Spontaneous formation of highly ordered nanostructures: thermal instability and mode selection in surface-capped polymer films.

We demonstrate a controllable formation process of wave-like patterns in thermally unstable surface-capped polymer films on a rigid substrate. Self-ordered wave-like structures over a large area can be created by applying a small lateral tension to the film, whereupon it becomes unstable. A clear mode selection process which includes creation, decay and interference between coexisting waves at different annealing conditions has been observed, which makes it possible to restrain the patterns which are formed finally. Our results provide a clear and new evidence of spinodal behaviour in such a film due to thermal instability. Furthermore, we show that the well-controlled patterns generated in such a process can be used to fabricate nanostructures for various applications.

Quasi-hexagonal ordered arrays of FePt nanoparticles prepared by a micellar method

Hexagonally ordered arrays of magnetic FePt nanoparticles on Si substrates are prepared by a self assembly of diblock copolymer PS-b-P2VP in toluene, a dip coating process and finally plasma treatment. The as-treated FePt nanoparticles are covered by an oxide layer that can be removed by a 40 s Ar+ sputtering. The effects of the sequence of adding salts on the composition distribution are revealed by x-ray photoelectron spectroscopy measurements. No particle agglomeration is observed after 600 degrees C annealing for the present ordered array of FePt nanoparticles, which exhibits advantages in patterning FePt nanoparticles by a micellar method. Moreover, magnetic properties of the annealed FePt nanoparticles at room temperature are investigated by a vibrating sample magnetometer.

Ordered Zinc Antimonate Nanoisland Attachment and Morphology Control of ZnO Nanobelts by Sb Doping

Hierarchical heterostructures of zinc antimonate nanoislands on ZnO nanobelts were prepared by simple annealing of the polymeric precursor. Sb can promote the growth of ZnO nanobelts along the [552] direction because of the segregation of Sb dopants on the +(001) and (110) surfaces of ZnO nanobelts. Furthermore, the ordered nanoislands of toothlike ZnSb2O6 along the [001](ZnO) direction and rodlike Zn7Sb2O12 along the [110](ZnO) direction can be formed because of the match relation of the lattice and polar charges between ZnO and zinc antimonate. The incorporation of Sb in a ZnO lattice induces composition fluctuation, and the growth of zinc antimonate nanoislands on nanobelt sides induces interface fluctuation, resulting in dominance of the bound exciton transition in the room temperature near-band-edge (NBE) emission at relatively low excitation intensity. At high excitation intensity, however, Auger recombination makes photogenerated electrons release phonon and relax from the conduction band to the trap states, causing the NBE emission to gradually saturate and redshift with increasing excitation intensity. The green emission more reasonably originates from the recombination of electrons in shallow traps with doubly charged V-O** oxygen vacancies. Because a V-O** center can trap a photoactivated electron and change to a singly charged oxygen vacancy V-O* state, its emission intensity exhibits a maximum with increasing excitation intensity.

Optical properties of highly ordered AlN nanowire arrays grown on sapphire substrate

Highly ordered AlN nanowire arrays were synthesized via a simple physical vapor deposition method on sapphire substrate. The nanowires have an extremely sharp tip < 10 nm, with the average length around 3 mu m. Raman spectroscopy analysis on the AlN nanowire arrays revealed that the lifetime of the phonons is shorter than that in bulk AlN. The transmission spectra of the AlN nanowires showed a blueshift similar to 0.27 eV at the absorption edge with that of the bulk AlN, which is closely related to the small size of the nanowires. (c) 2005 American Institute of Physics.

Ordered InAs quantum dots with controllable periods grown on stripe-patterned GaAs substrates

GaAs (001) substrates are patterned by electron beam lithography and wet chemical etching to control the nucleation of InAs quantum dots (QDs). InAs dots are grown on the stripe-patterned substrates by solid source molecular beam epitaxy, A thick buffer layer is deposited on the strip pattern before the deposition of InAs. To enhance the surface diffusion length of the In atoms, InAs is deposited with low growth rate and low As pressure. The AFM images show that distinct one-dimensionally ordered InAs QDs with homogeneous size distribution are created, and the QDs preferentially nucleate along the trench. With the increasing amount of deposited InAs and the spacing of the trenches, a number of QDs are formed beside the trenches. The distribution of additional QDs is long-range ordered, always along the trenchs rather than across the spacing regions.

Enhanced infrared absorption of spatially ordered quantum dot arrays

We have investigated the intersubband absorption for spatially ordered and non-ordered quantum dots (QDs). It is found that the intersubband absorption of spatially ordered QDs is much stronger than that of non-ordered QDs. The enhanced absorption is attributed to the improved size uniformity concurrent with the spatial ordering for the growth condition employed. For the FTIR measurement under normal incidence geometry, using a undoped sample as reference can remove the interference effect due to multiple reflections. (c) 2006 Elsevier B.V. All rights reserved.

