Synthesis, simulation & spectroscopy: physical chemistry of nanocrystals

Experiments on nanocrystalline semiconductors form a wide and rapidly expanding field of research. This chapter concentrates on two very different topics within this field. In the first part, pair formation of dopant ions in nanocrystals is discussed. After a general introduction on the influence of pair formation on the luminescence properties, pair formation in nanocrystals is discussed. Due to a difference between the connectivity for sites in the bulk and at the surface, the fraction of dopant pairs depends on the crystallite size. Simulations of the statistical distribution of dopant pair states in a nanocrystal as a function of crystal structure, size and dopant concentration are presented. A closed form approximation for the results of the simulations is derived and the validity is tested. The work presented can be used to estimate dopant pair concentrations in the case of random substitution or a lower limit for the pair concentration if preferential pair formation occurs. The second part of the chapter discusses the luminescence of a single nanocrystalline semiconductor particle. The absence of inhomogeneous broadening and other ensemble averaging effects has provided exciting new insight into the luminescence and quenching mechanisms. The linewidth, blue shift and bleaching of the luminescence of single CdSe/ZnS core/shell nanocrystals are shown and discussed. Finally, potential applications of nanocrystals as luminescent labels in biological systems are presented and a few challenges for future research are discussed

Probabilities for dopant pair-state formation in a nanocrystal: simulations and theory

For certain dopants, luminescence measurements allow one to distin- guish between single-ion and pair-state dopant emission in a (semicon- ductor) host. In a bulk crystal the concentration of each of these dopant- states can be calculated from the dopant fraction present in the material, and is found to correlate with luminescence measurements. However, for a nanocrystalline host-lattice, these concentrations cannot be calculated due to the difference in coordination numbers for ions at the surface (a substantial fraction in nanocrystals) and in the bulk. Here simulations of dopant pair-state distributions are presented for a zincblende nanocrystal. The probability of finding at least one pair-state in the nanocrystal and the percentage of dopants forming part of a pair-state were calculated on the basis of a statistical average of 1 . 105 simulations for the same crystal size and dopant concentration. Furthermore, the distribution of nanocrystal lattice positions over the surface and the bulk of the crystal are computed from the simulations and found to agree well with a first- order theory. Finally, a closed-form approximation of the probabilities (valid in any crystal lattice) and a rigorous upper bound for the error in the approximation are discussed

Luminescence of nanocrystalline ZnSe:Mn2+

The luminescence properties of nanocrystalline ZnSe:Mn^(2+) prepared via an inorganic chemical synthesis are described. Photoluminescence spectra show distinct ZnSe and Mn^(2+) related emissions, both of which are excited via the ZnSe host lattice. The Mn^(2+) emission wavelength and the associated luminescence decay time depend on the concentration of Mn2` incorporated in the ZnSe lattice. Temperature-dependent photoluminescence spectra and photoluminescence lifetime measurements are also presented and the results are compared with those of Mn^(2+) in bulk ZnSe

Synthesis and Photoluminescence of Nanocrystalline ZnS:Mn^(2+)

The influence of the synthesis conditions on the properties of nanocrystalline ZnS:Mn2+ is discussed. Different Mn2+ precursors and different ratios of the precursor concentrations [S2-]/[Zn2+] were used. The type of Mn2+ precursor does not have an effect on the luminescence properties in the synthesis method described. On going from an excess of [Zn2+] to an excess of [S2-] during the synthesis, the particle diameter increases from 3.7 to 5.1 nm, which is reflected by a change in the luminescence properties. Photoluminescence measurements also showed the absence of the ZnS defect luminescence around 450 nm when an excess [S2-] is used during the synthesis. This effect is explained by the filling of sulfur vacancies. The ZnS luminescence is quenched with an activation energy of 62 meV, which is assigned to the detrapping of a bound hole from such a vacancy

