Hoichang Yang

Selective Gas Transport Through Few-Layered Graphene and Graphene Oxide Membranes

Gas Separations When gas separation membranes are made thinner, they usually allow permeating gases to pass through faster. However, a thinner membrane may be poorer at separating between gas species. Kim et al. (p. 91) examined the permeability and selectivity of layered graphene and graphene oxide membranes. Gas molecules diffuse through defective pores and channels that form between the layers. Controlling these structures tuned the properties of the membranes to allow the extraction of carbon dioxide from other gases. Li et al. (p. 95) describe membranes as thin as 1.8 nanometers made from only two to three layers of graphene oxide. Small defects within the layers allowed hydrogen to pass through, separating it from carbon dioxide and nitrogen. Stacked graphene and graphene oxide membranes prepared with gas flow channels exhibit tunable gas separation performance. Graphene is a distinct two-dimensional material that offers a wide range of opportunities for membrane applications because of ultimate thinness, flexibility, chemical stability, and mechanical strength. We demonstrate that few- and several-layered graphene and graphene oxide (GO) sheets can be engineered to exhibit the desired gas separation characteristics. Selective gas diffusion can be achieved by controlling gas flow channels and pores via different stacking methods. For layered (3- to 10-nanometer) GO membranes, tunable gas transport behavior was strongly dependent on the degree of interlocking within the GO stacking structure. High carbon dioxide/nitrogen selectivity was achieved by well-interlocked GO membranes in high relative humidity, which is most suitable for postcombustion carbon dioxide capture processes, including a humidified feed stream.

Silicon Atom Substitution Enhances Interchain Packing in a Thiophene‐Based Polymer System

A new silole-containing low bandgap polymer is synthesized by replacing the 5-position carbon of PCPDTBT with a silicon atom (PSBTBT). Through experiments and computational calculations, we show that the material properties, particular the packing of polymer chains, can be altered significantly. As a result, the polymer changes from amorphous to highly crystalline with the replacement of the silicon atom.

Control of the nanoscale crystallinity and phase separation in polymer solar cells

Grazing-incidence x-ray diffraction and atomic force microscopy were performed on bulk heterojunction regioregular poly(3-hexylthiophene) (RR-P3HT) [6,6]-phenyl-C71-butyric acid methyl esters spin-cast films with different film processing conditions to correlate the crystalline nanostructure of P3HT with the corresponding solar cell performance. The increase in long wavelength absorption for solvent annealed films is related to highly conjugated crystal structure of RR-P3HT phase-separated in the active layer. Upon thermal annealing, the solvent annealed 50-nm-thick device shows high solar cell performance with fill factor up to 73% and power conversion efficiency of 3.80%.

Large-scale organic nanowire lithography and electronics

Controlled alignment and patterning of individual semiconducting nanowires at a desired position in a large area is a key requirement for electronic device applications. High-speed, large-area printing of highly aligned individual nanowires that allows control of the exact numbers of wires, and their orientations and dimensions is a significant challenge for practical electronics applications. Here we use a high-speed electrohydrodynamic organic nanowire printer to print large-area organic semiconducting nanowire arrays directly on device substrates in a precisely, individually controlled manner; this method also enables sophisticated large-area nanowire lithography for nano-electronics. We achieve a maximum field-effect mobility up to 9.7 cm(2) V(-1) s(-1) with extremely low contact resistance (<5.53 Ω cm), even in nano-channel transistors based on single-stranded semiconducting nanowires. We also demonstrate complementary inverter circuit arrays comprising well-aligned p-type and n-type organic semiconducting nanowires. Extremely fast nanolithography using printed semiconducting nanowire arrays provide a simple, reliable method of fabricating large-area and flexible nano-electronics.

Solubility-driven thin film structures of regioregular poly(3-hexyl thiophene) using volatile solvents

Using atomic force microscopy and grazing-incidence x-ray diffraction, the authors found that the use of volatile CH2Cl2 not only offers an advantage in minimizing regioregular poly(3-hexyl thiophene) (RR P3HT) film deposition time, but also directs the desirable parallel orientation of π-π stacking planes of RR P3HT with respect to solid substrates, for both spin and drop castings. The substrate temperature effects have been investigated to support claims that the substrate-induced crystallization of RR P3HT preseeds the parallel oriented crystals prior to spin casting, when the warm CH2Cl2 solution is loaded on a “cold” substrate held at room temperature.

