Krzysztof Nauka

Crystal structure of κ-In2Se3

Structural properties of single-phase films of {kappa}-In{sub 2}Se{sub 3} and {gamma}-In{sub 2}Se{sub 3} were investigated. Both films were polycrystalline but their microstructure differed considerably. The a-lattice parameter of {kappa}-In{sub 2}Se{sub 3} has been measured. Comparison between these two materials indicates that {kappa}-In{sub 2}Se{sub 3} has a significantly larger unit cell ({Delta}c = 2.5 {+-} 0.2 % and {Delta}a = 13.5 {+-} 0.5%) and a structure more similar to the {alpha}-phase of In{sub 2}Se{sub 3}.

Structural and electronic properties of amorphous and polycrystalline In2Se3 films

Structural and electronic properties of amorphous and single-phase polycrystalline films of γ- and κ-In2Se3 have been measured. The effect of deposition conditions on the film phase was studied extensively. The stable γ phase nucleates homogeneously in the film bulk and has a high resistivity, while the metastable κ phase nucleates at the film surface and has a moderate resistivity. The microstructures of polycrystalline hot-deposited and postannealed, cold-deposited γ films are quite different but their electronic properties are similar. The increase in the resistivity of amorphous In2Se3 films upon annealing is interpreted in terms of the replacement of In–In bonds with In–Se bonds during crystallization. Great care must be taken in the preparation of In2Se3 films for electrical measurements as the presence of excess chalcogen or surface oxidation may greatly affect the film properties.

Multilayer structured polymer light emitting diodes with cross-linked polymer matrices

Currently, there is great interest in manufacturing multilayer polymer light emitting diode (PLED) structures via low-cost solution-based spin-casting or printing methods. The difficulty with this approach is that solvent from freshly deposited films often dissolves the underlying layers. This letter demonstrates that fully operational multilayer PLED structures can be fabricated via a solution process by embedding the hole transport material in cross-linked inert polymer matrices that protect the functional material while subsequent layers are deposited using the same solvent. The resulting devices exhibited greatly improved quantum efficiency compared with devices that did not employ cross-linked polymer matrices.

Electron-beam detection of bits reversibly recorded on epitaxial InSe/GaSe/Si phase-change diodes

We demonstrate a data read-back scheme based on electron-beam induced current in a data storage device that utilizes thermal recording onto a phase-change medium. The phase-change medium is part of a heterojunction diode whose local charge-collection efficiency depends on the crystalline or amorphous state of a bit. Current gains up to 65 at 2 keV electron beam energy have been demonstrated using InSe/GaSe/Si epitaxial diodes. Fifteen write–erase cycles are obtained without loss of signal contrast by using a protective cap layer and short write pulses. 100 write–erase cycles have been achieved with some loss of contrast. Erasure times for the bits are longer than in similar polycrystalline In–Se media films. Possible reasons for the long erasure times are discussed in terms of a nucleation- or growth-dominated recrystallization. Prospects for extension to smaller bit sizes using electron-beam writing are considered.

Interpretation of the C1s XPS Signal in Copper Phthalocyanine for Organic Photovoltaic Device Applications

Copper phthalocyanine (CuPc) belongs to a class of small molecules offering particularly interesting advantages when employed in organic electronic devices. Because of its advantageous attributes like high thermal stability, inertness when exposed to acids or alkalis, relatively high electron conductivity, color and light fastness it has been employed in polymer photovoltaic devices as a unipolar dopants complementing the buckminsterfullerene (C60) acceptors and as a conductive buffer. Other organic applications include ambipolar OFETs and non-linear optics structures. X-ray photoelectron spectroscopy (XPS) has been commonly employed to monitor the quality of thin CuPc films. Although XPS analyses of CuPc have been done for over forty years there has not yet been agreement regarding interpretation of the major C1s signal, particularly in the case of non-stoichometric CuPc composition. This work presents systematic studies of the C1s signal of thin film deposits, fabricated using commercially available CuPc materials. It was found that composite C1s CuPc signal consists of five components: two related to the principal C positions within the CuPc macrocycle (C-C in 6-membered ring, C-C-N in 5-membered ring), two associated with shake-up transitions accompanying principal C transitions, and one due to mostly aliphatic impurities. Detailed analysis showed that the magnitude of shake-up peaks was approximately equal 10% to 12% of their principal transitions, in agreement with the theoretical calculations. Correspondingly, the C1s signal originating from the non-CuPc impurities quantitatively agreed with the IR attenuated total reflectance (ATR-IR) measurement of the C-H aliphatic vibrations originating from these impurities present within the CuPc layer. The proposed C1s interpretation has been successfully tested for a large number of commercial CuPc materials and provides a guideline for a routine XPS analysis of the CuPc in organic photovoltaic devices.

