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3 Sep 2012

Volume 101, Issue 10, Articles (10xxxx)

Issue Cover Spotlight Figure

Appl. Phys. Lett. 101, 103101 (2012); http://dx.doi.org/10.1063/1.4748099 (5 pages)

Massimo Cuscunà, Annalisa Convertino, Emiliano Zampetti, Antonella Macagnano, Alessandro Pecora, Guglielmo Fortunato, Laura Felisari, Giuseppe Nicotra, Corrado Spinella, and Faustino Martelli
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Electron spin resonance signature of the oxygen vacancy in HfO2

R. Gillen, J. Robertson, and S. J. Clark

Appl. Phys. Lett. 101, 102904 (2012); http://dx.doi.org/10.1063/1.4751110 (4 pages) | Cited 3 times

Online Publication Date: 6 September 2012

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The oxygen vacancy has been inferred to be the critical defect in HfO2, responsible for charge trapping, gate threshold voltage instability, and Fermi level pinning for high work function gates, but it has never been conclusively identified. Here, the electron spin resonance g tensor parameters of the oxygen vacancy are calculated, using methods that do not over-estimate the delocalization of the defect wave function, to be gxx = 1.918, gyy = 1.926, gzz = 1.944, and are consistent with an observed spectrum. The defect undergoes a symmetry lowering polaron distortion to be localized mainly on a single adjacent Hf ion.
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76.30.Mi Color centers and other defects
61.72.jd Vacancies
71.38.-k Polarons and electron-phonon interactions
71.20.Ps Other inorganic compounds
71.55.Ht Other nonmetals

Fixed charge in high-k/GaN metal-oxide-semiconductor capacitor structures

Junwoo Son, Varistha Chobpattana, Brian M. McSkimming, and Susanne Stemmer

Appl. Phys. Lett. 101, 102905 (2012); http://dx.doi.org/10.1063/1.4751466 (3 pages) | Cited 1 time

Online Publication Date: 7 September 2012

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The location and nature of fixed charge states in high-k/GaN metal-oxide-semiconductor capacitor structures are characterized by analyzing flatband voltage shifts in high-frequency capacitance-voltage measurements. It is shown that a significant fixed, positive sheet charge forms at Al2O3/GaN interfaces, but not at HfO2/GaN interfaces. Furthermore, an interface dipole is created at HfO2/Al2O3 interfaces, which causes an abrupt shift in the flat band voltage as HfO2 is introduced to form HfO2/Al2O3 bilayer dielectrics. The observed dependence of the flatband voltage shift on the relative thicknesses of the dielectrics comprising the bilayer dielectrics is discussed.
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73.40.Qv Metal-insulator-semiconductor structures (including semiconductor-to-insulator)
84.32.Tt Capacitors
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On-chip fabrication of ultrasensitive NO2 sensors based on silicon nanowires

Massimo Cuscunà, Annalisa Convertino, Emiliano Zampetti, Antonella Macagnano, Alessandro Pecora, Guglielmo Fortunato, Laura Felisari, Giuseppe Nicotra, Corrado Spinella, and Faustino Martelli

Appl. Phys. Lett. 101, 103101 (2012); http://dx.doi.org/10.1063/1.4748099 (5 pages) | Cited 1 time

Online Publication Date: 4 September 2012

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We report a very simple, robust, and reliable on-chip fabrication method of a chemoresistive sensor based on silicon nanowires (NWs). Our method permits the use of nanowires without the need of their removal and transfer to a support different from the growth substrate. Our method, completely based on the silicon technology platform, exploits nanowires directly grown onto a selected area, over and between pre-patterned, interdigitated electrodes defined on oxidized silicon. The fabricated sensor is capable to detect NO2 down to a few ppb levels operating at room temperature. The sensor characteristics benefit of the presence of self-welded nanowires.
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07.07.Df Sensors (chemical, optical, electrical, movement, gas, etc.); remote sensing
81.07.Gf Nanowires

A few-electron quadruple quantum dot in a closed loop

Romain Thalineau, Sylvain Hermelin, Andreas D. Wieck, Christopher Bäuerle, Laurent Saminadayar, and Tristan Meunier

Appl. Phys. Lett. 101, 103102 (2012); http://dx.doi.org/10.1063/1.4749811 (4 pages)

