Solar-to-Chemical Energy Conversion with Photoelectrochemical Tandem Cells.

Efficiently and inexpensively converting solar energy into chemical fuels is an important goal towards a sustainable energy economy. An integrated tandem cell approach could reasonably convert over 20% of the sun's energy directly into chemical fuels like H2 via water splitting. Many different systems have been investigated using various combinations of photovoltaic cells and photoelectrodes, but in order to be economically competitive with the production of H2 from fossil fuels, a practical water splitting tandem cell must optimize cost, longevity and performance. In this short review, the practical aspects of solar fuel production are considered from the perspective of a semiconductor-based tandem cell and the latest advances with a very promising technology - metal oxide photoelectrochemical tandem cells - are presented.

Novel hetero-layered materials with tunable direct band gaps by sandwiching different metal disulfides and diselenides.

Although bulk hexagonal phases of layered semiconducting transition metal dichalcogenides (STMD) such as MoS2, WS2, WSe2 and MoSe2 exhibit indirect band gaps, a mono-layer of STMD possesses a direct band gap which could be used in the construction of novel optoelectronic devices, catalysts, sensors and valleytronic components. Unfortunately, the direct band gap only occurs for mono-layered STMD. We have found, using first principles calculations, that by alternating individual layers of different STMD (MoS2, WS2, WSe2 and MoSe2) with particular stackings, it is possible to generate direct band gap bi-layers ranging from 0.79 eV to 1.157 eV. Interestingly, in this direct band gap, electrons and holes are physically separated and localized in different layers. We foresee that the alternation of different STMD would result in the fabrication of materials with unprecedented optical and physico-chemical properties that would need further experimental and theoretical investigations.

High color rending index and high-efficiency white organic light-emitting diodes based on the control of red phosphorescent dye-doped hole transport layer.

We have investigated the transport characteristics of red phosphorescent dye bis(1-(phenyl)isoquinoline) iridium (III) acetylanetonate (Ir(piq)2acac) doped 4,4',4"-tri(N-carbazolyl)triphenylamine (TCTA), and found that the increasing doping ratio was facilitated to improve the ability of hole transporting. A high color rendering index (CRI) and high-efficiency WOLED was achieved by employing Ir(piq)2acac doped TCTA film as an effective red emissive layer due to the generation of charge transfer complex (CTC) at the interface. The relative proportion in red: green: blue emission intensity can be controlled by the CTC concentration to obtain high CRI WOLEDs. The WOLED with an optimal red dye doping concentration of 5 wt% exhibits a high CRI of 89 and a power efficiency of 31.2 lm/W and 27.5 lm/W at the initial luminance and 100 cd/m², respectively. The devices show little variation of the Commission Internationale de I'Eclairage coordinates in a wide range of luminance.

Dependence of efficiencies in GaN-based vertical blue light-emitting diodes on the thickness and doping concentration of the n-GaN layer.

We investigate the dependence of various efficiencies in GaN-based vertical blue light-emitting diode (LED) structures on the thickness and doping concentration of the n-GaN layer by using numerical simulations. The electrical efficiency (EE) and the internal quantum efficiency (IQE) are found to increase as the thickness or doping concentration increases due to the improvement of current spreading. On the contrary, the light extraction efficiency (LEE) decreases with increasing doping concentration or n-GaN thickness by the free-carrier absorption. By combining the results of EE, IQE, and LEE, wall-plug efficiency (WPE) of the vertical LED is calculated, and the optimum thickness and doping concentration of the n-GaN layer is found for obtaining the maximum WPE.

Random CNT network and regioregular poly(3-hexylthiophen) FETs for pH sensing applications: a comparison.

