Carlo Carraro

Strategies for the International Protection of the Environment

This paper provides a general framework for studying the profitability and stability of international agreements to protect the environment in the presence of trans-frontier or global pollution. N countries are assumed to bargain on emission control. Each country decides whether or not to coordinate its strategy with other countries. A coalition is formed when both profitability and stability (no free riding) conditions are satisfied. The analysis shows that such coalitions exist but that only a small number of countries decide to cooperate. The paper thus explores the possibility of expanding such coalitions through transfers that induce other countries to cooperate. It is shown that large stable coalitions exist when low environmental interdependence exists and/or when the environmental damage functions are near-separable with respect to domestic and imported emissions. It is also shown that there are cases in which environmental negotiations can achieve substantial emission control even if countries behave non-cooperatively.

Strong interlayer coupling in van der Waals heterostructures built from single-layer chalcogenides

Significance A new class of heterostructures consisting of layered transition metal dichalcogenide components can be designed and built by van der Waals (vdW) stacking of individual monolayers into functional multilayer structures. Nonetheless, the optoelectronic properties of this new type of vdW heterostructure are unknown. Here, we investigate artificial semiconductor heterostructures built from single-layer WSe2 and MoS2. We observe spatially direct absorption but spatially indirect emission in this heterostructure, with strong interlayer coupling of charge carriers. The coupling at the hetero-interface can be readily tuned by inserting hexagonal BN dielectric layers into the vdW gap. The generic nature of this interlayer coupling is expected to yield a new family of semiconductor heterostructures having tunable optoelectronic properties through customized composite layers. Semiconductor heterostructures are the fundamental platform for many important device applications such as lasers, light-emitting diodes, solar cells, and high-electron-mobility transistors. Analogous to traditional heterostructures, layered transition metal dichalcogenide heterostructures can be designed and built by assembling individual single layers into functional multilayer structures, but in principle with atomically sharp interfaces, no interdiffusion of atoms, digitally controlled layered components, and no lattice parameter constraints. Nonetheless, the optoelectronic behavior of this new type of van der Waals (vdW) semiconductor heterostructure is unknown at the single-layer limit. Specifically, it is experimentally unknown whether the optical transitions will be spatially direct or indirect in such hetero-bilayers. Here, we investigate artificial semiconductor heterostructures built from single-layer WSe2 and MoS2. We observe a large Stokes-like shift of ∼100 meV between the photoluminescence peak and the lowest absorption peak that is consistent with a type II band alignment having spatially direct absorption but spatially indirect emission. Notably, the photoluminescence intensity of this spatially indirect transition is strong, suggesting strong interlayer coupling of charge carriers. This coupling at the hetero-interface can be readily tuned by inserting dielectric layers into the vdW gap, consisting of hexagonal BN. Consequently, the generic nature of this interlayer coupling provides a new degree of freedom in band engineering and is expected to yield a new family of semiconductor heterostructures having tunable optoelectronic properties with customized composite layers.

Self-assembled monolayers as anti-stiction coatings for MEMS: characteristics and recent developments

Despite significant advances in surface micromachining technology, stiction remains a key problem, severely limiting the realization and reliability of many micro-electro-mechanical systems (MEMS) devices. In this article, we focus on self-assembled monolayers as release and anti-stiction coatings for MEMS. Their formation mechanism, the microstructure coating process, and the characteristics of the coated microstructures are described, followed by a discussion of the current limitations, areas for improvements and recent progress for this coating technology.

WITCH - A World Induced Technical Change Hybrid Model

The need for a better understanding of future energy scenarios, of their compatibility with the objective of stabilizing greenhouse gas concentrations, and of their links with climate policy, calls for the development of hybrid models. Hybrid because both the technological detail typical of Bottom Up (BU) models and the long run dynamics typical of Top Down (TD) models are crucially necessary. We present WITCH � World Induced Technical Change Hybrid model�� a neoclassical optimal growth model (TD) with energy input detail (BU). The model endogenously accounts for technological progress, both through learning curves affecting prices of new vintages of capital and through R&D investments. In addition, the model captures the main economic interrelationships between world regions and is designed to analyze the optimal economic and environment policies in each world region as the outcome of a dynamic game. This paper provides a detailed description of the WITCH model, of its baseline, and of the model calibration procedure.

Hole selective MoOx contact for silicon solar cells

Using an ultrathin (∼ 15 nm in thickness) molybdenum oxide (MoOx, x < 3) layer as a transparent hole selective contact to n-type silicon, we demonstrate a room-temperature processed oxide/silicon solar cell with a power conversion efficiency of 14.3%. While MoOx is commonly considered to be a semiconductor with a band gap of 3.3 eV, from X-ray photoelectron spectroscopy we show that MoOx may be considered to behave as a high workfunction metal with a low density of states at the Fermi level originating from the tail of an oxygen vacancy derived defect band located inside the band gap. Specifically, in the absence of carbon contamination, we measure a work function potential of ∼ 6.6 eV, which is significantly higher than that of all elemental metals. Our results on the archetypical semiconductor silicon demonstrate the use of nm-thick transition metal oxides as a simple and versatile pathway for dopant-free contacts to inorganic semiconductors. This work has important implications toward enabling a novel class of junctionless devices with applications for solar cells, light-emitting diodes, photodetectors, and transistors.