Photoelectrochemical Characterization of Nanocrystalline ZnS :Mn^(2+) Layers

Measurements of the photoelectrochemical properties of nanocrystalline ZnS electrodes doped with Mn^(2+) are presented and discussed. The observation of both anodic and cathodic photocurrent is direct evidence for the nanocrystalline nature of the system. In-situ photoluminescence measurements showed stable Mn^(2+) related photoluminescence over a large potential range. Due to the unfavourable kinetics of electron and hole transfer across the interface between the nanocrystallites and solution, it is concluded that recombination accounts for most of the charge carriers generated by illumination. Breakdown of the ZnS into elementary Zn and S^(2-) in solution was also observed at negative potential. This breakdown introduces new non-radiative decay paths and is responsible for the slow luminescence decrease as a function of operating time

Oxidation and annealing of thin FeTi layers covered with Pd

The hydrogen storage material FeTi has the disadvantage to lose its sorption capacity in contact with impurities such as O and H O. A possibility to overcome this problem is to coat it with an anti-corrosive layer which is permeable for hydrogen. In this study we prepared FeTi layers covered with a 4 or 20 nm thin Pd layer. We used ion beam and sputter profiling techniques, X-ray photoelectron spectrometry and scanning probe techniques to investigate the response of these bi-layers upon annealing up to 3008C in vacuum, air and 10y5 mbar O . The layered structure remains intact up to 150 °C. At 2008C in air and O , Fe and some Ti move towards the Pd surface where they form oxide regions. At higher temperatures thicker oxide regions, presumably along the Pd grains, are formed. These processes are more pronounced for the case of 4 nm Pd. A model is presented to explain the observed phenomena. We conclude that up to 1508C 4 nm of Pd is sufficient to act as a protective layer. For a temperature of 2008C, 20 nm Pd may still provide sufficient protection against oxidation

Optical and electrical doping of silicon with holmium

2 MeV holmium ions were implanted into Czochralski grown Si at a fluence of 5.5*10^14 Ho/cm^2. Some samples were co-implanted with oxygen to a concentration of (7±1)*10^19 cm^(-3). After recrystallization, strong Ho segregation to the surface is observed, which is fully suppressed by co-doping with O. After recrystallization, photoluminescence peaks are observed at 1.197, 1.96 and 2.06 lm, characteristic for the 5-I-6 --> 5-I-8 and 5-I-7 --> 5-I-8 transitions of Ho^(3+). The Ho^(3+) luminescence lifetime at 1.197 lm is 14 ms at 12 K. The luminescence intensity shows temperature quenching with an activation energy of 11 meV, both with and without O co-doping. The observed PL quenching cannot be explained by free carrier Auger quenching, but instead must be due to energy backtransfer or electron hole pair dissociation. Spreading resistance measurements indicate that Ho exhibits donor behavior, and that in the presence of O the free carrier concentration is enhanced by more than two orders of magnitude. In the O co-doped sample 20% of the Ho^(3+) was electrically active at room temperature

Energy backtransfer and infrared photoresponse in erbium-doped silicon p-n diodes

Temperature-dependent measurements of the photoluminescence (PL) intensity, PL lifetime, and infrared photocurrent, were performed on an erbium-implanted silicon p - n junction in order to investigate the energy transfer processes between the silicon electronic system and the Er 4 f energy levels. The device features excellent light trapping properties due to a textured front surface and a highly reflective rear surface. The PL intensity and PL lifetime measurements show weak temperature quenching of the erbium intra-4 f transition at 1.535 mm for temperatures up to 150 K, attributed to Auger energy transfer to free carriers. For higher temperatures, much stronger quenching is observed, which is attributed to an energy backtransfer process, in which Er deexcites by generation of a bound exciton at an Er-related trap. Dissociation of this exciton leads to the generation of electron-hole pairs that can be collected as a photocurrent. In addition, nonradiative recombination takes place at the trap. It is shown for the first time that all temperature-dependent data for PL intensity, PL lifetime, and photocurrent can be described using a single model. By fitting all temperature-dependent data simultaneously, we are able to extract the numerical values of the parameters that determine the ~temperature-dependent! energy transfer rates in erbium-doped silicon. While the external quantum efficiency of the photocurrent generation process is small (1.831026) due to the small erbium absorption cross section and the low erbium concentration, the conversion of Er excitations into free e - h pairs occurs with an efficiency of 70% at room temperature