Dioxide Current Distribution Layers on the Performance of GaN-Based Vertical LED Power Chips
Yi Chun Chou 2,Li Ping Chou 2,Wei Yu Yen 2,Fu Pang Chen 2,Ho-Ching Ni 1,Hsin-Yueh Lin 1,Sheng Wen Wen 1,Shih-Hung Yang 1,Chien-Ming Su 1,Bo-Hao Huang 1, Jyh-Rong Gong 1
1 Physics National Chung Hsing University Taichung Taiwan,2 High Power Optoelectronics Inc Taichung Taiwan,2 High Power Optoelectronics Inc Taichung Taiwan1 Physics National Chung Hsing University Taichung TaiwanShow Abstract
GaN-based vertical structure LED (VLED) power chips show several advantages including good current spreading, high light extraction efficiency and reduced forward voltage. However, the GaN VLED power chips also suffer from loss of light absorption by n-electrode and current spreading limitation in lateral direction. These problems were previously minimized by p-GaN plasma etching, p-GaN ion bombardment and silicon dioxide (SiO2) layer deposition on p-GaN of the GaN VLED structure. Nevertheless, the above-mentioned approaches also cause certain drawbacks including reverse leakage current (IR) increment and poor adhesion of SiO2 layer on reflective mirror layer.
In this paper, instead of direct deposition of a SiO2 layer on top of the p-GaN of a GaN VLED structure, a SiO2 layer was deposited right underneath the reflective mirror layer of a GaN VLED structure. In this way, SiO2 layer was employed to serve as a current distribution layer (CDL) underneath the n-electrode of a GaN VLED power chip so as to limit current flowing in vertical direction and to enable current distribution in lateral direction of the GaN VLED power chip. In this case, SiO2 CDLs having various thicknesses and widths were processed on GaN VLED structures by employing plasma enhanced chemical vapor deposition (PECVD) and lithography with a typical power chip size of 55 x 55 mil2. It was found that the best GaN-based VLED performance occurred in an SiO2 CDL with thickness and width being 500 Å and 50 µm, respectively. Under an injection current of 350 mA, the forward voltage (VF) and light output power (LOP) of the best GaN VLED were 2.97 V and 672 mW, respectively, with its wall-plug efficiency (WPE) being 64.6% which shows an 8.7% enhancement in WPE compared to the WPE of a GaN VLED power chip having the same structure without CDL.
Key words : GaN vertical structure light-emitting diode (VLED), current distribution layer, LED power chip
9:00 PM - EP10.4.05
Fabrication of Triangular Shaped ZnO Nanostructure on Graphene Oxide
Sundaram Chandrasekaran 1,Yiseul Song 1,Miri Seo 1,Seung Hyun Hur 1
1 Univ of Ulsan Ulsan Korea (the Republic of),Show Abstract
Multifarious ZnO morphologies (Hollow spheres and Bucky bowls) and Graphene Oxide/ZnO hybrid structures (nanorod and triangle shaped) were fabricated through a facile hydrothermal method. The complex structural evolution of the ZnO and GO-ZnO hybrid structures was carefully analyzed through both experimental and analytical means (deformation models), we controlled the morphology of the ZnO and verified their growth mechanism. Dislocation driven growth is a favored mechanism and is also common effect for GO-ZnO nanorods and GO-ZnO triangles. The oxygen vacancies (VO) created by dislocation growth in GO-ZnO strongly influence the structure and photoelectrochemical (PEC) performance of the ZnO. The GO-ZnO triangle-shaped composites exhibit the maximum overall water splitting photocurrent density of ~1.517 mA/cm-2 vs RHE with the highest enhancement in the Incident Photon to Current conversion Efficiency (IPCE) of 10.41% under UV light irradiation. In addition, the PEC performance of these photoelectrodes was enhanced by increasing the oxygen defects to some extent and then decreased with the further addition oxygen defects, which indicates that an optimum amount of oxygen vacancies will improve the PEC efficiency of the GO-ZnO composites.
9:00 PM - EP10.4.06
Controlling the Space Distribution of Composition and Electronic Structure in Two Dimensional Layered Semiconductor
Xidong Duan 1,Honglai Li 1,Anlian Pan 1,Ruqin Yu 1,Xiangfeng Duan 2
1 Hunan University Changsha China,2 University Of California,Los Angeles Los Angeles United StatesShow Abstract
Two-dimensional layered semiconductors such as MoS2
have attracted considerable interest in recent times as semiconductor after Si. Much like the traditional semiconductor technique, controlling the space distribution of composition and electronic structure of two dimensional semiconductor material is essential to copnstruct all modern electronic and optoelectronic devices, including transistors, p–n diodes, photovoltaic/photodetection devices, light-emitting diodes and laser diodes. With a relatively small lattice mismatch (∼4%) between MoS2
, it is possible to produce coherent MoS2
heterostructures through a lateral epitaxial process. Our studies indicate that simple sequential growth often fails to produce the desired heterostructures because the edge growth front can be easily passivated after termination of the first growth and exposure to ambient conditions. To retain a fresh, unpassivated edge growth front is important for successive lateral epitaxial growth. To this end, we have designed a thermal CVD process that allows in situ switching of the vapour-phase reactants to enable lateral epitaxial growth of single- or few-layer TMD lateral heterostructures. We used this technique to realize the growth of compositionally modulated MoS2
lateral heterostructures. The WS2
lateral heterostuctures with both p- and n-type characteristics can also allow us to construct many other functional devices, for example, a CMOS inverter. The voltage gain of CMOS inverter reaches as large as 24.
To precisely control the band gap of these two-dimensional layered semiconductors is of central importance for creating optoelectronic devices with tunable spectral responses. Considering similarities in the atomic structure of many two dimensional semiconductor, such as MoS2
, it is possible to create a mixed alloy system with a tunable band gap by alloy composition. A simple one-step chemical vapor deposition approach was developed for the simultaneous growth of alloy MoS2x
） triangular nanosheets with completely tunable composition and band gap. Also, lateral composition graded atomic layered 2D MoS2(1−x)
nanosheets have been successfully synthesized using a simple moving source thermal evaporation method by an improved CVD route. Both microstructure and spectral characterizations demonstrate that the achieved nanosheets are highly crystallized, with the composition being continuously tuned from the pure MoS2
at the center (x = 0) to a highly Se doped ternary alloy at the edge (x = 0.68).References
1.Xidong Duan, Anlian Pan,Ruqin Yu, Xiangfeng Duan,et,a..l Nature Nanotechnology 9,2014,1024-1030.
2.Honglai Li, Xidong Duan,Anlian Pan, Xiangfeng Duan et,al.. J. Am. Chem. Soc. 2014, 136, 3756−3759
3.Honglai Li, Qinglin Zhang, Xidong Duan, Anlian Pan, Xiangfeng Duan et,al.. J. Am. Chem. Soc. 2015, 137, 5284−5287
9:00 PM - EP10.4.07
Hybrid MoS2–CdSe Nanocrystal Phototransistors with Ultrafast Photoresponse
HyunSoo Ra 1,DoHyuns Kwak 1,WoongYun Kim 2,Gyu Tae Kim 2,Min-Sang Lee 3,Jong-Soo Lee 1
1 DGIST Daegu Korea (the Republic of),2 School of Electrical Engineering Korea University Seoul Korea (the Republic of)3 Ecolumy Co., Ltd. Daegu Korea (the Republic of)Show Abstract
Molybdenum disulfide (MoS2) transition metal dichalcogenide (TMDC) semiconductors have attracted increasing attention for electronic and optoelectronic applications because of their high electron mobility, high photosensitive, and tunable bandgap. Recently, a 0-D/2-D hybrid device was reported to have a high responsivity R (≤107 A/W) and detectivity D* (≤1015 Jones). However, a critical issue of the 0-D/2-D hybrid device is its long decay time (≥300 ms) induced from trap and leakage of residual carriers at the interfaces of hybrid devices. This issue remains a major challenge for the development of hybrid devices.
