Model Systems For Fundamental Studies Research Articles

Ultrafast Charge Transfer Cascade in a Mixed-Dimensionality Nanoscale Trilayer.

Innovation in optoelectronic semiconductor devices is driven by a fundamental understanding of how to move charges and/or excitons (electron-hole pairs) in specified directions for doing useful work, e.g., for making fuels or electricity. The diverse and tunable electronic and optical properties of two-dimensional (2D) transition metal dichalcogenides (TMDCs) and one-dimensional (1D) semiconducting single-walled carbon nanotubes (s-SWCNTs) make them good quantum confined model systems for fundamental studies of charge and exciton transfer across heterointerfaces. Here we demonstrate a mixed-dimensionality 2D/1D/2D MoS2/SWCNT/WSe2 heterotrilayer that enables ultrafast photoinduced exciton dissociation, followed by charge diffusion and slow recombination. Importantly, the heterotrilayer serves to double charge carrier yield relative to a MoS2/SWCNT heterobilayer and also demonstrates the ability of the separated charges to overcome interlayer exciton binding energies to diffuse from one TMDC/SWCNT interface to the other 2D/1D interface, resulting in Coulombically unbound charges. Interestingly, the heterotrilayer also appears to enable efficient hole transfer from SWCNTs to WSe2, which is not observed in the identically prepared WSe2/SWCNT heterobilayer, suggesting that increasing the complexity of nanoscale trilayers may modify dynamic pathways. Our work suggests "mixed-dimensionality" TMDC/SWCNT based heterotrilayers as both interesting model systems for mechanistic studies of carrier dynamics at nanoscale heterointerfaces and for potential applications in advanced optoelectronic systems.

Morphological design of PDVB‐based particles

AbstractPolymeric microspheres with complex architectures find application as stationary phases for size exclusion separation, solid supports for surface reactions, catalysis, and delivery systems. The microparticles are commonly made from divinylbenzene by precipitation polymerization or template swelling. Upon combining both methods and applying photopolymerization, we discovered new morphological features that are not solely explicable by the individual processes. Firstly, three types of templates were synthesized by precipitation polymerization: noncrosslinked, slightly crosslinked, and slightly crosslinked with noncrosslinked polystyrene fractions. All template types were swollen with divinylbenzene, which was polymerized by photoinitiation, followed by template extraction. Changes in particle size were tracked by light microscopy, while SEM revealed submicron features like hollow cores, toluene‐insoluble walls, and surface grooves from the collapse upon drying of the toluene‐swollen corona. Our investigations indicated that the corona results from chain transfer and crosslinking of the template during photoinitiation. Higher precrosslinking of the template reduced the core cavity volume of the extracted particle, while more mobile polystyrene chains led to larger cavities. These architectures are promising model systems for fundamental studies of particle interactions, aggregation, and structure formation in the field of colloidal physics.

Tunable nonlinear optical mapping in a multiple-scattering cavity

Nonlinear disordered systems are not only a model system for fundamental studies but also in high demand for practical applications. However, optical nonlinearity based on intrinsic material response is weak in random scattering systems. Here, we propose and experimentally realize a highly nonlinear mapping between the scattering potential and the emerging light of a reconfigurable multiple-scattering cavity. A quantitative analysis of the degree of nonlinearity reveals its dependence on the number of scattering events. The effective order of nonlinear mapping can be tuned over a wide range at low optical lower. The strong nonlinear mapping enhances output intensity fluctuations and long-range correlations. The flexibility, robustness, and energy efficiency of our approach provides a versatile platform for exploring such nonlinear mappings for various applications.

