Sustainability and renewable energies – a critical look at photovoltaics

The current status of photovoltaic modules from wafer‐based Si and thin films of Si, CuInGaSe₂ (CIGS), CdTe and III–V compound semiconductors in terms of efficiencies and market volumes is shortly summarized and evaluated with respect to sustainability. We discuss the critical raw materials such as gallium, indium and germanium. These materials are either incorporated into the absorber layers or used as substrate materials, i.e. Ge in the case of III–V semiconductor based cells. On a 100 GW per year production level the availability of indium and tellurium will limit the growth of the respective photovoltaic technology. Alternative materials are on the horizon, such as the kesterite‐type materials which offer the possibility to replace In and Ga in CuInGaSe₂, but it may take a long period of time for modules of these materials to reach the production‐level efficiencies of today's established technologies. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Excellence – energy – ethics

Dear pss readers, As the previous year approached its end, news on three excellent prize winnings and nominations of pss authors and editors reached us: Gerhard Abstreiter of TU Munich will receive the Stern–Gerlach Medal 2014, the highest prize of the German Physical Society for experimental physics, honoring his work on low‐dimensional electron systems in semiconductor hetero‐ and nanostructures. His Review@RRL on InGaAs nanowires on silicon is opening the 2014 volume of pss (RRL) [1]. It is a welcome addition to our recent successful Focus Issue on Semiconductor Nanowires [2]. Our long‐term Editorial Advisory Board member, Wiley author and Guest Editor, Rainer Waser of RWTH Aachen and Research Centre Jülich, is one of the 11 winners of the highly prestigious Leibniz Prize for his outstanding research on nanoelectronics, especially oxides, ferroelectrics and resistive switching [3, 4]. Last but not least, one of the three nominated teams for the German Future Prize has been led by Wolfgang Schnick , LMU Munich, and Peter J. Schmidt , Philips Lumileds Aachen. Their groundbreaking work on new phosphor materials in white light emitting diodes (LEDs) for solid‐state lighting [5] goes back to a highly‐cited pss (a) article from 2005 [6] (see figure). The technology is now being commercialized and expected to enable energy savings on a grand scale in the coming years. Speaking of energy, research results related to this global challenge have been important throughout the year, touching areas such as thermoelectrics [7], efficiency of organic LEDs [8] and photovoltaics [9]. The latter field is even better represented since the recent introduction of our section rrl solar, covering solar cell materials or device development and characterization (see Editorial [10]). The full‐paper sister journals pss (a) and (b) presented an unprecedented number of high‐profile special issues in 2013 [11–15]. With heartfelt gratitude we look back onto fruitful collaboration with highly engaged guest editors, who helped bring to light issues such as the “Advanced Concepts for Silicon Based Photovoltaics” [11], the “Quantum Criticality and Novel Phases” [12], the “Disorder in Order: A special issue on amorphous materials” [13], the “Substrate Interactions in Heterogeneous Catalysis” [14], and the “Quantum Transport at the Molecular Scale” [15] among other interesting topical issues and sections. Both contributors and fine articles are too numerous to do justice to all of them here. We must restrict ourselves to a general invitation to browse this content, only hinting a few possible starting points, such as topological insulators [16], molecular electronics [17] and quantum phase transitions [18].

     

The elemental composition of Stradivari's musical instruments: new results through non‐invasive EDXRF analysis

