In the current talk we will use two examples to demonstrate the importance of using surface science studies to identify catalysts and electrocatalysts. Our research approaches involve parallel efforts in density functional theory (DFT) calculations, surface science experiments on model systems, and synthesis and evaluation of supported catalysts under thermochemical or electrochemical conditions. We will first use water electrolysis to demonstrate the feasibility of using monolayer Pt on tungsten carbide (WC) to achieve the same activity as bulk Pt. We will present DFT calculations of similar electronic and chemical properties between monolayer Pt/WC and Pt, synthesis and characterization of monolayer Pt/WC films, and electrochemical evaluation of the activity and stability of Pt/WC for water electrolysis. Comparing to the leading Pt electrocatalyst, the monolayer Pt/WC represents a reduction by a factor of ten in Pt loading [1,2].
We will then use the conversion of biomass-derived oxygenates to illustrate the advantages of using bimetallic catalysts. Bimetallic catalysts often show unique activity and selectivity over their parent metals due to the electronic modification and strain effect [3,4]. We will present our results on the characterization of Ni/Pt bimetallic surfaces and catalysts under in-situ reaction conditions, further highlighting the importance of using the combined approaches of DFT calculations, surface science experiments, and reactor evaluations [5,6].
[1] D.V. Esposito, S.T. Hunt, K.D. Dobson, B.E. McCandless, R.W. Birkmire and J.G. Chen, “Low-Cost
Hydrogen Evolution Catalysts Based on Monolayer Platinum on Tungsten Monocarbide Substrates”, AngewandteChemie International Edition, 49 (2010) 9859-9862
[2] D.V. Esposito, S.T. Hunt, Y.C. Kimmel and J.G. Chen, “A New Class of Electrocatalysts for Hydrogen Production from Water Electrolysis: Metal Monolayers Supported on Low-Cost Transition Metal Carbides”, Journal of the American Chemical Society, 134 (2012) 3025-3033. [3] W. Yu, M.D. Porosoff and J.G. Chen, “Pt-based Bimetallic Catalysis: From Model Surfaces to
Supported Catalysts”, Chemical Reviews, (2012) DOI: 10.1021/cr300096b.
[4] D.A. Hansgen, D.G. Vlachos and J.G. Chen, “Using First Principles to Predict Bimetallic Catalysts
for the Ammonia Decomposition Reaction”, Nature Chemistry, 2 (2010) 484-489.
[5] M. Salciccioli, W. Yu, M.A. Barteau, J.G. Chen, D.G. Vlachos, “Differentiation of O-H and C-H
Bond Scission Mechanisms of Ethylene Glycol on Pt and Ni/Pt Using Theory and Isotopic Labeling Experiments”, Journal of the American Chemical Society, 133 (2011) 7996-8004.
[6] W. Yu, M.A. Barteau and J.G. Chen, “Glycolaldehyde as a Probe Molecule for Biomass-derivatives:
Reaction of C-OH and C=O Functional Groups on Monolayer Ni Surfaces”, Journal of the American Chemical Society, 133 (2011) 20528-20535.

While recent efforts to enhance photon-to-O₂ conversion of photoanodes for water splitting have attracted much attention, linking the resulting apparent catalytic activities to the microscopic processes of electron-hole dynamics and interfacial hole transfer remains difficult. Herein, we investigate the kinetics of photo-excited carriers within the space charge layer of Nb-doped SrTiO₃ anodes under working conditions by means of time-resolved optical spectroscopy. By probing the dependence of the dynamics on various variables, including electrochemical potential, electrolyte composition, and doping concentration, we are able to quantify the kinetics of electron-hole separation in the space charge layer, and monitor hole transfer across the SrTiO₃/electrolyte interface. We discuss the relevance of our results in regards to the early steps in water oxidation at the SrTiO₃ interface, as well as to the design of novel photoanodes.

The typical photoanode in dye- and quantum dot- sensitized solar cells is composed of a wide band gap semiconductor, which acts as electron transporter for the photoelectrochemical system. Anatase TiO2 nanoparticles are one of the most used oxides and are able to deliver the highest photoconversion efficiency in this kind of solar cells, but intense research in the last years was also addressed to ZnO and other composite systems. Modulation of the composition and shape of nanostructured photoanodes is key element to tailor the physical chemical processes regulating charge dynamics and, ultimately, to boost the efficiency of the end user device, by favoring charge transport and collection, while reducing charge recombination.
We investigated several systems: (i) TiO2 nanoparticles / ZnO nanowires [1]; (ii) Multiwall carbon nanotubes (MWCNTs) / TiO2 nanoparticles [2]; (iii) TiO2 nanotubes [3-4]; (iv) Hierarchically self-assembled ZnO sub-microstructures [5]. Both dye molecules and semiconducting quantum dots were applied as light harvesters. Possible tailoring of structure and morphology of the photoanodes, and their implication in improving the functional properties of these kinds of excitonic solar cells will be discussed.
References:
[1] A. Vomiero, I. Concina, M.M. Natile, E. Comini, G. Faglia, M. Ferroni, I. Kholmanov, G. Sberveglieri, Applied Physics Letters 95 (2009) 193104.
[2] K.T. Dembele, R. Nechache, L. Nikolova, A. Vomiero, C. Santato, S. Licoccia, F. Rosei J. Power Sources 233 (2013) 93-97.
[3] A. Vomiero, V. Galstyan, A. Braga, I. Concina, M. Brisotto, E. Bontempi, G. Sberveglieri, Energy and Environmental Science 4 (2011) 3408-3413.
[4] V. Galstyan, A. Vomiero, I. Concina, A. Braga, M. Brisotto, E. Bontempi, G. Faglia, G. Sberveglieri, Small 7 (2011) 2437-2442.
[5] N. Memarian, I. Concina, A. Braga, S. M. Rozati, A. Vomiero, G. Sberveglieri, Angewandte Chemie In Ed 50 (2011) 12321-12325.

Formic acid (HCOOH) is a simple molecule that is an abundant product of biomass processing and can serve as an internal source of hydrogen for oxygen removal and upgrading of biomass to chemicals and fuels. In addition, HCOOH can be used as a fuel for low temperature direct fuel cells. We present a systematic study of the HCOOH decomposition reaction mechanism starting from first-principles and including reactivity experiments and microkinetic modeling. In particular, periodic self-consistent Density Functional Theory (DFT) calculations are performed to determine the stability of reactive intermediates and activation energy barriers of elementary steps. In addition, pre-exponential factors are determined from vibrational frequency calculations. Mean-field microkinetic models are developed and calculated reaction rates, orders, etc are then compared with experimentally measured ones. These comparisons provide useful insights on the nature of the active site, most-abundant surface intermediates as a function of reaction conditions and feed composition. Trends across metals on the fundamental atomic-scale level up to selectivity trends will be discussed. Finally, we identify from first-principles alloy surfaces, which may possess better catalytic properties for selective dehydrogenation of HCOOH than monometallic surfaces, thereby guiding synthesis towards promising novel catalytic materials.

Studying and improving activity and stability of earth-abundant catalysts at the light absorber/electrolyte interface to drive hydrogen production reaction is an important challenge for solar fuel generation in artificial photosynthesis. We have used X-ray Absorption Spectroscopy (XAS) and micro-focused X-ray Fluorescence (MXRF) mapping at the Advanced Light Source (ALS) to characterize molecular surface-linked catalysts at the interface. The electronic and structural properties of the metal core or ligand were probed before, during, and after electro- and photoelectro-chemical (PEC) operation with ex- and in-situ detection techniques. Using Co K-edge XAS, we have shown that a Co(dmgH)2PrCl molecular catalyst remains electronically and structurally intact after attachment through the pyridine ligand to a polymer chain linked to a light absorbing surface [1]. Using the Co fluorescence signal, we have also studied the stability of a cobaloxime-modified photocathode after rigorous PEC operation.
[1] Krawicz, Yang, Anzenberg, Yano, Sharp, Moore, Photofunctional Construct that Interfaces Molecular Cobalt-based Catalysts for H2 Production to a Visible-light Absorbing Semiconductor, Journal of the American Chemical Society, (submitted)

Ceria (CeO2) is one of the most active oxide electro-catalysts for hydrogen oxidation and water-splitting reactions at elevated temperatures. The surface concentration of oxygen vacancies and Ce3+ species were found to be significantly greater than the bulk, and can plausibly explain ceria&’s high electro-catalytic activity. Furthermore, the activities of ceria are expected to depend strongly on the surface termination, as already observed in nanostructured ceria with different facets. To better understand electrochemical reaction mechanisms and the structure-property relationship on the ceria surface, we investigated how the surface crystal structure of doped ceria differed from its bulk crystal. Specifically, we employed in-situ surface X-ray diffraction (SXRD) to study the atomic arrangements of (100)-and (111)-terminated ceria surfaces. We successfully fabricated atomically flat ceria films on yttria-stabilized zirconia substrates by pulsed laser deposition and confirmed that these films had well-defined step terraces using atomic force microscopy. We correlated the surface structure under both reducing and oxidizing atmosphere to the different catalytic activities of the (100)- and (111)-terminated films.

Thermochemical water-splitting is a two-step process where in step-1, redox material is heated at higher temperature creating oxygen vacancies, whereas in step-2, this partially reduced material is exposed to steam leading to H2 generation by scavenging the oxygen. As these steps are performed at very high temperatures, materials experience grain growth. As a result, surface area and porosity decrease, which translate into lower H2 volume generation with increase in thermochemical water-splitting cycles. Therefore, there is a need to thermally stabilize the redox materials with the viewpoint of mitigating the grain growth and achieving steady H2 production. In this study, Ni-ferrite nanoparticles were synthesized using the sol-gel method and later these nanopartices were utilized to prepare porous core-shell nanoparticles with Y2O3 stablized ZrO2 (YSZ). The H2 generation ability of core-shell Ni-ferrite/YSZ nanoparticles was investigated by performing five consecutive thermochemical cycles in the Inconel packed-bed reactor where water-splitting and regeneration steps were carried out at 650o-1100oC. Specific surface area (SSA) and porosity of these materials were analyzed before and after the thermochemical water-splitting reaction using BET surface area analyzer. Additionally scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to characterize the grain growth. The results suggest grain growth mitigation in core-shell Ni-ferrite/YSZ nanoparticles that resulted in relatively steady H2 volume in multiple thermochemical water-splitting cycles. The H2 volume observed with core-shell ferrite nanoparticles was higher than ferrite nanoparticles. Characterization of core-shell nanoparticles and the results obtained on H2 volume generation with ferrite and core-shell ferrite nanoparticles will be presented.

Mg can store up to ~7 wt.% hydrogen and is an appealing candidate as light weight and low cost hydrogen storage material. However hydrogen desorption in Mg hydride typically requires unfavorably high temperature of ~ 573 K due to its high thermodynamic stability, whereas the target operation temperature of fuel cells in automobiles is ~ 373 K or less. Furthermore the kinetics of H sorption in Mg is typically slow. Here we investigate hydrogen sorption behavior of Mg/Nb multilayers. Stress-induced orthorhombic Mg hydride (O-MgH2) is thermodynamically destabilized at ~ 373 K or lower. Such drastic destabilization arises from large tensile stress in single layer O-MgH2 bonded to rigid substrate, or compressive stress due to large volume change incompatibility in Mg/Nb multilayers. Ab inito calculations were performed to analyze the influence of interfaces on H sorption in Mg films. These studies provide insight on the mechanisms that may expedite the kinetics of H sorption in Mg.

Conjugated organic polymers, possessing intrinsic properties of large surface areas, high thermal and chemical stabilities, and low skeleton density, exhibit potential applications in gas storage and separation. Based on special spirocyclic or propeller-like monomers, various organic microporous polymers were prepared through a wide variety of C-C coupling reactions and further characterized at the molecular level by 13C CP/MAS NMR spectrum, as well as other techniques. All the obtained polymers are chemically stable. Thermal analysis shows that the materials are stable up to 350 °C under nitrogen. The fluorescent emission of the obtained conjugated polymers is tunable, ranging from 440 to 600 nm depending on the molecular structure of the monomer and coupling strategy. According to the obtained nitrogen physisorption isotherms, the Brunauer-Emmett-Teller (BET) specific surface area for these polymers varies between 700 and 2200 m<2> g<-1>. Adsorption isotherms show the polymers possess nice adsorption capacity to hydrogen and carbon dioxide, showing a good gas separation of carbon dioxide over methane.

Bulk-heterojunction (BHJ) solar cells based on phase-separated blends of organic materials of donor and acceptor (fullerene derivative) have been in continuous development over the past two decades.(1)-(3) Recently, solution-processable small molecule-based BHJ solar cells exhibiting comparable power conversion efficiency (PCE) of near 7% to the polymers with high potential under simple and optimized processing conditions.(4)-(6) The small-molecule donors have attractive features, including relatively simple synthesis and purification steps, mono-dispersity, and improved batch-to-batch reproducibility.
Here, we demonstrated solution-processed small-molecule p-DTS(FBTTh2)2:PC71BM BHJ solar cells with a PCE over 8 %. The fill factor (FF) is sensitive to the thickness of a calcium buffer layer between the BHJ active film and the Al cathode; for 20 nm Ca thickness, the FF is 73%, the highest value reported for an organic solar cell. The maximum external quantum efficiency exceeds 80%. After correcting for the total absorption in the cell through the normal incidence reflectance measurements, the internal quantum efficiency approach 100% in the spectral range of 600 to 650 nm and well over 80% across the entire spectral range from 400 to 700 nm. Analysis of the current-voltage (J-V) characteristics at various light intensities provides information on the different recombination mechanisms in the BHJ solar cells with various thicknesses of the Ca layer. Also, we can fabricate improved efficiency of small-molecule solar cells with PCE of 8.24% with enhanced JSC and FF using a low sheet resistance of ITO (5 Omega;/square) substrate (ITO thickness of 450 nm) which exhibits transmittance of 90% at 550 nm. The increased JSC and FF originate from the reduced series resistance (Rs). In summary, the PCE and FF of small molecule based bulk-heterojunction solar cells can be increased by optimized thickness of Ca interlayer and low sheet resistance of ITO substrate.
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(6) A. K. K. Kyaw, D. H. Wang, V. Gupta, J. Zhang, S. Chand, G. C. Bazan, A. J. Heeger, Adv. Mater. DOI: 10.1002/adma.201300295.

To greatly improve the electrocatalytic activity for methanol oxidation, high-quality exfoliated graphene decorated with uniform Pt nanocrystals (NCs) (3 nm) have been prepared by a very simple, low-cost and environmentally benign process. During the entire process, no surfactant and no halide ions were involved, which not only enabled very clean surface of Pt/graphene leading to excellent conductivity, but also greatly improved the electrocatalyst tolerance to carbon monoxide poisoning (Pt/graphene, If/Ib= 1.197), compared to commercial Pt/C (If/Ib= 0.893) catalysts. To maximize the electrocatalytic performance and minimize the amount of precious Pt, Pt-M/graphene (M=Pd, Co) hybrids have also been prepared, and these hybrids have much larger electrochemically active surface areas (ECSA), which are 4 (PtPd/graphene) and 3.3 (PtCo/graphene) times as those of commercial Pt/C. The PtPd/graphene and PtCo/graphene hybrids also have remarkably increased activity toward methanol oxidation (If/Ib= 1.218 and 1.558). Furthermore, density functional theory (DFT) simulations demonstrate that an electronic interaction occurred between Pt atoms and graphene, indicating that graphene substrate plays a crucial role in regulating the electron structure of attached Pt atom, which confirmed that the increased efficiency of methanol oxidation was due to the synergetic effects of the hybrid structure.

To improve the performance of organic photovoltaic cells, mechanisms of charge transfer/transport and charge separation have been widely investigated. These studies need acculate information on the electronic structure responsible to the processes. In general, however, it is not easy to reveal the electronic structure not only at metal-molecule interfaces but also of the molecular film itself due to large structural anisotropy of the molecule and their orientation in the film. Ionization energy (E_i) of the film depends on the molecular orientation, crystal structure and packing density [1]. In this study, we investigated the electronic structure and the molecular orientation of picene (C₂₂H₁₄) films prepared on SiO₂ and graphite (HOPG) substrates by using ultraviolet photomission spectroscopy (UPS) and metastable atom electron spectroscopy (MAES) [2].
Picene molecules were vacuum deposited step-by-step (upto ~10nm) on clean SiO₂ and HOPG under UHV. The deposition rate was ~1.7 Å/min on the SiO₂ and ~1.1 Å/min on the HOPG. All experiments were conducted at RT (293K).
We clarified using MAES from monolayer to multilayer that the molecules stand upright on the SiO₂ (long axis of the molecule is normal to the surface), while they lie flat on the HOPG (short axis of the molecule is slightly tilted w.r.t. the surface). Normal-emission UPS spectra of the standing and lying films are largely different due to the orientation dependent photoelectron angular distribution and intermolecular interaction. The valence band features of the lying film correspond well to the density-of-states from DFT of an isolated molecule, while those of the standing film represent similarlity with the result of the single crystal [3].
We determined E_i for the HOMO onset and peak after peak fitting analyses of the HOMO features. The E_i of the standing film at the peak position (E_i^PEAK= 6.06eV) is smaller by 0.74eV than that of the lying film (E_i^PEAK = 6.80eV), giving the energy difference ΔE_i^PEAK = 0.74 eV (ΔE_i^ONSET = 0.66 eV). These ΔE_i are much greater than that of pentacene (ΔE_i^PEAK= 0.43eV [4]) as well as other pi-conjugated molecules (ΔE_i^PEAK = 0.40eV for DIP and 0.40eV for CuPc). Effects of electronic coupling at the picene/substrate interfaces on the UPS features can be neglected because of weak molecule-substrate interaction. We also suggest that for picene the molecular dipole and energy-band dispersion do not contribute much on ΔE_i. Hence we can evaluate main origin of orientation dependent E_i by considering surface electrostatic potential produced by local dipoles (>C^-H⁺) in picene. At the conference, we will report results of the electrostatic potential culculation and gas phase spectra of picene to discuss the orientation dependence of E_i.
Refferrence
[1] S. Duhm et al, Nature Mater. 7, 326 (2008).
[2] Y. Harada et al, Chem. Rev. 97, 1897 (1997).
[3] Q. Xin et al, Phys. Rev. Lett. 108, 226401 (2012).
[4] H. Fukagawa et al, Phys. Rev. B 73, 245310 (2006).

On the CO2 activation a theoretical study is presented. On the basis of the quantum chemical calculations, the order of magnitude in the activation is described. Taking as a reference of the CO2 molecule in dilute gas phase or in vacuum, the activated states in electronic structure (wave functions) as well as in geometrical changes are described by the degree of modifications relative to the reference state.
The DFT calculations are performed to visualize this degree of activation as a function of the environmental effect on CO2; gas phase, liquid phase (especially as a function of pH), and coordination on transition metals. Special attention is paid on the activation by abundant metals such as Zn. The experimental and theoretical examples of reactions of the activated CO2 are presented in hydrothermal condition with transition metal atoms.

Excitonic solar cells (XSCs) are appealing candidates as alternative devices for solar energy conversion, being in principle low cost, rather environmental friendly and suitable for exploiting also the near infrared (NIR) region of solar spectrum. 1
Nanoparticulate TiO2 is currently the most known and exploited semiconductor metal oxide (SMO) applied as photoanode in both dye- and quantum dot-sensitized solar cells (DSCs and QDSCs), but other SMOs, such as ZnO and SnO2, are attracting a broad interest, due to higher electron mobility and band structure suitable for NIR-absorbing light harvesters, respectively. 2,3,4
Strategies devoted to either completely inhibit or at least reduce recombination of photogenerated free charges, which is still one open issue in photoconverison efficiency, are still requested.
In this work, nano-SnO2, with different particles sizes (4 to 20 nm) and tunable amount of oxygen vacancies, was applied as main active layer in photoanodes for XSCs together with ZnO (as polidisperse nano- and micro-structures or hierarchical structures) in order to engineer the global band structures involved in charge injection.
A careful optimization of different parameters, such as relative layered composition of ZnO@SnO2 interfaces and dye loading time, allows demonstrating the highly beneficial role of ZnO as blocking layer towards the exciton recombination at SnO2-light harvester-electrolyte interfaces, thus reaching high functional performances in terms of injected photocurrent and photo conversion efficiency (JSC=14.78 mA/cm2 PCE=4.96%).
Strategies successful in optimizing the functional performances of XCSs (both DSCs and QDSCs) through material interface engineering will be presented and discussed.
1 B. O&’Regan, M. Gratzel, Nature 1991, 353, 737
2 E. Guillén, L. M. Peter, J. A. Anta J. Phys. Chem. C, 2011, 115, 2262
3 N. Memarian, I. Concina, A. Braga, SM Rozati, A Vomiero, G Sberveglieri, Angew Chem 2011, 51, 12321
4 A. Hossain, JR Jenning, Z.Y. Koh,Q. Wang, ACS Nano, 2011, 5, 3172

Energy level alignment at organic/metal interfaces determines the performance of organic devices. To clarify the energy level alignment it is desired to obtain precise information of electronic states, in particular the highest occupied molecular orbital (HOMO) related states, at a complicated interface that consists of functional molecules. We studied the electronic structure and geometric structure of diindenoperylene (DIP: C₃₂H₁₆) monolayer (ML) on a Cu (111) surface by using angle-resolved ultraviolet photoelectron spectroscopy (ARUPS). It is known that DIP molecules form two different 2-dimensional lattice structures on Cu (111) [1].
A Cu (111) single crystal was cleaned by repeated Ar-ion sputtering and annealing cycles. The DIP ML film was prepared on the Cu (111) substrate by vacuum evaporation. ARUPS spectra were then measured at photon incidence angle α=45°, hv=28 eV and T=295 K at UVSOR facility (BL8B) at the Institute for Molecular Science. To study geometric structure as well as HOMO derived interface states we measured azimuthal angle (phi;) dependence by rotating the sample around the surface normal and photoelectron emission angle (theta;) dependence of the spectra of the ML.
ARUPS spectra of the DIP (ML)/Cu (111) show two prominent bands in the sp-band region of Cu (111). The first band is broad and located at binding energy (E_B: from the Fermi level) of ~0.7eV, which is ascribed to the former LUMO with electrons transferred from the substrate as in the case of PTCDA [2]. The second band consists of two features that are located at E_B=1.5 and 1.8 eV. We observed these two features show different phi; dependences at theta;=37°. Assuming that these two features come from DIP HOMO and using the two molecular adsorption geometries observed with STM (the short-range and the long-range ordered structures) [1], we computed phi; dependences of the photoelectron intensity from the two features for the two geometries, and compared with the observed phi; dependences. The agreement between the observed and computed phi; dependences is obtained when the 1.5-eV feature is originated from the long-range ordered structure and the 1.8-eV feature is from the short-range ordered structure. We can thus ascribe both of these two features at 1.5 and 1.8 eV to HOMO-derived states.
[1] D. G. de Oteyza et al, Phys.Chem.Chem.Phys. 11, 8741 (2009)
[2] S. Duhm et al, Org. Electro. 9, 111 (2008)

Transmission electron backscatter diffraction (t-EBSD) is a new technique of materials characterization using a standard scanning electron microscope (SEM). By changing the sample-detector geometry so that electrons pass through a thin specimen prior to entering a commercial EBSD detector, an interaction volume significantly smaller than that typically associated with conventional EBSD can be achieved. As a result, electron diffraction data in the form of Kikuchi patterns can be collected from extremely fine-scale films and particles, down to the sub-10 nm dimensional scale. In conventional EBSD, incident electrons are thought to undergo Kikuchi scattering in the top 10 to 40 nanometers of a film, thereby requiring a film at least that thick to produce a diffraction pattern, while achieving a lateral spatial resolution in the typical range of 20 nm to 35 nm parallel to the tilt axis and 80 nm to 90 nm perpendicular to the tilt axis. Here we present recent results and electron scattering simulations associated with probing the sampling limits of the t-EBSD technique for semiconductor industry-relevant films as thin as 5 nm, while maintaining single-nanometer lateral spatial resolution.

