Marian Tzolov

Size-tunable infrared (1000–1600 nm) electroluminescence from PbS quantum-dot nanocrystals in a semiconducting polymer

Nanocomposites consisting of PbS nanocrystals in a conjugated polymer matrix were fabricated. We report results of photo- and electroluminescence across the range of 1000 to 1600 nm with tunability obtained via the quantum-size effect. The intensity of electroluminescence reached values corresponding to an internal quantum efficiency up to 1.2%. We discuss the impact of using different-length capping ligands on the transfer of excitations from polymer matrix to nanocrystals.

Infrared absorption properties of carbon nanotubes synthesized by chemical vapor deposition

We present an infrared (IR) optical absorbance study of highly uniform nanotubes grown by chemical vapor deposition in the self-assembled porous matrix in alumina. For unambiguous IR spectral measurement, nanotubes were extracted from their growth template, purified, and evenly dispersed on a reflecting substrate. The findings, which are consistent with previous results from conduction studies, reveal that the nanotubes are semiconducting with a band gap of ∼100meV. This suggests the potential of nanotube arrays for IR electro-optical device applications.

Quantum dots in a metallopolymer host: studies of composites of polyferrocenes and CdSe nanocrystals

The photoluminescence (PL) of composite materials based on ferrocene-based organometallic polymers and CdSe nanocrystals (nc-CdSe) has been investigated in solution and in thin films. The polymers studied, poly(ferrocenylmethylphenylsilane) (PFMPS), poly(ferrocenylphenylphosphine) (PFP), and poly(ferrocenylphenylphosphine sulfide) (PFP-S), all quench the PL of the nc-CdSe. In solution, the relative quenching effects are solvent dependent, and are in the order PFMPS < PFP in toluene but in the order PFP < PFP-S < PFMPS in THF. Stern–Volmer analysis is consistent with coordination of PFP to the nc-CdSe. In the thin films, the relative quenching strength of the polymers is PFMPS ∼ PFP-S < PFP. Photoinduced absorption spectroscopy was employed to study the nature of the PL quenching.

Optical control over photoconductivity in polyferrocenylsilane films

We report the study and elucidate the origin of the photoconductivity of polyferrocenylsilanes achieved through photooxidation performed by ultraviolet irradiation in the presence of chloroform. The persistence over months of the changes in the optoelectronic properties allowed more detailed studies of the charge photogeneration process. The photocurrent spectrum mimics that of the absorption indicating that the photooxidized material is not a mechanical mixture of oxidized and unoxidized polymer units. Photomodulation spectroscopy revealed the existence of long-lived photoexcited states with a lifetime in the millisecond range. They have been interpreted as trapped excitons at the oxidized sites where the polymer is deformed due to the presence of the chloroform derived counter ions. Because of the relatively long lifetime of the trapped excitons they can dissociate and the formed charge carriers can be separated in an externally applied electric field. The effect of the polymer chain deformation around the oxidized unit extends over the neighboring polymer units. In light of the potential applications of this class of polymers in various electronic and photonic devices, the clarification of such a basic process as the photocurrent generation will be a key factor for further technological development.

Fabrication of highly ordered anodic aluminium oxide templates on silicon substrates

The controlled fabrication of highly ordered anodic aluminium oxide (AAO) templates of unprecedented pore uniformity directly on Si, enabled by new advances on two fronts - direct and timed anodisation of a high-purity Al film of unprecedented thickness (50 mum) on Si, and anodising a thin but pre-textured Al film on Si, has been reported. To deposit high-quality and ultra-thick Al on a non-compliant substrate, a prerequisite for obtaining highly ordered pore arrays on Si by self-organisation while retaining a good adhesion, a specially designed process of e-beam evaporation followed by in situ annealing has been deployed. To obtain an AAO template with the same high degree of ordering and uniformity but from a thin Al film, which is not achievable by the self-organisation alone, pre-patterning of the thin Al surface by reactive ion etching using a freestanding AAO mask that was formed in a separate process was performed. The resultant AAO/Si template provides a good platform for integrated growth of nanotube, nanowire or nanodot arrays on Si. Template-assisted growth of carbon nanotubes (CNTs) directly on Si was demonstrated via a chemical vapour deposition method. By controllably removing the AAO barrier layer at the bottom of the pores and partially etching back the AAO top surface, new CNT/Si structures were obtained with potential applications in field emitters, sensors, oscillators and photodetectors.