During recent decades, many researchers have tried to understand the main influences on the extraordinary sound and beauty of the masterpieces made by the ancient violin makers. This is still a challenge for many others today. Mainly because of a lack of written historical documents, the rediscovery of some of the ancient violin‐making processes was made possible thanks to scientific analyses performed on their materials by means of diagnostic techniques. However, understanding which substances were adopted is a very hard task, because the analyses are influenced by many factors: for example, alterations, wear, retouches, and the heterogeneity of materials. This paper presents some new EDXRF results collected on eight‐stringed musical instruments made by Antonio Stradivari between 1669 and 1734 (“Clisbee” 1669, “Hellier” 1679, “Ford‐Rougemont” 1703, “Joachim‐Ma” 1714, “Russian Federation” viola 1715, “Cremonese” 1715, “Vesuvius” 1727, and “Scotland” 1734) and now preserved at the Museo del Violino in Cremona. A brief comparison with a modern violin made by Simone Ferdinando Sacconi (“Hellier copy” 1941), one of the most eminent violin makers of the 20th century and one of the greatest experts on Stradivari's work, is also provided. This represents the first comparative analysis of a wide number of ancient musical instruments made by the same violin maker over an extended period. A non‐destructive and non‐invasive approach was followed to (a) understand the elemental composition differences between the best conserved and most worn‐out surfaces; (b) check if there are elemental similarities among the finishing materials of violins made in different years by the same violin maker; (c) give new suggestions about the materials used. To distinguish the best conserved areas from the worn‐out ones, a preliminary investigation by UV‐induced fluorescence photography was performed. In addition, stereomicroscopic observations and Fourier transform infrared spectroscopy (FTIR) analyses were performed on selected areas to validate the hypotheses. The results, in some cases comparable with previous research on Stradivari instruments, have increased the pool of information about materials and treatments adopted in the Stradivari workshop.     

A long road to a bright future

Dear pss readers, “This year's Nobel Prize in Physics recognizes the pioneering work of Professors Isamu Akasaki, Hiroshi Amano and Shuji Nakamura for the breakthrough developments in GaN materials and devices that enabled the field of solid state lighting. Blue LEDs have revolutionized display technology and enabled high efficiency white light sources.” said our Editorial Advisory Board member Jim Speck in response to our enthusiastic congratulations to his colleague at University of California, Santa Barbara. It was certainly an exceptional moment for a journal that has extensively documented GaN‐related research for decades. Isamu Akasaki and Hiroshi Amano, then both Meijo University and Nagoya University, published in pss on dislocations and stress in AlGaN/GaN thin films, e.g. [1, 2], Hiroshi Amano repeatedly acted as a guest editor [3–5], and all three laureates together have more than a 150 articles in pss and continue to publish on the topic. Obviously, though these contributions have gathered more than 1400 citations to date, not all are “breakthroughs”. Most are documenting continuous achievements in an important and highly specialized area. But together they witness the exceptional persistence required to master the growth of GaN to the degree of perfection that enabled blue LEDs. It is an example for the amount of patient, high‐level work that is necessary to turn a brilliant idea into a widespread invention, and pss is proud to accompany such progress along the way. Regarding today's promising trends in materials physics, we would like to showcase some of the special issues and topical sections that appeared in pss sister journals in 2014: “Oxide materials might form the basis for the next technological revolutions” according to J. M. Knaup, T. Frauenheim, P. Broqvist, and S. Ramanathan, the guest editors of the pss (RRL) Focus issue “Functional Oxides” [6]. These materials' enhanced complexity proposes both a greater challenge and greater benefits for future electronics and energy applications, and the 3 Reviews and 20 Letters offer computational and experimental perspectives on the current understanding. A topical section “Nanoscaled Magnetism and Applications”, guest‐edited by A. Zhukov, features contributions on magnetocaloric materials, spin relaxation, magnetization processes and magnetic nanostructures in pss (a) [7]. Another bustling topic in pss (a) are the recurring annual diamond and nanocarbon special issues, this time focused on “Advances on Diamond Surfaces and Devices” edited by B. Rezek, M. Nesládek, and K. Haenen [8], and kicked‐off by a Feature Article “Photoelectron emission from lithiated diamond” by K. M. O'Donnell, L. Ley, and others [9].