Online Publication Date: 4 September 2012

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We report the realization of a quadruple quantum dot device in a square-like configuration where a single electron can be transferred on a closed path free of other electrons. By studying the stability diagrams of this system, we demonstrate that we are able to reach the few-electron regime and to control the electronic population of each quantum dot with gate voltages. This allows us to control the transfer of a single electron on a closed path inside the quadruple dot system. This work opens the route towards electron spin manipulation using spin-orbit interaction by moving an electron on complex paths free of electrons.
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85.35.Gv Single electron devices

Electronic structure of graphene oxide and reduced graphene oxide monolayers

D. S. Sutar, Gulbagh Singh, and V. Divakar Botcha

Appl. Phys. Lett. 101, 103103 (2012); http://dx.doi.org/10.1063/1.4749841 (5 pages) | Cited 2 times

Online Publication Date: 4 September 2012

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Graphene oxide (GO) monolayers obtained by Langmuir Blodgett route and suitably treated to obtain reduced graphene oxide (RGO) monolayers were studied by photoelectron spectroscopy. Upon reduction of GO to form RGO C1s x-ray photoelectron spectra showed increase in graphitic carbon content, while ultraviolet photoelectron spectra showed increase in intensity corresponding to C2p-π electrons (∼3.5 eV). X-ray excited Auger transitions C(KVV) and plasmon energy loss of C1s photoelectrons have been analyzed to elucidate the valence band structure. The effective number of (π+σ) electrons as obtained from energy loss spectra was found to increase by ∼28% on reduction of GO.
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73.22.Pr Electronic structure of graphene
81.05.ue Graphene
79.60.Bm Clean metal, semiconductor, and insulator surfaces
82.80.Pv Electron spectroscopy (X-ray photoelectron (XPS), Auger electron spectroscopy (AES), etc.)
79.20.Uv Electron energy loss spectroscopy
71.45.Gm Exchange, correlation, dielectric and magnetic response functions, plasmons

Nanoparticle formation in a cavitation bubble after pulsed laser ablation in liquid studied with high time resolution small angle x-ray scattering

Shyjumon Ibrahimkutty, Philipp Wagener, Andreas Menzel, Anton Plech, and Stephan Barcikowski

Appl. Phys. Lett. 101, 103104 (2012); http://dx.doi.org/10.1063/1.4750250 (4 pages) | Cited 4 times

Online Publication Date: 4 September 2012

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We investigated nanoparticle formation after pulsed laser ablation in liquid using time-resolved small angle x-ray scattering. Laser ablation of a gold target in water induces a cavitation bubble in which two different particle species could be identified at maximum bubble extension: (i) primary particles of about 8–10 nm diameter, which show a smooth concentration gradient starting from the target and can also be found outside the cavitation bubble in the free liquid and (ii) secondary particles in the range of 45 nm diameter which have highest concentration in the upper part of the cavitation bubble but do not penetrate into the liquid.
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81.16.-c Methods of micro- and nanofabrication and processing
47.55.dp Cavitation and boiling
47.55.db Drop and bubble formation
79.20.Eb Laser ablation
78.70.Ck X-ray scattering

Electroluminescence from quantum dots fabricated with nanosphere lithography

L. Yu, S. Law, and D. Wasserman

Appl. Phys. Lett. 101, 103105 (2012); http://dx.doi.org/10.1063/1.4751341 (4 pages)

Online Publication Date: 5 September 2012

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We demonstrate strong carrier confinement in, and electroluminescence (EL) from, quantum nanostructures fabricated from epitaxially grown quantum wells (QWs) using a top-down nanosphere lithography, dry-etch, mass-transport, and overgrowth fabrication process. Optically active nano-pillars with diameters as small as 90 nm are fabricated, and narrow linewidth (18 meV) electroluminescence from a fabricated diode structure is observed, with an emission blue-shift of over 37 meV from the original quantum well sample luminescence. The results presented offer the potential for low-cost, large-area patterning of quantum nanostructures for optoelectronic applications.
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81.07.Ta Quantum dots
81.16.Nd Micro- and nanolithography
81.65.Cf Surface cleaning, etching, patterning
81.07.St Quantum wells
78.67.Hc Quantum dots
78.60.Fi Electroluminescence