BACKGROUND: Nowadays, there is a tremendous need for cheap disposable sensing devices for medical applications. Materials such as Carbon Nanotubes (CNTs) and regioregular P3HT are proven to offer a huge potential as cost-effective and solution processable semiconductors for (bio)sensing applications. METHODS: CNT-based field-effect transistors (CNT-FETs) as well as regioregular P3HT-based ones (P3HT-FETs) are fabricated and operated in the so-called electrolyte-gated configuration. The active layer of the P3HT-FETs consists of a spin-coated regioregular P3HT layer, which serves on one hand as the active sensing element and on the other hand as passivation layer for the transistor's metal contacts. The active layer of the nanotube transistors consists of a randomly distributed single walled CNT-network (>90% semiconducting tubes) deposited from a CNT-ink solution by spin-coating. RESULTS: We compare both devices concerning their stability in aqueous environment and their response when exposed to buffers with different pH. We found that even if P3HT shows lower stability its pH sensitivity is reproducible even after long-term measurements. CONCLUSION: CNT-FETs and P3HT-FETs offer different advantages and drawbacks concerning their stability in solution and the ease of fabrication. A discussion of their different sensing mechanisms as well as sensitivity is given here. GENERAL SIGNIFICANCE: This work reports on fast and cost-effective production of solution processable thin-film transistors based on carbon nanotubes and regioregular P3HT and demonstrates their suitability as reliable pH sensors. This article is part of a Special Issue entitled Organic Bioelectronics - Novel Applications in Biomedicine.

Integration of photoswitchable proteins, photosynthetic reaction centers and semiconductor/biomolecule hybrids with electrode supports for optobioelectronic applications.

Light-triggered biological processes provide the principles for the development of man-made optobioelectronic systems. This Review addresses three recently developed topics in the area of optobioelectronics, while addressing the potential applications of these systems. The topics discussed include: (i) the reversible photoswitching of the bioelectrocatalytic functions of redox proteins by the modification of proteins with photoisomerizable units or by the integration of proteins with photoisomerizable environments; (ii) the integration of natural photosynthetic reaction centers with electrodes and the construction of photobioelectrochemical cells and photobiofuel cells; and (iii) the synthesis of biomolecule/semiconductor quantum dots hybrid systems and their immobilization on electrodes to yield photobioelectrochemical and photobiofuel cell elements. The fundamental challenge in the tailoring of optobioelectronic systems is the development of means to electrically contact photoactive biomolecular assemblies with the electrode supports. Different methods to establish electrical communication between the photoactive biomolecular assemblies and electrodes are discussed. These include the nanoscale engineering of the biomolecular nanostructures on surfaces, the development of photoactive molecular wires and the coupling of photoinduced electron transfer reactions with the redox functions of proteins. The different possible applications of optobioelectronic systems are discussed, including their use as photosensors, the design of biosensors, and the construction of solar energy conversion and storage systems.

[18F]fluoromisonidazole and a new PET system with semiconductor detectors and a depth of interaction system for intensity modulated radiation therapy for nasopharyngeal cancer.

PURPOSE: The impact of a new type of positron emission tomography (New PET) with semiconductor detectors using 18F-labeled fluoromisonidazole (FMISO)-guided intensity modulated radiation therapy (IMRT) was compared with a state-of-the-art PET/computed tomography (PET/CT) system in nasopharyngeal cancer (NPC) patients. METHODS AND MATERIALS: Twenty-four patients with non-NPC malignant tumors (control group) and 16 patients with NPC were subjected to FMISO-PET. The threshold of the tumor-to-muscle (T/M) ratio in each PET scan was calculated. The hypoxic volume within the gross tumor volume (GTVh) was determined using each PET (NewPETGTVh and PET/CTGTVh, respectively). Dose escalation IMRT plans prescribing 84 Gy to each GTVh were carried out. RESULTS: The threshold of the T/M ratio was 1.35 for New PET and 1.23 for PET/CT. The mean volume of NewPETGTVh was significantly smaller than that of PET/CTGTVh (1.5±1.6 cc vs 4.7±4.6 cc, respectively; P=.0020). The dose escalation IMRT plans using New PET were superior in dose distribution to those using PET/CT. Dose escalation was possible in all 10 New PET-guided plans but not in 1 PET/CT-guided plan, because the threshold dose to the brainstem was exceeded. CONCLUSIONS: New PET was found to be useful for accurate dose escalation in FMISO-guided IMRT for patients with NPC.

