Zhou Mi
Email Address
phyzhoum@nus.edu.sg
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Publication Solution-processed conjugated polymer organic p-i-n light-emitting diodes with high built-in potential by solution- and solid-state doping(2009) Sivaramakrishnan, S.; Zhou, M.; Kumar, A.C.; Chen, Z.-L.; Png, R.-Q.; Chua, L.-L.; Ho, P.K.H.; PHYSICS; CHEMISTRYPolymer p-i-n homojunction light-emitting diodes (LEDs) comprising p -doped poly(dioctylfluorene-alt-benzothiadiazole) (F8BT) hole-injection, intrinsic F8BT emitter, and n -doped F8BT electron-injection layers have been demonstrated. A thin F8BT film was photocrosslinked and bulk p-doped by nitronium oxidation, then overcoated with an F8BT layer which was then surface n-doped by contact printing with naphthalenide on an elastomeric stamp. These LEDs exhibit high built-in potential (Vbi =2.2 V), efficient bipolar injection, and greatly improved external electroluminescence efficiency compared to control devices without the p-i-n structure. A modulated photocurrent technique was used to measure this Vbi, which systematically improves with diode structure. © 2009 American Institute of Physics.Publication Determination of the interface δ-hole density in a blue-emitting organic semiconductor diode by electromodulated absorption spectroscopy(2010-09-13) Zhou, M.; Png, R.-Q.; Sivaramakrishnan, S.; Chia, P.-J.; Yong, C.-K.; Chua, L.-L.; Ho, P.K.H.; PHYSICS; CHEMISTRYThe hole density at the interface of a poly(fluorene-alt-triarylamine) (TFB) with p-doped poly(3,4-ethylenedioxythiophene) (PEDT) anodes has been determined from electromodulation of the TFB subgap polaron band. At 295 K, this δ-hole density σOSC is approximately 1× 1012 cm-2, for which the current-voltage characteristics indicate an Ohmic contact. However at 30 K, σOSC falls to 2× 1011 cm-2, and the contact characteristics approach the injecting-blocking boundary. The PEDT/TFB vacuum offset inside the device is inferred to be 0.1 eV, and so the Fermi level is not as deeply pinned in the TFB gap as suggested by ultraviolet photoemission measurements. © 2010 American Institute of Physics.Publication Role of δ-Hole-doped interfaces at ohmic contacts to organic semiconductors(2009-08-06) Zhou, M.; Chua, L.-L.; Png, R.-Q.; Yong, C.-K.; Sivaramakrishnan, S.; Chia, P.-J.; Wee, A.T.S.; Friend, R.H.; Ho, P.K.H.; PHYSICS; CHEMISTRY; NUS NANOSCIENCE & NANOTECH INITIATIVEAn electromodulated absorption spectroscopy study of the contact between an organic semiconductor (OSC) poly(2,5-dialkoxy-p-phenylenevinylene) and p-doped poly(3,4-ethylenedioxythiophene) electrodes of different work functions (φvac) reveals direct evidence for the formation of a hole-doped layer at the OSC interface in equilibrium with high-φvac electrodes. When the hole density at this interface exceeds a few 1011cm-2, degenerate "bandlike" polaron states emerge. This appears to be crucial to furnish efficient carrier injection into the bulk of the OSC to achieve Ohmic injection. The gap measured by ultraviolet photoemission between the electrode Fermi level and the OSC transport level (typically pinned at 0.6eV) does not reflect the true injection barrier. © 2009 The American Physical Society.Publication High-performance polymer semiconducting heterostructure devices by nitrene-mediated photocrosslinking of alkyl side chains(2010-02) Png, R.-Q.; Chia, P.-J.; Tang, J.-C.; Liu, B.; Sivaramakrishnan, S.; Zhou, M.; Khong, S.-H.; Chan, H.S.O.; Burroughes, J.H.; Chua, L.