Flexible, transparent dielectric capacitors with nanostructured electrodes

Sophie Sorel, Umar Khan, and Jonathan N. Coleman

Appl. Phys. Lett. 101, 103106 (2012); http://dx.doi.org/10.1063/1.4750059 (5 pages)

Online Publication Date: 5 September 2012

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We have prepared flexible, transparent, dielectric capacitors by spraycasting very thin networks of single walled nanotubes (SWNTs) or silver nanowires (AgNWs) onto either side of free-standing polymer films. Impedance spectroscopy showed these structures to behave as a capacitor in combination with a series resistance. Those capacitors with SWNT electrodes displayed optical transmittance between 57% and 74%, capacitances ranging from 0.4 to 1.1 μF/cm2 and series resistances ranging 400 Ω/◻-10 kΩ/◻. However, using AgNW electrodes gave similar transmittance and capacitance but series resistance as low as 60 Ω/◻. Finally, the properties of these capacitors were invariant under flexing.
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84.32.Tt Capacitors

Synthesis and upconversion luminescence of N-doped graphene quantum dots

Ming Li, Wenbin Wu, Wencai Ren, Hui-Ming Cheng, Nujiang Tang, Wei Zhong, and Youwei Du

Appl. Phys. Lett. 101, 103107 (2012); http://dx.doi.org/10.1063/1.4750065 (3 pages) | Cited 4 times

Online Publication Date: 5 September 2012

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A hydrothermal approach was developed for the synthesis of N-doped graphene quantum dots (N-GQDs) by cutting N-doped graphene. The N-GQDs obtained have a N/C atomic ratio of ca. 5.6% and diameter of 1–7 nm. The photoluminescence (PL) properties of the N-GQDs were investigated. It was found that the N-GQDs possess bright blue PL and excellent upconversion PL properties.
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78.55.Hx Other solid inorganic materials
78.67.Hc Quantum dots
68.65.-k Low-dimensional, mesoscopic, nanoscale and other related systems: structure and nonelectronic properties
81.07.Bc Nanocrystalline materials
81.16.Rf Micro- and nanoscale pattern formation

Single photon emission in the red spectral range from a GaAs-based self-assembled quantum dot

Ł. Dusanowski, A. Golnik, M. Syperek, M. Nawrocki, G. Sȩk, J. Misiewicz, T. W. Schlereth, C. Schneider, S. Höfling, M. Kamp, and A. Forchel

Appl. Phys. Lett. 101, 103108 (2012); http://dx.doi.org/10.1063/1.4750241 (4 pages)

Online Publication Date: 5 September 2012

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Low temperature micro-photoluminecence and second-order single photon correlation experiments were performed on individual self-assembled In0.47Al0.34Ga0.19As/Al0.3Ga0.7As/GaAs quantum dots emitting in the range of 680–780 nm. Emission lines originating from exciton, biexciton, and charge exciton confined in the same dot could be identified. The derived exciton fine structure splitting is ∼ 125 μeV, whereas the biexciton and charge exciton binding energies are ∼ 4 and ∼ 9 meV, respectively. The photon correlation statistics measured for the exciton emission exhibited a clear antibunching with the value of gX−X2(0) = 0.30±0.05, confirming unambiguously that such quantum dots act as true single photon quantum emitters.
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78.67.Hc Quantum dots
68.65.Hb Quantum dots (patterned in quantum wells)
71.35.Pq Charged excitons (trions)
78.55.Cr III-V semiconductors

Morphology dependence of radial elasticity in multiwalled boron nitride nanotubes

H.-C. Chiu, S. Kim, C. Klinke, and E. Riedo

Appl. Phys. Lett. 101, 103109 (2012); http://dx.doi.org/10.1063/1.4751346 (5 pages) | Cited 1 time

Online Publication Date: 6 September 2012

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We report on the measurement of the radial modulus of boron nitride nanotubes (BN-NTs) with various sizes and thicknesses. These BN-NTs are radially much stiffer than previously reported thinner and smaller BN-NTs. Here, we show the key role of the morphology of the nanotubes in determining their radial rigidity; in particular, we find that the external and internal radii, Rext and Rint, have a stronger influence on the radial modulus than the nanotube’s thickness. The radial modulus decreases nonlinearly with 1/Rext until reaching, for a large number of layers and a large radius, the transverse elastic modulus of bulk h-BN.
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81.40.Jj Elasticity and anelasticity, stress-strain relations
61.46.Fg Nanotubes
81.07.De Nanotubes
62.20.de Elastic moduli