The logo of our journal assembled with blue GaN‐on‐Si LEDs (courtesy of Armin Dadgar, Otto‐von Guericke‐Universität, Magdeburg, Germany)     

Ambipolar inverters with natural origin organic materials as gate dielectric and semiconducting layer

Thin film electronics fabricated with non‐toxic and abundant materials are enabling for emerging bioelectronic technologies. Herein complementary‐like inverters comprising transistors using 6,6′‐dichloroindigo as the semiconductor and trimethylsilyl‐cellulose (TMSC) films on anodized aluminum as bilayer dielectric layer are demonstrated. The inverters operate both in the first and third quadrant, exhibiting a maximum static gain of 22 and a noise margin of 58% at a supply voltage of 14 V. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Carbon‐Coated Supraballs of Randomly Packed LiFePO4 Nanoplates for High Rate and Stable Cycling of Li‐Ion Batteries

One of the key strategies used to obtain high‐rate Li‐ion battery is the reduction of the Li‐ion path length inside the active materials and the enhancement of the ionic diffusion outside the active materials. It is demonstrated that electrochemical performance can be improved significantly at high C‐rates using carbon‐coated spherical aggregates or “supraballs” of randomly packed olivine LiFePO₄ (LFP) nanoplates as cathode active materials. 258 nm LFP nanoplates with 30 nm thickness are synthesized through a high‐temperature solvothermal method, in which short lithium‐ion channels are formed perpendicular to the top or bottom planes. These thin nanoplates are formed into carbon‐coated “supraballs” through a spray‐drying and thermal annealing process, in which nanoplates are not stacked but randomly packed due to relatively fast drying. Internal and external nanoplate ion diffusion is therefore enhanced simultaneously due to the optimal molecular crystalline structure and interparticle pore structures of the nanoplates. Indeed, the initial capacity of the carbon‐coated supraballs is 162 mAh g^?1 (173.34 mAh cm^?3) at 0.1 C and more than 80% is retained (≈130.91 mAh g^?1) at 50 C. Furthermore, they offer durable cycling stability (>500 cycles) at 1 C without compromising their capacity.     

Uplifting the Iwasawa

The Iwasawa manifold is uplifted to seven‐folds of either G₂ holonomy or SU(3) structure, explicit new metrics for the same having been constructed in this work. We uplift the Iwasawa manifold to a G₂ manifold through “size” deformation (of the Iwasawa metric), via Hitchin's Flow equations, showing also the impossibility of the uplift for “shape” and “size” deformations (of the Iwasawa metric). Using results of Dall'Agata and Prezas, Phys. Rev. D 69 , 066004 (2004) [arXiv:hep‐th/0311146] [1], we also uplift the Iwasawa manifold to a 7‐fold with SU(3) structure through “size” and “shape” deformations via generalisation of Hitchin's Flow equations. For seven‐folds with SU(3)‐structure, the result could be interpreted as M5‐branes wrapping two‐cycles embedded in the seven‐fold (as in [1]) ‐ a warped product of either a special hermitian six‐fold or a balanced six‐fold with the unit interval. There can be no uplift to seven‐folds of SU(3) structure involving non‐trivial “size” and “shape” deformations (of the Iwasawa metric) retaining the “standard complex structure” ‐ the uplift generically makes one move in the space of almost complex structures such that one is neither at the standard complex structure point nor at the “edge”. Using the results of Konopelchenko and Landolfi, J. Geom. Phys. 29 , 319 (1999) [arXiv:math.DG/9804144] [2], we show that given two “shape deformation” functions, and the dilaton, one can construct a Riemann surface obtained via Weierstraß representation for the conformal immersion of a surface in R ^l, for a suitable l, with the condition of having conformal immersion being a quadric in CP ^l‐1.     

Quadratic corrections to three‐point functions

Following the recent progress on the calculation of three‐point correlators with two “heavy” (with large quantum numbers) and one “light” states at strong coupling, we compute the logarithmic divergent terms of leading bosonic quantum corrections to correlation functions with “heavy” operators corresponding to simple string solutions in AdS₅ × S⁵. The “light” operator is chosen to be the dilaton. An important relation connecting the corrections to both the dimensions of “heavy” states, and the structure constants is recovered.     

