Project

Discipline

nanotechnology; ab initio simulations; surface effects; transport ; organic nanowires; defects; 1D nanostructures

Lead
Lay summary
The development of experimental techniques capable of manipulation and analysis at the nanoscale has determined in the last years the enormous fortune of nanotechnology. One of the most fascinating aspects is the manifestation in realistic systems of theoretically predicted (but often unexpected) quantum effects that lead to remarkable and unusual properties of matter at the 10-100 nm scale. In particular, the technology for the fabrication of organic nanostructures designed to become microelectronic devices is now mature, and the concepts governing the growth and stability of nano-derivatives of molecular crystals are now accessible. The range of forecasted applications is broad: catalysis, solar cells, nanosensors, lasers, device miniaturization, etc. The choice of p-conjugated molecules as building blocks for such organic nanostructures is central in the definition of this emerging new field and has clear advantages: ease of synthesis at low cost and big scale, processing in solution and possibility to control the properties of the devices by molecular design. In our laboratory at Empa a methodology was recently developed to fabricate single-crystalline 1D nanostructures with high density, quality and purity. The growth method is based on a plasma vapor deposition process of organic molecules on tailored-roughness substrate surfaces.  Such substrate-supported nanowires are ideal systems for fundamental studies such as: understanding of complex self-assembly mechanisms for organic molecules into crystalline structures, elucidating transport mechanisms, addressing the role of nanoscale domains in microstructures, defining of the relationship between molecular/crystalline structure and corresponding electronic properties.  The generalization of this growth methodology to different molecules provides a straightforward way to study the relation between the chemical structure of the molecules and the final morphology and structure of the wires. atomistic simulations, in connection to the experiments that are being conducted in our laboratory. ab initio The aim of the present project is to understand the structural and electronic properties of selected nanowires belonging to different molecular precursors by means of The project will be focused towards two main goals: The investigation of the geometric and electronic properties of ideal models of nanowires based on Cu-phtalocyanines and Co-phtalocyanines. Understanding of the transport properties of the nanowires   The successful outcome of the present project will represent a considerable step forward in the understanding of the electronic properties of molecular nanowires. Although transport theories are developed since several years, due to their complexity in relation to systems such as molecular nanowires, there are no detailed investigations of the transport properties of similar systems. The results obtained within our project will represent a step forward in the understanding of complex transport phenomena. Moreover, the collaboration with developers of transport theory codes will allow to tune and develop future instruments, more suitable for application in direct connection with experiments.

Direct link to Lay Summary

Last update: 21.02.2013

Active participation

Title	Year

Number	Title	Start	Funding scheme

The development of experimental techniques capable of manipulation and analysis at the nanoscale has determined inthe last years the enormous fortune of nanotechnology. One of the most fascinating aspects is the manifestation in realistic systems of theoretically predicted (but often unexpected) quantumeffects that lead to remarkable and unusual properties of matter at the 10-100 nm scale.In particular, the technology for the fabrication of organic nanostructures designed to become microelectronic devices is now mature, and the concepts governing the growth and stability of nano-derivatives of molecular crystals are now accessible. The range of forecasted applications is broad: catalysis, solar cells, nanosensors, lasers, device miniaturization, etc. The choice of p-conjugated molecules as building blocks for such organic nanostructures is central in the definition of this emerging new field and has clear advantages: ease of synthesis at low cost and big scale, processing in solution and possibility to control the properties of the devices by molecular design. In our laboratory at Empa a methodology was recently developed to fabricate single-crystalline 1D nanostructures with high density, quality and purity. The growth method is based on a plasma vapor deposition process of organic molecules on tailored-roughness substrate surfaces. Such substrate-supported nanowires are ideal systems for fundamental studies such as: understanding of complex self-assembly mechanisms for organic molecules into crystalline structures, elucidating transport mechanisms, addressing the role of nanoscale domains in microstructures, defining of the relationship between molecular/crystalline structure and corresponding electronic properties. The generalization of this growth methodology to different molecules provides a straightforward way to study the relation between the chemical structure of the molecules and the final morphology and structure of the wires.The aim of the present project is to understand the structural and electronic properties of selected nanowires belonging to different molecular precursors by means of ab initio atomistic simulations, in connection to the experiments that are being conducted in our laboratory. The project will be focused towards two main goals:The investigation of the geometric and electronic properties of ideal models of nanowires based on Cu-phtalocyanines and Co-phtalocyanines. For the model geometries the starting point will be the data collected from experimental measurements; van der Waals corrections, based on the Grimme parameterization will be included in our model. For the electronic properties the role played by surface effects and possible defects (as suggested by experiments) in the molecular crystal will be discussed;The evaluation of transport properties in the band conductance regime and in the hopping regime. On the one hand, we will simply rely on the band structure determined from our first goal; on the other hand we will also compute transfer integrals and reorganization energies to be included in proper Monte Carlo simulations.The methodology developed in the present project will be of fundamental relevance for the future research activity of our laboratory in the field of molecular based nanostructures.Summary of the research tasks1)Characterization of the geometrical and electronic properties of MPcs crystals.For two representative systems of interest, given the crystalline structure obtained by XDS measurements, we will compare theoretical and measured equilibrium geometries, compute the band structure of the corresponding crystalline phase and the correct Hubbard parameter U. We will perform phonon calculations for isolated molecules and, if possible, evaluate the effects of molecule-molecule interactions on the phonon spectra.The outcome of this task will be of general relevance for the scientific community: the correct Hubbard parameters will represent a reference value for future calculations.The final objective of the first task will be the investigation of surface effects, confinement effects and the inclusion of defects in the bulk and at the surface of the model nanowires.2)Transport properties: evaluation of transfer integrals, reorganization energy and evaluation of carriers mobilityThe second task is devoted to characterize the transport properties of the systems investigated in task 1. Carriers mobility for band-like conduction mechanism will be evaluated from the band structure calculations completed in the first task. The second step will be the evaluation of quantities necessary to compute mobility in the hopping regime as requested by Marcus-Hush formula (Marcus 1956, Hush 1958): transfer integrals as well as the reorganization energywill be computed.The final step will be collection of all the information to compute the charge carriers mobility in the incoherent regime of conductivity via Monte Carlo approaches. The results obtained from this task should allow understanding the origin of the unexpected conductibility of the nanowires obtained at Empa. The methodology developed within the first two tasks will be fundamental for future applications to the graphene based devices engineered at Empa. 3)Carriers mobility for a macroscopic nanowireIn case that the first two tasks are completed successfully the final efforts will be devoted to compute stress effects on the electronic properties of the systems and to transfer the results to a realistic model for a nanowire. By means of large scale molecular dynamics simulations the stress field in a realistic model of nanowire will be computed. Integrating the mobility across the wire will provide information on the macroscopic properties of the system. The additional methodology that could be developed in this last task is extremely appealing in view of its transfer to other sys-tems that will be investigated at Empa.