Frequency and Q factor control of nanomechanical resonators

Johannes Rieger, Thomas Faust, Maximilian J. Seitner, Jörg P. Kotthaus, and Eva M. Weig

Appl. Phys. Lett. 101, 103110 (2012); http://dx.doi.org/10.1063/1.4751351 (4 pages) | Cited 1 time

Online Publication Date: 6 September 2012

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We present an integrated scheme for dielectric drive and read-out of high-Q nanomechanical resonators that enable tuning of both the resonance frequency and quality factor with an applied dc voltage. A simple model for altering these quantities is derived, incorporating the resonator's complex electric polarizability and position in an inhomogeneous electric field, which agrees very well with experimental findings and finite element simulations. Comparing two sample geometries demonstrates that careful electrode design determines the direction of frequency tuning of flexural modes of a string resonator. Furthermore, we show that the mechanical quality factor can be voltage reduced sixfold.
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85.85.+j Micro- and nano-electromechanical systems (MEMS/NEMS) and devices
07.10.Cm Micromechanical devices and systems

Controlling the doping of single-walled carbon nanotube networks by proton irradiation

D. Walker, C. J. Mann, C. J. Panetta, D. R. Alaan, A. R. Hopkins, and S. H. Liu

Appl. Phys. Lett. 101, 103111 (2012); http://dx.doi.org/10.1063/1.4751551 (5 pages)

Online Publication Date: 6 September 2012

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We demonstrate the controlled desorption of adventitious dopants on networks of single-walled carbon nanotubes (SWNTs) with 100 keV proton irradiation. Networks of sorted metallic, semiconducting SWNTs, and unsorted SWNTs were investigated. The removal of dopants was indicated by an increase in sheet resistances along with an increase in the absorption of the low energy absorption band of semiconducting SWNTs. Semiconducting and unsorted SWNT networks exhibited the largest change in their sheet resistance, which indicates the conductivity of unsorted SWNT networks is dominated by the tube-tube junctions of semiconducting SWNTs.
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73.63.Fg Nanotubes
68.43.Nr Desorption kinetics
61.82.Fk Semiconductors
61.72.up Other materials
61.82.Rx Nanocrystalline materials
61.80.Jh Ion radiation effects

Determination of ion track radii in amorphous matrices via formation of nano-clusters by ion-beam irradiation

M. Buljan, M. Karlušić, I. Bogdanović-Radović, M. Jakšić, K. Salamon, S. Bernstorff, and N. Radić

Appl. Phys. Lett. 101, 103112 (2012); http://dx.doi.org/10.1063/1.4751841 (4 pages) | Cited 2 times

Online Publication Date: 6 September 2012

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We report on a method for the determination of ion track radii, formed in amorphous materials by ion-beam irradiation. The method is based on the addition to an amorphous matrix of a small amount of foreign atoms, which easily diffuse and form clusters when the temperature is sufficiently increased. The irradiation causes clustering of these atoms, and the final separations of the formed clusters are dependent on the parameters of the ion-beam. Comparison of the separations between the clusters that are formed by ions with different properties in the same type of material enables the determination of ion-track radii.
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61.80.Jh Ion radiation effects
61.46.Bc Structure of clusters (e.g., metcars; not fragments of crystals; free or loosely aggregated or loosely attached to a substrate)

Investigation of fluorine three-dimensional redistribution during solid-phase-epitaxial–regrowth of amorphous Si

F. Panciera, K. Hoummada, M. Mastromatteo, D. De Salvador, E. Napolitani, S. Boninelli, G. Impellizzeri, F. Priolo, A. Carnera, and D. Mangelinck

Appl. Phys. Lett. 101, 103113 (2012); http://dx.doi.org/10.1063/1.4751254 (4 pages) | Cited 1 time