Efficient Visible Quasi‐2D Perovskite Light‐Emitting Diodes

Efficient quasi-2D-structure perovskite light-emitting diodes (4.90 cd A(-1) ) are demonstrated by mixing a 3D-structured perovskite material (methyl ammonium lead bromide) and a 2D-structured perovskite material (phenylethyl ammonium lead bromide), which can be ascribed to better film uniformity, enhanced exciton confinement, and reduced trap density.

Diffraction from rough surfaces and dynamic growth fronts

Designed both for experimentalists who study rough surfaces and the dynamics of thin film growth using diffraction techniques and for theorists who wish to learn of such rough surfaces and dynamic behavior in Fourier space, this monograph quickly brings the readers to forefront research in the area of the dynamics of interface growth. Graduate and advanced undergraduate students as well as those readers who have very little prior knowledge of diffraction can pick up the subject matter with little difficulty.This monograph gives a brief review and summary at the end of each chapter. After the introduction of the elementary theory of diffraction in Chapter I, Chapter II discusses the various parameters and correlation functions that are essential in describing a rough surface. In Chapter III, the authors not only show analytical forms of the diffraction structure factor for both rough crystalline and non-crystalline surfaces, but also outline the methods of extracting the interface width, the lateral correlation length and the roughness parameter from the diffraction structure factor. To present the basic physical concepts underlying the scaling hypothesis during dynamic growth, in Chapter IV, a detailed description of the dynamic scaling properties of the height-height correlation function, the height difference function and the structure factor is given. The structure factor from a dynamic growth front is derived in Chapter V. An example of a quantitative measurement of the dynamic growth front of an epitaxial system is also given in this chapter. In Chapter VI, a particular type of rough surfaces having a diverging interface width associated with an equilibrium surface roughening transition is discussed. A comparison of the diffraction characteristics from divergent and non-divergent interface is also summarized.

Nano‐tailoring the Surface Structure for the Monolithic High‐Performance Antireflection Polymer Film

Both single-layer and multilayer antirefl ection (AR) coatings between a bulk substrate and air have been used extensively to reduce surface refl ection. These coating methods utilize the destructive interference between the lights refl ected from the interfaces to minimize the total intensity of the refl ected light. [ 1 , 2 ] However, the AR characteristics obtained with the coating methods can only be observed for a limited wavelength range and for an incidence angle close to the normal incidence. These drawbacks of the conventional coating methods can be suffi ciently overcome either by the graded refractive index (RI) layers method or by the antirefl ective structure (ARS) method. [ 3 , 4 ] The graded RI layers method and ARS method utilize optical impedance matching at two interfaces at air and substrate by making the RI of the AR layer vary gradually from the RI of air(n o ) to that of the substrate(n sub ) along the normal direction. In contrast with the coating methods, the graded RI layers method and ARS method were theoretically predicted to show broadband AR behavior of up to one order of magnitude and omnidirectional AR for an incidence angle of up to 80 degrees. [ 3 ]

Real time in situ X-ray diffraction studies on the melting memory effect in the crystallization of β-isotactic polypropylene

The melting memory effect during the crystallization and heating of semi-crystalline polymers was clearly demonstrated using β-isotactic polypropylene (β-iPP). Differential scanning calorimetry and real-time in situ X-ray diffraction using a synchrotron radiation source were employed to investigate the role of the newly formed α-form crystals via phase transformation from the metastable β-form during the melting process, and to elucidate the memory effect of these new α-form crystals during the crystallization process. The evolution of the memory effect in β-iPP during the crystallization and melting processes is ideally based on the existence of locally ordered α-form in the melt. We monitored the role of this local order by preparing the melt state using a range of hold temperatures and hold times. It was found that the final melt temperature and hold time greatly affect the crystallization behavior during cooling and the phase transformation behavior during heating. Lower hold temperatures and shorter hold times lead to samples rich in α-modification, whereas longer hold times generate samples rich in β-modification during crystallization. At higher hold temperatures even a short hold time is sufficient to destroy the local order in the melt, and the resulting sample exhibits more β-modification. The results are explained on the basis of the existence of local order in the amorphous melt along with external nucleating agent during the crystallization process.

Density control of single-walled carbon nanotubes using patterned iron nanoparticle catalysts derived from phase-separated thin films of a polyferrocene block copolymer

Single-walled carbon nanotube (SWNT) density and bundle size has been controlled by a simple one step CVD growth process using a polyferrocenylsilane block copolymer as the pre-organized catalyst source.