Fatty acid binding site of mitochondrial uncoupling protein UCP2 as probed by EPR spectroscopy of spin-labeled fatty acids

Electron paramagnetic resonance (EPR) spectroscopy of spin-labeled fatty acids was used to investigate their interaction withEscherichia coli-expressed human mitochondrial uncoupling protein UCP2 refolded from inclusion bodies in nonaethylene glycol monododecyl ether (C₁₂E₉) micelles. 5-DOXYL-stearic acid and 4-PROXYL-palmitic acid bound to UCP2 exhibited additional clearly separated h_+1I, h_?1I “immobile” peaks in the low- and high-field region, respectively, separated by 42 and 44 Gauss, and extensively reduced h_+1M, h_?1M “mobile” peaks, separated by about 30 G, whereas with 7-DOXYL-stearic acid the I and M peaks were smoothed together into one wide peak. Competition of 4-PROXYL-palmitic acid with added palmitic acid, arachidonic acid, and all-cis-8,11,14-eicosatrienoic acid and of 7-DOXYL-stearic acid with arachidonic acid was indicated by the disappearance of the h_+1I, h_?1I “immobile” peaks, whereas redistribution in micelles without protein was indicated by the rising of the h_+1M, h_?1M “mobile” peaks. In conclusion, a competition of palmitic, arachidonic, and eicosatrienoic acid within a putative fatty acid binding site was observed for mitochondrial uncoupling protein UCP2. This finding together with the observation of EPR spectra of highly immobilized probes exclusively in the presence of the recombinant UCP2 suggest the existence of a fatty acid binding site on UCP2 which is a prerequisite of the fatty acid cycling mechanism as previously postulated for UCP1.     

Optical characterization of atmospheric‐pressure plasma needle

The atmospheric‐pressure plasma needle is a promising source that can be used efficiently for different industrial applications. A radio frequency (RF) (13.56 MHz) generator was used to generate a He–O₂/Ar mixture plasma. The ground‐state oxygen atomic density [O] was calculated as a function of discharge parameters by “actinometry”. The Ar‐I (2p₁ → 1s₂) line at 750 nm and the O‐I (³P → ³S) line at 844 nm were used to estimate the [O] atomic density. The rotational temperature T _R of He–O₂/Ar mixture was measured from the rotational levels of the “first negative system” (FNS) by using the “Boltzmann plot”. The effect of discharge parameters on the atomic oxygen density [O] and the gas temperature was monitored. These results show that [O] density increases with RF power and O₂ concentration, but decreases with the gas flow rate. Whereas the gas temperature increases with increase in the input RF power, it decreases with increase in the gas flow rate and O₂ concentration in the mixture. Since the [O] atomic density contributes to plasma‐based biomedical applications, the proposed optimum conditions for plasma‐based decontamination of heat‐sensitive materials in the present study are 0.6% oxygen, 500 sccm flow rate, and 26 W RF power.     

Effects of mobile phone exposure on time frequency fine structure of transiently evoked otoacoustic emissions

Mobile phones have become very commonly used worldwide within a short period of time. To date there is only limited knowledge about interaction between electromagnetic fields (EMFs) emitted by mobile phones and the auditory function. Moreover, there is widespread concern that there may be potential for harm. The aim of this study was to assess potential subtle changes in cochlear function by measuring the temporal and spectral fine structure of transiently evoked otoacoustic emissions (TEOAE) in normal hearing subjects after exposure to EMFs emitted by Global System for Mobile Communication (GSM) mobile phones. TEOAEs were recorded in 27 healthy young adults before and after 10 min of real or sham exposure in a double-blind design. TEOAE data were analyzed both globally (broadband analysis) and using the Wavelet Transform (analysis of the time-frequency fine structure). The broadband analysis revealed no significant effect on TEOAEs related to exposure, confirming results of previous studies; in addition, no significant change was detected in the analysis of the elementary wavelet components, suggesting that the temporal and spectral fine structure of TEOAEs is not affected by 10 min exposure to low-intensity EMFs emitted by GSM mobile phones.     