-L.; Friend, R.H.; Ho, P.K.H.; PHYSICS; CHEMISTRY; DATA STORAGE INSTITUTEHeterostructures are central to the efficient manipulation of charge carriers, excitons and photons for high-performance semiconductor devices. Although these can be formed by stepwise evaporation of molecular semiconductors, they are a considerable challenge for polymers owing to re-dissolution of the underlying layers. Here we demonstrate a simple and versatile photocrosslinking methodology based on sterically hindered bis(fluorophenyl azide)s. The photocrosslinking efficiency is high and dominated by alkyl side-chain insertion reactions, which do not degrade semiconductor properties. We demonstrate two new back-infiltrated and contiguous interpenetrating donor-acceptor heterostructures for photovoltaic applications that inherently overcome internal recombination losses by ensuring path continuity to give high carrier-collection efficiency. This provides the appropriate morphology for high-efficiency polymer-based photovoltaics. We also demonstrate photopatternable polymer-based field-effect transistors and light-emitting diodes, and highly efficient separate-confinement-heterostructure light-emitting diodes. These results open the way to the general development of high-performance polymer semiconductor heterostructures that have not previously been thought possible. © 2010 Macmillan Publishers Limited. All rights reserved.Publication Mn-doped thiolated Au25 nanoclusters: Atomic configuration, magnetic properties, and a possible high-performance spin filter(2011-04-04) Zhou, M.; Cai, Y.Q.; Zeng, M.G.; Zhang, C.; Feng, Y.P.; ELECTRICAL & COMPUTER ENGINEERING; PHYSICSWe report an ab inito investigation on the ground-state atomic configuration, electronic structures, magnetic, and spin-dependent transport properties of Mn-doped Au25 nanoclusters protected by thiolate. It is found that the most stable dopant sites are near surfaces, rather than the center position of the nanoparticles. Transport calculations show that high- performance spin filters can be achieved by sandwiching these doped clusters between two nonmagnetic Au electrodes. The nearly perfect spin filtering originates from localized magnetic moments of these clusters that are well protected by ligands from the presence of electrodes. © 2011 American Institute of Physics.Publication Band-like transport in surface-functionalized highly solution-processable graphene nanosheets(2008-09-17) Wang, S.; Chia, P.-J.; Chua, L.-L.; Zhao, L.-H.; Png, R.-Q.; Sivaramakrishnan, S.; Zhou, M.; Goh, R.G.-S.; Friend, R.H.; Wee, A.T.-S.; Ho, P.K.-H.; PHYSICSSubstochiometric Graphite oxide (GO) nanosheets were surface-functionalized and purified to show excellent dispersity at the single-sheet level in organic solvents, sufficient for spin-coating and printing onto various substrates. It is found that the sheets are ca. 18-Å thick due to the combined thickness of the basal plane and the alkyl chains. No measurable dc conductance is obtained for anneal temperature below 180°C, which suggests the film conductivity less than 1×10-1 S cm-1. The sp 2 fraction of carbon atoms gives the fraction of carbons connected into the π-electron network responsible for the electronic and optical properties of the nanosheets. It is also found that mobile charge carriers are field-induced and confined on the most extended sections of these graphenites.Publication Direct evidence for the role of the madelung potential in determining the work function of doped organic semiconductors(2009-03-02) Chia, P.-J.; Sivaramakrishnan, S.; Zhou, M.; Png, R.-Q.; Chua, L.