Electric-Field-Assisted Assembly of Perpendicularly Oriented Nanorod Superlattices

Abstract

We observe the assembly of CdS nanorod superlattices by the combination of a DC electric field and solvent evaporation. In each electric field (1 V/um) assisted assembly, CdS nanorods (5 X 30 nm) suspended initially in toluene were observed to align perpendicularly to the substrate. Azimuthal alignment along the nanorod crystal faces and the presence of stacking faults indicate that both 2D and 3D assemblies were formed by a process of controlled super crystal growth.KEYWORDS Nanorod, Superlattice, assembly 2 Self assembly or directed assembly of discrete nanostructures into organized patterns provides a new route to the formation of functional materials. Colloidal nanocrystals are suitable building blocks as they can be synthesized with size and shape control. 1 The assembly of symmetrical nanospheres and nanocubes into superlattices is known; and in the case of silver nanocrystals, 3 nm in diameter, an insulator to metal transition is observed to occur as a function of sphere size and interparticle separation. [2][3][4] In effect, the superlattice functions as a novel nanocrystal solid where it is possible to control the electronic coupling by manipulating the size and position of the quantum confined structural units. The coupling can further be modified through exchange of the insulating organic ligands on the nanocrystal surface with low barrier organics e.g. hydrazine. Talapin and Murray used this approach to convert poorly conducting PbSe nanocrystal solids into n-and p-channel field effect transistors. 5 The ability to direct anisotropic structures such as cylindrical nanorods into superlattices is less well developed but also interesting. In organized nanorod superlattices, it may be possible to simultaneously and independently optimize quantities which depend on the diameter (such as band gap) from quantities which depend on length (total absorption, cross section or conductivity).Superlattice formation with spherical nanocrystals is strongly correlated to size monodispersity and their entropy driven packing under slow evaporation conditions. While dimensional control and monodispersity in nanorods has been achieved, their organization into superlattices is restricted as both positional and orientational ordering is required during assembly. Some progress has been made in preferred nanorod alignment in single layers with nematic and semectic ordering achieved from gentle evaporation of low boiling point solvents. [6][7][8][9] The rods align parallel to substrate in small domain sizes.There is further evidence for the preferred orientation of anisotropic nanostructured-rods, tubes and wires along electric field lines. The strength of the interaction is greatest in metallic nanostructures followed by nanostructures with permanent dipole moments e.g. CdSe, CdS nanorods. 10,11 Anisotropic structures with low polarizability such as silicon nanowires and carbon nanotubes can also be induced to align when the electric field induced torque is greater than the thermal excitation energy (kT). 12,13 The ...

A multi-rate kinetic model for spontaneous oriented attachment of CdS nanorods

“…The rods have been shown to have considerable electric dipole moments, with the dipole moment, μ, proportional to the rod length. 28,29 We have, therefore, that …”

Interfacial liquid-state surface-enhanced Raman spectroscopy

“…O riented assemblies of functional colloidal nanoparticles [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] are important for both fundamental science and technological applications. Such assemblies have been realized on two-dimensional (2D) solid substrates by utilizing evaporation-based convection 6,7 , capillary interaction 8 , electric fields 9,10 , substrate templating 11,12 and surface tailoring of nanoparticles 13,14 .…”

“…Such assemblies have been realized on two-dimensional (2D) solid substrates by utilizing evaporation-based convection 6,7 , capillary interaction 8 , electric fields 9,10 , substrate templating 11,12 and surface tailoring of nanoparticles 13,14 . However, robust control over the orientation and position of nanoparticles in 3D solutions remains a formidable challenge, owing to thermal fluctuations and diffusion.…”

Water Dispersible Semiconductor Nanorod Assemblies Via a Facile Phase Transfer and Their Application as Fluorescent Biomarkers

“…This effect parallels that observed from nanorods assembled on a substrate where supercrystallisation was evident with the nanorods finding their preferred place on the growing assembly before locking in thus allowing the lowest energy consideration to be reached. 11,12,15 Clearly, the similar formation of highly ordered nanorod discs from randomly dispersed nanorods in solution suggests a similar phenomenon is occurring on phase transfer. Particle size analysis, optical microscopy ( Fig.…”

“…[3][4][5][6][7][8][9][10] Nanorods have been directed into perpendicularly ordered assemblies either by using external electric fields or by evaporation control at solid/liquid or liquid/air interfaces. [11][12][13][14] The aspect ratio and polydispersity of the rods in addition to solvent-evaporation dynamics plays a key role in achieving vertical alignment and close packing from solution. In effect, the gradual packing of randomly dispersed nanorods into a reducing solvent volume allows attractive dipole-dipole and Van der Walls forces between ligand capped nanorods to dominate the assembly behaviour at a substrate.…”