Transmission EBSD, performed at ~ 20 keV, provides information from large areas of ultrathin films, while requiring only little material volume, by capturing forward-scattered electrons in transmission with standard EBSD equipment. Monte Carlo simulations support experimental results on the maximum thicknesses that can be reliably characterized with this method, by giving estimates of the energy losses of the electrons as they pass through increasing film thicknesses, and practical limits of the technique are investigated in terms of the mass-thickness of the sample, regardless of composition.

For decades Czerny-Turner (CT) spectrographs have been used to measure Raman spectra of materials. Inherent in the design of the CT spectrograph are optical aberrations including astigmatism, coma, and spherical aberration. These aberrations can cause Raman spectral peaks to be measured which are poorly resolved, have a low signal-to-noise ratio (SNR), and an asymmetric peak shape. The Schmidt-Czerny-Turner (SCT) spectrograph uses a unique optical design that completely eliminates astigmatism at all wavelengths and at all points on the focal plane, and greatly reduces coma and spherical aberration. The net result is Raman peaks of materials with improved spectral resolution, better SNR, and a symmetric peak shape. The use of a SCT spectrograph to obtain improved Raman spectra on a variety of materials will be discussed.

One of the main current research goals in the solid oxide fuel cell (SOFC) industry is to reduce the operating temperatures of the SOFCs from 800-1000 °C to 600-800 °C. Reducing the operating temperature of SOFCs will obviate the need for expensive specialty alloys or ceramics for interconnects. However lower operating temperatures slow down the rates of electrochemical electrode reactions resulting in lower overall system efficiencies.
Using electrochemical impedance spectroscopy (EIS) on cells of the configuration LSCF (La1-xSrxCo0.2Fe0.8O3-δ)/ gadolinium-doped ceria (GDC) barrier layer/yttria-stabilized zirconia (YSZ)/porous LSM-YSZ counter electrode, the effect of Sr dopant concentration on the performance of LSCF cathodes was investigated. The effect of temperature on cathode performance was also investigated. The eventual goal is to obtain correlations between the electrochemical performance of LSCF cathodes and cation surface segregation effects.

Tetrathiafulvalene (TTF) is an organic semiconductor acting as an electron donor in charge-transfer complexes with an electron acceptor such as 7,7&’,8,8&’-tetracyanoquinodimethane (TCNQ) and also in metal-organic interfaces. On the contrary, we found that TTF was also able to act as an electron acceptor to the surface of silver nanoparticles (Ag NPs), which was consistent with the previous report to ZnO(0001) surface. Therefore it could be expected that TTF may act as an electron acceptor to induce partial positive charge on the surface of Ag NPs, similar to a well-known electron acceptor TCNQ.
We have previously reported various kinds of chemically modified Ag NPs as olefin carriers, which can interact with olefin gas specifically and reversibly but not with paraffin, consequently resulting in facilitated olefin transport. For instance, surface energy-level of Ag NPs was tuned to have partial positive charge on their surface by introducing electron acceptor such as p-benzoquinone (p-BQ) and TCNQ. The chemically activated Ag NPs have specific interactions only with propylene, resulted in the high selectivity of propylene over propane and high propylene permeance for the separation of olefin/paraffin mixture gases.
Herein, we adopted TTF to activate the surface of Ag NPs for olefin carriers, and their work function and binding energy were investigated by both UV photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS). It was interestingly found that TTF was able to act as an electron acceptor and induce surface positive charge of Ag NPs, which can interact with olefin specifically, resulting in the high separation performance for olefin/paraffin mixtures.

We have investigated the thickness dependence of air exposure of Copper Phthalocyanine (CuPc) and C60 layers on molybdenum trioxide (MoO3) with ultraviolet photoemission spectroscopy (UPS). It was found that after the air exposure, the WF of MoOx dropped severely. Meanwhile, with sim;10 ÅCuPc thin films covered on the top, there is no big change of the MoOx WF before and after the air exposure. We also noticed that sim;10 Å C60 thin film could also protect the WF drop of MoO3 from air exposure. Our ultraviolet photoemission spectroscopy date indicated the MoO3 surface which was covered by approximately two monolayers of organic thin films are not active to air.

Iron pyrite (cubic β-FeS2), an earth abundant and nontoxic semiconductor, has been attracting resurgent attention as a promising candidate for solar energy conversion due to its suitable band gap (0.95 eV indirect, 1.03 eV direct), high absorption coefficient (~6×10^5 cm^-1), and high mobility in single crystals (> 300 cm^2/V s). However, the application of pyrite for solar cells has been hindered by its low open circuit voltage (up to 200 mV) and thus low efficiency (~3%), which has been attributed to rich band gap states located at pyrite/electrolyte and pyrite/metal interfaces. To improve the performance of pyrite solar cell, the understanding and effective passivation of pyrite surface is urgently desired. Using the phase-pure single crystal pyrite nanorods (NRs) and other nanostructures as the model system, we develop chemical vapor deposition of epitaxial zinc sulfide (ZnS) on pyrite NRs to passivate surface states. The heteorepitaxial interface between pyrite and ZnS has been confirmed with detailed transmission electron microscopy study. Employing field effect transistor (FET), scanning probe microscopy (SPM) measurements and other surface analysis techniques, we investigate the changes of surface potential dominated by surface defects and other semiconductor characteristics upon the passivation treatment. These results can be important for improving pyrite single crystal and thin film structures for solar energy applications.

Bulk heterojunction (BHJ) polymer solar cells are an area of intense interest because of their flexibility, relatively low cost and ease of processing. However, in the active layer of a conventional BHJ solar cell, most of the donor or acceptor domains are isolated or far from the electrodes. This morphology leads to a long conduction path which causes possible recombination of the electron and hole pairs and lowers the efficiency of the cell.
Our approach is a very simple way to build up highly ordered columnar structures within the active layer by introducing a second polymer, poly(methyl methacrylate) (PMMA), which is immiscible with poly(3-hexylthiophene) (P3HT), causing lateral phase segregation into columnar structures. Furthermore, studies have shown that when nanoparticles are present in an immiscible blend, they preferentially segregate to the interfaces, allowing the interface to produce a template for the particle segregation. We therefore postulated that if one or both of the phases were photoactive polymers, then if carrier particles, such as [6,6]-Phenyl C61 butyric acid methyl ester (PCBM), were added, a structure for heterojunction solar cells could be formed where a direct pathway to the electrodes could be templated for the carrier particles. We first tested this concept by modeling the system using a Molecular Dynamics (MD) simulation, which defined the regime in phase space in which the parameters would be in the correct range for generating phase segregated structures and then characterized the actual samples, using a variety of complementary techniques to confirm the columnar structure. Finally, the devices were constructed using these structures, which showed a marked improvement in efficiency over the normal BHJ cells.

In atomic force microscope (AFM), force curves are widely used to investigate surface properties in the nanoscale. Besides having a proper calibration of the AFM, the experimental parameters have to be kept constant during the entire experiment. This is especially difficult for highly adhering samples due to tip contamination. In case of the appearance of tip contamination, the experimental parameters change and the results can not be used. The contamination of the tip happens because the interacting forces between tip and sample are too high. Since a lot of information about the surfaces properties is recorded in the first part of the force curve. Here a technique is presented, where only the first five to ten percent of the force curve need to be recorded, in order to get qualitative information about the hydrophobicity of one sample compared to a reference sample. This prevents the tip from contamination, because the force acting between the tip and the surface are kept so low, that the tip does not get contaminated.

Photoelectrochemical water splitting is one of the candidates for hydrogen gas generation from water. GaN is suitable for the photoelectrochemical electrode due to the band-edge energies. However, the n-type GaN has stability problem that the surface anodic corrosion during the photoelectrochemical reaction. There are some reports described the electrolyte dependences, that is, the flatband potential dependences on the pH of electrolytes [1], however, the details have not been investigated. We discuss the photoelectrochemical surface stabilities of n-type GaN dependent on the electrolytes in this report.
The working electrode was n-type GaN grown on (0001) sapphire substrates by metal-organic vapor phase epitaxy (MOVPE). The GaN layer was 1.0 µm n-type layer on 2.0 µm undoped layer. The carrier concentration of n-type GaN layer was 1.6×10¹⁷ cm^-3. The counter and reference electrode, which were made of Pt, and Ag/AgCl/NaCl, respectively, were used for electrochemical evaluations. The light intensity was controlled as 100 mW/cm² by using of 500 W Xe-lamp. The electrolytes were 0.5 mol/L H₂SO₄ (pH 0.8), 1.0 mol/L HCl (pH 0.2), 1.0 mol/L KOH (pH 13.8), and 1.0 mol/L NaOH (pH 13.8). The surface morphology of GaN was observed by Normarski microscope and Atomic force microscope (AFM).
The flatband potential in HCl obtained from Mott-Schottky plot shifted 0.1 V to positive direction compared with that in H₂SO₄. The potential usually changes along with the Nernstian relationship of water, because the dominant adsorbed materials at the electrode surface are H⁺, OH^-, and water. The results show that the potential do not only depend on pH, that is, the absorption materials on the GaN surface changing with the electrolytes.
Cyclic voltammetry with light illumination was evaluated in order to clarify the photoelectrochemical properties. The over potential decreased in HCl electrolyte compared with that in H₂SO₄ estimated from the anodic and cathodic turn-on voltage difference. The variation of anodic current of 1 to 3 cycles in H₂SO₄ was much larger than that of HCl, NaOH and KOH.
The GaN surface after the reaction in H₂SO₄ was changed to rough. In contrast, mirror-like surfaces were remained after the reaction in HCl, NaOH and KOH. From these results, the reaction mechanism in H₂SO₄ is expected to be different from that of the other solutions.
In summary, the selection of electrolyte affects the surface stability probably due to the difference of the adsorbed materials on the photoelectrode surface.
[1] K. Fujii, K. Ohkawa, J. Electrochem. Soc. 153 (2006) A468.

In recent years, organic photovoltaic, especially the polymer-fullerene bulk heterojunction solar cells, exhibit many promising features, such as intrinsic flexibility, low cost, and ease-fabrication. Because of the phase separation between two components, the disordered structure in the active layer is the critical problem limiting the power conversion efficiency (PCE) of BHJ solar cell. Therefore, an ordered inner structure is one possible solution to enhance the efficiency of the BHJ solar cells. Our research demonstrated a method to build up self-assembled vertical columns of the photoactive polymer, poly (3-hexylthiophene) (P3HT), within the active layer of BHJ solar cells via the spontaneous phase segregation in the polymer blends of P3HT, a non-active but well characterized polymer PMMA or PS and (6,6)-phenyl-C61-butyric acid methyl ester (PCBM) confined at the interface between PMMA/PS and P3HT. After selectively removed the PMMA or PS by solvent soaking, we obtained the highly ordered columns as P3HT template. This template was further utilized to fabricate real devices by filling a second photovoltaic polymer into the template. The as prepared devices were promising for exhibiting higher power conversion efficiency due to the shorter carrier transportation pathway and larger interface area between donor and acceptor. The columnar structured template is investigated under atomic force microscopy (AFM) and transmission electron microscopy (TEM). Neutron reflectometry was used to demonstrate the confinement of PCBM at the interface between P3HT and PMMA/PS in the active layer.

Polymer electrolytes are regarded as an essential component in proton exchange membrane fuel cells (PEMFC) because of its high ionic transport, permeability to fuels, stability against corrosive environment and high electrical resistance, etc,. Of particular interest, thin polymer electrolytes have been attracted widely due to its distinct physical properties by the interfacial confinement, free surface effects and specific interactions between the active surface and polymer chains. A study of proton transport on nanoscaled thin polymer electrolytes disclose the clear scenario between the structure and property relations in confined systems. Indeed, the proton conductivity, structural orientation and water uptake capability in confined polymeric systems are influenced by varying the nature of substrate surface, origin of interfacial effects, morphology of thin films, and characteristics of domain spacing, etc,.
To understand the correlation between proton transport and structural orientation in polyelectrolytes, we have prepared the nanostructured polyimide thin films. Spin-coating was used to prepare the polyimide thin films and the desired thickness has been achieved by controlling the polymer to solvent composition. Thus, the 40 nm thin and 250 nm thick films have obtained on the quartz substrate. Both the relative humidity (RH) and temperature dependent proton conductivities were measured on these nanostructured polyimide thin films. A 40 nm thin polyimide film showed a slight enhancement of proton conductivity and the obtained maximum conductivity is 4.5 Chi; 10-2 S cm-1 at 298 K and 95% RH. Based on the orientation kinetics of polymer chains, the proton transport property have investigated. It is expected that the polymer chains are highly oriented in the in-plane direction, when the polyimide system is confined into thin film. Furthermore, confinement to thicknesses at nanoscale can lead to the anisotropies in internal morphology of the films, which is favorable for higher proton transport. From the Arrhenius plot, the activation energies were estimated and the obtained values are 0.24 and 0.20 eV respectively for 40 nm and 250 nm polyimide thin films. Employing the infrared (IR) p-polarized multiple-angle incidence resolution spectrometry (p-MAIRS) technique, polymer orientation was investigated in both the in-plane and out-of-plane direction.

There is much demand for back side passivation films of crystal silicon (Si) based solar cells, with slimming down Si wafer. Thermal silicon oxide, which is fabricated by an annealing at high temperature, is generally used as a passivation film of Si based solar cells. However, high temperature process gives Si a damage with deterioration of crystallinity. Hence passivation films fabricated by low temperature process are greatly requested. Aluminum oxide (AlO_x) thin film which has a large amount of negative fixed charge at the AlO_x/c-Si interface is expected as a most suitable one [1].
Several vacuum-based deposition processes, such as atomic layer deposition (ALD) and plasma enhanced chemical vapor deposition (PECVD), have been employed for AlO_x thin films fabrication. In this study, AlO_x thin films were grown by non-vacuum mist CVD, which is a suitable technique for fabricating metal and oxide alloy thin films with simple configuration and environmental friendliness [2].
AlO_x thin films were deposited on both sides of p-type Si wafers under several conditions, namely samp.1-6. The carrier lifetime was measured and calculated by the microwave photo conductivity decay (mu;-PCD) method. The carrier lifetime of samp.4 was longest in those samples and was longer than that of sample treated with chemical passivation. The result suggests that the high quality AlO_x passivation layer grown by mist CVD as same as that grown by vacuum process is obtained.
In the conference, the information of mist CVD and the passivation performance of AlO_x thin film including fixed charge density and interface state density calculated from capacitance-voltage measurements will be reported and discussed in detail.
[1] J. Schmidt, et all., Prog. Photovolt: Res. Appl., Vol.16, Iss.6, (2008) pp.461-466
[2] T. Kawaharamura, T. Uchida, et all., AIP Advances, Vol.3, Iss.3, (2013) pp.032135

Oligothiophenes and their derivatives have attracted lots of attentions as they can be used to fabricate opto-electronic devices including OLEDs, OFETs and OPVs. The ability to functionalize oligothiophene backbone allows for the fine-tuning of the electronic properties1. These conjugated oligomers can be synthesized via Suzuki coupling and Grignard reaction, which have proven to be particularly advantageous in the large-scale production of oligothiophenes. Here we demonstrate synthesis and characterization of a series of conjugated oligothiolphene derivatives; terthiophene, tetrathiophen and pentathiophene, and their self-assemblies organized on the air-water interface. Our results suggest conjugation length-dependent properties (UV-Vis and Photoluminescence). Furthermore, we can control the thickness of oligothiophene multilayer and the molecular orientation in a self-assembled multilayered thin films by using Langmuir-Blodgett technique.2 This method has advantages of experimental simplicity, time-efficiency and low cost, and it also plays a crucial role in device fabriacation. Notably, the lambda;max and PL spectra of the LB films exhibit a blue shift with increasing conjugation length, contradicts the trend that we observed for solution spectra of the oligothiophenes. The structure of these self-assemblies and the impacts toward devices performance will also be discussed.

Wind carrying large amounts of sand and water droplets can erode the leading edge of a turbine blade and increase surface roughness. Superior hard and corrosion-resistant coatings of chromium nitride (CrN) have been prepared in reactive bipolar pulsed sputtering system. CrN coatings have also been prepared using dc generator in the same sputtering system under identical deposition conditions. The properties of these coatings are compared with the pulsed sputtered coatings. FE-SEM, AFM, potentiostat and nanoindentation tester have been used to characterize the coatings. We present in detail coatings (e.g., growth rate, morphology, surface roughness, corrosion resistance and nanohardness). The columnar growth of the deposited films could be suppressed by using the pulsed plasma without increasing the deposition temperature. Our studies show that CrN coatings with superior properties can be prepared using a bipolar pulsed sputtering.
ACKNOWLEDGMENTS
Following are results of a study on the "Leades INdustry-university Cooperation" Project, supported by the Ministry of Education, Science & Technology (MEST)

Direct Methanol Fuel Cells (DMFCs) are promising candidates to produce more clear and efficient power supplies due its high energy density, low pollution, fast recharge and room temperature operation. Therefore, Platinum is the best catalyst for methanol oxidation in DMFCs and due to its high cost, different Pt-based composite materials have been tested in the last couple of years for use as methanol oxidation reaction catalysts for replacing Pt. More recently, graphene and graphene derivatives appear as promising support materials for heterogeneous catalysts due to the excellent electronic, mechanical and optical properties largely exploited nowadays in numerous technological developments. In this way, we apply the simple and versatile self-assembly Layer-by-Layer (LbL) technique to synergistically combine distinct materials properties and produce highly uniform composite films, aiming the DMFC performance enhancement. Different nanostructured LbL architectures containing graphene and graphene oxide (GO) nanosheets together with platinum nanoparticles were fabricated and tested. GO was synthesized using the Hummers method being further reduced using hydrazine in the presence of poly(sthyrenesulfonate)-PSS, producing stable grapheme nanosheets in aqueous suspension (GPSS), suitable for LbL applications. Platinum nanoparticles (PtNP) were synthesized and applied in different ways: i) reduced and suspended in PSS; ii) anchored to the graphene nanosheets and suspended in PSS; and iii) in situ reduction of Pt salt by sodium borohydride. Materials were analyzed by X-Ray Diffraction and UV-vis spectroscopy, and the electrocatalytic performance of these composite LbL films for methanol oxidation was analyzed by cyclic voltammetry. It was verified long-term stability and the tolerance of the LbL films due to the accumulation of intermediates species into the electrode (electrode poisoning), relating the anodic peak current in the forward (If) and backward (Ib) peak. Some LbL architectures have shown a good methanol oxidation capability and good tolerance to the intermediate contamination. Hereafter, the better LbL structure will be tested in a DMFC.

ZnO/Si junction solar cells were prepared by r.f magnetron sputtering system. After deposition, the ZnO samples were annealed under different temperature from 400 °C to 1000 °C at air ambient. High conversion efficiencies ranging between 10.5% and 15.5% without using frontal grid contact have been carefully studied and tested for the annealed samples. The photovoltaic properties of solar cells grown under different operating temperature are characterized and analyzed. Furthermore, the stability conditions and reliability of these solar cells have been tested.