Defined core–shell particles as the key to complex interfacial self-assembly

The two-dimensional self-assembly of colloidal particles serves as a model system for fundamental studies of structure formation and as a powerful tool to fabricate functional materials and surfaces. However, the prevalence of hexagonal symmetries in such self-assembling systems limits its structural versatility. More than two decades ago, Jagla demonstrated that core-shell particles with two interaction length scales can form complex, nonhexagonal minimum energy configurations. Based on such Jagla potentials, a wide variety of phases including cluster lattices, chains, and quasicrystals have been theoretically discovered. Despite the elegance of this approach, its experimental realization has remained largely elusive. Here, we capitalize on the distinct interfacial morphology of soft particles to design two-dimensional assemblies with structural complexity. We find that core-shell particles consisting of a silica core surface functionalized with a noncrosslinked polymer shell efficiently spread at a liquid interface to form a two-dimensional polymer corona surrounding the core. We controllably grow such shells by iniferter-type controlled radical polymerization. Upon interfacial compression, the resulting core-shell particles arrange in well-defined dimer, trimer, and tetramer lattices before transitioning into complex chain and cluster phases. The experimental phase behavior is accurately reproduced by Monte Carlo simulations and minimum energy calculations, suggesting that the interfacial assembly interacts via a pairwise-additive Jagla-type potential. By comparing theory, simulation, and experiment, we narrow the Jagla g-parameter of the system to between 0.9 and 2. The possibility to control the interaction potential via the interfacial morphology provides a framework to realize structural features with unprecedented complexity from a simple, one-component system.

Bimetallic Nanoparticles as a Model System for an Industrial NiMo Catalyst.

An in-depth understanding of the reaction mechanism is required for the further development of Mo-based catalysts for biobased feedstocks. However, fundamental studies of industrial catalysts are challenging, and simplified systems are often used without direct comparison to their industrial counterparts. Here, we report on size-selected bimetallic NiMo nanoparticles as a candidate for a model catalyst that is directly compared to the industrial system to evaluate their industrial relevance. Both the nanoparticles and industrial supported NiMo catalysts were characterized using surface- and bulk-sensitive techniques. We found that the active Ni and Mo metals in the industrial catalyst are well dispersed and well mixed on the support, and that the interaction between Ni and Mo promotes the reduction of the Mo oxide. We successfully produced 25 nm NiMo alloyed nanoparticles with a narrow size distribution. Characterization of the nanoparticles showed that they have a metallic core with a native oxide shell with a high potential for use as a model system for fundamental studies of hydrotreating catalysts for biobased feedstocks.

Long-distance spin transport through a graphene quantum Hall antiferromagnet

Antiferromagnetic insulators (AFMI) are robust against stray fields, and their intrinsic dynamics could enable ultrafast magneto-optics and ultrascaled magnetic information processing. Low dissipation, long distance spin transport and electrical manipulation of antiferromagnetic order are much sought-after goals of spintronics research. Here, we report the first experimental evidence of robust long-distance spin transport through an AFMI, in our case the gate-controlled, canted antiferromagnetic (CAF) state that appears at the charge neutrality point of graphene in the presence of an external magnetic field. Utilizing gate-controlled quantum Hall (QH) edge states as spin-dependent injectors and detectors, we observe large, non-local electrical signals across a 5 micron-long, insulating channel only when it is biased into the nu=0 CAF state. Among possible transport mechanisms, spin superfluidity in an antiferromagnetic state gives the most consistent interpretation of the non-local signal's dependence on magnetic field, temperature and filling factors. This work also demonstrates that graphene in the QH regime is a powerful model system for fundamental studies of antiferromagnetic, and in the case of a large in-plane field, ferromagnetic spintronics.

