Acid-Assisted Ligand Exchange Enhances Coupling in Colloidal Quantum Dot Solids

Jo, Jea Woong,Choi, Jongmin,Garcí,a de Arquer, F. Pelayo,Seifitokaldani, Ali,Sun, Bin,Kim, Younghoon,Ahn, Hyungju,Fan, James,Quintero-Bermudez, Rafael,Kim, Junghwan,Choi, Min-Jae,Baek, Se-Woong American Chemical Society 2018 NANO LETTERS Vol.18 No.7

<P>Colloidal quantum dots (CQDs) are promising solution-processed infrared-absorbing materials for optoelectronics. In these applications, it is crucial to replace the electrically insulating ligands used in synthesis to form strongly coupled quantum dot solids. Recently, solution-phase ligand-exchange strategies have been reported that minimize the density of defects and the polydispersity of CQDs; however, we find herein that the new ligands exhibit insufficient chemical reactivity to remove original oleic acid ligands completely. This leads to low CQD packing and correspondingly low electronic performance. Here we report an acid-assisted solution-phase ligand-exchange strategy that, by enabling efficient removal of the original ligands, enables the synthesis of densified CQD arrays. Our use of hydroiodic acid simultaneously facilitates high CQD packing via proton donation and CQD passivation through iodine. We demonstrate highly packed CQD films with a 2.5 times increased carrier mobility compared with prior exchanges. The resulting devices achieve the highest infrared photon-to-electron conversion efficiencies (>50%) reported in the spectral range of 0.8 to 1.1 eV.</P> [FIG OMISSION]</BR>

Tailoring the Energy Landscape in Quasi-2D Halide Perovskites Enables Efficient Green-Light Emission

Quan, Li Na,Zhao, Yongbiao,Garcí,a de Arquer, F. Pelayo,Sabatini, Randy,Walters, Grant,Voznyy, Oleksandr,Comin, Riccardo,Li, Yiying,Fan, James Z.,Tan, Hairen,Pan, Jun,Yuan, Mingjian,Bakr, Osman American Chemical Society 2017 NANO LETTERS Vol.17 No.6

<P>Organo-metal halide perovskites are a promising platform for optoelectronic applications in view of their excellent charge-transport and bandgap tunability. However, their low photoluminescence quantum efficiencies, especially in low-excitation regimes, limit their efficiency for light emission. Consequently, perovskite light-emitting devices are operated under high injection, a regime under which the materials have so far been unstable. Here we show that, by concentrating photoexcited states into a small subpopulation of radiative domains, one can achieve a high quantum yield, even at low excitation intensities. We tailor the composition of quasi-2D perovskites to direct the energy transfer into the lowest-bandgap minority phase and to do so faster than it is lost to nonradiative centers. The new material exhibits 60% photoluminescence quantum yield at excitation intensities as low as 1.8 mW/cm(2), yielding a ratio of quantum yield to excitation intensity of 0.3 cm(2)/mW; this represents a decrease of 2 orders of magnitude in the excitation power required to reach high efficiency compared with the best prior reports. Using this strategy, we report light-emitting diodes with external quantum efficiencies of 7.4% and a high luminescence of 8400 cd/m(2).</P>

Unraveling the Role of Monoolein in Fluidity and Dynamical Response of a Mixed Cationic Lipid Bilayer

Singh, Priya,Sharma, Veerendra Kumar,Singha, Subhankar,Garcí,a Sakai, Victoria,Mukhopadhyay, Ramaprosad,Das, Ranjan,Pal, Samir Kumar American Chemical Society 2019 Langmuir Vol.35 No.13

<P>The maintenance of cell membrane fluidity is of critical importance for various cellular functions. At lower temperatures when membrane fluidity decreases, plants and cyanobacteria react by introducing unsaturation in the lipids, so that the membranes return to a more fluidic state. To probe how introduction of unsaturation leads to reduced membrane fluidity, a model cationic lipid dioctadecyldimethylammonium bromide (DODAB) has been chosen, and the effects of an unsaturated lipid monoolein (MO) on the structural dynamics and phase behavior of DODAB have been monitored by quasielastic neutron scattering and time-resolved fluorescence measurements. In the coagel phase, fluidity of the lipid bilayer increases significantly in the presence of MO relative to pure DODAB vesicles and becomes manifest in significantly enhanced dynamics of the constituent lipids along with faster hydration and orientational relaxation dynamics of a fluorophore. On the contrary, MO restricts both lateral and internal motions of the lipid molecules in the fluid phase (>330 K), which is consistent with relatively slow hydration and orientational relaxation dynamics of the fluorophore embedded in the mixed lipid bilayer. The present study illustrates how incorporation of an unsaturated lipid at lower temperatures (below the phase transition) assists the model lipid (DODAB) in regulating fluidity via enhancement of dynamics of the constituent lipids.</P> [FIG OMISSION]</BR>

Stacking Structures of Few-Layer Graphene Revealed by Phase-Sensitive Infrared Nanoscopy

Kim, Deok-Soo,Kwon, Hyuksang,Nikitin, Alexey Yu.,Ahn, Seongjin,Marti&#x301,n-Moreno, Luis,Garcí,a-Vidal, Francisco J.,Ryu, Sunmin,Min, Hongki,Kim, Zee Hwan American Chemical Society 2015 ACS NANO Vol.9 No.7

