Alignment Of Nanocrystals Research Articles

Alignment of Colloidal Graphene Quantum Dots on Polar Surfaces

Controlling the orientation of nanostructures with anisotropic shapes is essential for taking advantage of their anisotropic electrical, optical, and transport properties in electro-optical devices. For large-area alignment of nanocrystals, so far orientations are mostly induced and controlled by external physical parameters, such as applied fields or changes in concentration. Herein we report on assemblies of colloidal graphene quantum dots, a new type of disk-shaped nanostructures, on polar surfaces and the control of their orientations. We show that the orientations of the graphene quantum dots can be determined, either in- or out-of-plane with the substrate, by chemical functionalization that introduces orientation-dependent interactions between the quantum dots and the surfaces.

Mesoscopic Structures of Nanocrystals: Collective Magnetic Properties Due to the Alignment of Nanocrystals

Maghemite nanocrystals, deposited by slow evaporation on HOPG (highly oriented pyrolytic graphite) substrate, form mesoscopic structures which strongly depend on the nanocrystal coating. When the c...

One-Dimensional Ordering of Self-assembled Ge Dots on Photolithographically Patterned Structures on Si (001)

AbstractWe report spatial arrangement of self-assembled Ge quantum dots on patterned structures on Si (001) without using complicated lithography. Starting from stripes and mesas fabricated by conventional lithography and plasma etching, we prepare sinusoidal Si stripes with narrow ridges and mesas with humped edges via high-temperature annealing. Deposited Ge self-assembles into coherent nanocrystals that align along the narrow ridges of the stripes and the sloped mesa edges. Enhanced strain relief at the ridges due to elastic relaxation and the high step densities on the shallow slopes at the edges are likely causes of nanocrystal alignment.

Anisotropically Nanostructured Silicon as an Efficient Optical Retarder

Silicon (Si) is the materials basis for the majority of integrated electronic devices. However, a key limitation of its application for active and passive optical devices stems from its indirect band gap and highly isotropic cubic lattice structure. In this paper we demonstrate a new way of realisation of silicon-based optical retarders which can be integrated in optical communication lines. Lorentz has demonstrated that for cubic lattices only a weak dependence of the refractive index value (n) on the light wave vector direction exists [1]. For instance, in bulk Si the maximum value of birefringence for light propagating along the [110] direction was found to be extremely small: An = n [110] - n [100] = 5 x 10 -6 , where subscripts denote the direction of the electric field vector [2]. Obviously this value does not allow to perform silicon-based devices controlling of the polarisation state of light. Porosified (100) Si surfaces have isotropic in-plane optical properties due to the equivalence of their [010] and [001] crystallographic directions [3]. Preferential alignment of nanocrystals in [100] etching direction results in a larger dielectric constant for the light polarised along [100] direction [4,5]. Thus, birefringence can be observed for light propagating along the layer. However, for most of practical applications an in-plane birefringence is required. To achieve in-plane uniaxial symmetry we have employed the electrochemical etching of lower-symmetry (110) Si wafers. In Ref. [6], the selective crystallographic pore propagation and, respectively, alignment of nanocrystals in equivalent [010] and [100] directions tilted to the (110) surface plane has been demonstrated. According to effective medium predictions, the projection of those on the (110) plane ([110] direction) would result in an uniaxial surface symmetry with a dielectric constant e [110] being different from e [001] . Bulk Si is not an anisotropic crystal, but its porous modification becomes intrinsically uniaxial for this plane due to anisotropic dielectric nanostructuring. Since nanocrystals retain the diamond-like crystalline structure and their sizes (1 to 50 nm, depending on wafer and etching parameters used [7]) are much smaller than the wavelength of light, porous layers still can be considered as a continuous optical medium.