Preparation and AFM characterization of self-ordered porous alumina films on semi-insulated gaas substrate

Self-ordered porous alumina films on a semi-insulated GaAs substrate were prepared in oxalic acid aqueous solutions by three-step anodization. The I-t curve of anodization process was recorded to observe time effects of anodization. Atomic force microscopy was used to investigate structure and morphology of alumina films. It was revealed that the case of oxalic acid resulted in a self-ordered porous structure, with the pore diameters of 60-70 nm, the pore density of the order of about 10(10) pore cm(-2), and interpore distances of 95-100nm. At the same time the pore size and shape change with the pore widening time. Field-enhanced dissolution model and theory of deformation relaxation combined were brought forward to be the cause of self-ordered pore structure according to I-t curve of anodization and structure characteristics of porous alumina films. (c) 2006 Elsevier Ltd. All rights reserved.

Enhanced photoresponse from the ordered microstructure of naphthalocyanine-carbon nanotube composite film

Naphthalocyanine-sensitized multi-walled carbon nanotube (NaPc-MWNT) composites have been synthesized through the pi-stacking between naphthalocyanine (NaPc) and carbon nanotubes. The resultant nanocomposites were characterized with a scanning electron microscope (SEM), a transmission electron microscope (TEM), and by UV - vis absorption and photocurrent spectra. The long-range ordering was observed in the NaPc - MWNT composites by using a TEM. The enhancement in the absorption intensity and the broadening of the absorption wavelength observed in the composite films, which were due to the attachment of NaPc on the MWNT surface, is discussed based on the measured UV - vis absorption spectra. Furthermore, the photoconductivity of the poly( 3-hexylthiophene)(PAT6) - NaPc - MWNT composite film was found to increase remarkably in the visible region and broaden towards the red regions. These new phenomena were ascribed to the larger donor/acceptor (D/A) interface and the formation of a biconsecutive D/A network structure, as discussed in consideration of the photoinduced charge transfer between PAT6 and NaPc - MWNT.

Kinetic Monte Carlo simulation of spatially ordered growth of quantum dots on patterned substrate

We report the growth of well-ordered InAs QD chains by molecular beam epitaxy system. In order to analyze and extend the results of our experiment, a detailed kinetic Monte Carlo simulation is developed to investigate the effects of different growth conditions to the selective growth of InAs quantum dots (QDs). We find that growth temperature plays a more important role than growth rate in the spatial ordering of the QDs. We also investigate the effect of periodic stress on the shape of QDs in simulation. The simulation results are in good qualitative agreement with our experiment. (c) 2006 Elsevier Ltd. All rights reserved.

Corrugated surfaces formed on GaAs(331)A substrates: the template for laterally ordered InGaAs nanowires

Morphology evolution of high-index GaAs(331)A surfaces during molecular beam epitaxy (MBE) growth has been investigated in order to achieve regularly distributed step-array templates and fabricate spatially ordered low-dimensional nano-structures. Atomic force microscope (AFM) measurements have shown that the step height and terrace width of GaAs layers increase monotonically with increasing substrate temperature. By using the step arrays formed on GaAs(331)A surfaces as the templates, we have fabricated highly ordered InGaAs nanowires. The improved homogeneity and the increased density of the InGaAs nanowires are attributed to the modulated strain field caused by vertical multi-stacking, as well as the effect of corrugated surface of the template. Photoluminescence (PL) tests confirmed remarkable polarization anisotropy.

Lateral ordered InGaAs self-organized quantum dots grown on (311) GaAs by conventional molecular beam epitaxy

Self-assembled InxGa1-xAs quantum dots (QDs) on (311) and (100) GaAs surfaces have been grown by conventional solid source molecular beam epitaxy. Spontaneously ordering alignment of InxGa1-xAs QDs with lower In content around 0.3 has been observed on As-terminated (B type) surfaces. The direction of alignment orientation of the QDs formation differs from the direction of misorientation of the (311) B surface, and is strongly dependent upon the In content x. The ordering alignment becomes significantly deteriorated as the In content is increased to above 0.5 or as the QDs are formed on (100) and (311) Ga-terminated (A type) substrates.

Ordered InAs quantum dots in InAlAs matrix on (001) InP substrates grown by molecular beam epitaxy

InAs self-organized quantum dots in InAlAs matrix lattice-matched to exactly oriented (001) InP substrates were grown by solid source molecular beam epitaxy (MBE) using the Stranski-Krastanow mode. Preliminary characterizations have been performed using photoluminescence and transmission electron microscopy. The geometrical arrangement of the quantum dots is found to be strongly dependent on the amount of coverage. At low deposition thickness. InAs QDs are arranged in chains along [1(1) over bar0$] directions. Luminescence from the quantum dots and the wetting layer consisting of quantum wells with well widths of 1, 2, and 3 monolayers is observed. (C) 1998 American Institute of Physics.