<P>The stacking orders in few-layer graphene (FLG) strongly influences the electronic properties of the material. To explore the stacking-specific properties of FLG in detail, one needs powerful microscopy techniques that visualize stacking domains with sufficient spatial resolution. We demonstrate that infrared (IR) scattering scanning near-field optical microscopy (sSNOM) directly maps out the stacking domains of FLG with a nanometric resolution, based on the stacking-specific IR conductivities of FLG. The intensity and phase contrasts of sSNOM are compared with the sSNOM contrast model, which is based on the dipolar tip–sample coupling and the theoretical conductivity spectra of FLG, allowing a clear assignment of each FLG domain as Bernal, rhombohedral, or intermediate stacks for tri-, tetra-, and pentalayer graphene. The method offers 10–100 times better spatial resolution than the far-field Raman and infrared spectroscopic methods, yet it allows far more experimental flexibility than the scanning tunneling microscopy and electron microscopy.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/ancac3/2015/ancac3.2015.9.issue-7/acsnano.5b02813/production/images/medium/nn-2015-02813x_0007.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/nn5b02813'>ACS Electronic Supporting Info</A></P>

Cobalt Electrolyte/Dye Interactions in Dye-Sensitized Solar Cells: A Combined Computational and Experimental Study

Mosconi, Edoardo,Yum, Jun-Ho,Kessler, Florian,Go&#x301,mez Garcí,a, Carlos J.,Zuccaccia, Cristiano,Cinti, Antonio,Nazeeruddin, Mohammad K.,Gra&#x308,tzel, Michael,De Angelis, Filippo American Chemical Society 2012 JOURNAL OF THE AMERICAN CHEMICAL SOCIETY - Vol.134 No.47

<P>We report a combined experimental and computational investigation to understand the nature of the interactions between cobalt redox mediators and TiO<SUB>2</SUB> surfaces sensitized by ruthenium and organic dyes, and their impact on the performance of the corresponding dye-sensitized solar cells (DSSCs). We focus on different ruthenium dyes and fully organic dyes, to understand the dramatic loss of efficiency observed for the prototype Ru(II) N719 dye in conjunction with cobalt electrolytes. Both N719- and Z907-based DSSCs showed an increased lifetime in iodine-based electrolyte compared to the cobalt-based redox shuttle, while the organic D21L6 and D25L6 dyes, endowed with long alkoxy chains, show no significant change in the electron lifetime regardless of employed electrolyte and deliver a high photovoltaic efficiency of 6.5% with a cobalt electrolyte. Ab initio molecular dynamics simulations show the formation of a complex between the cobalt electrolyte and the surface-adsorbed ruthenium dye, which brings the [Co(bpy)<SUB>3</SUB>]<SUP>3+</SUP> species into contact with the TiO<SUB>2</SUB> surface. This translates into a high probability of intercepting TiO<SUB>2</SUB>-injected electrons by the oxidized [Co(bpy)<SUB>3</SUB>]<SUP>3+</SUP> species, lying close to the N719-sensitized TiO<SUB>2</SUB> surface. Investigation of the dye regeneration mechanism by the cobalt electrolyte in the Marcus theory framework led to substantially different reorganization energies for the high-spin (HS) and low-spin (LS) reaction pathways. Our calculated reorganization energies for the LS pathways are in excellent agreement with recent data for a series of cobalt complexes, lending support to the proposed regeneration pathway. Finally, we systematically investigate a series of Co(II)/Co(III) complexes to gauge the impact of ligand substitution and of metal coordination (tris-bidentate vs bis-tridentate) on the HS/LS energy difference and reorganization energies. Our results allow us to trace structure/property relations required for further development of cobalt electrolytes for DSSCs.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/jacsat/2012/jacsat.2012.134.issue-47/ja3079016/production/images/medium/ja-2012-079016_0016.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/ja3079016'>ACS Electronic Supporting Info</A></P>

Metal-Organic Frameworks Mediate Cu Coordination for Selective CO₂ Electroreduction

Nam, Dae-Hyun,Bushuyev, Oleksandr S.,Li, Jun,De Luna, Phil,Seifitokaldani, Ali,Dinh, Cao-Thang,Garcí,a de Arquer, F. Pelayo,Wang, Yuhang,Liang, Zhiqin,Proppe, Andrew H.,Tan, Chih Shan,Todorovic& American Chemical Society 2018 JOURNAL OF THE AMERICAN CHEMICAL SOCIETY - Vol.140 No.36

<P>The electrochemical carbon dioxide reduction reaction (CO<SUB>2</SUB>RR) produces diverse chemical species. Cu clusters with a judiciously controlled surface coordination number (CN) provide active sites that simultaneously optimize selectivity, activity, and efficiency for CO<SUB>2</SUB>RR. Here we report a strategy involving metal-organic framework (MOF)-regulated Cu cluster formation that shifts CO<SUB>2</SUB> electroreduction toward multiple-carbon product generation. Specifically, we promoted undercoordinated sites during the formation of Cu clusters by controlling the structure of the Cu dimer, the precursor for Cu clusters. We distorted the symmetric paddle-wheel Cu dimer secondary building block of HKUST-1 to an asymmetric motif by separating adjacent benzene tricarboxylate moieties using thermal treatment. By varying materials processing conditions, we modulated the asymmetric local atomic structure, oxidation state and bonding strain of Cu dimers. Using electron paramagnetic resonance (EPR) and in situ X-ray absorption spectroscopy (XAS) experiments, we observed the formation of Cu clusters with low CN from distorted Cu dimers in HKUST-1 during CO<SUB>2</SUB> electroreduction. These exhibited 45% C<SUB>2</SUB>H<SUB>4</SUB> faradaic efficiency (FE), a record for MOF-derived Cu cluster catalysts. A structure-activity relationship was established wherein the tuning of the Cu-Cu CN in Cu clusters determines the CO<SUB>2</SUB>RR selectivity.</P> [FIG OMISSION]</BR>