Online Publication Date: 6 September 2012

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The fluorine redistribution during partial solid-phase-epitaxial-regrowth at 650 °C of a preamorphized Si substrate implanted by F was investigated by atom probe tomography (APT), transmission electron microscopy, and secondary ions mass spectrometry. Three-dimensional spatial distribution of F obtained by APT provides a direct observation of F-rich clusters with a diameter of less than 1.5 nm. Density variation compatible with cavities and F-rich molecular ions in correspondence of clusters are in accordance with cavities filled by SiF4 molecules. Their presence only in crystalline Si while they are not revealed by statistical analysis in amorphous suggests that they form at the amorphous/crystal interface.
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81.15.Np Solid phase epitaxy; growth from solid phases
68.55.ag Semiconductors
61.72.U- Doping and impurity implantation
61.72.Qq Microscopic defects (voids, inclusions, etc.)
79.20.Rf Atomic, molecular, and ion beam impact and interactions with surfaces

Tailoring properties of carbon-nanotube-based foams by ion bombardment

S. Charnvanichborikarn, S. J. Shin, M. A. Worsley, and S. O. Kucheyev

Appl. Phys. Lett. 101, 103114 (2012); http://dx.doi.org/10.1063/1.4751268 (4 pages) | Cited 2 times

Online Publication Date: 6 September 2012

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Particle irradiation is an effective method for manipulating properties of individual carbon nanotubes (CNTs). This potential, however, remains unexplored for macroscopic assemblies of cross-linked CNTs. Here, we study structural and electrical properties of ultralow-density cross-linked CNT-based nanofoams exposed to ion irradiation at room temperature over a wide range of ion masses and fluences. For all irradiation conditions studied, the electrical resistance of nanofoams initially increases with a rate that scales with the number of ballistically generated displacements. This process is attributed to the buildup of defects in graphitic nanoligaments. Irradiation with Ne and heavier ions leads to a decrease in the electrical resistance at large fluences, which is attributed to radiation-induced foam densification. In addition, heavy-ion bombardment causes amorphization of CNTs and smoothing of ligament surfaces. These results demonstrate that ion bombardment can be used for tailoring density, ligament morphology, and electrical properties of CNT-based foams.
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73.63.Fg Nanotubes
81.07.De Nanotubes
61.48.De Structure of carbon nanotubes, boron nanotubes, and other related systems
61.80.Jh Ion radiation effects
82.70.Rr Aerosols and foams
68.35.bp Fullerenes

Ballistic thermoelectric properties in graphene-nanoribbon-based heterojunctions

Chang-Ning Pan, Zhong-Xiang Xie, Li-Ming Tang, and Ke-Qiu Chen

Appl. Phys. Lett. 101, 103115 (2012); http://dx.doi.org/10.1063/1.4751287 (4 pages)

Online Publication Date: 6 September 2012

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Ballistic thermoelectric properties in graphene-nanoribbon-based heterojunctions are investigated by using the nonequilibrium Green's function approach and the Landauer transport theory. The results show that the phonon thermal conductances have similar effects for the different heterojunctions, while the electron transport is highly sensitive to the geometry details of the heterojunctions. The fluctuation of electronic transmissions can strongly enhance the thermopower. We can obtain the high thermoelectric figure of merit ZT ∼ 0.6 at room temperature T = 300 K and ZT ∼ 0.9 at low temperature T = 100 K by optimizing the thermopower, together with suppression of phonon transport by mismatching interface structures.
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72.20.Pa Thermoelectric and thermomagnetic effects
73.23.Ad Ballistic transport
66.70.Lm Other systems such as ionic crystals, molecular crystals, nanotubes, etc.
63.22.Rc Phonons in graphene
73.63.Bd Nanocrystalline materials
68.35.Ct Interface structure and roughness

Formation and control of wrinkles in graphene by the wedging transfer method

V. E. Calado, G. F. Schneider, A. M. M. G. Theulings, C. Dekker, and L. M. K. Vandersypen

Appl. Phys. Lett. 101, 103116 (2012); http://dx.doi.org/10.1063/1.4751982 (3 pages) | Cited 1 time

Online Publication Date: 7 September 2012

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We study the formation of wrinkles in graphene upon wet transfer onto a hydrophilic target substrate, whereby draining of water appears to play an important role. We are able to control the orientation of the wrinkles by tuning the surface morphology. Wrinkles are absent in flakes transferred to strongly hydrophobic substrates, a further indication of the role of the interaction of water with the substrate in wrinkle formation. The electrical and structural integrity of the graphene is not affected by the wrinkles, as inferred from Raman measurements and electrical conductivity measurements.
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61.48.Gh Structure of graphene
68.35.bt Other materials
78.30.Na Fullerenes and related materials
78.67.Wj Optical properties of graphene
72.80.Vp Electronic transport in graphene