The international dimension of environmental policy

Environmental economics was the subject of a successful research programme in the 1960s and 1970s. This programme dealt with such a wide range of environmental issues and policy problems that Partha Dasgupta, in a recent survey of the subject [see Dasgupta (1990)], claimed the primary environmental issues are now ‘very cold’ as topics for analytical investigation, and ‘dead’ as research problems. The examples provided by Dasgupta, from the foundations of environmental taxes, to the informative characteristics of tradeable emission permits, to the notion of ‘sustainable development’, are certainly appropriate. In the last few years, however, scientists have highlighted a set of ‘new’ environmental problems such as ozone layer depletion, global warming, deforestation, the loss of bio-diversity which share some common features: close links with economic development; intrinsic uncertainty; a pervasive international dimension. These issues require further research and development as they raise new questions, or propose old questions in a new context. Many of these questions derive from the international dimension of environmental phenomena, an issue which is interesting both in terms of theory and policy. As a consequence, we think that problems originating from the international dimension of environmental phenomena and policies are neither cold nor dead, but deserve serious analysis which must be integrated with the existing analytical and policy tools. In this paper, we open by reviewing several major environmental

Silver Dendrites from Galvanic Displacement on Commercial Aluminum Foil As an Effective SERS Substrate

A silver galvanic displacement process on commercial aluminum foil has been carried out to produce cost-effective SERS substrates. The process is based on an extremely simple redox process where aluminum is oxidized while silver ions are reduced, yielding a final silver dendritic structure that offers a large surface area-to-volume ratio. XPS measurements confirmed the metallic nature of the formed dendrites. SERS substrates were fabricated by spreading of the dendrites on double side Scotch tape attached to a paper slide. Three different thiols were incubated to achieve SAM formation on the Ag dendrites and measured by Raman spectroscopy. The obtained spectra presented well resolved bands and provided valuable information regarding the orientation of the thiols. The high Raman intensity also demonstrates the high enhancement capacities of the produced silver structures. The overall method is cost-effective and allows the use of silver dendrite paste for the mass production of SERS-active substrates, including on flexible substrates and/or via screen printing.

Evidence of Structural Strain in Epitaxial Graphene Layers on 6H-SiC(0001)

The early stages of epitaxial graphene layer growth on the Si-terminated 6H-SiC (0001) are investigated by Auger electron spectroscopy (AES) and depolarized Raman spectroscopy. The selection of the depolarized component of the scattered light results in a significant increase in the C-C bond signal over the second order SiC Raman signal, which allows us to resolve submonolayer growth, including individual, localized C=C dimers in a diamondlike carbon matrix for AES C/Si ratio of approximately 3, and a strained graphene layer with delocalized electrons and Dirac single-band dispersion for AES C/Si ratio >6. The linear strain, measured at room temperature, is found to be compressive, which can be attributed to the large difference between the coefficients of thermal expansion of graphene and SiC. The magnitude of the compressive strain can be varied by adjusting the growth time at fixed annealing temperature.

Climate Policy and the Optimal Balance between Mitigation, Adaptation and Unavoided Damage

It has become commonly accepted that a successful climate strategy should compound mitigation and adaptation. The accurate combination between adaptation and mitigation that can best address climate change is still an open question. This paper proposes a framework that integrates mitigation, adaptation, and climate change residual damages into an optimisation model. This set-up is used to provide some insights on the welfare maximising resource allocation between mitigation and adaptation, on their optimal timing, and on their marginal contribution to reducing vulnerability to climate change. The optimal mix between three different adaptation modes (reactive adaptation, anticipatory adaptation, and investment in innovation for adaptation purposes) within the adaptation bundle is also identified. Results suggest that the joint implementation of mitigation and adaptation is welfare improving. Mitigation should start immediately, whereas adaptation somewhat later. It is also shown that in a world where the probability of climate-related catastrophic events is small and where decision makers have a high discount rate, adaptation is unambiguously the preferred option. Adaptation needs, both in developed and developing countries, will be massive, especially during the second half of the century. Most of the adaptation burden will be on developing countries. International cooperation is thus required to equally distribute the costs of adaptation.

The Role of R&D and Technology Diffusion in Climate Change Mitigation: New Perspectives Using the Witch Model

This paper uses the WITCH model, a computable general equilibrium model with endogenous technological change, to explore the impact of various climate policies on energy technology choices and the costs of stabilising greenhouse gas concentrations. Current and future expected carbon prices appear to have powerful effects on R&D spending and clean technology diffusion. Their impact on stabilisation costs depends on the nature of R&D: R&D targeted at incremental energy efficiency improvements has only limited effects, but R&D focused on the emergence of major new low-carbon technologies could lower costs drastically if successful – especially in the non-electricity sector, where such low-carbon options are scarce today. With emissions coming from multiple sources, keeping a wide range of options available matters for stabilisation costs more than improving specific technologies. Due to international knowledge spillovers, stabilisation costs could be further reduced through a complementary, global R&D policy. However, a strong price signal is always required.