The microenvironment in polymer solids with added plasticizers. An attempt to measure the size and distribution of free volume in poly(methyl methacrylate) solids

Various amounts of n-alkylbenzenes (C_n: C₆H₅-C_nH_2n+1 (n = 3-16)) were doped into poly(methyl methacrylate) (PMMA) films, and the emission and thermal properties of each film were measured in detail together with their solid-state ¹³C NMR spectra. The aim of the present work was to estimate the size distribution of free volume in amorphous regions of polymer solids heavily doped with plasticizers by using C_n as models of a plasticizer. The excimer fluorescence yields of C_n in PMMA were found to depend on both the amount of C_n and the length of the alkyl chains of C_n, although the fluorescence spectra of all of the C_ns in dilute fluid solution were almost the same. The quantitative analysis showed: (1) C_n with n ? 12 induces a phase separation in PMMA, in which almost all of the C_n molecules are in a separated phase, and thus they cannot penetrate regions in which PMMA chains are aggregated. This means that C_n with n ? 12 cannot enlarge the space between PMMA chains. (2) Smaller C_n (n < 11) can enter free volumes between PMMA chains that correspond to their molecular size, but they can enlarge them only to a limited extent. Thus, the amount by which plasticization can increase the free volume of PMMA is limited by the size of the dopant and the inherent free volume of the polymer matrix. (3) The efficiency of excimer formation was found to reveal the maximum amount of C_n that could fit in the free volume of PMMA. Thus our fluorescence measurements showed that PMMA solids that were plasticized to their limit had a free volume that was larger than the volume occupied by all the conformers of C₅ and smaller than the volume occupied by almost all the conformers of C₁₂. In conclusion, we were able to obtain information on plasticization and to demonstrate a method of monitoring microenvironments in polymer solids after they have been doped with plasticizers.     

Synthesis and characterization of 2D transition metal dichalcogenides: Recent progress from a vacuum surface science perspective

Layered transition metal dichalcogenides (TMDs) are a diverse group of materials whose properties vary from semiconducting to metallic with a variety of many body phenomena, ranging from charge density wave (CDW), superconductivity, to Mott-insulators. Recent interest in topologically protected states revealed also that some TMDs host bulk Dirac- or Wyle-semimetallic states and their corresponding surface states. In this review, we focus on the synthesis of TMDs by vacuum processes, such as molecular beam epitaxy (MBE). After an introduction of these preparation methods and categorize the basic electronic properties of TMDs, we address the characterization of vacuum synthesized materials in their ultrathin limit-mainly as a single monolayer material. Scanning tunneling microscopy and angle resolved photoemission spectroscopy has revealed detailed information on how monolayers differ in their properties from multi-layer and bulk materials. The status of monolayer properties is given for the TMDs, where data are available. Distinct modifications of monolayer properties compared to their bulk counterparts are highlighted. This includes the well-known transition from indirect to direct band gap in semiconducting group VI-B TMDs as the material-thickness is reduced to a single molecular layer. In addition, we discuss the new or modified CDW states in monolayer VSe₂ and TiTe₂, a Mott-insulating state in monolayer 1T-TaSe₂, and the monolayer specific 2D topological insulator 1T′-WTe₂, which gives rise to a quantum spin Hall insulator. New structural phases, that do not exist in the bulk, may be synthesized in the monolayer by MBE. These phases have special properties, including the Mott insulator 1T-NbSe₂, the 2D topological insulators of 1T′-MoTe₂, and the CDW material 1T-VTe₂. After discussing the pure TMDs, we report the properties of nanostructured or modified TMDs. Edges and mirror twin grain boundaries (MTBs) in 2D materials are 1D structures. In group VI-B semiconductors, these 1D structures may be metallic and their properties obey Tomonaga Luttinger quantum liquid behavior. Formation of Mo-rich MTBs in Mo-dichalcogenides and self-intercalation in between TMD-layers are discussed as potential compositional variants that may occur during MBE synthesis of TMDs or may be induced intentionally during post-growth modifications. In addition to compositional modifications, phase switching and control, in particular between the 1H and 1T (or 1T′) phases, is a recurring theme in TMDs. Methods of phase control by tuning growth conditions or by post-growth modifications, e.g. by electron doping, are discussed. The properties of heterostructures of TMD monolayers are also introduced, with a focus on lateral electronic modifications in the moiré-structures of group VI-B TMDs. The lateral potential induced in the moiré structures forms the basis of the currently debated moiré-excitons. Finally, we review a few cases of molecular adsorption on nanostructured monolayer TMDs. This review is intended to present a comprehensive overview of vacuum studies of fundamental materials' properties of TMDs and should complement the investigations on TMDs prepared by exfoliation or chemical vapor deposition and their applications.     