(Invited) Electrocatalysis of Hydrogen Evolution on Single Crystal Gold Electrodes Decorated By Palladium and Rhodium Nanoislands

Single crystal gold electrodes, Au(hkl), modified by submonolayer deposit of highly active metals from platinum group are convenient bimetallic model systems for fundamental studies of the electrocatalytic activity of various nanostructures for fuel cell reactions. Preparation of such bimetallic electrodes was carried out by spontaneous deposition method, which was monitored by chronopotentiometric measurement of open circuit potential. Surface characterization of the modified bimetallic electrodes, as well as base substrate electrodes, were performed ex situ by atomic force microscopy Simultaneously recorded topographic and phase images provided insight into different characteristics of the bimetallic surfaces, such as the dimensions of the deposited islands, occurrence of the preferential deposition along specific surface sites and the substrate coverage with deposited islands. Phase AFM images display an obvious chemical contrast between two materials, independent of the surface topography enabling thus quite accurate estimation of the surface coverage, particularly important in the cases when the conventional ways are not accurate enough or not applicable. X-ray photoelectron spectroscopy has shown the oxidation state of the surface constituents of the as prepared bimetallic electrodes. Pronounced catalytic activity for hydrogen evolution reaction (HER) of gold single crystals decorated by palladium nanoislands with submonolayer coverage has been reported so far for different Pd/Au(111) nanostructures in acid [1] and alkaline [2] media. Rh/Au(111) bimetallic electrodes, obtained by the same spontaneous deposition method have shown even better catalytic activity for HER in acid solution [3]. The activity was in agreement to the one theoretically predicted for Au(111) surface covered with Rh overlayers, which was comparable to that of Pt(111) [4]. It was shown that among different Rh/Au(111) nanostructures, the one with 50% Rh coverage has shown the highest activity for HER in acid media, starting at approx. 100 mV more positive potential than the most active Pd/Au(111) [1], and very close to the activity of Pt(111), although not exceeding it. Similar order of activity has been reported for HER in alkaline solution [2,5]. Comparison of the catalytic activity of the most active Pd/Au(111) and Rh/Au(111) in acid and alkaline media is shown in Fig. 1. The research is extended to the use of Au(100) electrode as the substrate for the same foreign metals deposition. The comparison of the catalytic activity of bimetallic surfaces consisting of Pd or Rh islands spontaneously deposited with the same coverage on differently oriented gold single crystals have shown that those bimetallic surfaces having Au(111) as substrate is more active than the ones deposited on Au(100) as substrate. The improved catalytic activity of the hereby explored bimetallic electrodes was interpreted by means of geometric and electronic effects of the substrates, which are responsible for different electrocatalytic properties with respect to both constitutive metals. We belive that these results contribute to the current search for the efficient catalyst for HER. [1] M.Smiljanić, I. Srejić, B. Grgur, Z. Rakočević, and S. Štrbac, Electrocatal. 3, 369 (2012). [2] M.Smiljanić, I. Srejić, B. Grgur, Z. Rakočević, and S. Štrbac, Electrochim. Acta 88, 589 (2013). [3] M.Smiljanić, I. Srejić, B. Grgur, Z. Rakočević, and S. Štrbac, Electrochem. Commun. 28, 37 (2013). [4] G. Soldano, E.N. Schulz, D.R. Salinas, E. Santos, and W. Schmickler, Phys. Chem. Chem. Phys. 13, 16437 (2011). [5] S. Štrbac, M. Smiljanić, and Z. Rakočević, J. Electrochem. Soc. 163, D3027 (2016). Figure 1

Study of the Bulk Charge Carrier Dynamics in Anatase and Rutile TiO2 Single Crystals by Femtosecond Time-Resolved Spectroscopy

Understanding of the fundamentals behind charge carrier dynamics of photocatalytic materials is still illusive, hindering progress in our quest for renewable energy. TiO2 anatase and rutile are the most understood phases in photocatalysis and serve as the best model systems for fundamental studies. The ultrafast charge carrier dynamics, especially on TiO2 anatase single crystals (the most active phase), are unresolved. Here, femtosecond time-resolved spectroscopy was carried out to explore the dynamics of photoexcited charge carriers’ recombination in the anatase single crystal, for the first time using pump fluence effects, and we compared it to that in the rutile single crystal. A significant difference in charge carrier recombination rates between both crystals is recorded. More specifically, we found that the time constants for carrier recombination are 2 orders of magnitude slower for anatase (101) when compared to those of rutile (110). Moreover, bulk defects introduced by reduction of the samples v...