In this presentation, we first report a superior functional hybrid phototransistor fabricated using bilayer MoS2 and colloidal CdSe NCs. The hybrid CdSe/MoS2 phototransistor was designed to enhance the photodetector performance of the pristine MoS2. The hybrid CdSe/MoS2 phototransistor exhibited enhanced photoresponsivity R, 2.5 × 105 A/W, photo-detectivity D*, 2 × 1014 Jones, and decay time τdecay, ~60 ms, compared with those of pristine MoS2. The fast response of the photocurrent rise (τrise) and decay (τdecay) times reported in our hybrid photodetector are orders of magnitude shorter than the values reported for other hybrid devices. To provide a detailed photodetection mechanism for this hybrid MoS2 phototransistor, we discussed these effects via energy band diagrams of n-n type heterojunctions at the interface between bilayer MoS2 and CdSe NCs by applying density functional theory (DFT) and examining the photoluminescence (PL) and absorption spectra of both materials.
9:00 PM - EP10.4.08
Ultraviolet Light Sensor Based on Graphene Quantum Dots/Reduced Graphene Oxide Hybrid Film
Tran Van Tam 1,Won Mook Choi 1
1 University of Ulsan Ulsan Korea (the Republic of),Show Abstract
Graphene quantum dots (GQDs) are considered as a promising green nanomaterial and an alternative to traditional fluorescent nanocrystals for applications in photocatalysts, sensors, and bioimaging. Therefore, integrating the excellent optical and electrical properties of GQDs and superior physical properties of graphene by the formation of hybrid structures should help generate unique physical properties and offer a potential pathway for constructing all graphene-based material devices with high performance. Here, we developed a novel ultraviolet (UV) sensor based on GQDs/reduced graphene oxide (RGO) hybrids, showing the high performances of photoresponsivity and detectivity. The GQDs prepared by the simple bottom-up approach show high quantum yield and can produce the photogenerated carriers due to the strong absorption of UV light.
GQDs were first synthesized by the carbonization of citric acid (CA) through the hydrothermal method with an average diameter of 4.4 nm and high crystallinity, suggesting that CA has been successfully transformed into high quality GQDs after hydrothermal treatment. For the UV sensor fabrication using GQDs/RGO hybrid, RGO film was prepared on a SiO2/Si substrate using the spin-coating of GO and subsequent thermal reduction, and the GQDs solution was then sprayed onto the prepared RGO film. Under UV illumination, while the RGO film show no detectable current change, the GQDs/RGO hybrids clearly exhibits a current increase with the linear and symmetric I-V curves due to the Ohmic contact between GQDs and RGO. We further investigated the time-resolved photoresponse of the GQDs/RGO hybrids and the GQDs/RGO hybrids demonstrate good photoresponse and repeatability. As the UV light is on, the current increases sharply and reaches the saturation value. When the light source is off, the current recovers to its initial value. Notably, the UV sensor device using GQDs/RGO hybrids shows a high photo responsibility of 8.7×102 A/W and an excellent specific detectivity of 7.7×1013 Jones at a low operating voltage.
These high performances of the hybrids UV sensor are attributed to the strong absorption of UV light in GQDs, an efficient charge transfer at the GQDs/RGO junction, and a high electron mobility of RGO. The electrons of GQDs are excited to the conduction band under UV light illumination, and the photogenerated electrons transfer to the RGO film at the GQDs/RGO junction and flow through RGO to the Au electrode. Therefore, the recombination of photogenerated electron-hole pairs is reduced and the photocurrent of devices is enhanced. Our novel approach based on the solution-processed UV sensor using all carbon materials suggests an alternative strategy for designing the promising optoelectronic applications.
9:00 PM - EP10.4.09
Triboelectricity-Assisted Transfer of Graphene for Flexible Optoelectronic Application
Shuo Liu 1,Qingliang Liao 1,Shengnan Lu 1,Xiaohui Zhang 1,Zheng Zhang 1,Guangjie Zhang 1,Yue Zhang 1
1 University of Science amp; Technology Beijing Beijing China,Show Abstract
In this work, we developed a novel triboelectricity-assisted polymer-free method for the transfer of large-area chemical vapor deposited graphene films. With the assistance of the electrostatic force from friction generated charges, graphene sheet was successfully transferred from copper foils to flexible substrates. Characterization results confirm the presence of high quality graphene with less defects and contaminations, as compared to the graphene transferred by the conventional PMMA-mediated transfer process. What’s more, the graphene samples possess outstanding electrical transport capability and mechanical stability, which is manifested by acting as an electron transfer matrix in a flexible graphene/ZnO hybrid flexible photodetector. Our results show broad application potential of this transfer method in future flexible electronics and optoelectronics.
9:00 PM - EP10.4.10
Large-Scale Atomically Thin 2D Van der Waals p-n Heterojunction Devices for Optoelectronics
Chandan Biswas 1,Gustavo Lara 2,Anupama Kaul 1
1 Department of Metallurgical, Materials and Biomedical Engineering, Department of Electrical amp; Computer Engineering University of Texas at El Paso El Paso United States,2 Department of Electrical amp; Computer Engineering University of Texas at El Paso El Paso United StatesShow Abstract
Semiconducting p-n junctions are the key building blocks for the electronic and optoelectronic devices. Carrier transport across the junction occurs by diffusion and drift processes and results in a depletion region, with a built-in potential characteristic of the semiconductor material properties and doping. It is possible to develop two dimensional (2D) ultra-thin p-n heterojunction by using van der Walls materials, which have received significant attention since the mechanical exfoliation graphene from parent graphite. Such van der Waals p-n heterojunction composed of p- and n-type semiconductors are predicted to exhibit completely different charge transport properties compared to their bulk heterojunction counterparts. So far the size of such p-n heterojunction and its practical implementations were limited by the size of the 2D materials to few tens of microns. Here, we show atomically thin 2D van der Waals p-n heterojunction synthesized by chemical vapor deposition technique over millimeter length scales. Atomically thin p–n heterojunctions were fabricated using van der Waals assembly of transition-metal dichalcogenides (TMDs) such as p-type WSe2 and n-type MoS2. We believe these atomically thin millimeter scale p−n heterojunction devices highlight the attractive prospects and opportunities of these materials to impact numerous optoelectronic device platforms, such as photodetectors, ultra-fast rectifiers, photoluminescence devices, electroluminescence devices, photovoltaics, as well as spin- and valley-polarized light emitting diodes and on-chip lasers.
9:00 PM - EP10.4.11
Novel One-Pot Route for Growth of Graphene/Amorphous Carbon Heterostructure via Chemical Structure Control
Beomjin Park 1,Jaesung Park 4,Jin Gyeong Son 6,Yong-jin Kim 3,Seong Uk Yu 8,Hyo Ju Park 9,Dong-Hun Chae 4,Jinseok Byun 1,Gumhye Jeon 1,Sung Huh 5,Seoung-Ki Lee 10,Artem Mishchenko 3,Seung Hyun 1,Tae Geol Lee 6,Sang Woo Han 5,Jong-Hyun Ahn 10,Zonghoon Lee 9,Chanyong Hwang 11,Kostya Novoselev 3,Kwang Kim 8,Byung Hee Hong 2,Jin Kon Kim 1
1 POSTECH Pohang Korea (the Republic of),2 Department of Chemistry Seoul National University Seoul Korea (the Republic of),3 School of Physics and Astronomy University of Manchester Manchester United Kingdom,4 Center for Nanometrology Korea Research Institute of Standards and Science Daejeon Korea (the Republic of)5 Department of Chemistry and KI for the NanoCentury KAIST Daejeon Korea (the Republic of),6 Center for Nanosafety Metrology Korea Research Institute of Standards and Science Daejeon Korea (the Republic of)2 Department of Chemistry Seoul National University Seoul Korea (the Republic of),3 School of Physics and Astronomy University of Manchester Manchester United Kingdom7 Department of Chemistry Pohang University of Science and Technology Pohang Korea (the Republic of),8 Department of Chemistry Ulsan National Institute of Science and Technology Ulsan Korea (the Republic of)9 School of Materials Science and Engineering Ulsan National Institute of Science and Technology Ulsan Korea (the Republic of)4 Center for Nanometrology Korea Research Institute of Standards and Science Daejeon Korea (the Republic of)5 Department of Chemistry and KI for the NanoCentury KAIST Daejeon Korea (the Republic of)10 School of Electrical and Electronic Engineering Yonsei University Seoul Korea (the Republic of)3 School of Physics and Astronomy University of Manchester Manchester United Kingdom6 Center for Nanosafety Metrology Korea Research Institute of Standards and Science Daejeon Korea (the Republic of)11 Center for Nanometrology Korea Research Institute of Standards and Science Daejeon Korea (the Republic of)8 Department of Chemistry Ulsan National Institute of Science and Technology Ulsan Korea (the Republic of)2 Department of Chemistry Seoul National University Seoul Korea (the Republic of)Show Abstract
Precise patterning of graphene is highly important for tailor-made and sophisticated two-dimensional nanoelectronic and optical devices. However, graphene-based heterostructures have been synthesized by delicate multi-step chemical vapor deposition methods, limiting preparation of versatile heterostructures. Here, we report single-step growth of graphene/amorphous carbon (a-C) heterostructures from a solid source of polystyrene through selective photo-crosslinking process. Graphene is successfully grown from neat polystyrene regions, while crosslinked polystyrene regions turn into a-C because of a large difference in their thermal stability. Since the electrical resistance of a-C is at least two orders of magnitude higher than that for graphene, the charge transport in graphene/a-C heterostructure occurs through the graphene area. Measurement of the quantum Hall effect in graphene/a-C lateral heterostructures clearly confirms the reliable quality of graphene and well-defined graphene/a-C interface. The ability to directly synthesize patterned graphene from polymer pattern opens up new possibilities for the preparation of versatile heterostructures.