The optical and electrical properties of sputtered vanadium oxides and co-sputtered mixed transition metal oxides were studied as a function of temperature. The effect of depositing the metal oxides on very thin layer dielectric (RF sputtered SiO2 and Si) instead of directly on quartz substrate was investigated as well. The atomic composition of the films and the surface morphology were determined with Rutherford Backscattering Spectroscopy (RBS), X-Ray Diffraction (XRD), Energy Dispersive Spectroscopy (EDS), Scanning Electron Micrograph (SEM) and Atomic Force Microscopy (AFM)

Formation of a stable Solid Electrolyte Interface (SEI) between the anode and electrolyte material of a lithium ion battery (LIB) is a key factor in the performance of these batteries. Materials combinations which optimize battery performance and stability in demanding applications are the focus of major efforts as researchers attempt to correlate desirable battery properties with the presence of various chemical species in the SEIs after cycling. Often, however, the analysis is limited to the few outermost nanometers of the SEI. The greater photon energies available at the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory permit performance of Hard X-ray Photoemission Spectroscopy (HAXPES) at beamline X24A. HAXPES experiments significantly extend the photoemission probing depth to yield important information regarding the growth and composition of the SEI and a better understanding of LIBs.
Silicon is a promising anodic material for use in LIBs because its theoretical energy storage capacity is ten times that of more thoroughly investigated carbonaceous anodes. A recent study by Lucht, et al. has compared the addition of ethylene carbonate (EC) or fluoroethylene carbonate (FEC) solvent to the base electrolyte material in coin cell LIBs. Test cells containing FEC retained capacity better and showed more consistent efficiency for repeated cycling than those using EC. Analysis of the LIB anodes after cycling by TEM, EDX, TOF-SIMS, FT-IR and NMR indicate that the solvents added to each battery&’s electrolyte influence SEI formation and its maturation under cycling.
The present study investigates SEI formation and composition for LIBs using electrolytes containing LiPF6 salt and EC or FEC additives with a binder-free silicon nanoparticle anode in lithium ion coin cells. Utilizing the HAXPES technique, we are able to access information about the SEI composition down to the substrate anodic silicon. Using multiple photon energies up to 6 keV and the concomitant ability to probe more tightly bound core electrons than traditional XPS studies allows us to present depth-dependent characterization of the SEIs in order to display the unique properties of SEI growth and composition for each electrolyte additive.

Conjugated polymers have received considerable attention as organic semiconductors, with applications ranging from solar cells and organic light-emitting diodes (OLEDs) to organic field-effect transistors (OFETs). In addition to desirable opto-electrical properties, thermal and oxidative stability and mechanical toughness (resistance to fracture) are also important in large-scale fabrication of devices using these polymers. In this study, thin films of polyacetylene (PA) and poly(p-phenylenevinylene) (PPV) were prepared using the precursor approach. The precursor films, cast on glass substrates by spin-coating, were converted to the corresponding conjugated polymers by heating at different temperatures. The effect of temperature on the extent of reaction was studied using gravimetric and spectroscopic (infrared) techniques. Nanoindentation was used to measure the mechanical properties of the films at the sub-micron level. Reduced modulus, storage modulus, loss modulus, and loss factor (tan δ) were characterized through quasi-static and dynamic nanoindentation, using a three-faceted diamond Berkovich tip. The reduced modulus of PPV was found to be about 4 GPa with no significant change over the range of annealing temperatures. In contrast, PA showed a considerable decrease in the values of reduced modulus as the annealing temperature was increased. Similar trend was observed in storage modulus, that is, no significant decrease in storage modulus in case of PPV, but a decrease in storage modulus of PA with an increase in the annealing temperature.

Sulfonated aromatic polymers such as poly(p-phenylene)s have been widely investigated as polymer electrolyte membrane (PEM) materials for fuel cell applications due to their high thermal stability, high proton conductivity, and low cost. Sulfonated aromatic polymers, however, have low mechanical properties under hydrous conditions. The excess water sorption causes large and undesirable dimensional change of PEMs. Therefore, the control of water uptake for the PEMs based on poly(phenylene) derivatives is one of critical demands for fuel cell applications.
In order to control their water uptake, we synthesized sulfonated poly(phenylene)s having alkyl groups on the side chain. The sulfonated monomer, 2-hexyl-5-(2,5-dichlorobenzoyl)-benzene sulfonic acid 2,2-dimethyl-1-propyl ester, was polymerized via nickel-catalyzed coupling polymerization to obtain the polymer (NS-PHBP). NS-PEtBP, NS-PBuBP, which have ethyl and butyl groups, were also synthesized with the same procedure of NS-PHBP. The weight-average molecular weights of NS-PEtBP, NS-PBuBP, and NS-PHBP, were 126,000, 101,000, and 178,000 g mol^-1, respectively. S-PHBP, S-PEtBP, and S-PBuBP, were synthesized by the deprotection of neopentyl groups of corresponding NS-type polymers with diethylamine hydrobromide. The ion exchange capacity (IEC) of S-PHBP was 2.91 meq g^-1, while those of S-PEtBP and S-PBuBP were 3.47 meq g^-1 and 3.16 meq g^-1. The each sulfonation level calculated from the elemental analysis of the polymers was consistent with the theoretical value. The thermal stability of the polymers was investigated by using TG-DTA. The polymers showed no weight loss up to 200 - 210 °C. The water uptake of S-PHBP was 27% under 90%RH at 80 °C, whereas those of S-PEtBP and S-PBuBP were 45% and 31%. The ratio of freezable water molecules in membranes hydrated at 100%RH, determined by DSC measurements, decreased from 29% to 9% by increasing the alkyl side chain length. This suggested that the introduction of alkyl groups suppressed the sorption of free water. The proton conductivity of these polymers increased with increasing relative humidity and was about 10^-1 S cm^-1 at 90%RH and 80 °C.

Perfluorosulfonic acid polymers such as Nafion^® are the most promising polymer electrolytes for proton exchange membrane fuel cells (PEMFCs) because of their high proton conductivity and high chemical stability. However, the practical use of such polymers suffers from high cost, relatively low glass transition temperature, and high gas permeability. These issues have stimulated the research to develop alternative polymer electrolyte membranes, especially, acid-functionalized aromatic hydrocarbon polymers over the past decade. Typical aromatic ionomers have aryl or alkyl sulfonic acid groups to induce high proton conductivity. The conductivities of these ionomers at high temperature and low humidity are lower than that of Nafion^®, because aryl or alkyl sulfonic acid groups have lower acidity compared to super acidic groups such as perfluoroalkyl sulfonic acid. In this study, we report the synthesis of novel poly(phenylene)-based electrolyte having perfluoroalkyl sulfonic acid groups and investigate the effect of acidity for electrolyte properties by comparing with poly(phenylene)s containing aryl or alkyl sulfonic acid groups.
Three kinds of poly(phenylene)-based polymer electrolytes with perfluoroalkylsulfonic groups (FES-PPBP), benzenesulfonic groups (S-PPBP), and propylsulfonic groups (S-PrPBP) were synthesized via nickel-catalyzed coupling polymerization. The weight-average molecular weights (M_w) of FES-PPBP, S-PPBP, and S-PrPBP were 78.9 kg mol^-1, 119 kg mol^-1, and 178 kg mol^-1, respectively. The ion exchange capacities (IECs) of FES-PPBP, S-PPBP, and S-PrPBP were 2.12, 2.82, and 3.16, respectively. FES-PPBP, S-PPBP, and S-PrPBP exhibited lyotropic nematic liquid crystal behavior in dimethyl sulfoxide. FES-PPBP showed hydration numbers lambda; of 2-9 at 30-90%RH and 80 °C, which were the almost same lambda; values of S-PPBP and higher lambda; values than those of S-PrPBP. We employed the pulsed field gradient nuclear magnetic resonance (PFG-NMR) technique to investigate the self-diffusion coefficient of water in these membranes. The water diffusion coefficient in FES-PPBP membranes was 6.33×10^-10 m² s^-1 at 30 °C and 90%RH, which was higher than those of S-PPBP and S-PrPBP. The proton conductivity measured by impedance analyzer at 80 °C, and 30%RH of FES-PPBP was higher than S-PPBP and S-PrPBP, despite of the lower IEC values. These results showed the introduction of perfluoroalkyl sulfonic acid groups induces high proton conductivity without high sulfonation level.

The membrane electrode assemblies (MEAs) are composed of an ionomer, catalyst layer, and diffusion layer. Since there are no suitable hydrocarbon ionomers, which are used for catalytic layers, Nafion solution is used in most cases. If electron conductivity and redox properties can be given by using π-conjugated polymers as the ionomer, the improvement in catalyst activity or durability can be expected. In this study, we synthesized amphiphilic diblock copolymers composed of poly(thiophene) and poly(phenylene) electrolyte and evaluated their properties.
Block copolymers (NS_mHT_n) with π-conjugated units were synthesized by the polymerization of 1,4-dibromo-2,5-di-[4-(2,2-dimetyl-propoxysulfonyl)phenyl]proxybenzene (NS-DBPrB) and 2,5-dibromo-3-hexylthiophene (DBrHT) via catalyst transfer polycondensation. The m and n mean the polymerization degree of the hydrophilic and hydrophobic units, respectively. The number-average molecular weights and polydispersity indexes of NS_mHT_n were 10.8 - 18.3 kg mol^-1 and 1.10 - 1.25, respectively. S_mHT_n were synthesized by the deprotection of neopentyl groups of NS_mHT_n with diethylamine hydrobromide. The ion exchange capacity (IEC) values of S_mHT_n were determined by back titration to be in the range of 1.13 and 2.39 meq g^-1. The optical properties of NS_mHT_n and S_mHT_n was investigated by UV-vis-NIR absorption spectroscopy. The absorption bands corresponding to the phenylene and thiophene based bones were observed at around 300 nm and 500 nm, whereas S_mHT_n showed the absorption bands corresponding to the bipolaron bands at around 800 nm and 2000 nm. This suggested that the P3HT units were doped by the sulfonate groups of phenylene units . The PEFC performance was evaluated using the MEAs fabricated with Nafion^® 211 membranes and S₈HT₈₂, S₉HT₂₉ or S₁₈HT₂₃ ionomers. S₉HT₂₉ showed the best performance among three hydrocarbon ionomers. The limiting current density was 2420 mW cm^-2, and the maximum power density was 794 mW cm^-2 at 80 °C and 80%RH under 0.1 MPaG. The performance of MEAs with S_mHT_n were better than those of Nafion ionomers at 30%RH and 80 °C. Impedance measurements were carried out by using a impedance analyzer. The charge transfer resistance of the MEAs with Nafion ionomers was lower than those of MEAs with S_mHT_n ionomers at 80 %RH and 80 °C, while MEAs with S_mHT_n ionomers showed lower charge transfer resistance value than that of MEAs with Nafion ionomers at 30 %RH and 80 °C. The electrochemically active surface area (ECA) values were derived from cyclic voltammograms. ECA values decreased as relative humidity decreased. The ECA values of MEAs with S₈HT₈₂, S₉HT₂₉ and S₁₈HT₂₃ were better than that of MEA with Nafion at 30%RH and 80 °C.

Lots of efforts have been focused on harvesting the energy of wind or gas flow, mainly based on three commonly used mechanisms for converting mechanical energy to electrical energy: electromagnetic, piezoelectric and electrostatic mechanisms. In this work we propose a different approach that can convert gas flow energy to electric energy by using the triboelectric effect, in a structure integrating at least two conductive parts (i.e. electrodes) and one non-conductive part. The gas flow induces vibration of the cited parts. Therefore, the frequent attaching and releasing between a non-conductive layer with at least one electrode generates electrostatic charges on the surfaces, and then a electron flow between the two electrodes. Blowing natural air or other impure gas flows containing dust increase the generated charges since the friction produced at the interface between dust and other materials will increase the triboelectric charging. This approach allows detecting the concentration of dust in the gas flow without the need of any powering.
We demonstrate our concept by the development of some prototypes. For instance, a rectangular PET sheet with the dimension of 12mm×40mm, which is fixed from one of the shorter sides, serves as a movable part. It is placed, with two 0.25 mm spacers, between two Plexiglass sheets covered by copper tape, which are used as a frame and electrodes, respectively. The sheet is able to oscillate by blowing air through the channel, and then due to both charge transferring and inducing, a LED is continuously lightened up. As mentioned before, the intensity of generated voltage signals differ by flowing gases containing dust.
The polarity and the amount of those generated charges depend on the place of those materials in the triboelectric series as well as roughness, humidity, temperature, etc. Air is listed as the most positive triboelectric material in some triboelectric series [1]. Since pure gas cannot charge any surfaces, then we believe that the contaminated particles are able to charge the surfaces by coming into contact with them. However, depending on the dust types (i.e. material of particles), the electrodes or the non conductive material can be covered by an appropriate layer in order to detect and maximize the amount of generated charges.
Reference
1. Adams, Charles K. Nature's electricity. Tab Books, 1987.

As the primary source material for optoelectronic devices, silicon (Si) has unique advantages of being less toxic and naturally abundant on Earth. These advantages have led to a great deal of attention to nanostructured Si for various applications such as biological labeling, light emitting devices, transistors, and solar cells. The nonthermal plasma technique provides an effective method for synthesis of well-crystallized Si nanocrystals (Si NCs) with large production yield and excellent size control. Liquid thermal hydrosilylation with alkene ligands enables to form colloids of Si NCs well-dispersed into solvents with high-efficiency PL by utilizing. The NC size can be tuned such that one can obtain short-wavelength absorption and photoluminescence (PL) which spans across the visible-to-infrared region of the solar spectrum. This prompts an alternative potential use of Si NCs as a luminescent concentrator, which can improve absorption of solar cells by absorbing UV light and down-shifting it to a lower energy light via photoluminescence. This process can enhance the incident light intensity in the main absorption region of a solar cell and therefore increase the amount of the photons to be effectively absorbed. Aiming at UV and air stable photoluminescence of Si NCs, in this work, we investigate the UV and air stability of highly-efficient luminescence of Si NCs synthesized using nonthermal plasma technique and the effects of hydrogen gas-passivation and phosphorous (P) doping on it. After hydrosilylation, the Si NCs can be well dispersed into common solvent such as toluene with high-efficiency photoluminescence quantum yield (PLQY) of 73%. The Si NCs exhibit photobleaching effect within the first 4 hours of 365 nm UV irradiation and retain a PL quantum yield (PLQY) of 52% of the initial value irrespective of further irradiation. PLQY of the UV-irradiated Si NCs can recover with time, similar to the Staebler-Wronski effect. Gas-phase passivation of Si NCs by hydrogen afterglow injection plays a crucial role for improving PLQY and PL stability against UV or air exposure. Furthermore, phosphorous doping of Si NCs improves PLQY and leads to UV-stable Si NCs, as a result of suppression of surface defects and reduction of weak chemical bonds.

Multilayer films of poly(allylamine hydrochloride) (PAH) with poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) were assembled onto Nafion® 212 membranes using the layer-by-layer (LbL) technique to check how ultrathin films of PAH/PEDOT:PSS influence the methanol permeability and resistance of Nafion® membranes. The LbL assembly was chosen due to its simplicity and versatility to produce thin films with controllable thickness and morphology, taking the advantages of nanoscale engineering to create tailored nanostructures and functional surfaces with fine control in the film composition. The PEDOT:PSS choice was based on its chemical stability and possibility to increase the proton conduction due to the presence of sulfonic groups in the PSS structure, quite important for proton transfer in Nafion® membranes. Here, UV-vis and impedance spectroscopy were used to investigate the nanostructured PAH/PEDOT films onto a Nafion® 212 membrane, which was also characterized by voltammetric measurements to check the effect of the LbL films in the methanol permeation. Our results indicated good adherence and linear growth of the LbL films onto Nafion® 212, and also good action as a methanol barrier. Therefore, we believe that the LbL assembly requires some important charged groups to water permeation present at the Nafion® surface, consequently affecting the membrane resistance.

Electrochemical reactions on solid surfaces directly underpin the functionality of batteries, [1-3] fuel cell, [4-6] sensors, and multiple other electrochemical devices. In many cases, the direction and mechanism of reaction is controlled by the nanoscale details of surface structure, On the nanometer scale, localized electrochemical reactions enable technologies such as scanning probe microscopy (SPM) based nanofabrication,[7-9] data storage, and charge writing in oxide electronic devices.[10,11] However,the basic premise of electrochemistry holds that application of a negative bias induces local reduction and a positive bias induces local oxidation. We demonstrate by SPM that the reverse happens in (001) oriented epitaxial CeO2 thin films i.e. application of positive bias to an SPM probe can induce local reduction, a behavior we refer to as “electrochemical field effect”. Two different regimes are observed: a moderate bias/negligible current regime, where an anomalous redox processes occurs (2CeO2+3H2O --- 2Ce(OH)3+1/2 O2) and a high bias/sizeable current regime, where standard redox processes take place. The standard anodic/cathodic behavior is recovered in the high bias regime, where a sizeable transport current flows between the tip and film. The role of water in this process is explored by scanning transmission electron microscopy - electron energy loss spectroscopyanalysis. We believe that this behavior can be universal for electrochemical processes in nanoconfined volumes. Our study gives insight into the mechanism of the tip-induced electrochemical reactions as mediated by electronic currents as well as allows nano-structures and devices to be explored at the surface.
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[5]E.V. Tsipis, V.V. Kharton, J. Solid State Electrochem. 2011,15, 1007.
[6]V.S. Bagotsky, Wiley Press: 2009.
[7]R.Garcia, N.S. Losilla, J. Martinez, R.V. Martinez, F.J. Palomares, Y. Huttel, M. Calvaresi, F. Zerbetto F. Appl. Phys. Lett. 2010, 96, 143110.
[8]M. Tello, R. Garcia, Appl. Phys. Lett. 2001, DOI: 10.1021/nl049544f.
[9]Garcia, R.; Calleja, E.; Perez-Murano, F. Appl. Phys. Lett. 1998, 72, 2295.
[10]Y.W. Xie, C. Bell, T. Yajima, Y. Hikita, H.Y. Hwang, Nano Letters 2010, DOI: 10.1021/nl1012695.
[11]A. Kumar, T.M. Arruda, Y. Kim, I. N. Ivanov, S. Jesse, C. W. Bark, N. C.Bristowe, E. Artacho, P.B. Littlewood, C.B. Eom, S.V. Kalinin, ACS Nano 2012, 6, 3841.

Significant and steady improvements have been recorded over the past decade in the efficiency of organic light-emitting diodes and solar cells, or in charge carrier mobility in organic field-effect transistors. Interfaces between active organic layers and inorganic electrodes or other organic films or dielectrics control charge injection and collection, charge separation, and charge transport through devices, and a better understanding and control of these interface materials and structures has played a key role in the process. In this talk, we first review the most important mechanisms that control the energetics of organic/electrode interfaces, and show why low barrier contacts to wide band gap, low electron affinity or high ionization energy organic semiconductors are challenging. We then focus on three methods / materials that provide solutions to this challenge. The first involves transition metal oxides (TMO), such as molybdenum and tungsten tri-oxides (MoO3, WO3), which exhibits good n-type conductivity and very high work function (> 6.5 eV) [1]. A detailed analysis of organic/TMO interface energetics and the resulting hole injection process is given. The second involves the realization of efficient electron injection/harvesting electrodes. A “universal” lowering of the work function of many different types of electrodes is implemented via application of an ultra-thin film of the polymer polyethylenimine ethoxylated (PEIE) [2]. Finally, we discuss the role of chemical doping (n- and p-type) in making high conductivity, low energy barrier contacts to organic thin films [3].
[1] J. Meyer et al., Adv. Mat. 24, 5408 (2012)
[2] Y. Zhou et al., Science 336, 327 (2012)
[3] Y. Qi et al., J. Am. Chem. Soc. 131, 12530 (2009)

For the majority of large pi-conjugated molecules, the Fermi level of supporting coinage-metal substrates comes to lie well within their electronic energy gap. In some cases, however, the Fermi level is seen to cross into one of the frontier molecular orbitals, thus giving rise to substantially charged monolayers. Moreover, if the Fermi level comes to lie right within a manifold of frontier-orbital derived electronic states, the molecular monolayer can be regarded as having lost its intrinsic semiconducting properties and inherited the metallic character of the substrate. Results from a combined multi-technique experimental and theoretical study [1] on a particularly clear-cut case - pentacenequinone and pentacenetetrone on the (111) surfaces of Au, Ag, and Cu - suggest a common traits among many systems that exhibit this intriguing interfacial electronic structure: The effective conjugation length of the concerned organic semiconductors increases through interaction of specific chemical substituents with the metal surfaces. The ensuing reduction of the molecular energy gap is asymmetric and aids the respective frontier orbital in overcoming the counteracting phenomenon of Fermi-level pinning. These insights open new strategies for surface and interface engineering at hybrid interfaces in organic electronics. There, the knowledge-based design of metallic molecular interlayers harbors the potential for eliminating undesired energy barriers for charge-carrier injection and for simultaneously stimulating the desired growth mode of a subsequently deposited active material.
[1] G. Heimel, S. Duhm, I. Salzmann, A. Gerlach, A. Strozecka, J. Niederhausen, C. Bürker, T. Hosokai, I. Fernandez-Torrente, G. Schulze, S. Winkler, A. Wilke, R. Schlesinger, J. Frisch, B. Bröker, A. Vollmer, B. Detlefs, J. Pflaum, S. Kera, K. J. Franke, N. Ueno, J. I. Pascual, F. Schreiber, and N. Koch, Nature Chem. 5, 187-194 (2013).

The electronic structure of organic/metal (O/M) interfaces is crucial for Organic Electronics (e.g. organic photovoltaic cells), in which charge carrier injection at the interface strongly influences the device performance. One remaining issue in this research field is a formation of charge-transfer (CT) states at the O/M interfaces. While it is known that the CT state is caused by an electron-transfer from the metal substrate to the former lowest unoccupied molecular orbital (LUMO) of the adsorbate, it is not well understood until recently why the CT state forms unexpectedly in specific O/M systems. Heimel et al. succeeded to explain the mechanism leading to the formation of CT states in terms of surface-induced aromatic stabilization [1]; the stabilization of the π-conjugation of the adsorbed molecules due to an electron donation from the substrate atoms to specific substituents (e.g. C=O) within the molecules lowers the LUMO level, and subsequently induces the CT. This phenomenon apparently requires such a specific functional group with hetero-atoms, which also makes the molecule-substrate bonding distance shorter. However, it is not yet known whether both conditions are necessary for the stabilization.
In this paper, we will show that even functional groups without hetero-atoms yield a surface-induced aromatic stabilization. For this purpose, we studied the interface electronic structure of a perylene derivative (DIP: C32H16) [2] and perylene (C20H12) monolayers on Ag and Cu single crystal surfaces by means of angle-resolved ultraviolet photoelectron spectroscopy with synchrotron-light/radiation. We found the formation of CT states for DIP on both Ag and Cu surfaces, but not for perylene molecules. Comparing these results with other perylene derivatives (PTCDA: C32H8O6) [3], we will discuss the structural requirements of organic molecules for the formation of CT states.
[1] G. Heimel, et al., Nature Chem. 5 (2013) 187.
[2] C. Bürker, et al., Phys. Rev. B 87 (2013) 165443.
[3] S. Duhm, et al., Org. Electronics 9 (2008) 111.