-L.; Friend, R.H.; Ho, P.K.H.; PHYSICS; CHEMISTRYThe work function of a model degenerately doped organic semiconductor p-doped poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonic acid) can be systematically tuned over an eV-wide range by exchanging excess matrix protons with spectator cations, without altering the organic semiconductor doping level or polaron density. Ultraviolet photoelectron spectroscopy reveals this to arise not from an interface dipole, but from a bulk effect due to a shift in the Madelung potential set up by the local counter- and spectator-ion structure at the polaron sites. Electrostatic modeling of this potential is in agreement with the observed shift in carrier energetics. The spectator cations also cause a systematic shift in electron-phonon coupling and carrier delocalization, as revealed by infrared and Raman phonon modes, and charge-modulated absorption, which can be related to disorder in this potential. © 2009 The American Physical Society.Publication Direct spectroscopic evidence for a photodoping mechanism in polythiophene and poly(bithiophene-alt-thienothiophene) organic semiconductor thin films involving oxygen and sorbed moisture(2009-12-11) Zhuo, J.-M.; Zhao, L.-H.; Png, R.-Q.; Wong, L.-Y.; Chia, P.-J.; Tang, J.-C.; Sivaramakrishnan, S.; Zhou, M.; Ou, E.C.-W.; Chua, S.-J.; Sim, W.-S.; Chua, L.-L.; Ho, P.K.-H.; PHYSICS; CHEMISTRYDirect infrared spectroscopic evidence has been obtained for photodoping of high mobility regioregular poly(3-alkylthiophene) and poly[2,5-bis(3- alkyllthiophen-2-yl) thieno (3,2-b)thiophene] with the attendant formation of hydroxide counter-ions. This reveals the central role of dissolved water, explains the key features of degradation of the electrical characteristics of organic semiconductors in the ambient (see figure), and points to possible strategies to further improve their stability. © 2009 WILEY-VCH Verlag GmbH & Co. KGaA.Publication Effective work functions for the evaporated metal/organic semiconductor contacts from in-situ diode flatband potential measurements(2012-07-02) Zhou, M.; Png, R.-Q.; Khong, S.-H.; Sivaramakrishnan, S.; Zhao, L.-H.; Chua, L.-L.; Friend, R.H.; Ho, P.K.H.; PHYSICS; CHEMISTRYThe diode built-in potentials (V bi) of several polymer organic semiconductor (OSC) thin films [(2,5-dialkoxy-substituted poly(p- phenylenevinylene), poly(9,9-dialkylfluorene), poly(9,9-dialkylfluorene-alt- phenylene(N-phenyl)iminophenylene), and poly(9,9-dialkylfluorene-alt- benzothiadiazole)] sandwiched between p-doped poly(3,4-ethylenedioxythiophene) (PEDT:PSSH) and evaporated metal contacts have been measured by bias-dependent electromodulated absorption (EA) spectroscopy of the Stark-shifted π-π band. From these values and the vacuum-level offsets at the PEDT:PSSH contacts evaluated by sub-gap EA spectroscopy, the following effective work functions for the buried evaporated metal contacts have been obtained: Al 3.4 ± 0.1, Ag 3.7 ± 0.1, Au 4.4 ± 0.1, and Ca 2.4 ± 0.1 eV. These work functions are smaller than those of the clean metal surfaces by up to 0.8 eV, and are substantially independent of the OSC in the absence of charge transfer. © 2012 American Institute of Physics.Publication Electromigration of the conducting polymer in organic semiconductor devices and its stabilization by cross-linking(2007) Png, R.-Q.; Chia, P.-J.; Sivaramakrishnan, S.; Wong, L.-Y.; Zhou, M.; Chua, L.-L.; Ho, P.K.-H.; PHYSICSX-ray photoelectron spectroscopy (XPS) measurement of the ratio of poly(3,4-ethylenedioxythiophene) (PEDT) to polystyrenesulfonate (PSS) reveals accumulation of PEDT+ at the interface between the PEDT:PSSH hole-injection layer and the organic semiconductor during diode operation. This ionic drift of PEDT+ occurs even at low fields of 1 V cm-1, which will have an impact on the operational stability of the characteristics of organic light-emitting diodes. XPS and Raman spectroscopy indicate that dedoping of PEDT+ does not occur significantly in hole-only devices. Cross-linking at the 1 mol % level can stabilize the conducting polymer sufficiently against electromigration. © 2007 American Institute of Physics.