9:00 PM - EP10.4.13
High Performance Near-Infrared Photodetector Based on MoS2/Black Phosphorus/WSe2 Heterojunction
Hao Li 1,Lei Ye 1,Jianbin Xu 1
1 Chinese Univ of Hong Kong Shtain Hong Kong,Show Abstract
The remarkable properties of graphene, a two dimensional sheet of carbon atoms, have brought tremendous attention and interest to other 2D materials including atomically thin 2D transition metal dichalcogenides (TMDCs), Black Phosphorous (BP) and hexagonal Boron Nitrides (hBN). These two dimensional materials exhibit different electronic properties, ranging from semi-metallic graphene and semiconducting transition metal dichalcogenides and black phosphorous to insulating boron nitride, due to the diversity in their energy bandgaps. The recent discovery of the narrow-bandgap black phosphorous  serendipitously bridges the energy gap between zero-bandgap graphene and the relatively large-bandgap TMDCs,making 2D material a promising candidate for optical application across a wide range of the electromagnetic spectrum including ultraviolet light (hBN, ~6eV), visible light (TMDCs, 1 ~ 2.5eV), near infrared light (BP, 0.3~2eV)and mid infrared light (graphene, 0eV) . A triple-layer heterostructure device is hence designed and fabricated by us, where BP is utilized as an active material for infrared detection and WSe2 and MoS2 are selected respectively for p-type and n-type channels. Besides the excellent quantum efficiency provided by the built-in electric field, the photo current is further amplified by the BJT-like npn-structure, inducing remarkable photo-responsivity and normalized photocurrent to dark current ratio.
 Li, Likai, et al. "Black phosphorus field-effect transistors." Nature nanotechnology 9.5 (2014): 372-377.
 Xia, Fengnian, et al. "Two-dimensional material nanophotonics." Nature Photonics 8.12 (2014): 899-907.
 Zhang, Wenjing, et al. "Ultrahigh-gain photodetectors based on atomically thin graphene-MoS2 heterostructures." Scientific reports 4 (2014).
 Huang, Chunming, et al. "Lateral heterojunctions within monolayer MoSe2–WSe2 semiconductors." Nature materials 13.12 (2014): 1096-1101.
9:00 PM - EP10.4.14
Synthesis of Surfactant Free SnS Nanoplates and Morphologe Control in an Aqueous Solution
Heeseung Yang 1,Sunghwan Cho 1,Jun Hyuk Song 1,Unyong Jeong 2
1 Material Science and Engineering Yonsei University Seoul Korea (the Republic of),2 Material Science and Engineering POSTECH Pohang Korea (the Republic of)Show Abstract
The synthetic route to produce SnS nanoplates with the Pbnm crystal structure without any surfactant. The process is very quick, environment-friendly and accomplished in a mild aqueous condition at low temperature. The synthesis involves two steps, formation of an intermediate tin oxide hydroxide (Sn6O4(OH)4) gel used as precursor and its chemical transformation into SnS nanoplates in the presence of sulfide precursor. we found that addition of small amount of PVP during the chemical transformation results in the formation of cubic SnS with the Cmcm crystal structure. We found self etching effect of SnS nanoplates as increasing reaction time. We discuss about the development of the crystal structures on the basis of density functional theory (DFT) calculations on the structure-energy relationship of SnS nanostructures. The optical properties of the SnS nanoplates and nanocubes are investigated respectively.
9:00 PM - EP10.4.15
High Quality AlGaN Nanowires for Deep UV-LED Using MOCVD Growth Technique
San Kang 1,Ji-Hyeon Park 1,Jae-kwan Sim 1,Daeyoung Um 1,Seung kyu Lee 1,Da som Lee 1,Taek-Soo Jang 1,Cheul-Ro Lee 1
1 Chonbuk National University Jeonju City Korea (the Republic of),Show Abstract
Aluminum gallium nitride (AlGaN) has attracted significant attention for deep ultraviolet (DUV) light emitting diodes (LEDs) and laser diodes (LDs), and is positioned to replace conventional mercury-based light sources for air purification, sterilizers, and bio-chemical detection. To date, however, the growth of high quality AlGaN nanowire has proved to be much more difficult, compared with gallium nitride (GaN) nanowire, because of its intrinsic properties. In this letter, we demonstrate that these critical challenges can be addressed by employing two-step metal-organic chemical vapor deposition (MOCVD) growth process, the AlGaN nanowires are directly grown on the low-temperature AlGaN seed that formed on low-cost Si substrate. The process temperature was varied between 710 and up to 1000 °C, which is a principal advantage in order to optimize the material properties of the high quality AlGaN nanowires. The doping process becomes significantly more complex with increasing the Al content and the electrical resistivity is high for n-type AlGaN doped by silicon and p-type AlGaN doped by magnesium. The materials characterization involved extensive implementation of field-emission scanning electron microscopy (FE-SEM), high-resolution transmission electron microscopy (HR-TEM), photoluminescence (PL), cathodoluminescence (CL) and x-ray diffraction (XRD) measurements. By carefully running synthesis conditions a record IQE of 80% can be realized with AlGaN nanowires, which is nearly equivalent compared to high quality GaN nanowires. The 365 nm emitting AlGaN nanowire LEDs (UV-A) were achieved, with a turn on voltage of about 8 V, which is significantly lower than the commonly observed 20-40 V. In the end, high performance AlGaN nanowire LEDs with emission wavelengths covering the UV-B/C bands will be also demonstrated.
9:00 PM - EP10.4.16
Few-Layer Graphene Sheets Encapsulated Silver Nanoparticles: A Hybrid Platform for Enhanced Light Matter Interaction
Rishi Maiti 1,Subhrajit Mukherjee 1,Tridib Sinha 1,Samit Ray 1
1 IIT Kharagpur Kharagpur India,Show Abstract
Graphene, is viewed as a potential passive constituent of future optoelectronic devices due to its high electron mobility and optical transmittance (~97.7%) over a broad wavelength range but active graphene-based photonic devices have found limited applications due to its relatively inefficient interaction with light. Therefore an enhanced light matter interaction is required for their use in active photonic devices. We report a rapid, efficient and simple non-hazardous approach to synthesize large area graphene-Ag0 hybrid plasmonic nanostructures that show enhanced light-matter interaction. The novel hybrid combines the synergistic effect of the composite where Ag0 nanoparticles are stabilized in graphene matrix. The formation of highly stable Ag NPs with an average size of 40 nm is observed within the graphene layers. The Raman intensity of G band of graphene-Ag0 hybrid exhibits an enhancement by a factor of 20 over the control sample. The position of the G-band is up-shifted from 1583.6 cm-1 to 1586 cm-1, and 2D-band downshifted from 2738 cm-1 to 2715 cm-1 for the G-Ag0 hybrid, as compared to the control graphene sample indicating Ag NPs induced n-type doping of graphene-Ag0 nanocomposites due to metal-graphene interaction via electron transfer. Using optical simulation, we have studied the interaction of light with embedded Ag nanoparticles resulting in a strong amplification of the electric field in the graphene metal interface. We have fabricated Au/G-Ag0/Au lateral photodetector exhibiting high responsivity and spectral selectivity in the visible wavelength. The study exploits the synergistic effect of the graphene-metal nanocomposite, which may lead to the potential application of graphene based photonic devices.