Particle localization and hyperuniformity of polymer‐grafted nanoparticle materials

The properties of materials largely reflect the degree and character of the localization of the molecules comprising them so that the study and characterization of particle localization has central significance in both fundamental science and material design. Soft materials are often comprised of deformable molecules and many of their unique properties derive from the distinct nature of particle localization. We study localization in a model material composed of soft particles, hard nanoparticles with grafted layers of polymers, where the molecular characteristics of the grafted layers allow us to “tune” the softness of their interactions. Soft particles are particular interesting because spatial localization can occur such that density fluctuations on large length scales are suppressed, while the material is disordered at intermediate length scales; such materials are called “disordered hyperuniform”. We use molecular dynamics simulation to study a liquid composed of polymer‐grafted nanoparticles (GNP), which exhibit a reversible self‐assembly into dynamic polymeric GNP structures below a temperature threshold, suggesting a liquid‐gel transition. We calculate a number of spatial and temporal correlations and we find a significant suppression of density fluctuations upon cooling at large length scales, making these materials promising for the practical fabrication of “hyperuniform” materials.     

The potential of luminescence signals from electronic components for accident dosimetry

This study investigated the optically stimulated luminescence of a large number of electronic components extracted from both old and new generation mobile phones and chip modules of phone cards. Most resistors and all chip modules studied present a linear dose response (R > 0.99) in the dose range investigated (200 mGy up to 6 Gy, respectively 10 Gy), while capacitors, inductors and integrated circuits generally have a non-linear growth (exponential or cubic). For our experimental setup, an average specific luminescence of ∼20,000 cts in 2 s/Gy (n = 10) and ∼6000 cts in 2 s/Gy (n = 14) was obtained for two types of chip modules with a relatively high degree of homogeneity (relative standard deviation of 23% and 31%) and a minimum detectable dose of 7 mGy for immediate measurement. The investigated signals show small sensitivity changes (generally <10%) after repeated cycles of irradiation and readout. Preliminary fading measurements are presented. It can be concluded that most mobile phones and phone card components have a significant potential as retrospective luminescence dosimeters.     

Atomic classification of 6D SCFTs

We use F‐theory to classify possibly all six‐dimensional superconformal field theories (SCFTs). This involves a two step process: We first classify all possible tensor branches allowed in F‐theory (which correspond to allowed collections of contractible spheres) and then classify all possible configurations of seven‐branes wrapped over them. We describe the first step in terms of “atoms” joined into “radicals” and “molecules,” using an analogy from chemistry. The second step has an interpretation via quiver‐type gauge theories constrained by anomaly cancellation. A very surprising outcome of our analysis is that all of these tensor branches have the structure of a linear chain of intersecting spheres with a small amount of possible decoration at the two ends. The resulting structure of these SCFTs takes the form of a generalized quiver consisting of ADE‐type nodes joined by conformal matter. A collection of highly non‐trivial examples involving E₈ small instantons probing an ADE singularity is shown to have an F‐theory realization. This yields a classification of homomorphisms from ADE subgroups of into E₈ in purely geometric terms, matching results obtained in the mathematics literature from an intricate group theory analysis.     

Nanostructured thermoelectric oxides with low thermal conductivity

Complex metal oxides, such as e.g. perovskite‐type phases are developed as potential functional materials to improve the efficiency of thermoelectric converters. Among those, cobaltates with p‐type conductivity and n‐type manganates are considered for the realisation of a ceramic thermoelectric converter. Sintered pellets with the composition AMO_3–δ (A = Ln, RE; M = Co, Mn, Ni, Ti) and “Ca₃Co₄O₉ derivates” were synthesized and characterised concerning their thermoelectric properties in a broad temperature range. It was found that the Seebeck coefficient and the electrical conductivity do not depend on the dimensions of the crystallites, while the heat conductivity can be substantially lowered by decreasing the size of the crystalline domains in these systems. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

First-principles studies: Thiolated Au2Cr and Au6Cr clusters

The structural, energetic and magnetic properties of thiol-passivating Au₂Cr and Au₆Cr clusters are investigated by performing first-principles calculation based on density functional theory. We find that the adsorption of thiolate is energetically more favorable than thiol and that the thiolates favor “top” site adsorption on Cr atom in one methanethiolate adsorbed Au₂Cr cluster while they favor “bridge” site adsorption on top of the middle point of Cr–Au bonds in three methanethiolates adsorbed Au₂Cr cluster. In thiol-passivating Au₆Cr cluster, the thiol favor “top” site adsorption on top of atom Au while the thiolate favor “bridge” site adsorption on top of the middle point of Au–Au bonds. The energetics of the reactions indicates that these thiol-passivating Au₂Cr or Au₆Cr clusters can be used as hydrogen storage materials. There are large and positive spin populations on atom Cr. The spin populations of atoms Au, S and H are attributed to both the spin delocalization and the spin polarization mechanisms.     