Photocurrent response of the carbon nanotube-silicon heterojunction array

A highly ordered array of parallel, identical carbon nanotubes is grown non-lithographically in a bottom-up fabrication approach to form a heterojunction with a silicon substrate. Evidence of a space-charge separated region at the nanotube–silicon interface is present in the form of diode rectification and a closed-circuit zero-bias photocurrent in response to infrared light. Because carbon nanotubes are narrow bandgap semiconductors, their heterojunction with silicon was analysed spectrally via Fourier transform infrared photocurrent spectroscopy with the aim of investigating the suitability of this structure for infrared (IR) detector applications. IR photoresponse shows signs of temperature-dependent activation that is complex but consistent with estimates of the heterojunction barrier height. Considering the many interesting benefits and properties of carbon nanotubes, these results despite their earliness suggest that nanotube–silicon heterojunction systems could form the foundation for a new kind of infrared detection device.

Control over exciton confinement versus separation in composite films of polyfluorene and CdSe nanocrystals

Composite films of polyfluorene derivative poly(9,9-di-(2-ethylhexyl)-fluorenyl-2,7-diyl) and cadmium selenide nanocrystals were investigated using photomodulation spectroscopy exciting only the nanocrystal phase. Efficient charge separation resulting in hole injection into the polymer was observed in films in which the nanocrystals had been stripped of surface trioctylphosphine oxide passivating groups. The resulting induced absorption band centered at 1.2 eV was assigned to bipolarons or π-dimers formed on the polymer in the near vicinity of the polymer/nanocrystal interface. The intensity dependence of this band suggests bimolecular recombination, supporting the interpretation of the band as due to charge separation. The measured wide distribution of lifetimes for the photogenerated states confirms the glassy nature of the polymer.

Arrayed carbon nanotube infrared properties and potential applications

Carbon nanotubes are found to have versatile properties ranging from exceptional mechanical strength to semiconductor behavior with varying band gap, ranging from 0 eV to ~1 eV. In this work, we have explored the applicability of the carbon nanotubes for optical and IR sensing. Our platform is a vertical integration of highly uniform carbon nanotube arrays with silicon, forming a heterojunction structure. The heterojunction structure exhibits very good current rectification, voltage dependent capacitance and photocurrent response, all suggestive of an electronic diode function. However, we found, that the photogeneration in the first generation test devices is so far dominated by the silicon part of the heterojunction. Of the possible reasons for the elusive mid-IR photocurrent expected in the carbon nanotube is the presence of a thin barrier layer at the heterojunction. Further optimization of the devices is possible by modifying the technology to avoid the barrier layer formation and by improving the quality of the aluminum oxide matrix.

Hybrid nanostructures for mid-infrared to near-infrared detection

We will present our advance in the utilization of a non-lithographic approach for formation of periodic nanosized arrays and formation of hybrid structures suitable for light detection. We explore a self-organization process for formation of periodical nanopores in anodized aluminum oxide, the transfer of this pattern, and the subsequent growth within the pores. This approach was successfully demonstrated for a system having carbon nanotubes as kernel. The carbon nanotubes by themselves are very attractive for detector applications. It is theoretically predicted and experimentally proven that their band gap is adjustable in broad spectral range, their charge carrier mobility is high, and their thermal and mechanical properties are unmatched by other materials. The nanotemplate we use for growth of the nanotubes allows their controlled placement in a regular array, without restriction of the curvature of the surface to be covered. There are no principal limitations for scaling of the process. The third element of our approach is the integration with silicon which provides the compatibility with the well elaborated silicon technology. We will demonstrate the suitability of these structures for light detection.