Beneficial Effect of Surface Decorations on the Surface Exchange of Lanthanum Strontium Ferrite and Dual Phase Composites

Perovskites within the (La,Sr)(Fe,Co)O3 class of materials show variations in the oxygen stoichiometry depending on temperature and oxygen activity and can potentially be used as catalysts, electrodes in high-temperature solid oxide fuel cells, gas sensors or for oxygen transport membranes. These perovskites possess a mixed ionic and electronic conductivity (MIEC), which can be highly beneficial for the processes on oxygen electrode surfaces. The oxygen transport through a MIEC is determined by the rate of the oxygen exchange over the gas-solid interface and the diffusivity of oxide ions and electrons (or holes) in the bulk. The oxygen exchange process over the surface in general involves several reaction steps, O2 adsorption, dissociation, charge transfer and incorporation of ionic species. The Co-free end member of the material class; LSF (e.g. (La0.6Sr0.4FeO3-δ ) is fairly low cost and chemically stable in both mildly reducing and oxidizing atmosphere. The electronic conductivity is excellent (283 S/cm at 800 °C) but the ionic conductivity especially at low temperature is limited (0.014 S/cm, 800 °C). Due to these properties the material is a candidate for use in composite membranes in combination with a better ionic conducting material like CGO. Such systems are also excellent model systems for fundamental studies of the oxygen exchange process.The aim of this study is to characterize the oxygen transport properties of a dual-phase composite system as a function of temperature, and to elucidate the origin of the enhancement of the surface exchange reaction that has been reported to occur in such systems1by increasing the amount of ionic conductor. Additionally, the effect of surface decorations (desired) and impurities (undesired) on the surface exchange was studied to illustrate the importance of controlled conditions for the sample preparation and measurements. Electrical Conductivity Relaxation (ECR) was used to study the surface exchange reaction and the oxygen ion diffusion of single phase (La0.6Sr0.4)0.98FeO3-δ and dual phase composites of (La0.6Sr0.4)0.98FeO3-δ – Ce0.9Gd0.1O1.95 (LSF-CGO). From the relaxation curves, fitted by using Fick’s Laws of diffusion with appropriate boundary conditions, both the oxygen surface exchange coefficient (k ex) and the oxygen chemical diffusion coefficient (D chem) was derived. The results of ECR clearly show that the CGO surface is involved in the surface exchange reactions on the composite and that by increasing the fraction of CGO the surface exchange is significantly enhanced. The involvement of CGO in the surface exchange can arise from fast exchange in the triple phase boundaries and/or the spillover of oxygen ions from LSF to CGO, where they incorporate. Additional effects can also originate from the fact that LSF scavenges impurities from CGO and therefore activate the CGO surface for the surface exchange reaction as proposed by Druce et al. 2. This was separately studied by coating of the surface of the single phase LSF and the composite with different elements/oxides regarded as potential impurities. However, opposite to the trivial anticipation a strong positive influence of secondary phases (not normally considered to be good oxygen reduction catalysts) on the surface exchange coefficient was observed. For example depositing CaO/CaCO3 on the surface led to an enhancement of the oxygen incorporation rate for 3.4 times at 750 °C.The study concludes that the formation of a composite of MIEC and ionic conductor and controlled coating with a range of secondary phases on the surface (e.g. CaO, …) is highly beneficial for the oxygen reduction process.1. Hu, B.; Wang, Y.; Xia, C., J. Power Sources 2014, 269, 180-1882. Druce, J.; Kilner, J. A., J. Electrochem. Soc. 2014 , 161, F99-F104.

