Marek Oszajca

Synthesis and properties of ZnTe and ZnTe/ZnS core/shell semiconductor nanocrystals

We report the synthesis of spherical ZnTe nanocrystals and the successive coating with a ZnS shell to afford core/shell quantum dots. These nanocrystals can represent alternatives to cadmium-based quantum dots but their preparation and properties are challenging and relatively unexplored. The effect of various synthetic parameters on the reaction outcome was investigated, and the resulting nanocrystals were characterized by TEM, EDX, XPS, and spectroscopic measurements. The optical data indicate that these core/shell quantum dots belong to type I, i.e., both the electron and the hole are confined within the ZnTe core. Both the ZnTe core and ZnTe/ZnS core/shell quantum dot samples absorb in the visible region and are not luminescent. The ZnS shell preserves the optical properties of the core and improves the chemical and photochemical stability of the nanoparticles in air equilibrated solution, whereas they appear to be quite fragile in the solid state. XPS results have evidenced the distinct nature of core and core/shell QDs, confirming the formation of QDs with shells of different thicknesses and their evolution due to oxidation upon air exposure. Anodic photocurrent generation was observed when an ITO electrode functionalized with ZnTe/ZnS nanocrystals was irradiated in the visible region in a photoelectrochemical cell, indicating that the quantum dots perform spectral sensitization of the electron injection into the ITO electrode. Conversely, cathodic photocurrent generation was not observed; hence, the QD-modified electrode performs electrical rectification under a photon energy input.

Nanoparticles with logic and numeracy: towards 'computer-on-a-particle' optoelectronic devices

The review focuses on semiconductor nanoparticles and hybrid materials obtained by immobilisation of various molecular species on nanoparticulate semiconductors. These materials constitute unique systems combining collective properties of solids with structural diversity of molecules which show distinctive photoelectrochemical properties. Theoretical models of electronic interactions between molecules and semiconductor surfaces have been presented. Additionally, the review summarises the idea of small particles that can work as electronic devices. These devices are able to sense the environment and communicate with other devices and with the user. The devices are based on surface modified wide-band gap semiconductors and the photoelectrochemical photocurrent switching effect. This effect has created a new platform for novel chemical switches, logic gates and other information processing devices. The mechanism of photocurrent switching is discussed with respect to the type of surface complex-support interaction. Photoelectrochemical properties of multicomponent photoelectrodes based on wide band gap nanocrystaline semiconductors modified with various molecules were investigated. The review presents some examples of hybrid materials working as logic devices, including reconfigurable ones and simple arithmetic systems together with mechanistic problems related to nanoscale information processing.

Unconventional Computing Realized with Hybrid Materials Exhibiting the PhotoElectrochemical Photocurrent Switching (PEPS) Effect

Increasing demand for high computational power and high density memories enforces rapid development of microelectronic technologies. However, classical, silicon-based electronic elements cannot be miniaturized infinitely. Therefore, in order to sustain rapid development of information processing devices, new approaches towards future computing devices are needed. These approaches encompass either search for new material technologies or new information processing paradigms. In this chapter we present our contribution to the field including both approaches. We introduce classical, Boolean logic devices based on different materials and nanoscale implementations of ternary logic, fuzzy logic and neuromimetic computing.

Supramolecular assemblies of semiconductor quantum dots and a bis(bipyridinium) derivative: luminescence quenching and aggregation phenomena

We have synthesized CdSe and CdSe–ZnS core–shell luminescent nanocrystal quantum dots and studied their interaction with a ditopic bis(bipyridinium) compound in solution. The latter strongly quenches the luminescence of the quantum dots by a static mechanism, indicating that the nanocrystal and molecular components undergo association in the ground state. Photoexcitation of these inorganic–organic hybrids causes an electron-transfer process from the conduction band of the nanocrystal to the LUMO of the molecule. The ability of the bipyridinium-type species to trigger association of the quantum dots is evidenced by spectrofluorimetric titrations and DLS measurements in solution, and confirmed by TEM experiments on surfaces. The quantum dot–molecule complexes can be disassembled in solution by addition of a calixarene host capable of encapsulating the bipyridinium units of the molecular connector. Our results demonstrate that supramolecular chemistry offers convenient ways to control the aggregation of semiconductor nanocrystals, a crucial task for the generation of nanostructured arrays with well defined properties.

Unconventional molecular scale logic devices

Abstract: Semiconducting nanoparticles offer a versatile platform for various logic devices. Wide band gap semiconductors modified with molecular species are materials with unique optical and electronic properties. The most intriguing property of such systems is photoelectrochemical photocurrent switching (PEPS) effect. The polarity of photocurrent generated within these materials depends on many variables (light, electrode polarization, redox processes). Materials showing the PEPS effect can be used for construction of simple logic gates and other devices.