Scanning probe microscopy and spectroscopy of colloidal semiconductor nanocrystals and assembled structures

Colloidal semiconductor nanocrystals become increasingly important in materials science and technology, due to their optoelectronic properties that are tunable by size. The measurement and understanding of their energy levels is key to scientific and technological progress. Here we review how the confined electronic orbitals and related energy levels of individual semiconductor quantum dots have been measured by means of scanning tunneling microscopy and spectroscopy. These techniques were originally developed for flat conducting surfaces, but they have been adapted to investigate the atomic and electronic structure of semiconductor quantum dots. We compare the results obtained on colloidal quantum dots with those on comparable solid-state ones. We also compare the results obtained with scanning tunneling spectroscopy with those of optical spectroscopy. The first three sections provide an introduction to colloidal quantum dots, and a theoretical basis to be able to understand tunneling spectroscopy on dots attached to a conducting surface. In sections 4 and 5 , we review the work performed on lead-chalcogenide nanocrystals and on colloidal quantum dots and rods of II-VI compounds, respectively. In section 6 , we deal with colloidal III-V nanocrystals and compare the results with their self-assembled counter parts. In section 7 , we review the work on other types of semiconductor quantum dots, especially on Si and Ge nanocrystals

Зимостойкость некоторых видов рода Sorbus L. в Донбассе

Изучены анатомические особенности годичных побегов Sorbus persica Hedl., S. domestica L. и S. aucuparia L. Определены толщина тканей и степень лигнификации ксилемы. Наиболее развиты покровные и механические ткани у S. aucuparia, годичные побеги полностью вызревшие. У S. persica наблюдается отставание в развитии тканей и степени одревеснения побегов по сравнению с S. aucuparia. Отмечена высокая зимостойкость S. domestica и S. aucuparia и более низкая – S. persica. К перспективным видам отнесены S. domestica и S. aucuparia. Вид – S. persica нецелесообразно вводить в ассортимент декоративных растений юго-востока Украины.The anatomical peculiarities of annual spears of Sorbus persica Hedl., S. domestica L. and S. aucuparia L. were studied. The tissue depth and xylem lignification rate were defined. The tissues of S. aucuparia are most developed and annual spears are fully mature for vegetative period. S. persica lags in terms of tissue development and spear lignification rate compared to S. aucuparia. S. domestica and S. aucuparia revealed winter resistance at high level, whereas S. persica demonstrated it at low level. S. domestica and S. aucuparia are treated as perspective, while S. persica is unreasonable for introduction to the assortment of decorative plants of south-east of Ukraine

Self-assembly of colloidal nanocrystals as route to novel classes of nanostructured materials

Colloidal crystallisation is the only way to obtain three-dimensional ordered materials in which semiconductor, metallic, and magnetic nanocrystals are in close contact. It is expected that the quantum mechanical and dipolar interactions between the nanocrystal units can lead to unseen physical phenomena and materials. Here we review the development of this new and exciting field. We first compare nanocrystal superlattices with regular atomic solids regarding their mechanical strength and opto-electronic properties. We describe how nanocrystal superlattices have been obtained from colloid suspensions in several ways. The thermodynamic driving force for colloidal crystallisation is discussed in terms of inter-particle interactions in a good solvent and entropy. We compare the binary superlattices that have been obtained by solvent evaporation with the predictions of the hard-sphere model and show that semiconductor nanocrystals in a good solvent can behave as hard spheres. Finally, we discuss the quantum mechanical and dipolar interactions in nanocrystal superlattices and review recent studies of the opto-electronic and magnetic properties of novel superlattice materials

Nanoelectronics: from droplets to devices

Single-electron behaviour has been observed in devices that can be made by simply trapping gold nanoparticles between two droplets of liquid metal

Self-assembly of colloidal nanocrystals as route to novel classes of nanostructured materials

Colloidal crystallisation is the only way to obtain three-dimensional ordered materials in which semiconductor, metallic, and magnetic nanocrystals are in close contact. It is expected that the quantum mechanical and dipolar interactions between the nanocrystal units can lead to unseen physical phenomena and materials. Here we review the development of this new and exciting field. We first compare nanocrystal superlattices with regular atomic solids regarding their mechanical strength and opto-electronic properties. We describe how nanocrystal superlattices have been obtained from colloid suspensions in several ways. The thermodynamic driving force for colloidal crystallisation is discussed in terms of inter-particle interactions in a good solvent and entropy. We compare the binary superlattices that have been obtained by solvent evaporation with the predictions of the hard-sphere model and show that semiconductor nanocrystals in a good solvent can behave as hard spheres. Finally, we discuss the quantum mechanical and dipolar interactions in nanocrystal superlattices and review recent studies of the opto-electronic and magnetic properties of novel superlattice materials

Reappraisal of variable-range hopping in quantum-dot solids

The temperature dependence of the electrical conductivity of assemblies of ZnO nanocrystals, studied with an electrochemically gated transistor is very accurately described by the relation ln σ = ln σ0 − (T0/T)x with x = 2/3 over the entire temperature range from 7 to 200 K, independent of charge concentration and dielectric environment. These results cannot be explained by existing models but are supported by results on Au nanocrystals where an identical temperature dependence was observed (Zabet-Khosousi et al., Phys. Rev. Lett. 2006, 96 (15), 156403). We propose an adaptation of the Efros−Shklovskii variable-range hopping model by introducing an expression for nonresonant tunneling based on local energy fluctuations, which yields exactly the temperature dependence that is observed experimentally