A. K. Geim, K. S. Novoselov, Nature Mater. 2007, 6(3), 183.
F.Bonaccorso, Z. Sun, T. Hasan, A. C. Ferrari, Nature Photon. 2010, 4, 611–622.
9:00 PM - EP10.4.17
Tunable MoS2 Quantum Dots Based Optoelectronics Devices on Silicon Platform
Subhrajit Mukherjee 1,Rishi Maiti 1,Ajit Katiyar 1,Soumen Das 1,Samit Ray 1
1 IIT Kharagpur West Bengal India,Show Abstract
Molybdenum disulfide (MoS2), a layered structure material, has some interesting features, such as, fairly large bandgap (∼1.2 - 2.2 eV), moderate mobility at ambient temperature (∼100 cm2V−1s−1), make it very promising candidate for optoelectronic applications. In this report, size tunable, luminescent MoS2 quantum dots (QDs) have been successfully synthesized through well-controlled, yet inexpensive solvent assisted sono-chemical exfoliation method, followed by gradient centrifugation where the diameter of QDs could be varied within the range of ∼2 to 25 nm. Size tunable absorption and photoluminescence (PL) properties of MoS2 below a critical dimension have been systematically observed and exhibited a red shift in energy with the increase of dot size. All the QD dispersions exhibited a strong absorption peak due to quantum size effect, along with two relatively weaker peaks at lower energy due to interband transition. The PL spectra have also shown an intense size dependent emission with the peak centered at 2.2-2.4 eV, originated from quantum confinement. On the other hand, relatively weaker peaks at ∼1.97 eV and ∼ 1.82 eV are attributed to the direct band-edge transitions. The energy difference (150-180 meV) between two low energy peaks is correlated to the valence band splitting, in excellent agreement with theoretically reported results.
We have also fabricated inorganic p-n heterojunction photodetector (PD) as well as light emitting diode (LED) using colloidal n-type MoS2 QDs integrated with p-type Si. The PDs have shown very strong responsivity determined by the size of the QDs with optical selectivity in the visible to NIR wavelength range, which can be operated at a low applied bias at room temperature. The fabricated photodetector exhibited a high photo-to-dark current ratio (ION/IOFF) of ~103, with the peak responsivity and detectivity have been estimated to be ~850 mA/W and ~1011 cm.Hz1/2/W, respectively, at an applied bias of -2 V, for 2 nm dia. QDs. The responsivity of the device exhibited a dependency on QDs size and it’s enhanced by 4.5 times as the QDs size decreased from 7 nm to 2 nm. The devices also exhibited excellent stability and reproducibility with moderate response speed. The prepared QDs based photodetector is found to be attractive due to their unique properties arising from the geometry. Furthermore, the n-MoS2/p-Si heterojunction also displayed electroluminescence (EL) characteristic with white light emissions spectra from ~400 nm to ~850 nm and the EL intensity increased with increasing forward bias. A bluish-white light emission could be directly observed by the naked eye, when sufficient forward bias is applied across the device. The EL emission could be detectable around 15 V and increases continuously at room temperature. Our work demonstrates that the great potential of a lithography-free, efficient approach for the fabrication PDs and LEDs on existing Si technology using colloidal MoS2 QDs.
9:00 PM - EP10.4.18
Growth of Layered In2Se3 Nanosheet Directly on Graphene Surface
Fan Yang 1,Eui Sang Song 1,Bin Yu 1
1 SUNY Polytechnic Institute CNSE Albany United States,Show Abstract
Two-dimensional heterostructures represent a group of emerging functional nanostructures that could be useful in future-generation electronics, optoelectronics, and energy harvesting. Here we demonstrated the growth of layered In2Se3 nanosheet on the top of graphene film using chemical-vapor-deposition (CVD) approach. The physical morphology of In2Se3 nanosheet can be adjusted through critical processing parameters in CVD deposition and pre-growth treatments. Extensive material characterization, including SEM, XPS, Raman, and XRD, are conducted to confirm the structure and composition of the synthesized van der Waals 2D heterostructure.
9:00 PM - ep10.4.19
Monolayer Graphene/ Epitaxial Grown Germanium on Silicon Heterostructure Based Infrared Photodetector
Khurelbaatar Zagarzusem 2,Yeon-Ho Kil 1,Joung-Hee Kim 1,Kyu-Hwan Shim 1,Hyunjin Cho 3,Myung-Jong Kim 3,Chel-Jong Choi 1
1 School of Semiconductor and Chemical Engineering Chonbuk National University Jeonju Korea (the Republic of),2 School of Information and Communication Technology Mongolian University of Science and Technology Ulaanbaatar Mongolia,1 School of Semiconductor and Chemical Engineering Chonbuk National University Jeonju Korea (the Republic of)3 Institute of Advanced Composite Materials Korea Institute of Science and Technology Wanju-gun Korea (the Republic of)Show Abstract
Graphene, a single layer of carbon atoms, has recently emerged as a novel material with unique electrical and optical properties for novel opto-electronic applications such as terahertz detection field effect transistors , ultra-broadband photodetectors , high efficient light emitting diodes  and barristors . Most of these devices use of a Schottky barrier formed at a heterojunction between a semiconductor and the metallic graphene. Therefore, it is essential to understand the nature of carrier transport through Schottky barriers to design and optimization of graphene based novel devices.
Recently germanium (Ge) has received the much attention material for near infrared photo-detection due to its distinct properties including large absorption coefficient at near infrared frequencies, high electron conductivity compared to silicon (Si), low cost, and excellent compatibility of parallel processing with Si technology [5,6]. In addition, Ge growth on Si (Ge-on-Si) is more compatible with complementary metal-oxide-semiconductor (CMOS) circuitry and Si based monolithic integration on a same chip because of its relatively straightforward fabrication technique when compared with free-standing bulk Ge substrate . In present work, we fabricated infrared photodetectors with graphene-germanium-graphene structures on epitaxial grown Ge-on-Si substrate and investigated its opto-electrical parameters. The photodetectors operation explained by using band-alignment diagram and attributed to a charge transport mechanism between graphene and epitaxial Ge film on Si. The fabricated device shows dark current of 2.01 µA, photo responsivity R of about 0.35 A/W, detectivity D of 3.86×1010 Jones (cmHz1/2/W), and a quantum efficiency η of 28 % at -2 V reverse bias, which demonstrates the high value of the monolayer graphene electrode towards infrared detection. The integration of graphene electrodes into conventional Si based technology compatible Ge is therefore promising candidates for potential applications in optoelectronic circuits and low cost, high performance, and reliable photodetectors.
Seung Hyun Hur, University of Ulsan
Coskun Kocabas, Bilkent University
Barbaros Özyilmaz, National University of Singapore
Jang Ung Park, Ulsan National Institute of Science and Technology
Aldrich Materials Science
EP10.5: Optoelectronic Devices of 2D Materials II
Thursday AM, March 31, 2016
PCC North, 200 Level, Room 222 C
9:30 AM - EP10.5.01
Broadband Near-Unity Absorption in Monolayer MoS2 Heterostructures: Towards Ultrathin Photovoltaics in Transition Metal Dichalcogenides
Michelle Sherrott 1,Deep Jariwala 1,Artur Davoyan 1,Harry Atwater 1
1 California Inst of Technology Pasadena United States,Show Abstract
Two dimensional materials, such as graphene and atomically thin transition metal dichalcogenide (TMDC) semiconductors, have emerged as promising materials for nanophotonic and optoelectronic1 applications. Here we explore the potential use of such materials and their composite heterostructures for applications in photovoltaics. We show that a near-unity light absorption across the visible spectrum can be achieved in monolayer MoS2, as a test case for light trapping in two-dimensional semiconductor materials. We investigate several design strategies and underline promising platforms for future ultrathin and lightweight, high efficiency solar cells.
Monolayer sub-nm thick TMDC materials, e.g., molybdenum disulphide (MoS2), are direct bandgap semiconductors with an extraordinarily strong per-layer absorption, capturing 5-15% of visible light via single-pass absorption.2 Furthermore, high carrier mobility combined with an ultrathin structure makes TMDCs promising candidates for future photovoltaic applications. However, an efficient solar cell requires 100% light harvesting. Therefore broadband, angle-insensitive light-trapping designs are needed to realize high efficiency photovoltaic cells in two-dimensional TMDC materials.