In Vivo Targeted Cancer Theranostics by Core/Shell‐Structured Multifunctional Prussian Blue/PLGA “Nanococktails”

The construction of high‐performance nanotheranostic agent with Food and Drug Administration (FDA)‐approved materials for efficient treatment of breast cancer is still of great challenge. This work reports, for the first time, on the elaborate integration of two FDA‐approved materials together to construct a multifunctional core/shell‐structured “nanococktail” for cancer theranostics. The biocompatible Prussian blue nanoparticles with high photothermal‐conversion performance are coated by poly(lactic‐co‐glycolic acid) followed by further surface targeting engineering (folic acid conjugation). The anticancer drug paclitaxel is concurrently encapsulated into the nanocarrier with high efficiency and capacity. Especially, these “nanococktails” act as the desirable contrast agents for photoacoustic/magnetic resonance imaging dual‐mode diagnostic imaging, providing the potential for guidance and monitoring during the therapeutic process, which has been systematically demonstrated both in vitro and in vivo. Importantly, these “nanococktails” have demonstrated their high performance in synergistic in vivo photothermal therapy and chemotherapy against breast cancer tumor xenograft. This work not only provides a high‐performance theranostic “nanococktail” platform for efficient theranostic treatment of cancer but also paves a new way for the integration of various functional moieties together for realizing the specific diagnostic imaging‐guided and synergistic cancer therapy.     

Mobile-ion concentrations and mixed-alkali effects in β-alumina solid electrolytes

A theory of the mixed-alkali effect based on the weak electrolyte model is used in the analysis of conductivity isotherms in the Na/K β-alumina system. This gives mobile interstitialcy concentrations, [Na²⁺₂] or [Na⁺_i], which agree remarkably well with those obtained by Wolf from calculations based on changes in the Haven ratio, H_R. The high conductivity of β-alumina arises apparently from a high “defect mobility” rather than from the existence of quasi-liquid motions. Other consequences of the weak electrolyte theory are discussed.     

Ultrahigh‐Density Plasmonic‐Nanoparticle‐Sensitized Semiconductor Photocatalysts Profit from Cooperative Light Harvesting and Charge Separation Processes: Experiments,Simulations, and Multifunctional Plasmonics

Here, a controlled synthesis of remarkable 3D photocatalysts is presented that is composed of ultrahigh‐density unaggregated plasmonic Au nanoparticles (AuNPs) chemically bound to vertically aligned ZnO nanorod arrays (ZNA) through bifunctional molecular linkers. Experimental probes and electromagnetic simulations of electron transfer and localized plasmonic coupling processes are exploited to gain insight into the underlying light‐irradiation‐induced interactions in the 3D ZNA–AuNPs photocatalysts. Highly dense AuNPs on ZNA surfaces act as sinks for the storage of UV‐generated electrons, which promote the separation of charge carriers and create numerous photocatalytic reaction centers. Furthermore, 3D finite‐difference time domain simulation indicates that significant visible light confinement and enhancement around the ZNA–AuNPs interfacial plasmon “hot spots” contribute to efficient conversion of light energy to electron‐hole pairs. Significantly, in comparison with the bare ZNA, the 10‐nm‐sized AuNPs‐decorated ZNA exhibits 10.6‐fold enhanced photoreaction rate in the entire UV–vis region. Moreover, various novel hybrid structures based on the plasmonic AuNPs and diverse nanostructures (films, powdered nanorods, mesoporous, and nanotubes) or functional materials (multiferroic BiFeO₃, CuInGaSe₂ absorber layers, and photoactive TiO₂) are successfully constructed using the present synthesis methodology. It may stimulate the progress in materials science toward the synthesis of multifunctional plasmonic heterostructures or devices.