Interface engineering is gaining more and more importance in the development of organic bulk heterojunction solar cells. Addition of an organic interlayer in between the donor-acceptor blend and the aluminum top contact is an easy way to further increase the power conversion efficiencies.[1] In this work we studied interlayers based on an imidazolium-substituted polythiophene. The addition of such a layer in between the active layer (PCDTBT:PC71BM) and the aluminum top contact resulted in an increase in short circuit current.[2] In this study advanced morphological and electrical characterization techniques are introduced towards a better understanding of the origin of this effect. As Scanning Probe Microscopy (SPM) techniques have a lateral resolution of about 5-10 nm for imaging and 20-30 nm for electrical characterization, they are suitable to study the local morphology, mechanical properties and electrical properties. The coverage of the polythiophene layer was probed with PeakForce Quantitative Nano Mechanical mapping. PeakForce TUNA allows us to correlate morphology with nanoscale conductivity and Kelvin Probe Force Microscopy shows the change in surface potential when this layer is added. The combination of the aforementioned techniques allows us to correlate the local mechanical properties to either the polythiophene film or the underlying bulk heterojunction. Important observations are that adding this interlayer results in a decreasing surface potential difference and notably changes the nanoscale conductivity. These observations can be of important interest towards further developments in the field of efficiency enhancing interlayers.
[1] Seo, J. H., Gutacker, A., Sun, Y., Wu, H., Huang, F., Cao, Y., Scherf, U., et al. (2011). Improved high-efficiency organic solar cells via incorporation of a conjugated polyelectrolyte interlayer. Journal of the American Chemical Society, 133(22), 8416-9. doi:10.1021/ja2037673
[2] J. Kesters, T. Ghoos, H. Penxten, J. Drijkoningen, T. Vangerven, D. M. Lyons, B. Verreet, T. Aernouts, L. Lutsen, D. Vanderzande, J. Manca, W. Maes, Advanced Energy Materials 2013. DOI: 10.1002/aenm.201300049.

Organic Photovoltaic (OPV) represents a renewable energy source for their potential benefits of being light-weight, mechanically-flexible, and compatible with low-cost solution processing techniques. Knowledge of the electronic and chemical properties at the interfaces between dissimilar materials in OPV devices is critical to understand the factors affecting their performances. However, due to the inaccessibility, the buried interfaces between evaporated metal contacts and active layer remained not well studied. For poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) bulk heterojunction (BHJ) active layer, it was shown that annealing after the deposition of Al cathode significantly improves the device performance due to rearrangement in the BHJ morphology. Here we examine conventional P3HT:PCBM device performance with different metal (Al vs. Ag) cathodes without further annealing to investigate understand whether metal deposition by itself could induce BHJ morphology change and to study the relationship between interfacial properties and device characteristics. By adopting a controlled delamination method to reveal the buried metal-BHJ interfaces, we are able to perform direct measurements of work function, ionization potential, and spatial homogeneity on both metal and the organic sides of these interfaces. From tapping mode atomic force microscopy (AFM) and scanning kelvin force microscopy (SKFM), we demonstrate that thermal evaporation of metal without further annealing is sufficient to induce BHJ morphology change. With the photoelectron spectroscopy in air (PESA) and Kelvin probe (KP) measurements, we have determined the ionization potential and work function of these interfaces, which can be different from the top surface. In addition, x-ray photoelectron spectroscopy (XPS) will be applied to quantify the concentration of P3HT vs. PCBM at the air vs. metal interfaces and changes due to metal evaporation. The results from these direct measurements of buried interfaces allow us to correctly describe the spatial distribution of electric field and chemical species in the actual devices to understand the device behavior.

Transformative device technologies based on new organic electronic materials like OPVs and OLEDs exploit interfaces to control fluxes of charge and/or energy. The overall power conversion efficiency in an organic solar cell depends on the balance between the rates of exciton dissociation, recombination and separation at the donor acceptor interface. Inability to design, control and engineer these interfaces remains a key bottleneck in their widespread use for the next generation organic electronic devices. Here, we report three different strategies that allow us to completely suppress the exciplex (or charge transfer state) recombination between any donor and acceptor system. We observe that the photocurrent increases by 300 times and the power conversion efficiency increases by an order of magnitude simply by inserting a spacer layer in the form of an a) insulator b) Oliogomer or using a c) heavy atom at the donor-acceptor interface in a P3HT/C60 bilayer device. We find that using each of these strategies, the mechanism of photocurrent generation (or suppression of exciplex recombination) is completely different. Moreover, these strategies are applicable universally to any donor-acceptor interface and will be used to modify the donor-acceptor interfaces in a bulk heterojunction (BHJ) device. Finally, a simple transport model will be used to explain the change in the charge transfer and recombination rates and predict current-voltage curves.

Thin film bulk heterojunction (BHJ) systems exploiting fullerenes dispersed in conjugated polymers are popular material candidates for organic photovoltaics (OPVs) that can be easily chemically tuned to change the interactions and properties. The device efficiency is dependent on a number of factors including the inherent morphology of the BHJs, which depends on a number of parameters including the strength of polymer-fullerene interactions, degree of crystallinity, and interface segregation behaviour. To improve device efficiency and make OPVs commercially viable, it is essential to be able to understand and consequently control the morphology of BHJ films.
As part of a comprehensive study of morphological factors we are studying model systems composed of mixtures of different poly(3-alkyl thiophene)s (P3BT, P3HT and P3OT) and fullerenes (C60, PCBM and bis-PCBM). BHJ morphologies of these mixtures in full device configuration have been explored using a variety of techniques and correlated to their device performance. Ångstrom resolved composition depth profile analysis using Neutron Reflectivity (NR) has shown that varying amounts of fullerene segregate to both the top (Al) and bottom (PEDOT:PSS) electrodes. Analysis shows that the segregated excess fullerene at the PEDOT:PSS electrode is inversely related to the fill factor, whilst the excess is linearly related to the short-circuit current. Grazing incidence small angle neutron scattering (GISANS) has shown that the lateral phase domain morphology varies through the BHJ thickness. At the Al electrode interface, domains of tens of nanometers are observed but everywhere else in the film domain sizes are over hundreds of nanometers. Using grazing incidence wide angle X-ray scattering (GIWAXS) has shown that devices with the highest efficiency has the lowest overall total crystallinity. The crystals that do exist are for the most part randomly oriented relative to the electrode surfaces, with a small degree of orientation of the (100) planes of the poly(thiophenes) parallel to the substrate.
Using these design queues in the current work we are exploring the use of additives to control crystallization of the poly(thiophene). Given we observe a reduction in fill factor (FF) with increased fullerene at the PEDOT:PSS we are exploring the methods to modify the electrode interface to promote poly(alkylthiophene) segregation and hence increase FF.

Polyethylene dioxythiophene polystyrene sulphonate (PEDOT:PSS) produces organic photovoltaic (OPV) devices with a higher open circuit voltage (Voc) than any other hole transport layer. We will show that the increased Voc comes from a doping reaction between the bulk heterojunction polymer and the acidic group on the PSS at the layer interface. PSS phase separates to the top interface during coating and so is available for reaction with a subsequently deposited layer. However, since the PSS is electrochemically bonded into the PEDOT:PSS layer, the reaction can only occur if the PSS is able to mix with the polymer in the adjacent layer. For this reason, the extent of the doping reaction can be controlled by limiting the mixing between materials. For most conjugated polymers we show that miscibility with PSS occurs at 120-200C. Interestingly, the same polymers do not mix with PEDOT:PSS in this temperature range. We also measure whether the doping efficiency of the PSS and find that although the PSS mixes with many conjugated polymers, it will only dope polymers with a relatively low work function. The study is completed by showing that a stronger acidic dopant (Nafion) can replace PSS and is capable of doping conjugated polymers with high work functions. Understanding the details of interface doping will be necessary to further increase the efficiency of OPV devices.

The influence of additives, specifically 1,8-diiodooctane (DIO), on the stability, performance and morphology of conjugated polymer (P3HT)/fullerene (PCBM) based solar cells was investigated. Solar cell devices with DIO additive showed up to 25% higher photoconversion efficiency compared to devices without DIO. This efficiency improvement is primarily due to increase in fill factor (FF) and short circuit current (Jsc). Additives improved donor-acceptor interpenetration, resulting increased charge carrier mobility. The hole mobility in the active layer was 7.71x 10-5 m2/Vs, 9.69x10-5 m2/Vs and 1.00x10-4m2/Vs for pristine, 3% DIO and 4% DIO respectively. The carrier sweep times estimated from transient spectroscopy measurements indicated that addition of DIO decreased the carrier sweep time significantly. The solar cells were stored in a glovebox and their device characteristics were measured periodically over 42 days. Results showed that addition of DIO helped improvising the shelf-stability of the cells. The efficiency of cells decreased by 62% in case of pristine whereas the decrease was 22% and 51% for 3% and 4% DIO cells, respectively. The changes in donor-acceptor morphology, electrical conductivity of the active layer, its local photoconductivity, carrier sweep times and intensity-dependent current-voltage characteristics were studied in detail. Results indicate that the morphological stability and trap generation plays an important role in increased carrier recombination in aged cells.

The effects of confinement on the optical, electrical and structural properties of polymer:fullerene bulk heterojunction thin films is of great significance for novel thin film organic electronics. Particularly in organic photovoltaics, spatially dependent morphology such as the phase segregation of donor and acceptor materials within the photoactive layer can have significant effects on device design, performance and optimization. However, the effect of bulk heterojunction film thickness on phase segregation, which is of vital importance for ultra-thin (<50 nm) bulk heterojunction photovoltaics, is not well understood. To this end, we use variable angle spectroscopic ellipsometry and near edge X-ray absorption fine structure spectroscopy to experimentally measure the optical constants, vertical donor-acceptor volume fraction, and morphology of bulk heterojunction layers between 15 - 200 nm. We find that as the thickness of the bulk heterojunction is decreased from the thickness range of 100 - 200 nm to the ultra-thin regime of less than 50 nm, surface-energy effects at the interfaces play an increasingly important role in determining the bulk heterojunction morphology and phase segregation. In addition, we measure the change of the complex refractive indices and shift of absorption resonances relative to neat donor and acceptor films, which provides insight into the domain structure and degree of order in the interpenetrating donor-acceptor phases. Furthermore, we find for the first time that as the film thickness is decreased below 50 nm, less phase segregation is observed, and the film becomes structurally less affected by the addition of fullerenes into the polymer film. Lastly, we observe that the shift in the exciton bandwidth and of the singlet exciton peaks, 0-0 & 0-1, due to the addition of fullerenes diminishes in the ultra-thin regime (<50 nm) attributed to thin-film confinement of the bulk heterojunction.

There has been widespread interest in organic photovoltaic (OPV) devices over the last decade. OPV, which can be utilized in large area and flexible surfaces, is a promising low-cost alternative to solid-state solar cells. It has a significant foreground in industrial applications. However, one of the major limitations of OPV devices is the short lifetime and instability. As a key approach to extend lifetime of OPV devices and improve their stability, encapsulation limits the transmission of oxygen, water vapor and ultraviolet light to the device. Here, we represent an encapsulation method using single layer of organic polymer via initiated chemical vapor deposition (iCVD) at low surface temperatures (<30°C). This iCVD encapsulation was first performed directly on a widely used conducting polymer, poly(3,4-ethylenedioxythiophene) (PEDOT), which losses conductivity during annealing. Stability was evaluated at three temperatures, 30, 50, and 100°C. The results of conductivity change of PEDOT verses annealing time show that the encapsulation can help to maintain the conductivity of PEDOT up to 50 times longer than non-encapsulated samples during the annealing. The morphology and conformality of the encapsulation layer was characterized by AFM and SEM. The apparent activation energy for conductivity loss was 51kJ/mol. The thickness effect of encapsulation and possible degradation mechanism will be discussed. The transparent encapsulation layer also largely improves the conformality of PEDOT layer (The profile roughness parameter, Rq, reduces from 9.7 nm to 0.6 nm after the encapsulation of 200 nm film). This novel encapsulation method features with low-substrate-temperature and solvent-free, which prevent possible damages to OPV devices during the encapsulation process. We further test this iCVD encapsulation on an inverted OPV solar cell. The efficiency degradation between different encapsulation methods will be discussed.

The electronic and chemical structure of interfaces is of central importance for understanding and tailoring of materials, chemical processes, and electronic devices. Thus, over the past few decades, enormous experimental (and theoretical) insights into electronic and chemical structures of well-defined model surfaces have been sought and gained by a variety of approaches.
But what if the area of interest is buried, e.g., at an interface? What if no suitable, well-defined, and clean model surfaces are available? And how much do we know about the electronic and chemical structure of liquids or at liquid-solid interfaces?
The purpose of this talk is to demonstrate how a tool chest of soft x-ray spectroscopies (including lab-based techniques and approaches using high-brilliance synchrotron radiation) is uniquely suited to address such questions. Material systems to be discussed include compound semiconductors for thin film solar cells and photoelectrochemical water splitting, water, and water/solid interfaces, and it will be shown how soft x-rays can be utilized to shed light on their electronic and chemical properties.

CdTe/CdS-based thin-film solar cells are established candidates for cost-effective devices yielding high efficiencies.¹ To further improve their performance, a fundamental understanding of the chemical and electronic interface structure throughout the CdCl₂-treated CdTe/CdS/SnO₂:F/glass layer stack is necessary. Significant diffusion processes occur during manufacturing, particularly induced by the post-deposition CdCl₂ treatment^2,3, and thus most interfaces in the device cannot be considered abrupt junctions. Hence, it is impossible to use standard surface-science characterization approaches of step-wise material deposition and subsequent analysis to derive a model of the layered structure.
In this contribution, we describe a study of the buried CdS/SnO₂:F interface using a combination of surface-sensitive characterization methods and a reproducible cleaving process. To investigate the morphological, chemical, and electronic properties of the exposed surfaces after cleavage, we have employed atomic force microscopy (AFM), x-ray (XPS) and ultra-violet (UPS) photoelectron spectroscopy, as well as inverse photoelectron spectroscopy (IPES). Cleavage was achieved by gluing metal sheets to both sides of the layer stack to allow for a mechanical lift-off.
After the cleaving, we identify large smooth regions as well as smaller areas with islands on the freshly exposed surface of the “glass side” and corresponding holes on the back surface of the former top side. The average hole depth and island height corresponds to the CdS thickness. XPS measurements show that the “island side” predominantly consists of Sn and O, with small amounts of Cd, S, and Te. Correspondingly, the “hole side” consists largely of Cd and S with small amounts of Te (yet more pronounced than on the SnO₂:F side). This contrasting composition suggests that cleavage, to a large extent, occurs at the CdS/SnO₂:F interface (with small areas cleaving at the CdTe/CdS interface). Furthermore, significant quantities of Cl are detected on both sides of the interface, indicative of the interdiffusion processes induced by the CdCl₂-treatment. In our presentation, we will provide an in-depth chemical and electronic analysis of the deeply buried CdS/SnO₂:F interface and discuss the impact of non-perfect cleavage surfaces and the effects of diffusion on the derived electronic structure. Finally, we will relate our results to the performance of CdTe-based thin-film solar cells.
¹http://investor.firstsolar.com/releasedetail.cfm?ReleaseID=743398
²S. Pookpanratana et al., Appl. Phys. Lett. 97, 172109 (2010).
³N. Romeo et al., Solar Energy 77, 795 (2004).

We present a novel approach towards achieving high visible transmittance for vanadium dioxide (VO2) coated surfaces whilst maintaining the solar energy transmittance modulation required for smart-window applications.
Our method deviates from conventional approaches and utilises subwavelength surface structures, based upon those present on the eyeballs of moths, that are engineered to exhibit broadband, polarisation insensitive and wide-angle antireflection properties.
The motheye functionalised surface is expected to benefit from simultaneous super-hydrophobic properties that enable the window to self-clean.
We develop a set of design rules for the motheye surface nanostructures and, following this, numerically optimise their dimensions using parameter search algorithms implemented through a series of Finite Difference Time Domain (FDTD) simulations.
We select six high-performing cases for presentation, all of which have a periodicity of 130 nm and aspect ratios between 1.9 and 8.8.
Based upon our calculations the selected cases modulate the solar energy transmittance by as much as 23.1% whilst maintaining high visible transmission of up to 70.3%.
The performance metrics of the windows presented in this paper are the highest reported for VO2 based smart-windows.

Amorphous titanium oxide (TiOx) is deposited onto crystalline silicon using a low-temperature, metal-organic chemical vapor deposition (MOCVD) process. TiOx/Si heterojunction diodes fabricated using the process have been demonstrated to prevent hole transport while allowing electron transport [1]. Here we report on the electronic structure and chemical composition of the heterojunctions, studied using surface science techniques such as Ultraviolet and X-ray Photoelectron Spectroscopy (UPS, XPS) and Inverse Photoemission Spectroscopy (IPES). From UPS and IPES measurements, the TiO2 energy gap has been determined to be 4.6 eV, significantly larger than the 3.1-3.3 eV gap expected for crystalline TiO2. The heterojunction band line-up features conduction and valence band offsets of 0.12 eV and 3.39 eV, and an interfacial dipole of 0.75 eV with Fermi level pinning. Based on the large valence band offset, and the small conduction band offset, the heterojunction appears to be a good electron extractor and hole blocker. XPS studies indicate that the titanium is primarily in the +4 oxidation state and the film is composed of titanium dioxide. Changes in heterojunction electronic structure and chemical composition are observed with annealing in nitrogen. Electrically, the amount of band-bending in the silicon is reduced, implying that the silicon is better passivated. Chemically, an increase in the amount of nonstoichiometric SiOX is detected at the interface. Surface recombination velocity studies, which directly determine the electrical interface quality, have shown that the minority carrier lifetime increases with optimized annealing. Chemical changes due to annealing may be responsible for improvements in silicon surface passivation, and hence minority carrier lifetimes. TiOx/Si heterojunctions offer good hole-blocking and electron-extracting properties for electronic device applications, and improvements in electrical interface quality can be attained with optimized annealing conditions.

[1] S. Avasthi, W. E. McClain, G. Man, A. Kahn, J. Schwartz, and J. C. Sturm, Appl. Phys. Lett., 102, 203901 (2013)

Thermally-grown silicon dioxide has been extensively used to passivate surfaces of silicon solar cells for many years. However the high temperature oxidation process requires critical wafer cleaning and a substantial thermal budget. Furthermore, the high temperatures required are often detrimental to lower-cost, lower purity silicon wafers [1] that are increasingly being used for photovoltaic applications in order to reduce manufacturing costs. Recently, surface recombination velocities of less than 40 cm/s have been demonstrated on silicon wafers passivated with silicon dioxide layers that were electrochemically grown in nitric acid at room temperature [2]. The performance of this silicon surface passivation is comparable to that attained by the best thermal oxide [3], however the electrochemical method reported in [2] resulted in slow growth, non-uniform oxidation and was sensitive to the spacing between the electrodes.
In this paper we report the use of a light-induced anodisation (LIA) method to uniformly anodise the p-type silicon surfaces of silicon solar cells. The light-induced current of the cell results in the p-type surface becoming anodic, and when the cell is immersed in a suitable electrolyte, a uniform silicon dioxide layer results. The high uniformity of the formed oxide is possibly due to the uniform drift of the positive charge carriers in the silicon to the surface being anodised. The oxide grows at higher rate than that in nitric acid, and with a constant bias voltage the growth rate is positively correlated to the bias voltage. An oxide layer with thickness of 18 nm can be formed by anodising for 5 min with 15 V bias in 0.5 M sulphuric acid. After annealing in oxygen and forming gas at 400 degree C for 30 min each, an average implied open circuit voltage of 656 mV was measured by photo-conductance decay on 180 µm p-type 1-3 Ohm cm Cz silicon wafers, with a value of 653 mV being measured for the same wafers passivated by a thermally-grown oxide of the same thickness. The properties of the silicon dioxide layers formed by LIA have been characterized by photoconductance lifetime measurements, ellipsometry, x-ray photoelectron spectroscopy, deep level transient spectroscopy and Fourier transform infrared spectroscopy.
References:
1. Cousins, P.J. and J.E. Cotter, Minimizing lifetime degradation associated with thermal oxidation of upright randomly textured silicon surfaces. Sol. Energ. Mat. Sol. Cells, 2006. 90(2): p. 228-240.
2. Grant, N.E. and K.R. McIntosh, Low surface recombination velocity on (100) silicon by electrochemically grown silicon dioxide annealed at low temperature. IEEE Electron Device Lett., 2010. 31(9): p. 1002-1004.
3. Glunz, S.W., et al., Injection level dependent recombination velocities at the Si/SiO2 interface for various dopant concentrations. J. Appl. Phys., 1994. 75(3): p. 1611-1615.

In this presentation, we will review our studies of the structure and electronic properties of extended defects, such as dislocations and grain boundaries, in thin-film solar cell materials by the combination of aberration-corrected scanning transmission electron microscopy (STEM) and first-principles density-functional theory (DFT). The atomic structure and chemistry of extended defects were determined using Cs-corrected STEM. Based on the determined structure and chemistry, the electronic properties of the extended defects are studied by DFT calculations. The solar cell materials studied include CdTe, CuInSe2, and Cu2ZnSnSe4. We find that while some extended defects are very harmful to the device performance, some others are not. While some extended defects cause Fermi level pinning, some extended defects can actually be beneficial for solar cell performance. The electronic properties of the extended defects depend also on the host material. For example, the Sigma 3 boundaries are more harmful in CdTe than in CuInSe2. We will discuss why the extended defects behave differently in different solar cell materials.