First, we have demonstrated that a dielectric nano-antenna based motif resonantly enhances light absorption in monolayer MoS2. In particular, we consider a subwavelength scale unit cell comprised of a tapered TiO2 resonator formed on top of a monolayer of MoS2/SiO2/Ag back reflector heterostructure. Interplay between broadband resonances in the high index dielectric nano-antenna with the Fabry-Perot resonance in the low index SiO2 layer maximizes the electric field intensity and light absorption in the MoS2. This enhances the absorption of the structure to above 75% of incident visible light and absorption exceeds 90% at the A and B exciton peaks in MoS2.
Second, we demonstrate strong light-matter interaction with stacked two-dimensional absorber/spacer heterostructures. We show that a near-unity, broadband and angle insensitive absorption of visible light in a monolayer TMDC – few nm thick dielectric hexagonal boron nitride (hBN) is possible, with just a few heterostructure unit cells.
Lastly we have explored thin film interference-based light trapping approaches to absorption in few-layer thickness MoS2.3 We demonstrate that just 10nm thick MoS2 on a Ag back-reflector absorbs greater than 80% of the incident light between 500nm – 700nm with angle-insensitivity.
We will further discuss coupled optoelectronic models that account for light absorption and carrier collection, for future efficient ultrathin solar cell designs with TMDC absorber layers, and will discuss preliminary experimental investigations.
1. Jariwala, D. et al, ACS Nano 2014, 8, 1102–1120.
2. Li, Y. et al, Phys. Rev. B 2014, 90, 205422.
3. Kats, M. et al, Nat. Mater. 2013, 12, 20-24.
9:45 AM - *EP10.5.02
Self-Assembled Growth of Transition Metal Dichalcogenide Wires and Their Heterostructures
Jong-Hyun Ahn 1
1 Electrical and Electronic Engineering Yonsei University Seoul Korea (the Republic of),Show Abstract
Transition metal dichalcogenides (TMDs) with good electronic, mechanical, and optical properties have been studied for application in a variety of electronic and optoelectronic devices. In particular, strategies toward the formation of well-aligned TMD patterns with tunable thicknesses and periodicities are needed to construct complex, integrated devices and heterogeneous structures with new optoelectronic functionalities. In this talk, I present a solution-based synthetic method for self-aligned MoS2 and WS2 wire arrays and their heterostructures with controlled sizes and properties. The thicknesses and periodicities of the aligned wires can be precisely controlled by adjusting certain parameters, such as the internal flow, humidity and pH. These TMD wires were used as one-dimensional semiconducting materials in the construction of flexible, transparent electronic devices. In addition, WS2/MoS2 heterostructures show clear optical and structural modulation.
10:15 AM - EP10.5.03
Graphene-Enabled Active Microwave Devices
Osman Balci 1,Ertugrul Karademir 2,Semih Cakmakyapan 3,Nurbek Kakenov 1,Emre Polat 4,Ekmel Ozbay 1,Coskun Kocabas 1
1 Bilkent University Ankara Turkey,2 Trinity College Dublin Ireland3 University of California LA Los Angeles United States4 University of Glasgow Glasgow United KingdomShow Abstract
Graphene provides a unique platform to control light-matter interaction in a broad spectrum. Electrical control of microwaves in the free space has been an outstanding challenge. In this work, we describe a new approach to control microwaves using large-area active graphene devices. Our strategy relies on electrostatic tuning of the density of high mobility charge carriers in the order of 1014 cm-2 on an atomically thin graphene electrode which operates as a tunable metal in microwave frequencies. We synthesized large area single layer graphene (20x20 cm2) using chemical vapor deposition. Using large area graphene, we demonstrate a new class of active surfaces capable of real-time electrical control of reflection, transmission and absorption of microwaves. Using this approach, we fabricated switchable radar absorbing surfaces with tunable reflection suppression ratio up to 50dB with operation voltage of 3V. To increase the functionality of these devices, we incorporate these devices with metallic split ring resonators to fabricate graphene-enabled tunable metamaterials. In this device architecture, metallic resonators are capacitively coupled to the graphene electrodes that introduce voltage-controlled dissipation with modulation depth of 50dB and operation voltage of 3V. Large modulation depth, simple device architecture, and mechanical flexibility are the key attributes of the graphene-enabled active microwave surfaces that could find a wide range of applications ranging from active signal processing to adaptive camouflage.
10:30 AM - EP10.5.04
High Photosensitivity and Broad Spectral Response of Multi-Layered Germanium Sulfide Transistors
Rajesh Kumar Ulaganathan 1,Yi-Ying Lu 1,Chia-Jung Kuo 1,Srinivasa Reddy Tamalampudi 2,Raman Sankar 1,Fang Cheng Chou 1,Yit-Tsong Chen 1
1 National Taiwan University Taipei Taiwan,2 Academia Sinica Taipei TaiwanShow Abstract
In this paper, we report the optoelectronic properties of multi-layered GeS nanosheets (~28 nm thick)-based field-effect transistors (called GeS-FETs). The multi-layered GeS-FETs exhibit remarkably high photoresponsivity of Rλ ~ 206 AW-1 under illumination of 1.5 µW/cm2 at l = 633 nm, Vg = 0 V, and Vds = 10 V. The obtained Rλ ~ 206 AW-1 is excellent as compared with a GeS nanoribbon-based and the other family members of group IV-VI-based photodetectors in the two-dimensional (2D) realm, such as GeSe and SnS2. The gate-dependent photoresponsivity of GeS-FETs was further measured to be able to reach Rλ ~ 655 AW-1 operated at Vg = -80 V. Moreover, the multi-layered GeS photodetector holds high external quantum efficiency (EQE ~ 4.0 × 104 %) and specific detectivity (D* ~ 2.35 × 1013 Jones). The measured D* is comparable to those of the advanced commercial Si- and InGaAs-based photodiodes. The GeS photodetector also shows an excellent long-term photoswitching stability with a response time of ~7 ms over a long period of operation (>1 h). These extraordinary properties of high photocurrent generation, broad spectral range, fast response, and long-term stability make the GeS-FET photodetector a highly qualified candidate for future optoelectronic applications.
Keywords: Germanium sulfide, photodetector, photoresponsivity, external quantum efficiency, specific detectivity
10:45 AM - EP10.5.05
Substrate Temperature Induced Band Gap Tuning of MgZnO via Pulsed Metal Organic Chemical Vapor Deposition (PMOCVD)
Fikadu Alema 1,Brain Hertog 1,Oleg Ledyaev 1,Ross Miller 1,Andrei Osinsky 1,Winston Schoenfeld 2
1 Agnitron Technology Eden Prairie United States,2 CREOL, The College of Optics and Photonics, University of Central Florida, 4000 Central Florida Blvd., Orlando United StatesShow Abstract
We report on substrate temperature (TS) induced band gap tuning of MgZnO epitaxial films grown via pulsed metal organic chemical vapor deposition (PMOCVD) method. MgZnO films were grown on ZnO (~30 nm)/AlN (25 nm)/Al2O3 (0001) substrates by varying TS from 500 oC to 700 oC in a vertical rotating disk PMOCVD. The fractions of Zn and Mg introduced into the MOCVD reactor were set to 0.83 and 0.17, respectively, and remain unchanged for the growth of each film. The band gap of the films was estimated by UV-visible transmission and varies from 3.38 eV to 3.87 eV, corresponding to an Mg content between x=0.06 and x=0.27. Although the concentration of Mg introduced into the reactor for the growth of each film was constant, its content in the grown films has shown strong dependence on TS, leading to band gap tuning in MgZnO alloy. The highest Mg content of 0.27 (3.87 eV) was grown at 630 oC, indicating the requirement of an optimal TS to realize an efficient incorporation of Mg into the ZnO lattices. The origin of TS induced Mg incorporation (band gap tunability) into ZnO host is discussed in light of vapor pressure and parasitic reaction during the growth process. Cathodoluminescence (CL) measurement has shown variable spectral peak position due to the TS dependent Mg content in the films. No multi-absorption edge and CL band splitting were observed, signifying the absence of phase segregation which was further confirmed by XRD measurement. AFM analysis has shown a smooth surface morphology for the high Mg content films. MSM structured interdigitated photodetector was also fabricated on the films to characterize their photo response properties. Owing to the TS dependent Mg in the films, the devices exhibit response spectra whose peaks were ranging from 315nm to 365 nm with spectral rejection ratio more than an order of magnitude.