Ni80Fe20/Ni binary nanomagnets for logic applications

J. Ding and A. O. Adeyeye

Appl. Phys. Lett. 101, 103117 (2012); http://dx.doi.org/10.1063/1.4751259 (4 pages)

Online Publication Date: 7 September 2012

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We demonstrate the functionality of Ni80Fe20/Ni binary nanostructures cells fabricated using self-aligned shadow deposition technique in logic applications. Depending on the magnetic ordering of the cells, distinct dynamic states probed by broadband ferromagnetic resonance spectroscopy are realized. We show that the magnetic ordering can be manipulated to achieve logic operations by controlling the amplitude and the orientation of reset fields. Magnetic force microscopy and micromagnetic simulations were used to validate our experimental results.
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75.75.-c Magnetic properties of nanostructures
75.50.Tt Fine-particle systems; nanocrystalline materials
81.07.Bc Nanocrystalline materials
76.50.+g Ferromagnetic, antiferromagnetic, and ferrimagnetic resonances; spin-wave resonance
75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.50.Bb Fe and its alloys
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Degradation induced decrease of the radiative quantum efficiency in organic light-emitting diodes

Tobias D. Schmidt, Daniel S. Setz, Michael Flämmich, Bert J. Scholz, Arndt Jaeger, Carola Diez, Dirk Michaelis, Norbert Danz, and Wolfgang Brütting

Appl. Phys. Lett. 101, 103301 (2012); http://dx.doi.org/10.1063/1.4749815 (4 pages)

Online Publication Date: 4 September 2012

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The efficiency decrease during electrical operation of organic light-emitting diodes is a crucial issue for both applied and fundamental research. In order to investigate degradation processes, we have performed an efficiency analysis for phosphorescent state-of-the-art devices in the pristine state and after an accelerated aging process at high current density resulting in a luminance drop to less than 60% of the initial value. This loss in efficiency can be explained by a decrease of the radiative quantum efficiency of the light-emitting guest/host system from 70% to 40%, while other factors determining the efficiency are not affected.
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85.60.Jb Light-emitting devices
FREE

Single crystal field-effect transistors containing a pentacene analogue and their application in ethanol vapor detection

Guangyao Zhao, Huanli Dong, Lang Jiang, Huaping Zhao, Xiang Qin, and Wenping Hu

Appl. Phys. Lett. 101, 103302 (2012); http://dx.doi.org/10.1063/1.4750063 (3 pages)

Online Publication Date: 5 September 2012

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Organic field-effect transistors (OFETs) have attracted attention in the field of organic electronics. The conducting channel of OFETs is highly sensitive to out stimulus, so OFETs can be used as ultrasensitive sensors and actuators. Here, a new kind of OFET containing micro- and nanocrystals of a pentacene analogue is reported. The OFET exhibited a mobility of 0.8 cm2 V−1 s−1 and an on/off ratio of 1.7 × 107. Moreover, it displayed high sensitivity, selectivity, and reproducibility in the detection of ethanol gas, indicating its great potential as a sensor for ethanol gas.
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85.30.Tv Field effect devices
07.07.Df Sensors (chemical, optical, electrical, movement, gas, etc.); remote sensing
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Carrier doping to the organic Mott insulator by conjugating with tetrathiafulvalene

Y. Takahashi, Y. Nakagawa, K. Hayakawa, T. Inabe, and T. Naito

Appl. Phys. Lett. 101, 103303 (2012); http://dx.doi.org/10.1063/1.4750066 (4 pages) | Cited 1 time