NONCONJUGATED CONDUCTIVE POLYMERS

Conjugation is not a prerequisite for a polymer to be conductive. A polymer must have at least one double bond in the repeat to become conductive. Interaction with a dopant (e.g., electron acceptor) causes transfer of an electron from the double bond to the dopant creating a hole at the double bond site. Electrical conduction occurs via intersite hopping of holes. Various spectroscopic methods (FTIR, optical absorption, solid-state ¹³C NMR, etc.) along with electrical measurements have been used to elucidate the mechanism of conduction in specific nonconjugated conductive polymers. Examples of these polymers include 1,4-polyisoprene which has one double bond and three single bonds in the repeat. The conductivity of polyisoprene increases 100 billion times upon doping with iodine to a maximum value of 10 S/m. Polyisoprene (natural rubber) is used nonconjugated conductive polymers have a wide range of applications in antistatics, various sensors and optoelectronics.     

Tomographic analysis of dilute impurities in semiconductor nanostructures

We describe the application of pulsed-laser atom probe (PLAP) tomography to the analysis of dopants and unintentional impurities in Si and Ge nanowires grown by the vapor–liquid–solid mechanism. PLAP tomography was used to determine the concentration of phosphorous in Ge nanowires and B in Si nanowires, enabling comparisons of the atomic concentrations of the reactants with those of the reaction products. Oxygen impurities were also detected, but the contribution from background gas adsorption was not ruled out. Gold catalyst impurities were not detected, and an upper bound of 5 ppm was established. Intrinsic and extrinsic origins of the detection limits of dopants and other impurities are described in detail. A tapered nanowire geometry was found to improve the mass resolution and signal-to-noise ratio by increasing the tip cooling rate. Simulations of nanowire cooling under laser pulsing were used to validate this improved approach to PLAP analysis of nanowires.     

Slow Relaxation in Ice and Ice Clathrates and its Connection to the Low-Temperature Phase Transition Induced by Dopants

We have used dielectric spectroscopy (frequency range: 10 6 Hz-10 m 3 Hz) and differential scanning calorimetry down to a temperature of 77 K, to study the effect of various dopants on the molecular relaxation in hexagonal ice ( I h ) and some clathrate hydrates ( I c ). The nonelectrolytic dopants did not affect a large change in the dynamics. However, when I h and I c are doped with the alkali hydroxides: KOH, NaOH, LiOH and Ca(OH) 2 , a drastic fall in the relaxation times of the order of 10 7 -10 12 has been noticed. It appears that the alkali metal- and hydroxyl-ion pair exert a large influence on the orientational mobility of water molecules by way of polarization over a domain. The amount of phase that gets transformed to the low temperature (low- T ) ordered phase depends on the solid solubility of the dopant. The nature of the relaxation and variation of the static dielectric constant have been examined critically to get an insight into the actual mechanism responsible for the above phenomena. Our studies confirm the earlier views of Tajima et al . (1984) J. Phys. Chem. Solids , 45 , 1135, that the low- T phase transition which has so far escaped observation for kinetic reasons has now revealed itself by the catalytic action of the dopants.     