Experiments on the thermoelectric properties of quantum dots

Quantum dots (QDs) are good model systems for fundamental studies of mesoscopic transport phenomena using thermoelectric effects because of their small size, electrostatically tunable properties and thermoelectric response characteristics that are very sensitive to small thermal biases. Here we provide a review of experimental studies on thermoelectric properties of single QDs realized in two-dimensional electron gases, single-walled carbon nanotubes and semiconductor nanowires. A key requirement for such experiments is methods for nanoscale thermal biasing. We briefly review the main techniques used in the field, namely, heating of the QD contacts, side heating and top heating, and touch upon their relative advantages. The thermoelectric response of a QD as a function of gate potential has a characteristic oscillatory behavior with the same period as is observed for conductance peaks. Much of the existing literature focuses on the agreement between experiments and theory, particularly for amplitude and line-shape of the thermovoltage $V_{th}$. A general observation is that the widely used single-electron tunneling approximation for QDs has limited success in reproducing measured $V_{th}$. Landauer-type calculations are often found to describe measurement results better despite the large electron-electron interactions in QDs. More recently, nonlinear thermoelectric effects have moved into the focus of attention and we offer a brief overview of experiments done so far. We conclude by discussing open questions and avenues for future work, including the role of asymmetries in tunnel- and capacitive couplings in thermoelectric behavior of QDs.

Magnetically driven anisotropic structural changes in the atomic laminateMn2GaC

Inherently layered magnetic materials, such as magnetic ${M}_{n+1}A{X}_{n}$ (MAX) phases, offer an intriguing perspective for use in spintronics applications and as ideal model systems for fundamental studies of complex magnetic phenomena. The MAX phase composition ${{M}_{n}}_{+1}A{X}_{n}$ consists of ${M}_{n+1}{X}_{n}$ blocks separated by atomically thin $A$-layers where $M$ is a transition metal, $A$ an A-group element, $X$ refers to carbon and/or nitrogen, and $n$ is typically 1, 2, or 3. Here, we show that the recently discovered magnetic $\mathrm{M}{\mathrm{n}}_{2}\mathrm{GaC}$ MAX phase displays structural changes linked to the magnetic anisotropy, and a rich magnetic phase diagram which can be manipulated through temperature and magnetic field. Using first-principles calculations and Monte Carlo simulations, an essentially one-dimensional (1D) interlayer plethora of two-dimensioanl (2D) Mn-C-Mn trilayers with robust intralayer ferromagnetic spin coupling was revealed. The complex transitions between them were observed to induce magnetically driven anisotropic structural changes. The magnetic behavior as well as structural changes dependent on the temperature and applied magnetic field are explained by the large number of low energy, i.e., close to degenerate, collinear and noncollinear spin configurations that become accessible to the system with a change in volume. These results indicate that the magnetic state can be directly controlled by an applied pressure or through the introduction of stress and show promise for the use of $\mathrm{M}{\mathrm{n}}_{2}\mathrm{GaC}$ MAX phases in future magnetoelectric and magnetocaloric applications.

Heteroepitaxial hematite photoanodes as a model system for solar water splitting

Heteroepitaxial hematite thin film photoanodes were fabricated, characterized, and shown to be a model system for fundamental studies into solar water splitting.

Determination of the Nature of the Cu Coordination Complexes Formed in the Presence of NO and NH3 within SSZ-13

Ammonia-selective catalytic reduction (NH3-SCR) using Cu zeolites is a well-established strategy for the abatement of NOx gases. Recent studies have demonstrated that Cu is particularly active when exchanged into the SSZ-13 zeolite, and its location in either the 6r or 8r renders it an excellent model system for fundamental studies. In this work, we examine the interaction of NH3-SCR relevant gases (NO and NH3) with the Cu2+ centers within the SSZ-13 structure, coupling powder diffraction (PD), X-ray absorption spectroscopy (XAFS), and density functional theory (DFT). This combined approach revealed that, upon calcination, cooling and gas exposure Cu ions tend to locate in the 8r window. After NO introduction, Cu ions are seen to coordinate to two framework oxygens and one NO molecule, resulting in a bent Cu–nitrosyl complex with a Cu–N–O bond angle of ∼150°. Whilst Cu seems to be partially reduced/changed in coordination state, NO is partially oxidized. On exposure to NH3 while the PD data suggest the Cu...