At the border between biology and chemistry dye molecules are subject to an increasing interest in the field of renewable energies. Initially tailored for Dye Sensitized Solar Cells (DSSC), these molecules are now widely used in many emerging technologies including solid state DSSC, photo-catalytic systems for water-splitting or hydrogen production. One of their key roles within these devices is the transfer of the oxidation power: charges can be transferred from one molecule to another across a monolayer. It is their structure at the nanoscale that governs the properties of a full monolayer. Experiments can provide us with the characteristics of a device on a case-to-case basis. However, a clear link between the scales with good understanding of the dye structure to function relationship is missing.
In this work we introduce a multi-scale method to model hole diffusion dynamics through a monolayer of dye molecules anchored to a nanocrystalline film (as within a device). We treat the intermolecular charge transfer step as a non-adiabatic hopping process and calculate the hopping rate as a function of the electronic coupling (J) and the reorganization energy (lambda;). First we propose a numerical method [1] based on Density Functional Theory (DFT) to calculate the inner- and outer-sphere reorganization energies and show that the nature of the surrounding medium dominates lambda;, consistent with the high values of lambda; observed experimentally (lambda; ~ 1 V) . Next, we combine Molecular Dynamics (MD) and DFT to study the influence of the dyes dynamical arrangement at the nanoscale on J. Our approach explicitly account for the configurational disorder within the dye monolayer, which, at the best of our knowledge, has not yet been done. Finally we incorporate these parameters into a continuous time random walk, accounting for disorder, to estimate the hole diffusion coefficient at the scale of the dyes monolayer. The results show reasonable agreement with experimental measurements on a range of dye sensitized films. [2]
[1] Vaissier, V. Barnes, P. Kirkpatrick, J. and Nelson, J. PCCP 15, 4804-4814 (2013).
[2] Vaissier, V. Mosconi, E., Pastore M, Barnes, P. De Angelis F. and Nelson, manuscript in preparation

A typical amorphous-silicon(a-Si) based tandem solar cell requires a back reflector (BR) for efficient light absorption. The BR is comprised of a mirrored metal layer (Al and/or Ag) and a ZnO layer, which is interposed between silicon layers and the metal layer, serving as a buffer to enhance the optical reflection and prevent the metal element from migrating into the Si layers. In this work, we singled the ZnO out of the BR, and, carefully, explored how variations of its properties, such as transmission, thickness, etc, caused changes in performance of tandem a-Si/a-SiGe solar cell, such as fill factor, short circuit current, QE spectra, and energy conversion efficiency. Our experimental results reveal that, with variations of ZnO layer properties, the cell performance has an obvious changes, even in the shape of QE spectra. This study provides an insight on how the ZnO, as a buffer layer between reflective metal layer and silicon layer, influences the performance of a-Si/a-SiGe tandem solar cells.

Silicon-based solar cells dominate today&’s photovoltaic market, accounting for more than 85% of the world&’s production share. They are, however, limited by defects inherent to their growth process. Herein, we focus on one of the most deleterious defects in multicrystalline silicon (mc-Si): Dislocations. These defects are known to degrade solar cell device performance by serving as recombination centers for photogenerated charge carriers [1]. The strength of such recombination, however, has been reported to vary significantly between different dislocation clusters, even when separated by as little as a few millimeters [2,3].
Dislocations intersecting the wafer surface can be revealed upon chemical etching as etch pits of various geometrical shapes. Spatially resolved quantum efficiency maps show higher recombination activity at dislocation clusters containing a higher degree of etch pit shape variation However, the relation between etch-pit topology, the dislocation microstructure, and recombination strength have not been studied in much detail.
In this contribution, we investigate the correlation between dislocation electrical activity, etch pit shape distribution, and dislocation core structure. We determine the recombination strength of different dislocation clusters through spatially resolved laser beam-induced current (LBIC) measurements. We study the crystallographic nature of the dislocations by electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). Lastly, the shape variation of dislocation etch pits within different clusters is studied systematically and compared to the cluster&’s recombination activity. Quantitative results confirm our qualitative observation that higher dislocation recombination activity appears to be correlated to a higher degree of disorder in the geometrical eccentricity of etch pits
The understanding of dislocation electrical activity via chemically-treated surface analysis can provide an easy path towards inferring solar cell performance in a processing line, with the potential of evolving into a robust early-stage predictive method of dislocations&’ impact in efficiency, consequently shortening the feedback loop for industry&’s crystal-growth process control.
[1] C. Donolato, Journal of Applied Physics 84(5), 2656-2664 (1998).
[2] V.V.Kveder et al., Physical Review B Vol. 51, No. 16 (1995).
[3] Rinio et al., Phys. Status Solidi A 208, No. 4, 760-768 (2011).

The efficiency of thin-film solar cell absorbers based on the earth-abundant Cu2ZnSn(S,Se)4 (CZTSSe) kesterite material system has recently been increased to 11.1% [1]. Detailed device characterization of CZTSSe [2] and (Se-free) Cu2ZnSnS4 (CZTS) [3] solar cells reveals that the activation energy (Ea) of the dominant charge carrier recombination process is smaller than the absorber band gap (Eg). This is commonly interpreted as an indication for a high-rate recombination across the buffer/absorber interface [4,5], which would limit the device performance. The activation energy of a (S-free) Cu2ZnSnSe4 (CZTSe) device, however, was found to be in accordance with the CZTSe band gap derived from quantum efficiency measurements [6], indicative for a low-rate recombination in the absorber bulk.
In order to shed light on the charge carrier recombination mechanism in kesterite-based thin-film solar cells, we focus on the electronic buffer/absorber interface structure. To independently measure the position of the valence band maximum (VBM) and the conduction band minimum (CBM), we employ direct and inverse photoemission. With this information, we are able to derive the valence (VBO) and conduction (CBO) band offset in a purely experimental approach. As a result, we could identify a significant “cliff”-like CBO at the CdS/CZTS interface [7] in agreement with device characteristics [3]. This “cliff” in the conduction band reduces the energy barrier for a charge carrier recombination across the defect-rich buffer/absorber interface [5], leading to a Ea < Eg situation. In terms of solar cell performance, an aligned conduction band is thus preferred. For the solar cell device this would translate into an Ea asymp; Eg condition.
In this contribution, we will report the results of a CdS/CZTSe study (i.e., of the Se-based absorber system). Preliminary data analysis (neglecting interface-induced band bending) shows that the positions of the CBM for CdS and CZTSe (with respect to the Fermi level) are similar, suggesting an aligned conduction band in accordance with the recombination mechanism derived in Ref. [6]. In our presentation, we will paint a comprehensive model for the electronic structure (specifically the band alignment) of the CdS/kesterite interface and its impact on solar cell performance.
[1] T.K. Todorov et al., Adv. Energy Mater. 3, 34 (2013).
[2] O. Gunawan et al., Appl. Phys. Lett. 97, 233506 (2010).
[3] K. Wang et al., Appl. Phys. Lett. 97, 143508 (2010).
[4] M. Turcu et al., Appl. Phys. Lett. 80, 2598 (2002).
[5] R. Scheer, J. Appl. Phys. 105, 104505 (2009).
[6] A. Redinger et al., Thin Solid Films 535, 291 (2013).
[7] M. Bär et al., Appl. Phys. Lett. 99, 222105 (2011).

The advent of scanning probe microscopy has provided the unique ability to investigate matter with ultimate precision. Single atoms and molecules can today not only be imaged with unprecedented resolution but also probed by local spectroscopy, manipulated to assemble functional nanostructures and excited to induce chemical change. In the present talk I will present our recent efforts to push the limit of scanning probe microscopy and spectroscopy by exploiting ultralow temperatures and high magnetic fields as well as by developing novel vacuum interfaces for the controlled handling of large molecules with negligible vapor pressure. The experiments provide unprecedented microscopic details of functional molecular interfaces. Many new perspectives ranging from molecular electronics through molecular quantum electrodynamics to energy conversion are opened up by these developments.

Research into renewable energy has been a heavily investigated topic in recent years due to the recognition of our dependence on an ever diminishing supply of fossil fuels, as well as the negative effects fossil fuels have on the atmosphere. Solar Cells, particularly organic photovoltaic (OPVs) cells, are promising areas of research for efficiently harnessing some of the 1000 W/m2 of solar radiation incident on the earth&’s surface. Surface characterization of these OPVs can be challenging due to the domain sizes formed in an efficient bulk heterojunction OPV. Transmission electron microscopy (TEM) and atomic force microscopy (AFM) are widely used in the characterization of the surface morphology of OPVs; however, little information other than the size of the domains is collected. Nanothermal analysis (nanoTA) is an AFM based tool that gives a researcher the ability to determine the glass or melt transitions on the surface features of an OPV with nanoscale spatial resolution. When combined with Lorentz contact resonance (LCR), a nanomechanical spectroscopy technique with sub 50 nm spatial resolution, AFM no longer provides just morphological information but thermal and mechanical as well. Polymer blends of poly(3-hexylthiophene), (P3HT), and (6,6)-phenyl-C61-butyric acid methyl ester (PCBM) are a popular donor-acceptor material for bulk heterojunction OPVs. The size of the domains formed during the annealing process is crucial to the performance of the completed device. Using a combination of nanoTA and LCR it is possible to simultaneously investigate the effects of different annealing parameters on the thermal and mechanical performance of P3HT:PCBM blends. In addition, the topographical information provided by AFM is readily correlated with the LCR and nanoTA results. Finally, chemical information of the domains can be collected using AFM-IR, a technique that allows for 100 nm spatial resolution infrared spectroscopy.

We have developed a dual-mode Kelvin probe featuring two novel detection modes comprising Air Photoemission Spectroscopy (APS), which yields information on the absolute work function (Phi;) of a surface/thin film, and Surface Photovoltage Spectroscopy (SPS) which produces information relevant to spectroscopic characterization of solar cells. These measurement modes allow a full characterization of the electronic energy band diagram including valance band energy, fermi-energy and surface potential (band-bending) under standard conditions.
The traditional Scanning Kelvin probe (SKP) measures small changes (1-3 meV) in a non-contact fashion, using a vibrating tip. This system is extremely versatile, capable of automatic monitoring of changes in Phi; or sample fermi-level under ambient, controlled atmosphere and UHV environments. Using a combination of Visible/IR and deep UV illumination (1.2 - 7.0eV) this detection system has been used to characterize the work function of metallic and ionization potential of semiconducting thin films and TCO&’s utilised in device fabrication such as Au, Ag,, Al, Si, ITO, ZnO, TiO, Pedot, GaP. Other examples include the near fermi-level Density Of States (DOS) in Cobalt-Phylocyanine (CoPc).
The resulting tool is extremely useful for surface characterization of organic semiconductors and solar cells, allowing clarification of the energy band diagram and adsorption phenomena.

Studying nucleation and growth has been one of the major goals in materials science. Fundamental understanding of initial growth is essential for fabrication of nanomaterials with desired physical properties. Consequently, atomic level investigation on as-grown nuclei and local atomic arrangements around defects is required. Such high-resolution study along with crystallographic analysis could be performed using transmission electron microscopy (TEM). So far, the method growing nanomaterials directly on prefabricated thin membranes, such as Si3N4[1] and SiO2[2], has been developed for imaging as-grown nanomaterials by TEM to avoid damages during typical sampling process. Nevertheless, these membranes are not suitable for the atomic-scale investigation of initial growth mechanisms since a few tens of nanometers-thick-membranes contribute to significant electron beam scattering which might prevent clear imaging of nano-sized objects.
Here, we report on atomic-resolution observation of initial growth behavior using TEM by growing nanomaterials directly on graphene. Graphene exhibiting excellent electron beam transparency and high mechanical strength is an ideal supporting layer for TEM measurements by minimizing the background signal from the underlying membrane. In addition to merely sustaining nanomaterials as a support, graphene can be further used as a substrate for nanomaterials growth. The crystalline nature of graphene along with its electron beam transparency ultimately enables direct imaging of nanomaterials and allows us to systematically investigate the initial growth mechanisms. In this study, we chose ZnO as the growth material since graphene-based hybrid structures are promising candidates for fabricating next-generation optoelectronic and electronic devices[3].
Using “direct growth and imaging” method, we could clearly observe the initial states of nuclei and their evolution to widely coalesced grains as well as atomic configurations of various defects. In addition, orientational relationship of nuclei with graphene and the formation of epitaxial relationship during nanomaterials growth could be verified.
[1] A. W. Grant et al. Nanotechnology, 15, 1175 (2004)
[2] F. Enquist et al. Thin Solid Films, 145, 99 (1986)
[3] K. Chung et al. Science, 330, 655 (2010)

Surface-enhanced Raman Spectroscopy (SERS) is a powerful, non-destructive and ultra-sensitive characterization tool down to single molecular level. For chemical identification in the gas phase, successful and effective SERS is achieved by the use of roughened surface of deposited metal substrates or nanostructures (such as dendrites or nanoparticles). The benefit of applying the same approach to investigate atomic-scale interfaces in ultra-thin materials for photovoltaics is enormous. This ability is currently hindered by the complexity in creating an effective, easy-to-synthesize and rapid-to-deploy SERS substrate, while simultaneously maintaining the ability to probe non-invasively, non-destructively and reversibly the surface of interest. Here, we propose a simple, novel SERS platform that uses the natural roughness of commercially available Au microparticles (diameter between 0.5 and 1.5 um) to obtain the spectral enhancement. The reversibility of the application of the particles is achieved through deposition at atmospheric conditions through dispersion in inert gas and successful removed after SERS acquisition. We show that this novel microparticle-induced SERS platform (mp-SERS) can be used to identify and monitor the evolution of complex interfaces. A first example of such interfaces is the intercalated hydrogen layer in epitaxial graphene on SiC surfaces, which is cannot be detected by conventional Raman spectroscopy. Through mp-SERS, we monitor each step in the hydrogenation process, and its removal while at the same time probing the evolution of the graphene layer. We also show that mp-SERS can be used to probe the quality and extent of the CuO layer that forms in between CVD graphene and the copper substrate. The facile and rapid deployment of mp-SERS shows the potential for the characterization of complex and heterogeneous atomic-scale interfaces for emerging renewable energy materials such as purely two-dimensional photovoltaics, for fundamental investigations as well as for quality control.

A noncontact chemical and electrical measurement technique of XPS is performed to investigate a number of optical and electronic devices under operation. The main objective of the technique is to trace chemical and location specified surface potential variations as shifts of the XPS peak positions under operating conditions. Devices consisting of single and multi p-n junctions made out of Si. GaN and Graphene have been investigated under light illumination and/or under forward as well as reverse bias. The main advantage of the technique is its ability to assess element-specific surface electrical potentials of devices under operation based on the energy deviation of core level peaks in surface domains/structures. Detection of the variations in electrical potentials and especially their responses to the energy of the illuminating source under operation is also shown to be capable of detecting, locating, and identifying the chemical nature of structural and other types of defects.

Improvements in lithium-ion battery energy density, safety, and lifetime are crucial to the development of a reliable battery for transportation applications. One approach is to develop cathode materials with a higher operating potential and good environmental compatibility. Spinel-type LiNi0.5Mn1.5O4 with an average redox potential of 4.7 V vs. Li/Li+ and a capacity of 147 mAh/g is one of the most promising materials being considered, especially when compared to conventional cathodes, e.g. LiCoO2 and LiFePO4.
This high-energy density Ni-spinel is at the edge of the thermodynamic stability window of standard carbonate-based electrolytes. This leads to electrolyte oxidation reactions at the electrode/electrolyte interface, which govern the long-term performance and lifetime of the battery. A better fundamental understanding of these processes is essential to the development of new battery materials. It also requires the use of newly developed in-situ characterization tools and experimental methodologies.
In this work, the interfacial reactivity of LiNi0.5Mn1.5O4 with a carbonate-based electrolyte was probed using fluorescence and x-ray absorption spectroscopy. In situ fluorescence and x-ray absorption measurements were carried out during charge and discharge of a spectro-electrochemical cell with carbon- and binder-free LiNi0.5Mn1.5O4 electrodes. For the cycled cathode, a large fluorescence background was observed due to the presence of decomposition products. The fluorescence signal can be used to provide insight to the degradation processes, their dynamics, and the soluble reaction products that diffuse into the electrolyte. The X-ray absorption unveils structural and oxidation state changes. 1 Together, this data provide a more complete picture of the electrode and cell degradation mechanism.
The fluorescence intensity rise correlates with the beginning of the Ni2+ oxidation in LiNi0.5Mn1.5O4. This behavior is not observed for LiMn2O4 spinel based electrodes. This indicates that the oxidation change of nickel is primarily responsible for the observed oxidation of the electrolyte.2 X-ray absorption analyses corroborate these results by showing and quantifying the dissolution and migration of Ni2+ and Mn4+ species from the cathode to the anode.
The mechanisms of these side reactions and their impact on the Li-ion cell&’s electrochemical performance will be discussed.
Acknowledgement
This work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231 under the Batteries for Advanced Transportation Technologies (BATT) Program.
References
1. J. Cabana, M. Casas-Cabanas, F. O. Omenya, N. A. Chernova, D. Zeng, M. S. Whittingham and C. P. Grey, Chemistry of Materials, 24, 2952 (2012).
2. N. S. Norberg, S. F. Lux and R. Kostecki, Electrochemistry Communications. in press.

The atom probe tomography has widely been used owing to its high depth and spatial resolution to investigate 3-dimensional structure and chemical information of material. After introducing pulsed laser, the application area of APT is expanded including metals, semiconductors or oxide. A number of recent results have established that oxide films can be analyzed by laser assisted APT such as MgO, Al2O3, WO3 and thick SiO2. In semiconductor industry, therefore, APT has emerged as the only (the) technique of analyzing nano-scale devices of present-day.
The analysis of silicon dioxide of varying form and thickness in nano-device is an object of concern because it plays crucial role in device reliability and performance. There were several reports of successful ATP analysis of insulating oxides but not much works for silicon dioxide. Even those works for silicon dioxide are limited to just one layer film. In real devices silicon dioxide layer doesn&’t exist separately, it&’s mostly surrounded by semiconductor material. However, appropriate analysis, date reconstruction and interpretation of APT for silicon dioxide with semiconductor multi layers are still difficult due to different evaporation mechanism, lack in the understanding, of semiconductors and oxide. It is, therefore, essential that APT measurements are optimized to maximize the effectiveness of APT analysis of silicon dioxide with semiconductor material system.
In this study, Si(10nm)/ SiO2(10nm)/ Si/ SiO2hellip; multi-layer, which is simple but difficult to analysis semiconductor/ insulator heterojunction structure, was successfully analyzed by using laser assisted APT. Prior to analyze Si/SiO2 multi-layer, the experiments of thick silicon dioxide with poly-silicon film were performed to identify field evaporation dependence on laser conditions for silicon dioxide. And the appropriate condition for optimum performance of Si/SiO2 (semiconductor/ insulator) heterojunction multi-layer analysis is discussed such as laser-pulse energy, evaporation rate, voltage control and optimal evaporation conditions. To maximize the specimen survivability and minimize the reconstruction error of APT analysis of Si/SiO2 multi-layer field evaporation valance must be maintained during the experiment and careful examination is required.
The outcome of this research will greatly facilitate the application of APT technique to electronic nano-device analysis

Electrochemical and physical phenomena at the surfaces of transition metal oxides play the key role in multiple energy technologies ranging from fuel cells to separation membranes to sensors. Both structural and electronic aspects of these behaviors are of interest, and are uniquely accessible through high-resolution probe-based studies. In this presentation, I will present several examples of atomic scale studies of oxide surfaces enabled by combination of the in-situ Pulsed Laser Deposition growth with atomic resolution Scanning Tunneling Microscopy and Spectroscopy. Using this approach, we have visualized atomic structures and vacancy configuration in several functional oxides including manganites and ruthenates. The spatial inhomogeneities observed on the atomic- and near atomic level can be further correlated with local electronic properties accessed by continuous current imaging spectroscopy. Here, multivariate statistical analysis of CITS data using supervised- and non-supervised learning methods allows separation of individual responses and visualizing them in real space. We have further extended the applicability of local crystallographic mapping to scanning tunneling microscopy data, allowing for mapping surface electrochemistry and order parameter fields on the atomic level, as well exploring bias-induced transitions and atomic motion on the atomic level. We further explore these studies to probe in-situ the sub-10 nm scale ionic dynamics and electrochemical reactions using SPM techniques based on strain (electrochemical strain microscopy) and current-based first-order reversal curve methods. In these, the slow dynamic in current and electromechanical responses is linked to local ionic dynamics, allowing visualization of the latter with sub-10 nm resolution. The role of contact pressure in the tip-surface junction on these phenomena is explored.
This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences (BES), Materials Sciences and Engineering Division (SVK, PM, AT). ZG, AB, APB, and RKV are supported by the Center for Nanophase Materials Sciences which is sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy

Controlling interfaces is critical for optimizing performance of photovoltaic (PV) devices. In typical p/n heterojunction thin-film PV devices, the back-contact interface provides an ohmic contact to the absorber, and potentially can serve as an electron reflector to minimize recombination effects at the back surface. Back-contact material parameters that must be controlled include work function for metals, or carrier concentration, electron affinity, and bandgap in the case of p-type semiconducting contacts. The interface between the p-type absorber and the n-type buffer layer is responsible for carrier collection in the device, and must provide favorable conduction band alignment, with a low concentration of defects that might cause excessive interface recombination or inhibit quasi-Fermi level separation. Additionally, the window layer material must have as large a band gap as possible to maximize light transmission to the absorber. Critical material parameters for the window layer include band gap, electron affinity, carrier concentration, and crystal structure and lattice constant. Other important considerations for back-contact and window layer materials include thermal and chemical stability during processing and after the device is completed, as well as issues related to cost and availability of raw materials. To address these complex and interrelated issues, researchers at the National Renewable Energy Laboratory (NREL) have integrated a set of capabilities for studying chemical, structural and electronic properties of critical surfaces and interfaces in PV devices. These capabilities include x-ray and ultraviolet photoelectron spectroscopy (XPS/UPS) inverse photoelectron spectroscopy (IPES), scanning Auger electron spectroscopy (AES), low energy electron diffraction (LEED), scanning tunneling spectroscopy (STM), low-energy ion scattering spectroscopy (ISS), and temperature programmed desorption (TPD). These analytical capabilities are coupled to a surface-analysis cluster tool, which includes a physical vapor deposition tool for chalcogenide materials, as well as a nitrogen-purged glovebox for sample transfers and wet-chemical processing under a controlled ambient. Results of recent studies aimed at optimizing performance of PV devices based on the earth-abundant thin-film absorber Cu2ZnSn(S,Se)4 will be presented. Examples of interface studies related to other thin-film PV technologies such as CdTe will also be discussed.