11:30 AM - *EP10.5.06
Defect Controlled Synthesis of Emissive 2D Materials through Intercalation
Seokwoo Jeon 1
1 KAIST Daejon Korea (the Republic of),Show Abstract
Recent reports of emission from graphene and other 2D materials interest researchers because the new and low cost emissive materials with chemical stability and compatibility of solution process may replace inorganic, or organic, fluorescent materials. However, the emission origin is still not clear so further strategy to improve optical properties is needed. Our group has developed unique exfoliation and dispersion methods of 2D materials by the intercalation of alkali metal/organics. With the advantage of the methods generally excluding, or controlling, the oxidation of 2D materials, we have studied the origin of intrinsic emission (~ 400 nm) of graphene in depth. With those samples, I will introduce the role of oxidation in graphene quantum dots (GQDs) which forms isolated small sp2 carbon hexagons within a GQD, known as subdomains. Experimental evidences supported with calculation of the formation energy of subdomains and bandgap will give a guideline to improve emission properties. Successful demonstration of EL device and cell staining with the controlled GQD will be good examples for further use. Not only for the GQD but also for 2D chalcogenides, the intercalation strategy seems to work fine. Controlled bandgap widening of oxidized MoS2 from 1.8 to 2.6 eV was observed from exfoliated flakes. These examples will support the intercalative synthesis of 2D material is a key technique for optoelectronic applications.
12:00 PM - EP10.5.07
Effect of Vacuum Annealing and Chemical Doping on the Optical, Electrical and Valley Properties in Monolayer MoS2
Daeyoung Lim 1,DongHak Kim 1
1 KHU Yongin Korea (the Republic of),Show Abstract
We investigated the effects of chemical doping and vacuum annealing on the optical, electrical and valley properties in the 2D transition metal dichalcogenides such as MoS2. The photoluminescence (PL) intensity of monolayer MoS2 is dramatically increased, but that of MoSe2 is decreased after p-type chemical doping. The valley polarization of the monolayer MoS2 decreases fast also, while that of MoSe2 shows the opposite behavior. The PL enhancement in intrinsically n-type MoS2 can be explained by the p-doping induced suppression of the non-radiative trionic decay. On the other hand, monolayer MoSe2 is more intrinsic electrically and p-doping rather increases the trion formation and decay. The valley polarization change, however, seems to be better explained when both the exciton lifetime change and the additional intervalley scattering due to charged dopant impurities are considered. We also observed a similar PL enhancement and a valley polarization decrease in the vacuum-annealed MoS2. But, contrary to the previous explanations, our electrical measurement clearly shows that the PL enhancement is not due to the quenching of the trionic decay. The measured electron mobility shows an increase in the defect density with vacuum annealing, indicating that the PL enhancement is not due to the quenching of defects either. On the other hand, the valley polarization decrease seems to be related more to the increased scattering rate due to the annealing-generated defects, most likely S-vacancy than exciton lifetime change.
12:15 PM - EP10.5.08
Spontaneous and Strong Graphene n-Doping via Soda-Lime Glass and Its Application in Graphene-Semiconductor Junctions
D. M. Nanditha Dissanayake 3,Ahsan Ashraf 2,Dan Dwyer 4,Kim Kisslinger 1,Lihua Zhang 1,Yutong Pang 2,Harry Efstathiadis 4,Matthew Eisaman 2
1 Brookhaven National Laboratory Upton United States,3 Voxtel Inc. Eugene United States,1 Brookhaven National Laboratory Upton United States,2 Stony Brook University Upton United States4 SUNY Polytechnic Institute Albany United States1 Brookhaven National Laboratory Upton United StatesShow Abstract
Scalable and low-cost n-doping of pristine graphene has potential applications in a wide range of technologies such as batteries, sensors, fuel cells, microelectronic-circuits and could also serve as transparent, highly-conductive, and electrically-tunable junctions for optoelectronic applications. While p-doping of graphene is ubiquitous, sustainable n-doping of graphene remains a challenge. Previous attempts to chemically n-dope graphene have yielded low electron densities (less than 9.5x1012 cm-2) and are strongly susceptible to degradation over time. Furthermore, in most cases chemical doping causes adverse effects to the intrinsic functional properties of graphene. In this work, we demonstrate strong (1.33x1013 cm-2 electron density), robust, and spontaneous n-doping of graphene on the surface of a soda-lime-glass substrate. This is realized without any external wet/gas-phase chemical, high-temperature, or vacuum processes via surface-transfer doping from the sodium in the glass. We find that n-doping density reaches 2.11x1013 cm-2 when the graphene is transferred onto a p-type copper indium gallium diselenide (CIGS) semiconductor that itself has been deposited on soda-lime-glass. The high electron density is reached via surface-transfer doping from sodium atoms that diffuse from the glass through the CIGS and concentrate at the CIGS surface. We investigate this effect via aberration corrected high-resolution transmission electron microscopy, energy dispersive X-ray spectroscopy, time-of-flight secondary ion mass spectroscopy, current-voltage transconductance, and low temperature current-voltage measurements. Furthermore, we demonstrate an in-situ n-graphene/p-semiconductor Schottky junction with an ideality factor of 1.21 and strong photo response. The ability to achieve strong and persistent graphene n-doping on low-cost, industry-standard materials paves the way toward an entirely new class of graphene-based microelectronic and optoelectronics devices such as photodetectors, photovoltaics and sensors and electrochemical devices such as batteries and supercapacitors.
12:30 PM - EP10.5.09
Engineering van Hove Electronic Structure in Small-Angle Twisted Bilayer Graphene
Lujie Huang 1,Cheol-Joo Kim 1,Adam Tsen 2,Lola Brown 1,Marcos Guimaraes 3,Jiwoong Park 1
1 Chemistry and Chemical Biology Cornell University Ithaca United States,3 Applied Physics Cornell University Ithaca United States,2 Physics Columbia University New York United States3 Applied Physics Cornell University Ithaca United StatesShow Abstract
In twisted bilayer graphene (tBLG), the interlayer interaction induces additional van Hove singularities (vHS) and mini-gaps near the intersections between the Dirac cones of the two layers; this results in several electrical and optical phenomena at an energy level that monotonically increases with the twist angle θ. In tBLG with a small θ (below 5 degrees), the Fermi level can be tuned to reach the mini-gaps by electrical doping, enabling additional means (“conductance switching”) to control its electrical properties. As a result, small-θ tBLG could allow for novel tBLG-based applications with θ-dependent properties. However, there currently exist two limiting factors that limit its further development: scalable production of θ-controlled tBLG and a facile characterization method for small θ. In this talk, we first demonstrate a novel dry transfer method that can produce small-θ tBLG samples with controlled θ and high-quality interface. Second, we introduce a Raman-based characterization method that directly maps θ in small-θ tBLG samples based on our quantitative study of the R’ Raman band. Finally, we report the electrical conductivity of small-θ tBLG transistor with dual gates, which shows doping dependent conductance switching when the Fermi level is close to the mini-gaps. Our experiment is the first comprehensive electrical and optical characterization of angle-resolved small-θ tBLG that can be further generalized for developing novel electronic and optoelectronic devices based on multilayer 2D materials with the interlayer rotation as an additional tuning parameter.
EP10.6: 2D Materials for Flexible Optoelectronics
Thursday PM, March 31, 2016
PCC North, 200 Level, Room 222 C
2:30 PM - *EP10.6.01
Transparent Healthcare Devices Using Functionalized Graphene
Dae-Hyeong Kim 1
2 Center for Nanoparticle Research, Institute for Basic Science Seoul Korea (the Republic of),1 Seoul National Univ Seoul Korea (the Republic of),Show Abstract
Recent advances in soft electronics have attracted great attention due in large to the potential applications in personalized, bio-integrated healthcare devices. The mechanical mismatch between conventional electronic/optoelectronic devices and soft human tissues/organs causes many challenges, such as the low signal to noise ratio of biosensors because of the incomplete integration of rigid devices with the body, inflammations and excessive immune responses of implanted stiff devices originated from frictions and foreign nature to biotic systems, and the huge discomfort and consequent stress to users in wearing/implanting these devices. Ultraflexible and stretchable electronic and optoelectronic devices utilize the low system modulus and the intrinsic system-level softness to solve these issues. Here, we describe our unique strategies in the synthesis and functionalization of nanoscale two dimensional materials, their seamless assembly and integration, and corresponding device designs toward wearable and implantable healthcare devices. These implantable and wearable bioelectronic systems combine recent breakthroughs in unconventional soft electronics to address unsolved issues in the clinical medicine, which provides new opportunities the personalized healthcare.