Polymorph-Selective Preparation and Structural Characterization of Perylene Single Crystals

Organic semiconductors occurring in polymorphic structures represent excellent model systems for fundamental studies of optoelectronic excitations in different crystalline configurations. Perylene is an archetypal polycyclic aromatic hydrocarbon appearing in two polymorphs known as α- and β-phases which adopt different molecular packing motifs. However, the growth of high quality single crystals with appropriate sizes and polymorph selectivity remains challenging. In this study, we compare various approaches toward polymorph-selective perylene single-crystal growth. Though crystals of both polymorphs are obtained from toluene solution (either by cooling of a saturated solution or by evaporation of the solvents), they exhibit numerous defects and their size cannot be precisely controlled. Vapor deposition and resublimation favor the formation of α-crystals which can be rationalized by a newly identified thin-film phase that forms initially. Further, we demonstrate that organic molecular beam deposition ont...

Epitaxial Fe/Y2O3 interfaces as a model system for oxide-dispersion-strengthened ferritic alloys

The fundamental mechanisms underlying the superior radiation tolerance properties of oxide-dispersion-strengthened ferritic steels and nanostructured ferritic alloys are poorly understood. Thin film heterostructures of Fe/Y2O3 can serve as a model system for fundamental studies of radiation damage. Epitaxial thin films of Y2O3 were deposited by pulsed laser deposition on 8% Y:ZrO2 (YSZ) substrates with (100), (110), and (111) orientation. Metallic Fe was subsequently deposited by molecular beam epitaxy. Characterization by X-ray diffraction and Rutherford backscattering spectrometry in the channeling geometry revealed a degree of epitaxial or axiotaxial orientation for Fe(211) deposited on Y2O3(110)/YSZ(110). In contrast, Fe on Y2O3(111)/YSZ(111) was fully polycrystalline, and Fe on Y2O3(100)/YSZ(100) exhibited out-of-plane texture in the [110] direction with little or no preferential in-plane orientation. Scanning transmission electron microscopy imaging of Fe(211)/Y2O3(110)/YSZ(110) revealed a strongly islanded morphology for the Fe film, with no epitaxial grains visible in the cross-sectional sample. Well-ordered Fe grains with no orientation to the underlying Y2O3 were observed. Well-ordered crystallites of Fe with both epitaxial and non-epitaxial orientations on Y2O3 are a promising model system for fundamental studies of radiation damage phenomena. This is illustrated with preliminary results of He bubble formation following implantation with a helium ion microscope. He bubble formation is shown to preferentially occur at the Fe/Y2O3 interface.

Mechanistic Principles of Platinum Oxide Formation and Reduction

In polymer electrolyte fuel cells, the platinum catalyst in its active form is found predominantly in an oxidized state. Formation and reduction of surface oxide species determine both the electrocatalytic activity of the oxygen reduction reaction as well as the rate of corrosive Pt dissolution. Understanding of mechanisms and rates of oxide formation and reduction is therefore essential in view of both performance and durability. Pt(111) is the generic model system for fundamental studies in fuel cell electrochemistry and cyclic voltammetry at Pt(111) gives an unabated view of the oxide formation and reduction processes. The unresolved challenge is to develop an electrochemical kinetic model that allows the current response measured in cyclic voltammetry to be de-convoluted and interpreted in relation to independent spectroscopic, imaging and theoretical data. Accordingly, a kinetic model for Pt(111) oxide formation and reduction within the voltage range of 0.65–1.15 V is developed and evaluated against electrochemical, spectroscopic and computational studies. Considering the complexity of surface processes involved and the simplicity of the proposed model, the agreement with the extensive range of data is convincing. The model provides a comprehensive picture of surface electrochemical processes that occur at Pt(111) in the normal operational voltage range of the cathode catalyst for polymer electrolyte fuel cells in automotive applications.