Acoustic phonon-limited resistivity of spin-orbit coupled two-dimensional electron gas: the deformation potential and piezoelectric scattering.

We study the interaction between electron and acoustic phonons in a Rashba spin-orbit coupled two-dimensional electron gas using Boltzmann transport theory. Both the deformation potential and piezoelectric scattering mechanisms are considered in the Bloch-Grüneisen (BG) regime as well as in the equipartition (EP) regime. The effect of the Rashba spin-orbit interaction on the temperature dependence of the resistivity in the BG and EP regimes is discussed. We find that the effective exponent of the temperature dependence of the resistivity in the BG regime decreases due to spin-orbit coupling.

Variable magnification dual lens electron holography for semiconductor junction profiling and strain mapping.

Dual lens operation for electron holography, which was developed previously (Wang et al., Ultramicroscopy 101 (2004) 63-72; US patent: 7,015,469 B2 (2006)), is re-investigated for bright field (junction profiling) and dark field (strain mapping) electron holography using FEI instrumentation (i.e. F20 and Titan). It is found that dual lens operation provides a wide operational range for electron holography. In addition, the dark field image tilt increases at high objective lens current to include Si <004> diffraction spot. Under the condition of high spatial resolution (1 nm fringe spacing), a large field of view (450 nm), and high fringe contrast (26%) with dual lens operation, a junction map is obtained and strain maps of Si device on <220> and <004> diffraction are acquired. In this paper, a fringe quality number, N', which is number of fringe times fringe contrast, is proposed to estimate the quality of an electron hologram and mathematical reasoning for the N' number is provided.

Photovoltaic effect of light carrying orbital angular momentum on a semiconducting stripe.

We investigate the influence of a light beam carrying an orbital angular momentum on the current density of an electron wave packet in a semiconductor stripe. It is shown that due to the photo-induced torque the electron density can be deflected to one of the stripe sides. The direction of the deflection is controlled by the direction of the light orbital momentum. In addition the net current density can be enhanced. This is a photovoltaic effect that can be registered by measuring the generated voltage drop across the stripe and/or the current increase.

An innovative and efficient method to control the shape of push-pull membrane deformable mirror.

We carry out performance characterisation of a commercial push and pull deformable mirror with 48 actuators (Adaptica Srl). We present a detailed description of the system as well as a statistical approach on the identification of the mirror influence function. A new efficient control algorithm to induce the desired wavefront shape is also developed and comparison with other control algorithms present in literature has been made to prove the efficiency of the new approach.

Characterization and modeling analysis for metal-semiconductor-metal GaAs diodes with Pd/SiO2 mixture electrode.

Characterization and modeling of metal-semiconductor-metal (MSM) GaAs diodes using to evaporate SiO2 and Pd simultaneously as a mixture electrode (called M-MSM diodes) compared with similar to evaporate Pd as the electrode (called Pd-MSM diodes) were reported. The barrier height (φ(b)) and the Richardson constant (A*) were carried out for the thermionic-emission process to describe well the current transport for Pd-MSM diodes in the consideration of the carrier over the metal-semiconductor barrier. In addition, in the consideration of the carrier over both the metal-semiconductor barrier and the insulator-semiconductor barrier simultaneously, thus the thermionic-emission process can be used to describe well the current transport for M-MSM diodes. Furthermore, in the higher applied voltage, the carrier recombination will be taken into discussion. Besides, a composite-current (CC) model is developed to evidence the concepts. Our calculated results are in good agreement with the experimental ones.

Biotin-functionalized semiconducting polymer in an organic field effect transistor and application as a biosensor.

This report presents biotin-functionalized semiconducting polymers that are based on fluorene and bithiophene co-polymers (F8T2). Also presented is the application of these polymers to an organic thin film transistor used as a biosensor. The side chains of fluorene were partially biotinylated after the esterification of the biotin with corresponding alcohol-groups at the side chain in F8T2. Their properties as an organic semiconductor were tested using an organic thin film transistor (OTFT) and were found to show typical p-type semiconductor curves. The functionality of this biosensor in the sensing of biologically active molecules such as avidin in comparison with bovine serum albumin (BSA) was established through a selective decrease in the conductivity of the transistor, as measured with a device that was developed by the authors. Changes to the optical properties of this polymer were also measured through the change in the color of the UV-fluorescence before and after a reaction with avidin or BSA.