3:00 PM - EP10.6.02
Highly Transparent, Wearable Alcohol Sensors for Wireless Detection Using Metal Oxide-Metal Nanowire Hybrid Structures
Joohee Kim 1,So-Yun Kim 1,Gyu Jeong Jeong 1,Jang Ung Park 1
1 UNIST Ulsan Korea (the Republic of),Show Abstract
Electronic systems that enable bio-sensing of physiological conditions with wireless communication in wearable platforms are growing interests in the next-generation electronics. To realize hands-free detectors, recent advances are developed by incorporating bio-sensor into power transmission coil or Bluetooth modules to eliminate the use of onboard battery and cumbersome external connection.
In addition, for the fabrication electronics in wearable form remaining the key elements such as flexibility and transparency, recent research suggests considerable types of strategies compared to use of conventional ITO electrode, in which metal nanowires (NWs) electrode is a powerful candidate due to their mechanical flexibility, conductivity, and transparency. Also, for the fabrication of highly sensitive and selective alcohol sensor, metal oxide-based materials are suitable for detection of reducing or oxidizing gases by current measurements.
In this talk, we presented wearable, transparent, and wireless alcohol gas sensor based on hybrid structures using metal oxide materials and metal NWs. This alcohol sensors show excellent properties against mechanical loading which provides advantages to fabrication of wearable electronics. Also, the real-time sensing of alcohol gas exhibits high sensitivity by responding below 250 ppm and high-reliability in terms of time (~15 days). Furthermore, we demonstrated the real-time, wireless detector for monitoring the alcohol gas by integrating a resonant circuit or Bluetooth modules into the devices. We believe that the advance of these device suggest a substantial promise application in future electronics.
3:15 PM - EP10.6.03
Multifunctional Oxides for Integrated Manufacturing of Efficient Graphene Electrodes for Organic Electronics
Piran Ravichandran Kidambi 1,Christ Weijtens 2,John Robertson 3,Stephan Hofmann 3,Jens Meyer 2
1 MIT Cambridge United States,2 Philips Research Aachen Germany3 University of Cambridge Cambridge United KingdomShow Abstract
Using multi-functional oxide films, we report on the development of an integration strategy for
scalable manufacturing of graphene-based transparent conducting electrodes (TCEs) for organic
electronics. A number of fundamental and process challenges exists for efficient graphene-based
TCEs, in particular, environmentally and thermally stable doping, interfacial band engineering for
efficient charge injection/extraction, effective wetting, and process compatibility including
masking and patterning. Using complementary X-ray photoelectron spectroscopy (XPS) and Ultra-violet
photoelectron spectroscopy (UPS), we show that all of these challenges can be effectively addressed at
once by coating graphene with a thin (>10 nm) metal oxide layer. We demonstrate
graphene electrode patterning without the need for conventional lithography and thereby achieve
organic light emitting diodes with efficiencies exceeding those of standard indium tin oxide
reference devices on rigid and flexible substrates.
Sanders et al. Nanoscale (just accepted)
Kidambi et al. Applied Physics Letters (2015)
Kuruvila et al. Journal of Materials Chemistry C (2014)
Meyer et al. Scientific Reports (2014)
3:30 PM - *EP10.6.04
Laser Material Interactions for Flexible and Nanomaterial Applications
Keon Jae Lee 1
1 Department of Materials Science and Engineering KAIST (Korea Advanced Institute of Science and Technology) Daejeon Korea (the Republic of),Show Abstract
This seminar introduces recent progresses of laser material interactions that can extend the application of self-powered flexible electronics and two-dimensional (2D) materials. Laser technology is highly important for future flexible electronics because it can adopt high-temperature processes on plastics, which is essential for high-performance electronics, resulting from ultra-short time pulse duration. (e.g. LTPS process over 1000 °C). The first part will introduce self-powered flexible piezoelectric energy harvesting via inorganic-based laser lift off (ILLO). Energy harvesting technology converting external mechanical sources (such as vibration and bio-mechanical energy) to electrical energy is recently a highly demanding issue. The high-performance flexible thin film nanogenerator was transferred by ILLO from bulk substrates for self-powered biomedical devices such as pacemakers and brain stimulators. The second part will introduce flexible large scale integrations (LSI) and high-density 1S1R memristor devices via ILLO. Flexible memory is a crucial part of electronics for data processing, storage, and radio frequency (RF) communication. To fabricate flexible memristor arrays, we applied innovative transfer protocol of high density 1S1R memory with ILLO technology. The last part will discuss the laser material interaction for 2D materials. We observed a melt-mediated phase separation into two thin layers from SiC wafers via excimer laser. This technology is very useful to make new two dimensional materials.
Related References (from Keon’s group as corresponding authors)
 Nano Letters 11, 5438, 2011.  Nano Letters 10, 4939, 2010.
 Nano Letters 12, 4810, 2012.  Nano Letters 14, 7031, 2014
 Adv. Mater, 26, 2514, 2014.  Adv. Mater. 26, 4880, 2014
 Adv. Mater, 26, 7480, 2014  Adv. Mater. 24, 2999, 2012.
 Adv. Mater. 27, 3982, 2015  Adv. Mater. 27, 2866, 2015
 Energy Environ. Sci. 8, 2677, 2015  Energy Environ. Sci., 7, 4035, 2014
 ACS Nano 7, 11016, 2013  ACS Nano 9, 4120, 2015
 ACS Nano 7, 4545, 2013  ACS Nano 7, 2651, 2013.
 ACS Nano 8, 9492, 2014  ACS Nano 8, 7671, 2014
 ACS Nano, 9, 6587, 2015  Adv. Energy Mater. 3, 1539, 2013
 Adv. Energy Mater. 5, 1500051, 2015  Adv. Func. Mater. 24, 2620, 2014
 Adv. Func. Mater. 24, 6914, 2014  Nano Energy, 14, 111, 2015
 Nano Energy, 1, 145, 2012
Keon Jae Lee is an associate professor in Dept. of MSE at KAIST. He earned the Ph.D degree from University of Illinois Urbana Champaign, in 2006. His current research interests are self-powered flexible electronic system for bio, energy and electronic devices. He has coauthored over 60 SCI papers in journal including Science, Nature Materials, Nano Letters, Adv. Mater, Energy Environ. Sci, ACS Nano, Adv. Energy Mater., Adv. Func. Mater., Nano Energy. He has filed ~150 patents and more than 40 of these are licensed.
4:30 PM - *EP10.6.05
Stretchable Graphene Photodetector with Enhanced and Strain-Tunable Photoresponsivity
Pilgyu Kang 1,Michael Cai Wang 1,Peter Knapp 1,SungWoo Nam 1
1 Univ of Illinois-Urbana Urbana United States,Show Abstract
Graphene offers numerous advantages for flexible optoelectronics including outstanding mechanical strength, high carrier mobility, broadband absorption, high electrical and thermal conductivity, and gate-tunable carrier density. In addition, unlike other conventional semiconductor materials, graphene exhibits broadband absorption which could lead to the realization of a broadband photodetector. Due to its low optical absorptivity, graphene photodetector research so far has been focused on hybrid systems. However, hybrid systems require complicated integration and fabrication processes on conventional rigid substrates, and the formation of heterogeneous interfaces could result in the loss of high carrier transport velocity intrinsic to graphene. In this talk, I will present our work on a stretchable photodetector with enhanced and strain-tunable photoresponsivity. Our detector is based exclusively on graphene and engineers the two dimensional material into three dimensional (3D) structures. We achieve enhanced graphene photoabsorption by increasing its areal density with a buckled 3D structure, which simultaneously improves device stretchability to 200% strain. A c.a. 400% enhancement in photoresponsivity is achieved compared to the responsivity of a flat graphene photodetector. Furthermore, we demonstrate a new concept of strain-tunable photoresponsivity where a 200% applied tensile strain results in a 100% modulation in photoresponsivity. I will also discuss the capability of our stretchable photodetector as a highly flexible and conformal photodetector with enhanced photoresponsivity that can be applied to arbitrary shaped surfaces for potential applications in bio-implantable electronics. This approach to photoresponsivity enhancement and tunability can also be applied to other emerging two dimensional materials to achieve high performance and flexible applications. We believe that a highly stretchable and conformal photodetector with enhanced photoresponsivity will find broad applications in flexible electronics and stretchable optoelectronics in the future.