Xenon in l-Alanyl-l-Valine Nanochannels: A Highly Ideal Molecular Single-File System

Xenon-129 pulsed field gradient NMR and hyperpolarized xenon-129 spin tracer exchange NMR experiments were performed on polycrystalline l-Ala-l-Val (AV) and l-Val-l-Ala (VA) nanochannels under identical conditions of temperature and Xe pressure. Displacements of up to 3 μm were measured in specimens with mean crystallite lengths of around 50–100 μm. The combination of the two NMR techniques yielded the most definitive evidence for molecular single file diffusion to date, in which the time-scaling of the mean squared displacement is proportional to the square-root of time. The PFG-NMR echo attenuation data yield single-file mobility factors of 6 ± 0.7 × 10–13 m2 s–1/2 and 4.4 ± 0.2 × 10–13 m2 s–1/2. The results establish Xe in AV nanochannels, in particular, as an ideal experimental model system for fundamental studies of single-file diffusion dynamics.

Spin coated titanium–ruthenium oxide thin films

Substrates of different roughness spin coated with Ti0.7Ru0.3O2 films have been evaluated as model system for fundamental studies of the industrially and scientifically interesting (Ti,Ru)O2 based electrodes. The approach allowed for much more accurate control over the material synthesis than the traditionally used brush-, dip-, or spray-coating, on titanium-metal substrates. It moreover yielded well-defined samples suitable for basic studies of the surface properties that are of fundamental importance for understanding the electrochemical functionality of the electrode. We have compared the films on silicon substrates to films prepared by spin coating the same material on titanium-metal substrates. Samples have been characterized using atomic force microscopy (AFM), X-ray diffraction, scanning electron microscopy (SEM), and cyclic voltammetry. The samples displayed a uniformity of the films appropriate for AFM characterization. The smoother the substrate the less cracks in the coating. Using easily broken silicon wafers as substrate, a straightforward sample preparation technique was demonstrated for cross-section SEM. In addition, using high spinning velocities we have deposited the oxide films directly on silicon-nitride grids, thin enough to allow for studies with transmission electron microscopy without further sample preparation.

High‐Performance Ambipolar Diketopyrrolopyrrole‐Thieno[3,2‐b]thiophene Copolymer Field‐Effect Transistors with Balanced Hole and Electron Mobilities

Ambipolar OFETs with balanced hole and electron field-effect mobilities both exceeding 1 cm(2) V(-1) s(-1) are achieved based on a single-solution-processed conjugated polymer, DPPT-TT, upon careful optimization of the device architecture, charge injection, and polymer processing. Such high-performance OFETs are promising for applications in ambipolar devices and integrated circuits, as well as model systems for fundamental studies.

Engineering the band structure of one-dimensional hypersonic phononic crystals

Phononic crystals—the mechanical analogues of photonic crystals—have attracted increasing interest and have been widely studied within the past decade. The phononic dispersion relation at hypersonic frequencies can be directly measured by the powerful non-destructive technique of high resolution spontaneous Brillouin-light-scattering (BLS) [W. Cheng et al., Nature Mater. 2006, 5, 830]. Due to the vector nature of the elastic wave propagation, theoretical phononic band structures can be uniquely verified at low dimensionality, and hence 1D phononic crystals constitute model systems for fundamental studies. Such hybrid Bragg stacks, composed of alternating layers of silica and poly(methyl methacrylate) (PMMA), respectively, exhibit clear hypersonic phononic band gaps [Gomopoulos et al., Nano Lett. 2010, 10, 980]. In this paper, we report on the fabrication, characterization, and both experimental and theoretical dispersion diagrams along and normal to the periodicity direction of silica/PMMA multilayers. The width of the gap, the phonon frequencies, and their intensities near the first Brillouin zone are sensitive probes of the longitudinal moduli and elasto-optic constants of the individual layers and structural parameters. Mixing with layer modes under oblique incidence conditions allows access to the shear moduli of the two layers.