Engineering light: advances in wavelength conversion materials for energy and environmental technologies.

Upconversion photoluminescence (UC) occurs in optical materials that are capable of absorbing low energy photons and emitting photons of higher energy and shorter wavelength, while downconversion (DC) materials may absorb one high energy photon and emit two of lower energy for quantum yields exceeding unity. These wavelength conversion processes allow us to transform electromagnetic radiation so it may be more effectively utilized by light-capturing devices and materials. Progress in designing more efficient organic and inorganic photochemical conversion systems has initiated a recent surge in attempts to apply these processes for practical uses, including enhancement of many energy and environmental technologies. In this review, we introduce important concepts in UC and DC materials and discuss the current status and challenges toward the application of wavelength conversion to solar cells, photocatalysis, and antimicrobial surfaces.

Electronics and optoelectronics of two-dimensional transition metal dichalcogenides.

The remarkable properties of graphene have renewed interest in inorganic, two-dimensional materials with unique electronic and optical attributes. Transition metal dichalcogenides (TMDCs) are layered materials with strong in-plane bonding and weak out-of-plane interactions enabling exfoliation into two-dimensional layers of single unit cell thickness. Although TMDCs have been studied for decades, recent advances in nanoscale materials characterization and device fabrication have opened up new opportunities for two-dimensional layers of thin TMDCs in nanoelectronics and optoelectronics. TMDCs such as MoS(2), MoSe(2), WS(2) and WSe(2) have sizable bandgaps that change from indirect to direct in single layers, allowing applications such as transistors, photodetectors and electroluminescent devices. We review the historical development of TMDCs, methods for preparing atomically thin layers, their electronic and optical properties, and prospects for future advances in electronics and optoelectronics.

A stable solution-processed polymer semiconductor with record high-mobility for printed transistors.

Microelectronic circuits/arrays produced via high-speed printing instead of traditional photolithographic processes offer an appealing approach to creating the long-sought after, low-cost, large-area flexible electronics. Foremost among critical enablers to propel this paradigm shift in manufacturing is a stable, solution-processable, high-performance semiconductor for printing functionally capable thin-film transistors - fundamental building blocks of microelectronics. We report herein the processing and optimisation of solution-processable polymer semiconductors for thin-film transistors, demonstrating very high field-effect mobility, high on/off ratio, and excellent shelf-life and operating stabilities under ambient conditions. Exceptionally high-gain inverters and functional ring oscillator devices on flexible substrates have been demonstrated. This optimised polymer semiconductor represents a significant progress in semiconductor development, dispelling prevalent skepticism surrounding practical usability of organic semiconductors for high-performance microelectronic devices, opening up application opportunities hitherto functionally or economically inaccessible with silicon technologies, and providing an excellent structural framework for fundamental studies of charge transport in organic systems.

Disentangling neighbors and extended range density oscillations in monatomic amorphous semiconductors.

High energy x-ray diffraction measurements of pure amorphous Ge were made and its radial distribution function (RDF) was determined at high resolution, revealing new information on the atomic structure of amorphous semiconductors. Fine structure in the second peak in the RDF provides evidence that a fraction of third neighbors are closer than some second neighbors; taking this into account leads to a narrow distribution of tetrahedral bond angles, (8.5 ± 0.1)°. A small peak which appears near 5 Å upon thermal annealing shows that some ordering in the dihedral bond-angle distribution takes place during structural relaxation. Extended range order is detected (in both a-Ge and a-Si) which persists to beyond 20 Å, and both the periodicity and its decay length increase upon thermal annealing. Previously, the effect of structural relaxation was only detected at intermediate range, involving reduced tetrahedral bond-angle distortions. These results enhance our understanding of the atomic order in continuous random networks and place significantly more stringent requirements on computer models intending to describe these networks, or their alternatives which attempt to describe the structure in terms of an arrangement of paracrystals.