5:00 PM - EP10.6.06
Ultra-Thin Layered Ternary Semiconductor: For High Performance Photo-Transistor on Rigid and Flexible Substrate
Packiyaraj Perumal 1,Yang-Fang Chen 1
1 National Taiwan University Taipei Taiwan,Show Abstract
Two-dimensional (2D) ternary semiconductor are an emerging class of new materials, have attracted significant interest in recently as a family of few-layered materials owing to the huge potential for electronic and optoelectronic applications. Yet, in contrast to other types of metal dichalcogenides, 2D tin dichalcogenides semiconductor, is also an important layered compounds with high performance ultrathin channels and flexible optoelectronic devices. In general, Se doping in tindichalgogenide whether we could assemble 2D layered material with an atomic thickness, which may give them some interesting properties such as those of MoS2, still remains unresolved. Herein, we report the ternary semiconductor, making them great candidates for photo-transistor, unprecedentedly impressive. However, using exfoliation from bulk crystal, we establish the characteristics of few-layered in optical, atomic force microscopy, and transmission electron microscopy. The few-layered photo-transistor, fabricated on both a rigid SiO2/Si and a flexible polyethylene terephthalate (PET) substrate and examined the optoelectronic properties. Field-effect transistor (FET) were made from thin layers of 6 nm, which exhibit n-type semiconductor behavior. Furthermore,a photo-transistor was demonstrated to exhibit a high photoresponsivity and specific detectivity (D*) is achieved, as well. These optoelectronic figure of merits are significantly higher than those of recently reported photodetectors configured with other 2D crystals (GaS, GaSe,GeSe and InSe). It is worth emphasizing that the synthesized ultrathin nanosheets possess confined thickness, high responsivity, compatibility and flexibility with current technologies make the ternarysemiconductor photo-transistor a highly qualified candidate for next-generation optoelectronic applications.
5:15 PM - EP10.6.07
Photoconductive Characteristics of Copper Phthalocyanine-Stacked MoS2 Field Effect Transistors
Jinsu Pak 1,Jingon Jang 1,Kyungjune Cho 1,Tae-Young Kim 1,Jae-Keun Kim 1,Younggul Song 1,Woong-Ki Hong 2,Misook Min 1,Hyoyoung Lee 3,Takhee Lee 1
1 Seoul National University Seoul Korea (the Republic of),2 Korea Basic Science Institute Jeonju Korea (the Republic of)3 Sungkyunkwan University Seoul Korea (the Republic of)Show Abstract
Recently, transition metal dichalcogenide (TMD) two-dimensional (2D) materials have gained a considerable interest as a candidate for next generation nanoelectronics devices. Among the various TMD materials, molybdenum disulfide (MoS2) has been widely researched as 2D field-effect transistors (FETs) and exhibited the outstanding electrical properties. In contrast to graphene which lacks an energy band gap, MoS2 has a direct band gap of 1.8 eV as a single layer and an indirect band gap of 1.2 eV as a multilayer MoS2. Becuase of these band gap properties of MoS2, the photodetectors made with MoS2 have exhibited excellent photoreponse properties. Beyound the structure of MoS2 FET alone, it has been demonstrated the MoS2-based heterostructure through stacking other p-type materials results in higher photoreponsivity . If p-type materials are to be used for heterostructure MoS2-based photodetectors, organic materials can be a good choice because p-type organic semiconductor layers have been extensively studied and can be easily stacked on 2D films by spin-coating or deposition systems, and the thickness of the organic layer on 2D films can be accurately controlled.
In this study, we have investigated the electrical and photoresponsive properties of MoS2 FET-based photodetectors by stacking copper phthalocyanine (CuPc) layers on the MoS2 surface . We characterized and compared the shift of threshold voltage and the photodetection characteristics of MoS2 devices prior to and after the devices were stacked with p-type organic semiconductor (CuPc) layers on the MoS2 surface. In addition, we found the optimized CuPc thickness (~2 nm) on MoS2 surface for the best performance as a photodetector with a photoresponsivity of ~1.98 A/W, a detectivity of ~6.11 × 1010 jones, and an external quantum efficiency of ~12.57 %. Consequentially, we could enhance the photoresponsivity of devices throughout the wavelength ranges and under different intensity. Our study suggest that the MoS2 vertical hybrid structure with organic material can be promising as efficient photodetecting devices and optoelectronic circuits
 B. Radisavljevic, A. Radenovic, J. Brivio, V. Giacometti and A. Kis, Nature Nanotechnology, 6, 147 (2011).
 D. Kufer, I. Nikitskiy, T. Lasanta, G. Navickaite, F.H.L. koppens, and G. Konstantaos, Adv. Mater. 27, 176 (2015).
 Jinsu Pak, Jingon Jang, Kyungjune Cho, Tae-Young Kim, Jae-Keun Kim, Younggul Song, Woong-Ki Hong, Misook Min, Hyoyoung Lee, and Takhee Lee, submitted (2015).
5:30 PM - EP10.6.08
Graphene Based Multifunctional Capacitive Sensors and Their Performances
Kang Minpyo 1,Jong-Hyun Ahn 1
1 Yonsei university Seoul Korea (the Republic of),Show Abstract
Recently, considerable efforts have been devoted towards realization of wearable devices due to its unique capability of detecting multiple stimuli changes such as pressure, touch and temperature. This capability has potential to extends its applicability to human robotics and smart prosthetics. Wearable devices(e-skin) basically imitates the functions of human skin which consists of an integrated network of sensors that analyze information about tactile and thermal stimuli. Moreover, there has progress in developing an electronic skin with additional functionalities including chemical and biological sensing with great sensibilities. Implementing wearable devices requires contact with various substrates conformally during service which makes conformal property of flexible devices for mechanical behaviors and even reliability essential for realizing the technology. For realizing this wearable electronic devices which have conformality, it is important to determine suitable material. There are many candidates such as graphene, metal meshes and silver nanowires which have great electrical properties. But among those candidates, graphenes combines flexibility with conductivity and transparency, making it an obvious choice for research into flexible electronic devices.
In this research, our sensors have been demonstrated for several wearable applications including touch panels, proximity sensors and even sensing temperature and various chemical solutions which could be harmful. These various sensing capabilities originates from the capacitance change in the sensing point according to a disturbance of the fringing electric field. These multi-stimuli responses will be an effective way to realize next-generation wearable devices
5:45 PM - EP10.6.09
Controlled Growth of Graphene on Various Substrates via Mobile Hot-Wire-Assisted CVD
Jinwook Baek 1,Jinsup Lee 1,Hongkyung Kim 1,Myeongsoo Lee 1,Jaegil Choi 1,Joo Yub Kim 1,Seokwoo Jeon 1
1 KAIST Daejeon Korea (the Republic of),Show Abstract
In this work, we introduce two major strategies of graphene synthesis with mobile hot-wire (MHW) assisted chemical vapor deposition (CVD) system; increasing the graphene domain size with the formation and merging of graphene nanoclusters on Cu foil and growing graphene directly on dielectric substrates by controlling the partial vapor pressure of metal catalyst. For increasing graphene domain size, cooling late of graphene at designated locations can be controlled by movement of a MHW as an independent heat source over a Cu foil which is on a bottom heater. Graphene nanoclusters nucleated on a Cu foil before the movement of MHW are growing and merging into global graphene domain through external variables including the substrate temperature, wire scan speed, and the flow rate of carbon feed gas. The observation of global domain (~ 300 μm ) of graphene embedded with small domains (~ 10 nm) whose orientations have exclusively high-angle tilt boundary of approximately 30°, reveals the potential growth mechanism and recrystallization of graphene in our system. For the direct growth of graphene on dielectric substrates, partial vapor pressure of metal catalyst is the key factor in graphene synthesis. We utilize the catalytic metal as a mobile wire to control the partial vapor pressure of metal clusters, which transport the atomic carbon to the substrate and completes the graphene growth, resulting in continuous and highly crystalline graphene. This result represents a significant step forward in understanding the growth mechanism of CVD graphene and offers an insight on the transfer-free growth of graphene on arbitrary substrates.