Cu2ZnSnSSe4 (CZTSSe) thin-film solar cell is a promising material as a substitution for Cu(In,Ga)Se2 (CIGSe) because it has non-toxic, inexpensive and earth abundant elements like Zn and Sn rather than In and Ga. In order to increase the conversion efficiency of CZTSSe, a variety of growth methods are being challenged and efficiency larger than 8% is achieved. In our study, we fabricated CZTSSe thin-films by sputtering and subsequent selenization process in a furnace. Most of single crystal Si thin-film solar cell indicated that grain boundaries (GBs) are defect acting as a recombination region. However, polycrystalline thin-film solar cell such as CIGS and CdTe reported that GBs are not recombination region but minority carrier collection and provide current route to upper layer. Thus, we investigated local electrical properties such as GBs by using Kelvin probe force microscopy and conductive atomic force microscopy. These tools have developed into a powerful technique for investigating polycrystalline compound thin-film solar cells. From these results, we obtain surface potential difference of GBs and grain intra in CZTSSe thin-films. Morphological variations simultaneously observed with surface potential indicate that negative bending of potential mostly is found in the GBs of the high-efficient CZTSSe sample.

The III-V tandem GaAs:GaInP2 PV:PEC (photovoltaic: photoelectrochemical) cell has achieved 12.4% solar-to-hydrogen water splitting efficiency but is unstable in electrolyte solutions. This instability can be caused by dissolution of the GaInP2 epilayer or surface oxidation. The surface oxide adds a high series resistance and introduces surface states that increase carrier recombination, which both degrade the performance of the device. The goal of this research is to use photoluminescence (PL), XPS, and PEC characterization to investigate whether atomic layer deposition (ALD) Al2O3 and TiO2 coatings on p-GaInP2 1) enhance PEC performance and/or 2) stabilize the GaInP2 surface during operation in a PEC cell. Before ALD, GaInP2 is pretreated with a concentrated sulfuric acid etch that removes the native surface oxide and leaves an indium-rich surface oxide with a lower surface state density. The GaInP2 epilayer is then coated with a stable and conductive material using ALD. TiO2 is known to be stable in a wide range of electrolyte solutions and adds very little series resistance when used as a coating on a p-GaInP2 photocathode. Furthermore, ALD Al2O3 has been shown to remove surface states on other III-V materials in PV and metal-oxide-semiconductor devices. Preliminary measurements show a several fold PL enhancement after etching and ALD Al2O3 deposition. Furthermore, ALD TiO2 on p-GaInP2 shows an enhanced photocurrent onset (open-circuit) potential, which indicates that ALD TiO2 may also reduce surface state densities of GaInP2. With the optimization of a sub-nanometer ALD Al2O3 interfacial layer and a corrosion resistant ALD TiO2 over layer, we anticipate enhanced open-circuit potential and durability of p-GaInP2 as well as other III-V based solar water splitting photocathodes.

Interfaces within thermoelectric materials play key roles in controlling charge carrier densities and charge transport as well as thermal transport within thermoelectrics. We describe the growth, structure and physical effects of semi-metallic nanoparticles and their interfaces in thermoelectric semiconductor hosts. The semi-metallic nanoparticles are effective phonon scatterers and can contribute or deplete charge carrier concentrations from a semiconductor host, depending on the relative alignment of the semi-metal Fermi level and the semiconductor conduction band edges.
A broad range of epitaxial rare earth compounds in compound semiconductor hosts have now been grown, both by MBE co-deposition of semi-metals and semiconductors and by MBE island formation and subsequent semiconductor overgrowth. In many cases, TEM measurements show sharply defined embedded nanometer-scale structures, while ultra high vacuum scanning tunnel microscopy reveals the surface atomic motion that occurred during growth. Scanning tunnel spectroscopy shows the localized electronic structure. Dilute rare earth atoms act as electron donors in some cases, while higher concentrations of rare earths often produce sharply defined nanoparticles, nanorods and layers. The semi-metallic embedded particles and rods display strong surface plasmon infrared absorptions, which provide a sensitive probe of electron concentration and motion in the embedded nanostructures, even in the presence of appreciable size quantization.

Bulk heterojunction solar cells and thermoelectric composites both involve charge transport and thermal excitation/relaxation across interfaces between disordered matrix polymers on one hand, and more crystalline phases such as fullerenes for solar cells and heavy element semiconductors for thermoelectrics. It is challenging to probe these interfaces, as overlying bulk materials block access thereto. We employ scanning Kelvin probe microscopy (SKPM) to measure spontaneous and applied electric fields at these interfaces, formed by fabrication of side-by-side junctions where the interface extends to the sample surface. SKPM is performed simultaneously with lateral current flow. We find that these electric fields are substantial enough to strongly influence the performance of energy converting devices. Conversely, tuning these fields by application of polarizing voltages or by chemical design could positively impact performance. In addition to electron transport, there is growing evidence that thermal equilibration, or its inhibition, also contributes to energy transduction efficiency. The relationship of these thermal effects to local electric fields, and design principles for the relevant interfaces, will be discussed.

Thermoelectric devices, which can be used for the direct conversion of heat energy to electricity, have drawn intense interest as promising candidates for harvesting waste heat and solar thermal energy. Very recently, several groups reported high power factors and remarkable figure-of-merit values by using organic semiconductors. This shows that the performance of organic thermoelectrics could approach that of their inorganic counterparts. On the other hands, there are other concerns by using organic semiconductors for thermoelectric applications such as the environmental stability at high temperature, reliable measurement to determine the carrier density and carrier mobility, and anistropic thermoelectric properties due to preferred molecular orientations. In this presentation, we will report detailed studies on the thermoelectric properties of bench-mark conducting polymers PEDOT:PSS. The carrier transport properties were studied by using ion gel transistors combined with in situ UV-Vis-NIR spectroscopy.[1] The anistropic electrical and thermal conductivity, and the humidity dependent thermoelectric properties of PEDOT:PSS will be discussed. We will show the first demonstration that large area organic thermoelectric module can be actually used to power light emitting diodes.
[1] Q.S. Wei, M. Mukaida, Y. Naitoh and T. Ishida, Adv. Mater., 2013, 25, 2831.

Surface and interfacial phenomena are becoming increasingly important in the development of new materials for a wide range of renewable energy applications. In light of this, the identification of new nanoscale interfacial phenomena can provide new insight or techniques for manipulating the properties of energy based materials. We have recently demonstrated a unique interfacial electron transfer phenomenon that occurs in core-shell heterostructured nanoparticles composed of plasmonic metals such as gold, silver, platinum and copper. The phenomenon leads to enhancement of many of the material properties including resistance to oxidation or plasmonic enhancement. The understanding of the electron transfer mechanism leads to opportunities to create new materials for energy that can benefit from accentuated electronic properties such as in solar cell materials, solar catalysts or fuel cell catalyst materials. The presentation will focus on our recent results in the synthesis and characterization of core-shell nano-heterostructures with enhanced electronic interfacial properties and discuss the implications to creating new materials for energy. Techniques such as XRD, XPS, TEM, STEM-HAADF, EDS Elemental Mapping and others will be used to elucidate the particle structure and electronic properties.

A single molecule bridging two metal electrodes is the simplest device configuration that can be designed for molecular electronics.1 However, the detailed understanding of electronic transport through single molecule or an ensemble of self-assembled molecules embedded between two metallic leads is still a matter of controversy. Multiple factors influence the charge transport in the molecular junction, with particular attention to be given to the band states of the electrodes, molecular orbital energies, bias potential, molecular vibrational frequency and importantly molecule-electrodes electronic coupling.2 We here investigate electronic transport in an ensemble molecular junction embedding an alkylthiol derivative of a phenol substituted bithiophene by means of current vs voltage and temperature dependent measurements. Importantly and unlikely many other molecular junctions reported in literature, our system is defined by the design of a HOMO level being very close to the gold work function, therefore enabling a “good” electrode-molecule coupling even at low biases. Therefore, resonant conditions for electron transport can be found at room temperature and very low biases. From the temperature dependence of the electronic transport, we found a clear transition from incoherent resonant tunneling (strong coupling), where hopping regime shows activation energy of 0.1 eV, to a coherent inelastic tunneling (weak coupling). We also explored different configurations for the top-electrode of the junction: Au; PEDOT:PSS/Au and Ti-Pt conductive-Atomic Force Microscopy (c-AFM) tips. In all cases, we found a transition voltage, VT of 0.3 Volt. On the one side, the consistent presence of a similar VT in all tested configurations is indicative of a clear molecular signature in charge transport. On the other, we can assign VT to a transition between a hopping regime and a tunneling regime, evidencing the role of the bithiophene core and of the length of the alkyl chain in sizing the coupling between the molecule and the electrode and in enabling resonant conditions for hopping transport at room temperature.3
These findings represent a step forward to the rationalization of fundamental electronic processes in molecular junctions for the development of molecular based electronics.
References
[1] N.J. Tao, Nature nanotechnology 1, 173 (2006)
[2] A. Troisi & all, PNAS, 104, 14255 (2007)
[3] I. Baldea & all, Physica Status Solidi B, 1-14 (2012)
Corresponding author: [email protected]

Formation of oxygen vacancies by introducing various mixed-valent cation dopants is a common procedure to improve the cathode performance in solid oxide fuel cells. A generic computational procedure is developed in this work to predict the oxygen vacancy equilibrium concentrations at experimentally relevant temperatures and oxygen partial pressures for both bulk and surface oxide phases. The calculations are based on the first-principles density functional theory and a constrained free-energy functional. Quantitative agreements are found by direct comparisons to the thermogravimetry measurements for various strontium-doped lanthanum cobalt iron oxides. Our results indicate that the oxygen vacancies are energetically stabilized at surfaces for all temperatures and all oxygen partial pressures, while such surface stabilization effects become stronger at higher temperatures and lower oxygen partial pressures.

Nanoparticles of poly(3hexyl thiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) were synthesized employing simple mini-emulsion technique. The formation of the nanoparticles was confirmed by dynamic light scattering, atomic force microscopy (AFM) and scanning electron microscopy. The nanoparticles were assembled into thin films by alternatively spin-coating from the nanoparticle dispersions. The built-up of the film was monitored by absorption spectroscopy. The morphology of an annealed five bi-layered film of the nanoparticles was investigated by AFM. The distinct nanoscale co-continuous P3HT and PCBM phase was observed. Photovoltaic response of a bi-layered film was measured. Synthesis and characterization of the nanoparticles will be reported.

We report that the functionalities of dielectric surface can affect to the operational reliability of organic field-effect tranistors (OFETs). We introduced the polymer with methoxy- and chloro- functional groups onto the dielectric layer. Both dielectric surfaces modified with two polymers shows similar hydrophobicity and the surface energies. Also, pentacene semiconductor on both modified dielectric surfaces has same crystalline structure. To investigate the effect of the functionalities of dielectric surface in device, we studied ultravilolet photoelectron spectroscopy (UPS) and the operational reliability under sustained bias stress on both devices. The UPS results provide the energy band structures of dielectric surface with two functionalities and energetic mismatch between the HOMO levels of the semiconductor and dielectric surface. The bias stress stability exhibits that reliability is quite different as the functionalities of dielectric surface. We found that the energetic mismatch between semiconductor and dielectric surface related to charges transporting barrier for trapping into dielectric layer. Thus, the smaller energetic mismatch attributed to lower trapping barrier, resulting in more stable operation of OFETs.

Spray techniques are well known for being powerful and flexible tools for depositing a large variety of materials, allowing a fine modulation of material aspect according to the working parameters.1
Herein, we propose the application of spray deposition (SD) in two relevant steps of quantum dot solar cells (QDSCs) fabrication: i) the in situ generation and growth of QDSCs on a TiO2 scaffold 2 and ii) a highly reproducible fabrication of Cu2S counter electrodes, which is currently one bottle neck for QDSCs. 3
In both cases, SD reveales incredibly reliable especially from a reproducibility point of view, allowing in one case to deposit on TiO2 higher amount of QDs together with smaller nanocrystals as compared with the classical SILAR (successive ionic layer absorption and reaction) by immersion, and in the other case quickly delivering (few seconds) hierarchical structured Cu2S structures featuring high catalytic performances as cathodes in QDSCs.
In particular, we demonstrate that the application of SD to SILAR, requiring a reduced amount of chemicals for QD generation and growth, provides for a highly homogeneous coverage of the TiO2 photoanodes in the whole depth of the substrate. Evaluation of device performances shows improved functional properties, especially related to photoconversion efficiency and photocurrent density, both of them being almost two-fold the corresponding prepared by immersion SILAR.
Electrochemical analyses of the SD-generated Cu2S counter electrodes demonstrate highly improved functional performances as respect to Au cathodes, which resulted in increased photocurrent densities (above 15 mA/cm2) and fill factors (systematically higher than 40%). Photoconversion efficiencies exceeded 3% with associated impressive incident photo-to-current efficiencies (between 80% and 90%).
1. S. Che, S. Sakurai, K. Shinozaki N. Mizutani, J. Aer. Sci. , 1998, 29, 271
2. I. Concina, N. Memarian, G.S. Selopal, M.M. Natile, G. Sberveglieri, A. Vomiero, J. Power Sources, 2013, 240, 736
3. J. G. Radich, R. Dwyer, P. V. Kamat, J. Phys. Chem. Lett., 2011, 2, 2453

Outstand understanding for intrinsic stresses and defects evolution in photovoltaic (PV) devices became an essential part of new developments of these devices. In particular, Multi-Junction (MJ) solar cells depend on multi-layers thin film that may suffer from a high density of dislocations and other defects as a result of high lattice and thermal mismatch between different layers. In general, these defects act as scattering centers that impact the minority carrier lifetime, reduce thermal conductivity, and form easy pathways for impurity diffusion. Thus, they limit the performance, reliability, and lifetime of PV devices.
A three-dimensional multiple-slip dislocation model, specialized finite-element formulations and predictive failure models, were used to investigate new sub cell structure that collects photons in the energy range of 1.5 - 1 eV in MJ solar cells using numerical techniques; in particular, to address GaAs growth on Si substrates. The formulation is based on accounting for thermal and intrinsic stresses as a result for different processing conditions and microstructures. The computational framework and the constitutive formulation were validated with experimental results for MJ-PV devices. Furthermore, the formulation was used to investigate a recently developed technique, embedded void approach (EVA), which can be used to address both the high density of defects and the cracking/bowing of GaAs grown on Si substrates. The current work will lay the groundwork for more extensive use of silicon in MJ-PV devices.

A model for the self-assembly of Nafion onto Pt was elucidated through the correlation of peaks experimentally obtained using polarization modulated IR reflection absorption spectroscopy (PM-IRRAS) to a theoretical vibrational spectrum derived using density functional theory (DFT). The visualization of calculated normal mode animations showed that SO₃^-, CF₃ and backbone functional groups contribute to the group modes observed in the PM-IRRAS spectrum. Subsets of DFT calculated normal mode coordinates and relevant atomic partial charges were used to represent functional group contributions (e.g., stretching, bending, wagging, etc) to each normal mode. This provides a quantitative basis for assigning Nafion group modes. These quantitative assignments show that in previous studies the contribution of SO₃^- and CF₃ groups to theoretical peaks associated with PM-IRRAS bands are understated,¹ placing too much emphasis on role of the backbone in the self-assembly of Nafion on Pt.
1. Kendrick, I., et al., Elucidating the Ionomer-Electrified Metal Interface. J. Am. Chem. Soc., 2010. 132(49): p. 17611-17616.

Quantum dots (QDs) allow for manipulation of the position and energy levels of electrons at sub-10 nm length scales through control of material chemistry, size, and shape. They may influence the rate of charge transfer reactions and therefore assist in photovoltaic or photocatalytiic energy conversion processes. To realize these opportunities, we need to improve our understanding of the electronic structure within individual QDs.
It is known from optical studies that the bandgap of semiconductor QDs increases as their size decreases due to the narrowing of the quantum confinement potential. The mechanism of quantum confinement also indicates that the localized properties within individual QDs should depend on their shape in addition to their size, but direct observations of this effect have proven challenging due to the limited spatial resolution of measurement techniques at this scale and the ability to remove contributions from the surroundings. Here we present experimental evidence of spatial variations in the lowest available electron transition energy within a series of single electrically isolated QDs due to a dome-shaped geometry, measured using electron energy-loss spectroscopy in a (scanning) transmission electron microscope [(S)TEM-EELS]. We observe a consistent increase in the energy onset of electronic excitations from the lateral center of the dot toward the edges, which we attribute purely to shape. This trend is in qualitative agreement with a simple quantum simulation of the local density of states in a dome-shaped QD.
The measurements presented in this work provide direct evidence for the capability to tune the local electronic properties within individual nanostructures through shape control. This phenomenon has possible implications for devices that depend on electron dynamics near the surfaces of nanostructures, for example, through its effects on the local availability of optical transitions, the likelihood of tunneling events for photogenerated charge carrier extraction, and the availability of surface charge for chemical reactions.
[1] H. J. Jung*, N. P. Dasgupta* et al., Nano Lett. 13 (2), 176 (2013)

The surface of charged wet TiO2 anatase (001) functionalized by ruthenium ion at ambient temperatures is studied by computational modeling. Response of this model to photoexcitations at ambient temperatures is explored with Redfield density matrix equation of motion in the basis of Kohn-Sham orbitals. The parameters of the Redfield equation are on-the-fly non-adiabatic couplings for electronics degrees of freedom obtained along the ab initio molecular dynamics nuclear trajectories. The main results in this study are following: 1. Optical properties of the doped models such as light absorption intensity and transition energies can be tuned by modifying total charge; 2. Electron & hole relaxation rates depend on initial excitation; 3. In the doped model, excitations of lower energy provide quicker relaxation. Results of computational modeling and would benefit the understanding of mechanism of electron transfer process on the surface of ruthenium doped TiO2.

Harvesting solar energy for the production of clean fuel by a photoelectrochemical system represents a very attractive, yet a challenging task. This talk will review the recent efforts done to tailor metal oxide-based photocathode materials for the solar-driven hydrogen production. The materials will be classified into three categories: simple oxides, complex oxides and photocathodes used in p-n self-biased heterojunction cells. Generally, three strategies have been recommended to tailor p-type metal oxide semiconductors to meet the requirements for efficient solar-driven water splitting; namely (1) coating the p-type metal oxide either with a protective layer or a dye, (2) using co-catalyst and (3) merging the p-type material with an n-type photoanode with the proper optical and electrical properties. In the light of those strategies, the optical, structural and photoelectrochemical characteristics of such assemblies will be discussed.
Keywords: Metal oxides, hydrogen, photocathode, solar, diode, self-biased, water splitting
Reference:
Allam et al., Recent advances in the use of metal oxide-based photocathodes for
solar-driven hydrogen production, Int. J. Hydrogen Energy, 2013

The design of efficient and cost-effective catalysts for the oxygen evolution reaction (OER) is crucial for the development of electrochemical conversion technologies. Recent experiments show that perovskite transition-metal oxides can exhibit high electro-catalytic activity for OER. However, little is known about the atomic and electronic structures, thermodynamic stability, and resulting activity of perovskite surface reconstructions, which are caused by realistic environment during fabrication, measurement, and eventual device operation. In this work, we apply first-principles density function theory and ab initio thermodynamics to investigate the environment-dependent surface/interface structures of perovskite oxide heterostructure catalysts, particularly those based on LaMnO3. We develop a stability phase diagram and catalytic activity volcano for LaMnO3 heterostructures under realistic environment of alkaline fuel cells. Our work could lead to accurate identification of determinants on the activity of perovksites reconstructions, and rapid computations of the determinants as a function of environment such as temperature and pH.

Room temperature, solution processed chemical doping of organic semiconducting films is becoming increasingly important for manipulating key device parameters like lowering charge injection barriers and increasing the conductivity of materials. However, due to the low ionization energy required to dope materials with low electron affinity, reactivity of electron donating molecules (n-dopants) with oxygen or water in ambient conditions has so far limited the application of n-doped materials in devices. We investigated n-doping of the regioregular conjugated polymer P(NDI₂OD-T₂), an electron-transporting material which exhibits air stability and high mobility in organic thin-film transistors [1]. We present the solution-processed n-doping of P(NDI₂OD-T₂) with a strongly reducing air-stable organometallic dimer, RhCpCp*, with doping concentrations ranging from 0.03 to 0.001 molar ratio (MR) [2]. Effective n-doping is demonstrated by an eight orders of magnitude increase in room temperature conductivity and a concomitant decrease in the electron transport activation energy of thin films with increasing doping concentration via variable temperature current-voltage measurements. Fermi level shifts measured by direct and inverse photoemission spectroscopy confirm decreasing work function with increased doping concentration. The pronounced trend of conductivity and activation energy with doping concentration provides evidence for the passivation of deep electronic trap states in P(NDI₂OD-T₂) by the addition of n-dopants in direct analogy to established analysis of UHV evaporated n-doped C₆₀ [3].
[1] Yan, H.; Chen, Z.; Zheng, Y.; Newman, C. E.; Quin, J.; Dolz, F; Kastler, M. and Facchetti, A., Nature 2009, 457, 679.
[2] Guo, S.; Kim, S. B.; Mohapatra, S. K.; Qi, Y.; Sajoto, T.; Kahn, A.; Marder, S. R. and Barlow, S., Adv. Mater. 2012, 24, 699-703.
[3] Olthof, S; Mehraeen, S.; Mohapatra, S.K.; Barlow, S.; Coropceanu, V.; Brédas, J-L.; Marder, S. R. and Kahn, A., Phys. Rev. Lett. 2012, 109, 176601.