Interactions between organic photoconductor and plasma discharge within an electrophotographic environment

Structural and chemical compositional changes occurring within the surface region of an organic photoconductor electrically charged with Paschen discharge over an extended period of time have been investigated with the help of attenuated total reflectance Fourier transform infrared spectroscopy and x-ray photoelectron spectroscopy. Experimental conditions were selected to simulate interactions between the photoconductor and the plasma discharge occurring in a typical electrophotographic printing environment. A long time exposure of the photoconductor to energetic charged and neutral species, and UV photons impinging upon the photoconductor caused oxidation of the surface, substantial chemical bond breakage, and the reformation of bonding configurations within the region below the surface. These resulted in the formation of a thin surface region with properties different from those of the original photoconductor. Since this study closely follows a modern electrophotographic process, the obtained conclusion...

Chapter 8 Deep Level Defects in Epitaxial III/V Materials

Publisher Summary This chapter describes deep level defects in epitaxial III/V materials. Epitaxial III/V materials play an increasingly important role in a variety of electronic, optoelectronic, and optical applications. The development of new epitaxial growth processes and improvements to already existing techniques have enabled the growth of layers of a quality (expressed by the quantity of structural and electronic defects) previously unobtainable for bulk III/V crystals. Therefore, the III/V material-based devices are now frequently fabricated using homoepitaxial layers grown on the corresponding bulk substrates. But the even greater importance of III/V epitaxial processes stems from the fact that they facilitate the growth of heteroepitaxial structures with properties unavailable for devices fabricated using bulk crystal materials. The electronic properties of the III/V epilayers are in many instances, as in the case of bulk crystals, modified by the deep states present in their band gaps. Some of them are beneficial and have been successfully employed either to improve existing devices or to obtain structures with new characteristics.

Surface Molecular Vibrations as a Tool for Analyzing Surface Impurities in Copper Phthalocyanine Organic Nanocrystals

Comparison of the IR spectra of the Cu-phthalocyanine (CuPc) nanocrystals obtained using surface sensitive attenuated total reflectance (ATR) and bulk sensitive transmittance sample configurations revealed small but measurable changes of some vibrational frequencies of the molecules at the surface of nanocrystals with the outermost part of the surface CuPc molecules being the most affected. These changes are caused by electrostatic interactions between the polar components of the molecules on the surface of nanocrystals and external polar molecular species vicinal to the nanocrystals. The external polar species can be either chemically bonded to the CuPc nanocrystals surface or they can reside in its vicinity without forming a chemical bond with the nanocrystal. Molecular modeling (DMOL3 - Materials Studio and Gaussian calculations) of the impact of selected external polar species vicinal to a CuPc molecule on the CuPc molecular vibrations confirmed experimentally observed changes in the vibrational frequencies of the selected CuPc molecular bonding configurations and provided detailed information on the forces involved in these interactions. The population of external polar species vicinal to the CuPc surface can be modified by washing the nanocrystals or by introducing polar molecular additives miscible with the CuPc nanocrystals. Reduction in the number of external polar additives was accomplished by either centrifuging the aqueous dispersion of the nanocrystals or by organic solvent-based Soxhlet extraction, while their number was increased by soaking (followed by drying) the nanocrystals in high and low pH aqueous solutions containing SO 3- and OH- ions. These quantitative and qualitative modifications of the population of external polar species surrounding CuPc nanocrystals were reflected in the corresponding changes of the selected vibrational frequencies of the CuPc surface molecules providing an effective tool for not only recognizing the molecular species vicinal to a nanocrystal but also quantifying their concentration. Some of these modifications can also be observed with a naked eye in the form of noticeable color changes of the CuPc nancrystalline powder. This is due to the extremely high visible extinction coefficient of the CuPc nanocrystals causing that the impinging light is mostly absorbed/reflected within the surface region of the nanocrystals. Changes of the electronic structure within this region, caused by the interactions with the vicinal polar species, shift the vis absorption/reflection spectra changing the observed color of the nanocrystalline powder. Similar results were obtained for other molecular nanocrystals, including yellow chromophore molecules. Preliminary data indicate that the described analytical method of analyzing the molecular polar species vicinal to a molecular nanocrystal could find variety of applications ranging from molecular device fabrication to pharmaceutical materials.

Light Emission from Polymer‐Semiconductor Nanocrystal (PFO‐CdSe/ZnS) Structures

Electroluminescence was achieved from hybrid conjugated polymer (polyfluorene) — II‐VI semiconductor nanocrystal structures. The energy of the carriers injected into the polymer was transferred into the nanocrystals causing light emission with a photon energy corresponding to the nanocrystals’ bandgap. Undesirable polymer‐nanocrystal phase segregation and presence of defective nanocrystals was responsible for the relatively low emission efficiency from the hybrid devices.

Heteroepitaxy of InSe/GaSe on Si(111) Substrates

High quality growth of InSe on Si(111) was achieved by insertion of GaSe buffer layer. Rhombohedral polytypes were formed in both the InSe and GaSe layers. Twinning and stacking disorder was often detected in these materials due to their layered structure. Moreover, in samples with a thin GaSe layer, strong interdiffusion of indium into the GaSe layer was detected that resulted in the formation of an In x Ga y Se phase. The dominant threading defects present in these InSe/GaSe heterostructures were screw dislocations, which may act as nonradiative recombination centers.