First-principles study on doping of tetragonal ZnSe monolayers

Doping is a very important and effective method to be used to modulate the properties of two-dimensional (2D) materials. In this work, the electronic and magnetic properties of ultrathin tetragonal ZnSe monolayer doped by twenty different kinds of atoms neighboring Zn/Se were systemically investigated using first-principles calculations. Substitution at the Zn/Se sites was found to be easy if the monolayer was grown under Zn-/Se-poor conditions. Among non-metal dopants, only F atom is thermodynamically favored to replace Se atom, while a number of metal atoms (i.e. Ca, Sc, Ti, and Mn) are able to substitute Zn atom. It is suggested by theoretical calculations that pristine ZnSe monolayer inclines as an n-type semiconductor by certain doping. Our results open a new avenue for the modulation of the novel tetragonal ZnSe monolayer for a wealth of potential optoelectronic applications.     

Scintillation properties of PbWO4 crystals doped with the rare-earth ions

In PbWO₄(PWO) crystals grown by Czochralski method the influence of atmosphere of the growth (O₂, air) and doping with the rare-earth ions of different types (A³⁺=Lu³⁺, Gd³⁺,Tb³⁺,Eu³⁺ as well as doubly doped A³⁺–Li⁺) on light yield and luminescence decay were analyzed. PWO scintillator with the ultra-fast (τ=0.5 ns) main component of luminescence decay (87% of total light yield) was obtained using the O₂-growth atmosphere and doping by Eu₂O₃ at a concentration of 5000 ppm. It is concluded that the decrease of decay constant of the main scintillation component is the result of the resonant energy transfer between the centers of “blue” PWO luminescence (λ_max=420 nm) and the 4f–4f-transitions of Eu³⁺ ions in this spectral region.     

Defects in semiconductor nanostructures

Impurities play a pivotal role in semiconductors. One part in a million of phosphorous in silicon alters the conductivity of the latter by several orders of magnitude. Indeed, the information age is possible only because of the unique role of shallow impurities in semiconductors. Although work in semiconductor nanostructures (SN) has been in progress for the past two decades, the role of impurities in them has been only sketchily studied. We outline theoretical approaches to the electronic structure of shallow impurities in SN and discuss their limitations. We find that shallow levels undergo a SHADES (SHAllow-DEep-Shallow) transition as the SN size is decreased. This occurs because of the combined effect of quantum confinement and reduced dielectric constant in SN. Level splitting is pronounced and this can perhaps be probed by ESR and ENDOR techniques. Finally, we suggest that a perusal of literature on (semiconductor) cluster calculations carried out 30 years ago would be useful.      

High dose implantations of antimony for buried layer applicationsrrr

A computer simulation of low energy implantation is presented. The method of calculation takes into account the inelastic processes and vacancies in the crystal lattice.

The described simulation was used for a nitrogen implanted molybdenum single crystal. From the result of the calculation the possible positions of implanted nitrogen on the surface and in the bulk were obtained. The results of simulations are compared with those measured by SIMS and AES.     

Effects of different compositions from magnetic and nonmagnetic dopants on structural and electrical properties of ZnO nanoparticles-based varistor ceramics

In the current study, 20 nm zinc oxide (ZnO) nanoparticles were used to manufacture high-density ZnO discs doped with Mn and Sn via the conventional ceramic processing method, and their properties were characterized. Results show that the dopants were found to have significant effects on the ZnO varistors, especially on the shape and size of grains, which are significantly different for both dopants. The strong solid-state reaction in the varistor from the 20 nm ZnO powder during the sintering process may be attributed to the high surface area of the 20 nm ZnO nanoparticles. Although Mn and Sn do not affect the well-known peaks related to the wurtzite structure of ZnO ceramics, a few of the additional peaks could be formed at high doping content (≥2.0) due to the formation of other unknown phases during the sintering process. Both additives also significantly affect the electrical properties of the varistor, with a marked changed in the breakdown voltage from 415 V to 460 V for Sn and from 400 V to 950 V for Mn. Interestingly, the electrical behaviors of the varistors, such as breakdown voltage, nonlinear coefficient, and barrier height, are higher for Mn- than Sn-doping samples, and the opposite behaviors hold for hardness, leakage currents, and electrical conductivities. Results show that the magnetic moment and valence state of the two additive dopants are responsible for all demonstrated differences in the electrical characteristics between the two dopants.