Using Soxhlet extraction we have fractionated a broad molecular weight distribution of the low bandgap polycarbazole polymer Poly[N-9'-heptadecanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole) (PCDTBT) into three molecular weight fractions; low Mw (chloroform fraction), medium Mw (chlorobenzene fraction) and high Mw (dichlorobenzene fraction). Using neutron reflectivity we studied the vertical profile for different molecular weight fractions when blended with PC70BM and fabricated devices using the same processing parameters. We found that the average device power conversion efficiency varied from 3.7% to 5.9 % depending on the molecular weight of the fraction that was used, with the middle molecular weight fraction producing the highest efficiency devices (average PCE 5.9% and peak efficiency of 6.15%). We attribute this to the middle fraction having a thicker PCDTBT enrichment layer near the PEDOT:PSS interface.

Most of the electrochemistry processes occur within the thin layer of water or solution at the electrolyte/electrode interfaces, commonly denoted as the electrical double layer (EDL). In spite of some classic EDL theories, very limited experimental information is available about these solvent or solute species within such EDLs. We have used x-ray absorption spectroscopy (XAS) to study the water molecules within the interfacial region between bulk water and gold electrode. By comparing the fluorescence yield (FY) signal generated from bulk water and total electron yield (TEY) signal generated from the water within the interfacial region, we found that within the interfacial region, the structure of the water molecules resembles the structure of ice, evidenced by very weak pre-peak feature in oxygen K-edge spectra. We have also developed a mechanical chopper system to modulate the incident x-ray, installed on the Beam Line 8.0.1 of the Advanced Light Source in LBNL. With the modulated x-ray source and proper electronics, we can separate the tiny TEY signal from dominant faradaic current and characterize water molecules at such liquid/electrode interfaces under electrochemical conditions. [1] While positive bias is applied between the Au electrode and the counter electrode, the polar water molecules within the EDL can respond to the external electrical field and reorient their oxygen towards the gold surface, resulting in fewer broken/distorted hydrogen bonds; thus pre-peak feature in the oxygen K-edge spectrum is further suppressed. Under negative bias, the oxygen atoms were forced to orient away from gold, leading to more broken hydrogen bonds and thus stronger pre-peak features. This in-situ XAS technique can be potentially applied to other electrochemical systems, to investigate the orientation, intercalation and other important electrochemical phenomena related to solvent or solute species within EDLs.
[1]. J. J. Velasco-Velez, C. H. Wu, M. B. Salmeron, in preparation.

Several materials such as doped Zirconia (e.g. 8 mol. % YSZ) and doped Ceria (e.g. 10 mol. % GDC) have been tried as the electrolyte for optimization of Solid oxide fuel cells (SOFCs) for effective energy conversion, but these materials need high temperatures to operate with maximum efficacy because their ionic conductivity decreases by reducing temperature. In the present research, pellets of 8 mol. % YSZs with different CeO2 content (5 wt. % and 10 wt. %) were fabricated by both conventional sintering (CS) and spark plasma sintering (SPS). It was found that high density samples can be fabricated by SPS technique with a significantly short sintering time (1200oC, ~5 min) compared with CS (1450oC, 4h. Densification of ~98 % of the theoretical density was achieved for CeO2 doped 8YSZ SPS pellet while it is ~94 % for CeO2 doped 8YSZ CS pellets. The CeO2-YSZ nano-composites have been synthesized and tested at intermediate temperatures. The particle size for both the starting powders is in the sub-nano range. Phase and micro structural analysis indicated the formation of CeO2-YSZ solid-solution when fabricated using conventional sintering, while CeO2 forms nano-composite with 8YSZ when synthesized via SPS. The ionic conductivity measurement was performed using AC impedance spectroscopy in air from 300 to 700°C. We will discuss the conductivity behavior of the nano-composites of CeO2-8YSZ and compare the result with the solid-solution formation of similar compositions.

It has been shown for several small molecule organic semiconductor systems [1-2] that inserting a (n- or p-)doped transport layer at the contacts could significantly improve the efficiency of the device, in solar cells in particular. However, the concept of spatially confined doping has been barely tested on polymer-based devices, mainly due to difficulties in fabricating polymer multilayers and controlling dopant position in solution-deposited films. Our work here is intended to combine the concept of spatially-controlled molecular doping with the technique of soft lamination of polymer films [3]. We modify the hole-collection electrode of a polymer solar cell by inserting via lamination a p-doped polymer interlayer. P-doping of the interlayer is achieved by co-solution of the hole-transport polymer poly(3-hexylthiophene-2,5-diyl)(P3HT) and the soluble oxidizing molecule molybdenum tris-[1-trifluoroethanoyl-2-trifluoro-methylethane-1,2-dithiolene] (Mo(tfd-CO2Me)3). P-doping is investigated via ultra-violet photoemission spectroscopy, which shows the expected Fermi level shift toward to P3HT HOMO level, and current-voltage measurements, which show an enhancement of several orders of magnitude of the conductivity in the P3HT layer. Spatially-controlled interface doping of a device is then achieved by laminating an ultra-thin doped layer (20-30 nm) on top the active layer, following a technique described elsewhere [3]. We fabricated hole-only devices with the following structure: ITO/ Polyethylenimine, 80% ethoxylated (PEIE)/P3HT(150 nm)/doped P3HT (30 nm)/Ag, and recorded a large enhancement of hole injection at the P3HT/Ag interface. We further applied this method to inverted polymer-based solar cells comprising (i) a PEIE-modified ITO substrate, (ii) a bulk heterojunction (BHJ) active layer P3HT:6,6-phenyl-C61-butyric acid methyl ester (PCBM) or P3HT:Indene-C60 Bis-Adduct (ICBA),(iii) a laminated interlayer with doping concentration varying from 0 to 3.8 wt%, and (iv) a Ag contact. With the 3.8 wt% doped P3HT interlayer between the Ag contact and the active layer, the efficiency of the inverted solar cells dramatically improved compared to those without any modification interlayer, yielding the following characteristics: Voc=0.57 V; Jsc=7.95 mA/cm2; FF=0.64 and eta;=2.9% for P3HT:PCBM active layer; Voc=0.80 V; Jsc=7.7 mA/cm2; FF=0.63 and eta;=3.9% for P3HT:ICBA active layer. SIMS measurements performed to examine the diffusion of dopants into the BHJ show that the dopants are stable in their host material. We conclude that the combination of solution-based doping with soft lamination is a very promising technique to fabricate low barrier polymer contacts for a range of devices, in particular polymer-based photovoltaic devices.
1.Qi, Y., et al., Chem. Mat., 2009. 22(2): p. 524-531.
2.Tress, W. et al., Adv. Funct. Mat., 21(11): p. 2140-2149.
3.Shu, A.L., et al., Org. Electr., 14(1): p. 149-155.

Fuel cells attract significant attention as one of the future energy. One of the important materials, a proton conductive membrane, each proton-conductive membrane has a specific proton conductivity and activation energy. Normally they are understood as physical properties in the bulk case. This proton transport property can be modified by using the interface if we consider the thin film cases. The thin film has an interface, therefore to use this interface effectively gives a potential for improving the proton conducting property and other properties. These results can feedback to the development of the bulk membranes. A polymer orientation to the 1-dimensional, 2-dimensional, and 3-dimensional also gives the modified physical properties. However, their studies on the proton transport property are unexplored yet. In this paper, we studied on the relationship between the polymer orientation and characteristic of proton conduction using the interface.
We synthesized a sulfonated polyimide as a proton conductive material. This polymer has a rigid structure include aromatic ring so it is easy to make an oriented structure to use the π-π interactions between the polymer chains. The orientated sulfonated polyimide thin film was prepared by spin coating. We investigated structure of the polyimide thin films with p-polarized multiple-angle incidence resolution spectrometry (p-MAIRS) technique and DFT calculations. Both of the results show the polyimide has twisted structure in the film. The conductivity of the unoriented bulk sample decreased under high relative humidity (RH), however, the conductivity of the oriented thin film did not decrease under high RH and kept to increase with RH. The proton conductivity of the oriented thin film exhibited ca. 10-1 S cm-1 under 298 K and 95% RH.

The interaction of metal and support in oxide supported transition metal catalysts has been proved have extremely favorable effects on catalytic performance. Herein, meso- or macro-porous oxides were synthesized and loaded with size-controlled Pt nanoparticles. Through catalytic reaction studies of CO oxidation and hydrogenative reforming of n-hexane, it was demonstrated how different oxides, metals, and their interfaces give rise to different chemistry. In particular, the interface of Pt and mesoporous oxides (Co3O4, NiO, MnO2, Fe2O3, and CeO2) yielded a huge enhancement in CO oxidation rate, which was orders of magnitude greater than that of pure Pt or oxides. For hydrogenative reforming of n-hexane on supported Pt, the oxide supports (Al2O3, TiO2, Nb2O5, Ta2O5, and ZrO2) play a critical role to determine the activity and selectivity of the catalyst. In situ characterizations under catalytically relevant reaction conditions demonstrated a strong correlation between the oxidation state of the oxide support and the catalytic activity at the oxide-metal interface. Not only the remarkable enhancement of catalytic performance, but an in-depth mechanistic understanding about the redox chemistry occurring at the oxide-metal interface under catalytic conditions provides extraordinary advancements in the current knowledge in this field.

Highly fluorinated polymer brush gate dielectric is used for stable operation of Organic Field-Effect Transistors (OFETs). It was synthesized from simple synthesis mechanism called as para-fluorine click reaction. Thermally evaporated PTCDI-C13 based OFETs (Bottom gate, Top contact) are fabricated and continuous gate bias stress (-40V) is applied to evaluate the stability. The device showed much higher electrical stability in N2-purged glove box than the HMDS, crosslinked PVP gate dielectric based ones. It is because the charge trapping is reduced by the hydrophobic surface which can minimize the diffusion of polar contents including water vapor through the interface between the semiconductor and the gate dielectrics. This fluorine polymer brush gate dielectric based OFETs also enabled solution processable semiconductors TES-ADT with a moderately high mobility of 0.64cm2/Vs.

Artificial photosynthesis is a growing field of science with multiple applications including the generation of electricity and production of hydrogen. A heterogeneous approach to generating hydrogen is the photocatalytic water splitting at electrodes, where metal catalysts embedded in semiconductors serve as reactive surfaces for water adsorption and decomposition into oxygen and protons. We develop a reduced representation of the system via the flow of a fluid composed of water and ions over a solid surface of close packed particles with embedded metal catalysts. To investigate the transport phenomena that can arise from such electrochemical reactions, coarse-grained molecular dynamics simulations using the LAMMPS package are utilized to predict the nanoscale behavior of the system. The goal is to observe and characterize the adsorption dynamics of the fluid mixture to the metal catalysts through the measurement of properties such as diffusion coefficients, velocity profiles, radial distribution functions, and mean residence times. Our objective is to gain a better understanding of the transport phenomena to explain the kinetics and thermodynamics of the processes occurring during the flow of reactants over a catalytic surface using molecular dynamics simulations. These simulations can identify the role of molecular scale properties on the catalytic process, thereby allowing predictions of the water splitting performance. These investigations have the potential to be applied to other chemical processes that make use of fluid reactions with embedded catalysts on the support.

To date, studies of graphene-silicon (G-Si) Schottky junctions for photovoltaic purposes have focused exclusively on using graphene grown through chemical vapor deposition (CVD). CVD growth of graphene is energetically intensive and is not routinely available. One viable alternative to the CVD growth of graphene is solution processed graphene. Ultrasonication of flake graphite in water containing ionic or non-ionic surfactants gives rise to aqueous dispersions of few layer graphene of varying concentrations. This solution based graphene can then be readily processed into films of varying size, shape and thickness, leading to easy control of properties such as optical transparency, sheet resistance and conductivity.
In this study ultrasonication of flake graphite with the non-ionic surfactant Tween-60 resulted in aqueous dispersions of few layer graphene, which was confirmed by comparison of the Raman spectra of the original graphite and end dispersion. Films of varying thickness were then prepared and their optical transparencies, sheet resistance and conductivity measured. These films were then used to form G-Si Schottky junctions and their photovoltaic properties investigated. It was found that the performance (measured by the power conversion efficiency (PCE) of the cell) of the pristine cells could be improved by a factor of seven by further processing of the active area interface between silicon and graphene with HF and this improvement was found to occur over a number of days. While the overall PCE of these cells was low compared to CVD grown G-Si Schottky junctions, the open circuit voltage was found to be comparable. After optimal film thicknesses were found, in situ chemical doping of the graphene films was investigated in an attempt to increase cell PCE. It was found that the way of doping, ie. direct exposure to liquid or exposure to vapor, resulted in markedly different responses from cells. Several different dopants were found to have a positive, albeit brief, effect on cell efficiency raising the PCE to 0.5 %.

La-based perovskites such as La_xSr_1-xCo_yFe_1-yO_3-δ have been widely studied as cathode materials for solid oxide fuel cell (SOFC) devices due to their high levels of mixed ionic-electronic conductivity. The surface catalytic activity is a rate limiting step seen as a key obstacle to device commercialisation. The controlling factor is the oxygen reduction reaction, making the outermost surface chemistry vitally important. This remains largely unknown for the majority of materials.
In this study we investigate a range of 6 analogous La-based perovskites doped with Sr or Ca on the A-site, all with the general formula La_0.6(Sr,Ca)_0.4XO_3-δ, where X is Fe_0.8Co_0.2, Fe_0.8Ni_0.2 or Co. Each material is sintered to densities greater than 95% and polished to a 1 µm finish. Samples of each are then annealed for an extended period of time at a range of temperatures (400-1000 °C).
Low energy ion scattering (LEIS) analysis of the surfaces allows us to determine the surface chemistry with unparalleled sensitivity to just the first monolayer as opposed to the nanometre scale depth resolution possible in other techniques such as X-ray photoelectron spectroscopy (XPS). Through the systematic study of the surface evolution with temperature we reveal the temperature dependence of surface segregation in these perovskite structures and its dependence on A-site and B-site cations, previously unreported.
Utilising a ¹⁸O isotopic tracer exchange technique combined with LEIS and secondary ion mass spectrometry (SIMS) allows us to investigate and quantify surface exchange and the diffusion profile at a single temperature (800 °C) in each material with surface chemistries obtained at two different annealing temperatures (800 and 1000 °C). This allows the direct correlation of surface chemistry with oxygen reduction behaviour and bulk vacancy content from thermogravimetric analysis (TGA), providing a depth of understanding not previously possible.

Thin film solid oxide fuel cells (µ-SOFCs), which use electrodes and electrolyte films with thicknesses of the order of tens of nanometers, have demonstrated power densities above 0.5 W/cm2 at temperatures below 600°C. The decrease of the operating temperature of SOFCs opens up the possibility of using them as power sources in mobile applications, but also allows demonstrating new power source operational capabilities enabled by the use of non-traditional SOFC materials. In this respect, we have recently demonstrated that using vanadium oxide on the anode-side of µ-SOFCs allows storing energy to continue generating power for short time periods in the absence of fuel.
In the present contribution, we explore in detail the influence of the stoichiometry of as-deposited thin film vanadium oxide anodes on the performance of µ-SOFCs. Vanadium oxide films were deposited by reactive magnetron sputtering without external heating. Different oxygen concentrations in the sputtering chamber allowed obtaining different stoichiometries in the as-deposited films, as indicated by x-ray photoelectron spectroscopy and x-ray diffraction. The stoichiometry was varied from V metal to V2O5. V and VO films were crystalline while other as-deposited vanadium oxide stoichiometries were amorphous. µ-SOFCs with yttria-stabilized zirconia electrolyte, porous platinum cathode, and anodes with different vanadium oxide stoichiometries were fabricated and tested at temperatures between 150 and 440°C. Composition effects on open circuit voltage and peak power performance will be considered in depth in connection with the electrocatalytic activity of the oxide electrodes.

Solid oxide regenerative fuel cells (SORFC) have been introduced as a dual-function technology to produce electricity in the mode of a solid oxide fuel cell (SOFC) or hydrogen, when operating as a solid oxide electrolyzer cell (SOEC). They supply power to the grid during the period of high demand, and store energy in the form of chemical energy utilizing renewable energy sources or excess electrical grid capacity during off-peak hours. The two operating modes differ in the electric potential gradient and reacting environment at the electrode surface, which significantly influences the performance and long-term stability of SORFC. In general, SOEC exhibits inferior performance and stability in comparison with SOFC, which limits the commercial development and deployment of this technology. In this study, performance degradation and long-term stability of SORFC are investigated. The cells used in this research consist of Ni - yttria-stabilized zirconia (YSZ) hydrogen electrode, YSZ electrolyte, and two different types of perovskite-fluorite composite air electrodes, La1-xSrxCoyFe1-yO3 (LSCF) - gadolinia-doped ceria (GDC) and La1-xSrxMnO3 (LSM) - YSZ. Degradation mechanisms of the air and hydrogen electrodes were separately examined through electrochemical and microstructural investigations. It is shown that the primary cause for the degradation of the air electrode is the cation migration occurring at the perovskite electrode due to oxygen evolution reaction, while that for the hydrogen electrode is a high steam concentration, which accelerates coarsening of nickel and leads to gradual increase of the polarization resistance. Degradation of each electrode was suppressed by employing novel electrode materials and optimizing the microstructure of porous composite electrodes. Nano-scale catalysts were incorporated into the porous electrodes to further reduce the polarization resistance and enhance the cell performance.

The design of highly proton-conductive materials is essential for many applications in the field of solid-state ionics. One fundamental approach to creat highly proton-conductive materials is chemical modification. Usually, sulfonic acid groups are used as a proton conductive group because of their excellent proton conductivity. However, the high acidity of sulfonic groups restricts the polymer backbone to fluoro or aromatic groups, which requires high production costs. Therefore, a new method to produce highly proton-conductive materials has been sought for a long time.
In a Nafion membrane, protons are well known to be transported through nanochannels made by sulfonic acid groups. The nanochannels are created by phase separation with the amphiphilic character of Nafion. Therefore, the proton conductive group orientation is also important to improve proton conductivity. Our group found that the proton conductivity of the Nafion ultra thin film decreased extremely compared to that of the commercial membrane. In the thin film, sulfonic acid groups are highly orientated and isolated each other, which results in the poor conductive channel formation.[1]
Amino acid polymers take several hierarchical structures such as a-helix or b-sheets using hydrogen bonding between the amino acids. We speculated that these high order structures can contribute to improve the proton transport properties. Poly(aspartic acid) has free carboxylic acid groups at the side chains, and the protons at these groups are mobile during proton conduction. In this study, we investigated the proton transport properties of a partially protonated poly(aspartic acid)/sodium polyaspartate (P-Asp) thin film prepared by spin coating. We found a highly oriented secondary structure in the thin film on MgO(100) substrate acted as a proton conducting channel, induced the enhancement effect.[2]
[1] Y. Nagao, The Journal of Physical Chemistry C, 117, 3294 - 3297, 2013.
[2] Y. Nagao, J. Matsui, T. Abe, H. Hiramatsu, H. Yamamoto, T. Miyashita, N. Sata, H. Yugami, Langmuir, 29, 6798 - 6804, 2013.

Ionic flows on oxide surfaces play a major role in operation of fuel cells, batteries, catalysts and gas sensors. Understanding the mechanisms of the electronic/ionic charge generation and its dynamics is necessary for improvement of the device characteristics as well as expanding our fundamental knowledge. Techniques, allowing working at the nanoscale - Scanning Kelvin Probe Microscopy (SKPM), electric force microscopy (ESM) stand out as unique methods of direct probing the surface potential with a spatial resolution of ca. 100 nm, i.e. at the scale of grain boundaries, defects, inclusions and other hot spots of active devices. However, the said methods only allow detection of the total potential created by surface and bulk charges, electronic and ionic species. Selective detection of different types of charges is possible based on the differences in their mobility. Here we present time-resolved Kelvin Probe Force Microscopy technique (tr-KPFM) that maps the surface potential of an active device with both space and time resolution. We demonstrate separation of the surface and bulk charge species responses in Ca-substituted bismuth ferrite, where bulk oxygen vacancy motion is associated with a metal-insulator transition in this materials and the more mobile surface ions control the conductive channel formation. Tr-KPFM also allowed us probing surface ions dynamics in insulating materials, such as quartz and LiNbO3 ferroelectric, where charge transport cannot be measured directly.
Research was supported (E.S., S.J., S.V.K., I.K.) by the U.S. Department of Energy, Basic Energy Sciences, Materials Sciences and Engineering Division. This research was conducted at the Center for Nanophase Materials Sciences (E.S., S.J., S.V.K., I.K.), which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy.