Online Publication Date: 5 September 2012

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The electrical conductivity of the organic Mott insulator ET-F2TCNQ (ET = bis(ethylenedithio)tetrathiafulvalene, F2TCNQ = 2,5-difluoro-7,7,8,8-tetracyanoquinodimethane) crystal was found to be enhanced by conjugation with a tetrathiafulvalene (TTF) single crystal on its surface; surface sheet resistance decreased from 5 × l05 to 2 × 103 Ω/sq. The mechanism of this decrement was investigated through optical and atomic force microscopy measurements at the interface. When TTF was conjugated to the ET-F2TCNQ crystal, electron injection from TTF and complex formation between TTF and F2TCNQ occurred. Neutral ET molecules were consequently generated at the interface, and this charge doping broke the Mott insulating state.
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73.25.+i Surface conductivity and carrier phenomena
73.20.Fz Weak or Anderson localization
61.72.up Other materials
81.05.Lg Polymers and plastics; rubber; synthetic and natural fibers; organometallic and organic materials
72.80.Le Polymers; organic compounds (including organic semiconductors)
73.20.At Surface states, band structure, electron density of states
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Inverted top-emitting blue electrophosphorescent organic light-emitting diodes with high current efficacy

K. A. Knauer, E. Najafabadi, W. Haske, and B. Kippelen

Appl. Phys. Lett. 101, 103304 (2012); http://dx.doi.org/10.1063/1.4750141 (4 pages) | Cited 1 time

Online Publication Date: 5 September 2012

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Two different types of inverted top-emitting blue electrophosphorescent organic light-emitting diodes (OLEDs) are demonstrated that differ only in the choice of high electron mobility transport layers. The electron transport layer consists of either 1,3,5-tri(p-pyrid-3-yl-phenyl)benzene (TpPyPB) or 1,3,5-tri(m-pyrid-3-yl-phenyl)benzene) (TmPyPB). Devices with TpPyPB exhibit a current efficacy of 5.1 cd/A at 1259 cd/m2. OLEDs with TmPyPB show higher performance with a current efficacy of 33.6 cd/A at 1126 cd/m2. The difference in performance of OLEDs with TmPyPB is due to a combination of TmPyPB's higher triplet energy that decreases exciton transfer to the ETL and altered charge balance.
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85.60.Jb Light-emitting devices
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Electrical stabilities and carrier transport mechanisms of flexible organic bistable devices based on CdSe-InP core-shell nanoparticle/polystyrene nanocomposites

Dong Yeol Yun, Woo Seung Song, Tae Whan Kim, Sung Woo Kim, and Sang Wook Kim

Appl. Phys. Lett. 101, 103305 (2012); http://dx.doi.org/10.1063/1.4748873 (4 pages) | Cited 1 time

Online Publication Date: 5 September 2012

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Flexible organic bistable devices (OBDs) for the memory characteristics utilizing CdSe-InP core-shell nanoparticle/polystyrene nanocomposites were fabricated on indium-tin-oxide-coated polyethylene terephthalate substrates. Current-voltage measurements on Al/CdSe-InP nanoparticles embedded in polystyrene layer/indium-tin-oxide/polyethylene terephthalate devices without and with bending exhibit wide-range current hysteresis behaviours with ON/OFF ratios of 1 × 107 and 1 × 105, respectively. The endurance number of the ON/OFF switchings without bending was 1 × 105 cycles. The switching characteristics of the OBDs after bending were stable enough to distinguish the ON and OFF. The carrier transport mechanisms of the OBDs are described on the basis of the current-voltage results.
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84.30.Sk Pulse and digital circuits
85.35.-p Nanoelectronic devices
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Correlation between density of paramagnetic centers and photovoltaic degradation in polythiophene-fullerene bulk heterojunction solar cells

Reg Bauld, Leesa M. Fleury, Marima Van Walsh, and Giovanni Fanchini

Appl. Phys. Lett. 101, 103306 (2012); http://dx.doi.org/10.1063/1.4749813 (4 pages)

Online Publication Date: 6 September 2012

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We investigated the physical processes underlying the degradation of poly(3-hexyl-thiophene):phenyl-C61-butyric acid methyl ester (P3HT:PCBM) photovoltaics under harsh environmental conditions during a 70-70-70 test (70% humidity at 70 °C from 0 to 70 h) using a variety of analytical techniques aimed at monitoring moisture incorporation. While the total oxygen content did not significantly increase during the test, a limited fraction of oxygen forms paramagnetic centers in P3HT, PCBM and, more limitedly, P3HT:PCBM heterojunctions. A strong correlation exists between the density of paramagnetic centers and the decay in the AM 1.5 photoconversion efficiency of the devices.
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88.40.jp Multijunction solar cells
88.40.hj Efficiency and performance of solar cells
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