Many common perovskite structured Solid Oxide Fuel Cell (SOFC) cathode materials exhibit surface passivation through 'A site' segregation at common operating temperatures (~500-600^oC) [1]. These processes have a severely negative effect upon the oxygen reduction rates and therefore overall cell performance. Quantifying the true extent of these segregation effects has not been fully explored, however, it is clear that the properties of the surface are essential. Low Energy Ion Scattering (LEIS) enables compositional analysis of a single atomic layer on the outer surface of materials and as such is an essential tool in understanding cathode degradation. Of primary concern is the surface exchange of oxygen which is commonly measured using the Isotopic Exchange Depth Profiling (IEDP) method measured by Secondary Ion Mass Spectrometry (SIMS). Exchange equipment is, in the vast majority of cases, independent from analysis instruments and as such sample surfaces tend to be contaminated and even altered during sample transfer.
In this work we have developed oxygen exchange capabilities between 300 and 600^oC in the preparation chamber of a LEIS instrument. This has allowed common SOFC cathode materials to be annealed, exchanged and transferred directly into the LEIS instrument without entering an atmosphere with pressures higher than 10^-6 mbar, therefore minimising surface adhesion of any contaminants and removing the necessity for post-exchange cleaning steps. This avoids issues related to the commonly used oxygen plasma clean and its effect on oxygen isotope ratios on the surface monolayer which prove an obstacle in obtaining accurate surface exchange quantification.
The LEIS instrument in use is also adjoined to a SIMS instrument allowing sample transfer under vacuum. This has enabled the direct comparison of surface composition and surface exchange coefficient to be obtained by the two complementary techniques.
This novel experimental technique, combining highly surface sensitive LEIS and in-situ surface catalysis quantification allows a previously unobtainable accuracy in the determination of oxygen surface exchange measurements.
[1] M. Kubicek, A. Limbeck, T. Fromling, H. Hutter, and J. Fleig, "Relationship between cation segregation and the electrochemical oxygen reduction kinetics of La_0.6Sr_0.4CoO₃ thin film electrodes" Journal of The Electrochemical Society, vol. 158, no. 6, pp. B727-B734, 2011.

This study investigates surface segregation behavior and phase formation in LaxSr1-xCoyFe1-yO3-δ (LSCF), a commonly used cathode material for solid oxide fuel cells (SOFC). (100)-oriented LSCF thin films with varying ‘x&’ values were deposited on (110)-oriented NdGaO3 (NGO) substrates by pulsed laser deposition (PLD). The samples were annealed in atmospheres with various CO2 partial pressures at 800°C. Using synchrotron technique of total reflection x-ray fluorescence (TXRF), surface segregation in these thin films have been quantified. The morphological changes at the surface have been examined by AFM and SEM studies. The kinetics and thermodynamics of the segregation will be discussed in this presentation.

A Mn-doped yttria-stabilized zirconia (Mn-YSZ) interlayer was introduced between the strontium-doped lanthanum manganite (LSM) anode and electrolyte (YSZ) to suppress interfacial degradation in solid oxide electrolysis cells (SOEC). The durability of cells with the configuration LSM//YSZ/Mn-LSM//LSM was evaluated at 840°C at 0.8V for 500h. The Mn-YSZ interlayer was prepared by sol-gel coating method. The Mn-YSZ sol-gel interlayer modified the anode and electrolyte interface, mitigated the interfacial compound formation and anode delamination, and suppressed the long term degradation. The role of coating thickness and Mn dopant content was studied to improve the SOEC performance. Mechanism pertaining to mitigation of and anode delamination will be discussed.

Graphitic Carbon Nanofiber supported PtRu have been measured by the Lukehart Group to have higher activity as anode catalysts in Direct Methanol Fuel Cells than PtRu supported on other types of carbon. The reason for the higher activity as well as an atomistic understanding of the catalyst material is, however, missing. In the present work, density functional theory was used to investigate the carbon-metal interface for both monometallic Pt and the random alloy PtRu. The carbon support is a stacked cup structure with exposed edges of carbon sheets along the length of the fiber. This was modeled by one graphene sheet with zigzag or armchair termination interacting with one of several different crystal terminations of the metal surface. The strongest carbon-metal interaction was calculated for a carbon zigzag termination interacting with the (111) facet. This is in excellent agreement with TEM data recorded by the Lukehart Group. By consideration of the surface energy, Pt should segregate to the surface of the PtRu alloy. However, because carbon interacts stronger with Ru than with Pt, we find that the alloyed surface is the thermodynamically preferred interface for carbon supported PtRu particles. The character of the carbon-metal bond was investigated in detail by charge transfer and density of states analysis. It was found that there are several bonds formed to the surface and significant charge re-equilibration occurred upon binding, indicating a significant pathway for electron conduction from the catalyst particle to the carbon support. Furthermore, the core level binding energy shifts of carbon, platinum and ruthenium were found to provide valuable signatures of the interface, which would aid future high resolution x-ray photoemission core level spectroscopy (XPS) experiments of these systems. XPS experiments are currently under investigation.

Olefin isomerization can be achieved at the cathode of intermediate temperature fuel cells (250^oC -450^oC). The fuel cell electrolyte consists of a metal hydride foil (Pd) that supports a thin (<100µm) electronically insulating proton conducting layer (EIPC). Hydrogen is oxidized at the anode to protons that migrate through the Pd foil-EIPC composite, providing a flux of protons to the cathode. These protons spillover onto Pd catalysts at the cathode and acid catalyze olefin isomerization. The catalytic activity can be tuned by controlling the cell potential (i.e., the rate of proton spillover). In this work, 2-methyl-2-pentene is isomerized in a 250^oC intermediate temperature fuel cell. The EIPC is silica-doped ammonium polyphosphate.

The (La,Sr)CoO₃/(La,Sr)₂CoO₄ (LSC_113/214) hetero-structure is reported to show orders of magnitude improvement in oxygen reduction reaction (ORR) kinetics at around 500 ^oC. The interfaces are believed to be responsible for such high ORR activity. The investigation of the underlying mechanism has been ongoing in our research. The objective of this work is to understand of the role of the interface in determining the high reactivity to oxygen reduction. Such understanding can potentially guide the design of novel SOFC cathodes, as well as for other catalytic oxide systems using hetero-interfaces. We particularly focus on probing the local electronic structure, atomic structure and chemistry near the interface, features that governing the charge transfer process in ORR.
The structure of the LSC_113/214 multilayer system was confirmed by transmission electron microscopy. Energy dispersive spectroscopy and x-ray photoelectron spectroscopy with sputtering was performed to identify local compositional changes. Segregation of Sr or other cations at the interfaces between the different layers was not found as a reason of the high ORR reactivity. We assessed the local electronic structure of multilayer (ML) interfaces with high spatial resolution at elevated temperatures in oxygen gas environment by combining in situ scanning tunneling microscopy and spectroscopy (STM/STS) and grazing incidence focused ion beam (FIB). This approach for the first time overcame the challenges in accessing the buried layers and interfaces of ML systems using scanning probes at elevated temperatures. Our results show that while the LSC₁₁₃ in the ML structure behaves similar to its single-phase counterpart at high temperatures (200-300 ^oC), the LSC₂₁₄ is electronically activated through an interface coupling with LSC₁₁₃. Another approach we used to obtain local electronic structure at elevated temperature is x-ray photoemission spectroscopy with depth profiling. Using this technique, we confirmed a consistent change of the chemical composition across the multilayers, and traced the variation of the valance band edge and the Co oxidation state across the interface as a function of temperature.
Our result points out that a wide-band-gap oxide can be electronically activated by charge injection due to the defect states of a neighboring reducible oxide phase. This can be one key reason the ultra-fast ORR kinetics near the LSC_113/214 phases. Our approach can also be applied to other fields where the electronic structure near hetero-interfaces is an important determinant of performance, such as in electronics and catalysis.

The assignment of key IR bands of the sulfonated tetratfluoroethylene copolymer Nafion has been a source of confusion for decades. Specifically, the prevailing literature associates peaks at ~1060 cm^-1 and ~970 cm^-1 with the sulfonate group and the ether group nearest to the sulfonate group, respectively. The assignment of the Nafion spectrum by associating each peak to a single functional group is an oversimplification and precludes proper analysis. Time dependent IR transmission spectroscopy of hydrated Nafion during dehydration show the reversible disappearance of the 1061 cm^-1 and 969 cm^-1 peaks concurrent with the emergence of peaks at ~928 cm^-1 and ~1408 cm^-1. The first pair of group modes includes internal coordinates of dissociated sulfonate group with a local C_3V symmetry. The second pair of group modes includes coordinates of a protonated sulfonate group with C₁ symmetry. The C_3V group modes give way to the C₁ group modes as the membrane dehydrates. The DFT normal mode analysis confirms that the sulfonic acid/sulfonate site plays in dominant role in the C₁ and C_3V group modes respectively. This work clarifies the importance of assigning fluoropolymers peaks as group modes rather than traditional single functional group assignments as is often the case with the ~1061 cm^-1 and ~969 cm^-1 C_3V group modes.

Charge transfer between surfaces of two distinctly different materials through triboelectric effect is a well-known phenomenon that has various applications such as powder spray painting, electrophotography, electrostatic separation and energy harvesting. Recently, triboelectric nanogenerators (TENGs) have been invented for harvesting ambient mechanical energy based on the triboelectric effect coupled with electrostatic effect, and it has demonstrated unprecedented high output of its kind in both voltage and power density and efficiency, showing great promise for building self-powered portable electronics as well as possible large-scale energy harvesting.
Although the triboelectrification effect is known for thousands of years, a fundamental understanding about it is rather limited. Research has been conducted to characterize the triboelectrification process using various methods such as rolling sphere tool-collecting induced charges from rolling spheres on top of dielectric disk, and using atomic force microscopy (AFM) to measure surface electrostatic force or potential on surfaces contacted by micropatterned materials. However, these methods either lack an accurate control of the electrification process and/or cannot directly reveal the triboelectric interface, thus hardly achieving a quantitative understanding about the in-situ triboelectric process.
In this paper, we demonstrate an in-situ method to quantitatively characterize the triboelectrification at nano-scale via a combination of contact-mode AFM and scanning Kevin probe microscopy (SKPM). Benefited from the fact that the AFM system can precisely control the contact force, area, speed and cycles of the triboelectric process, systematical characterizations of the triboelectrification were realized including triboelectric charge distribution, multi-friction effect, as well as the subsequent charge diffusion on the dielectric surface. This methodology provides a powerful tool to investigate the parameters that are important in designing high performance TENGs. Furthermore, we demonstrated nano-patterning of surface charges using an AFM based triboelectrification process, which has promising applications for directed nano-assembly of charged nanostructures.
Reference
Y. S. Zhou, Y. Liu, G. Zhu, Z. Lin, C. Pan, Q. Jing, Z. L. Wang, Nano Lett., 2013, 13 (6), pp 2771-2776

Tungsten oxide (WOx) improves the activity of vanadium oxide (VOx) in the selective catalytic reduction of environmentally harmful nitric oxides. To better understand this synergy, we investigate the reduction-oxidation behavior of oxide-supported VOx and WOx species using a surface and interface science approach. We focus on monolayer and sub-monolayer VOx and WOx grown by atomic layer deposition on (0001)-terminated oxides with the corundum structure, namely α-Fe2O3 (hematite) and α-Al2O3 (sapphire). In particular, we illuminate the role of substrate reducibility on the surface morphology and chemistry of these model catalyst systems. In situ synchrotron X-ray measurements, including X-ray standing wave imaging, reveal changes in the positions of cations relative to the substrate lattice as the chemical environment is cycled between oxidizing and reducing conditions. X-ray spectroscopy shows chemical state shifts which correspond to changes in the position of surface cations. Density functional theory calculations support the experimental results, suggesting cation coordination environments and local symmetries for different chemical states. Further calculations provide structures and energetics of adsorbed molecules such as nitric oxide and ammonia, yielding insight into the mechanism of V-W synergy in selective catalytic reduction of nitric oxides.

As effective ways to convert mechanical motion to electrical power, the various transducers, such as electromagnetic inductive-transducer, piezoelectric-transducer, and capacitive-transducer have been developed. The electromagnetic inductive transducers defined by Faraday&’s law are dominant for electric energy harvesting from mechanical motion. Also, the piezoelectric transducer is becoming the emerging technology for self-powered portable device. Otherwise, in spite of good potential of energy transducing by variable capacitance, capacitive-transducer remains an infant state for energy harvesting, because it needs external bias-voltage sources for accumulating charges at electrodes or addition processes to let the dielectric have polarity, which means the passive transducer, or toxic liquid metals for operating. Also, their sources for energy harvesting are restricted by the only artificial intermittent stimulation like pushing or vibration.
To overcome the limitations, we propose a new active capacitive-transducer as an energy harvester to generate the considerably effective electric power from water&’s natural motion without any external bias-voltage sources or additional processes. Specially, we change the classical paradigm in capacitive-transducer; the capacitive-transducer must need the external bias-voltage source or additional processes for the energy harvesting from variable capacitance by mechanical motion. One of the significant limitations to apply capacitive-transducer to energy harvester is the non-practical usage. However, herein we introduce the novel active capacitive-transducer without any external bias-voltage sources.
We found that the overlapped area variation between the water and the dielectric layer can effectively induce the variation of electric charges, and then easily generates electricity. These active capacitive-transducers have a simple structure: substrate including the patterned transparent electrodes, spin-coated dielectric layers, and hydrophobic layers. Actually, through the motion of only one flowing tap-water droplet of 30 mu;l, we could successfully generate the electricity with a peak voltage ~3.1 V and peak current ~5.3 mu;A, which was enough to turn on a green LED. Also, using the energy harvesters based on this novel concept, we demonstrated the wide applicability toward natural water&’s motions, such as rain, rivers, and even sea waves as well. We believe that they can substantially inspire for new energy harvesting technology from ambient energy sources.

Effectively harvesting ambient mechanical energy is the key for realizing self-powered and stand-alone electronics, which not only addresses the limitations of traditional power supplies such as batteries but also brings about tremendous economic, environmental, and social benefits. We report a tri-electrode structured triboelectric nanogenerator (TENG) that is based on four sets of small-sized metal gratings and nanoparticle-enabled interface modification. By producing average effective power of 13.2 W and corresponding power density of 6.6 KW/m2 at motion velocity of 10 m/s, it can extremely effectively harvest energy from friction at conversion efficiency of 67 %. More importantly, general lighting regarding common applications was achieved by regular lights that were continuously powered by the tri-electrode TENG. This work firmly establishes the TENG as a practical technology of renewable energy generation for powering commonly used functional electronics.

Organic-inorganic hybrid sol-gel materials containing polar groups that can undergo reorientational polarization under an electric field provide a potential route to processable, high permittivity dielectric materials for energy storage. We have shown that sol-gel thin films based on neat 2-cyanoethyltrimethoxysilane (CNETMS) exhibit large permittivity (~20) and an extractable energy density of 7 J/cm³ at ~300 V/µm. In this work, we have investigated the influence of a thin polymeric film “blocking” layer deposited on the CNETMS on the breakdown statistics and the energy storage characteristics. Two polymeric materials, a fluoropolymer (CYTOP) and poly(p-phenylene oxide) (PPO), were examined as potential materials to block charge injection into neat CNETMS films. Blocking layers with thicknesses ranging from 20 nm to 176 nm were deposited on top of CNETMS film by spin casting. The bilayer films exhibit a decrease of permittivity as well as loss tangent as a result of the introduction of low-k organic layer at frequencies from 100 Hz to 1 MHz. Interestingly, PPO/CNETMS bilayer films show an order of magnitude reduction in leakage current compared to neat CNETMS films, which suggests the utility of PPO in blocking charge injection into neat CNETMS film. Energy density measurements using the charge-discharge method show an increase of 25% in extractable energy density with PPO/CNETMS bilayer films due to improved breakdown strength. Polarization-electric field (P-E) measurements show that PPO/CNETMS bilayer films exhibit comparable maximum extractable energy density to neat CNETMS. Additionally, unlike CYTOP/CNETMS bilayer, the dielectric loss of PPO/CNETMS bilayer films derived from P-E loops exhibits a plateau at fields from 200 to 500 V/µm, which suggests trapped charges in PPO and at the PPO/CNETMS interface at these fields undergo interfacial polarization along with reorientational polarization of neat CNETMS. The observed results will be discussed in regards to film morphology, partitioning of electric field, charge trapping, and loss of bilayer films. Our study indicates that: 1) the blocking layer provides enhanced breakdown strength to bilayer films, 2) the extractable energy density and energy extraction efficiency of bilayer films depend on the trapped charges in blocking layer and the interface between blocking and CNETMS layer.

Metal nitrides are of increasing interest as electrode materials for redox supercapacitor applications, with studies on TiN, VN and MoN all showing promising capacities.¹ A key recent paper shows capacities of 1340 F g^-1 at low cycling rates (2 mV s^-1) and 554 F g^-1 even at high rate (100 mV s^-1) in VN with aqueous KOH electrolyte.² The metal nitrides are highly conductive materials that store charge in an oxidised surface layer that can be replenished by corrosion of the underlying metal nitride. Hence synthesis of high surface areas and new compositions could lead to further improvements.
Since charge storage utilises the surface of the material, modification of the surface to optimise its composition could lead to improvements in capacity. In order to focus on this surface we have produced TiN and VN electrodes by direct ammonolysis of the metal foils. Grazing incidence X-ray diffraction studies show graded compositions with the MN phases at the surface and underlying M₂N and metal. These surfaces were subjected to electrochemical and thermal treatments and the surface composition has been characterised using X-ray photoelectron spectroscopy and atomic force microscopy. Electrochemical cycling shows variations in charge storage ability with electrochemical pre-treatment of the surface yielding the highest capacities. The capacity variations have been linked to the electrode surface chemistry.
1. D. Choi and P. N. Kumta, J. Electrochem. Soc., 2006, 153, A2298; R. A. Janes, M. Aldissi and R. B. Kaner, Chem. Mater., 2003, 15, 4431; C. Chen, D. Zhao, D. Xu and X. Wang, Mater. Chem. Phys., 2006, 95, 84.
2. D. Choi, E. Blomgren and P. N. Kumta, Adv. Mater., 2006, 18, 1178.

Nanostructured silicon-based alloys and composites have received great attention as a possible replacement of the conventional carbon-based anodes due to their higher lithium storage capacity. In many cases, however very little is known about their electrochemical properties and microstructure evolution during lithiation and delithiation, despite their importance for overcoming many technical hurdles faced in practical use. Using first principles-based atomistic modeling, we have explored the lithiation and delithiation behavior in various Si-based nanostructures and nanocomposites including nanowires and Si-C composites. This talk will present our recent progress, particularly focusing on addressing (1) the lithiation and delithiation mechanisms of Si near the surface and interface, with comparisons to those in bulk Si, and (2) how the surface and interface affects the performance of Si-based nanomaterials as Li-ion battery anodes, such as charging rate and capacity retention. Our study highlights that the presence of surfaces and interfaces alters the lithiation behavior considerably; for instance, the Li mobility along the surface or interface tends to be significantly enhanced by several factors. We will also present the surface and interface effects on the structural evolution, bonding mechanism, mechanical property, and voltage profile of lithiated Si nanowires and Si-C nanocomposites. The improved understanding may offer important guidance for the rational design of nanostructured Si-based alloys and composites in order to maximize their capacity retention and rate capability.

The lithation and delithation of silicon anodes occur at potentials beyond the window of stability of current commercial liquid electrolytes, leading to electrochemical decomposition of the electrolyte and formation of a solid-electrolyte interface (SEI) whose precise chemical composition is both dynamic and poorly understood. The massive volume expansion of silicon during lithiation leads to mechanical degradation with increased cycling, thus increasing the electrode surface area exposed to electrolyte. This increased surface area further promotes formation of the SEI, thus exacerbating lithium consumption. To characterize the chemical composition of the SEI more accurately, we employ anoxic XPS1 and ToF-SIMS techniques to prevent contamination by air and moisture, both of which substantially alter SEI composition. Results show the effect of different electrochemical treatments and surface chemistry on the composition of oligomeric and inorganic SEI products. Furthermore, we propose that conversion reactions play an important role in the formation of a true mixed oxide interphase within the SEI that affects lithiation and delithation of silicon.

1. K. W. Schroder, H. Celio, L. J. Webb, and K. J. Stevenson, J. Phys. Chem. C 2012 116, 19737-19747

laquo; There is plenty of room at the bottom raquo;. This visionary foresight of R. Feynman, introduced during a lecture at Caltech in 1959, was at the root of numerous scientific and technological developments, taking benefit of the "strange phenomena" occuring at the smallest scales. There remains however a lot to explore, in particular in the context of fluids at the nanoscales and their specific transport properties.
A major challenge for nanofluidics thus lies in building distinct and well-controlled nanochannels, amenable to the systematic exploration of their properties. Here we describe the fabrication and use of a hierarchical nanofluidic device made of a boron nitride nanotube that pierces an ultrathin membrane and connects two fluid reservoirs. Such a transmembrane geometry allows the detailed study of fluidic transport through a single nanotube under diverse forces, including electric fields, pressure drops and chemical gradients.
Using this device, we discover very large, osmotically induced electric currents generated by salinity gradients, exceeding by two orders of magnitude their pressure-driven counterpart. We show that this result originates in the anomalously high surface charge carried by the nanotube&’s internal surface in water at large pH, which we independently quantify in conductance measurements. The nano-assembly route using nanostructures as building blocks opens the way to studying fluid, ionic and molecule transport on the nanoscale, and may lead to biomimetic functionalities.
Our results furthermore suggest that boron nitride nanotubes could be used as membranes for osmotic power harvesting under salinity gradients, with unprecedented energy conversion. Applications in the field of sustainable energy harvesting will be discussed.

Oxides have shown high activity for the oxygen evolution reaction (OER), however a lack of fundamental understanding of the physical-chemical properties of these oxides has limited mechanistic understanding and optimal catalyst design. Although analytical and theoretical frameworks for the electronic structure of perovskites, such as e_g occupancy and O p-band center, have been shown to be good descriptors of OER activity, such frameworks have yet to be connected to experimental measurements of electron density of states (DOS). We present a comprehensive investigation of the perovskite family, AA&’BO₃, using a combination of soft X-ray absorption spectroscopy (XAS), X-ray emission spectroscopy (XES), and X-ray photoelectron spectroscopy (XPS). Through the combination of O K and M L absorption and emission spectra, we obtain detailed information regarding the oxygen and transition metal character of the occupied and unoccupied DOS. We experimentally identified trends in the energetic parameters relevant to the conductivity gap under the Zaanen-Sawatzky-Allen (ZSA) framework - including the on-site Coulombic repulsion (U_dd), charge-transfer gap (Δ_CT), transfer integral (t) - as a function of the perovskite chemistry, and establish trends with the perovskite surface reactivity for the OER.