Dissipated power in metal oxide nanowires (rNW<45 nm) often causes important self-heating effects and as a result, undesired aging and failure of the devices. Nevertheless, this effect can be used to optimize the sensing conditions for the detection of various gaseous species, avoiding the requirement of external heaters. In this letter, the sensing capabilities of self-heated individual SnO2 nanowires toward NO2 are presented. These proof-of-concept systems exhibited responses nearly identical to those obtained with integrated microheaters, demonstrating the feasibility of taking advantage of self-heating in nanowires to develop ultralow power consumption integrated devices.

The responses of individual ZnO nanowires to UV light demonstrate that the persistent photoconductivity (PPC) state is directly related to the electron-hole separation near the surface. Our results demonstrate that the electrical transport in these nanomaterials is influenced by the surface in two different ways. On the one hand, the effective mobility and the density of free carriers are determined by recombination mechanisms assisted by the oxidizing molecules in air. This phenomenon can also be blocked by surface passivation. On the other hand, the surface built-in potential separates the photogenerated electron-hole pairs and accumulates holes at the surface. After illumination, the charge separation makes the electron-hole recombination difficult and originates PPC. This effect is quickly reverted after increasing either the probing current (self-heating by Joule dissipation) or the oxygen content in air (favouring the surface recombination mechanisms). The model for PPC in individual nanowires presented here illustrates the intrinsic potential of metal oxide nanowires to develop optoelectronic devices or optochemical sensors with better and new performances.

Silicon nanowires—filamentary crystals with a very high ratio of length to diameter (see figure)—allow growth of the wurtzite crystalline phase, which is also semiconducting for silicon. The association of this phenomenon with the competition between surface energy and pressure effects occurring at diameters below 150 nm is shown.

Making more energy efficient technologies is still far from having those envisaged ubiquitous deployments (so called the Internet of Things, or the Industrial Internet), which will enable optimal industrial operation, and will contribute to improve the social welfare.Today, it is possible to build a device which features this industrial wireless performance, and is able to in-node analyze the acquired data. However, energy-dimensioning the device in order to meet the application requirements is not an easy task, especially when the reliability claimed for industrial applications faces up to the uncertainty introduced by energy harvesting.Modeling and dimensioning the energy consumption of an application at pre-deployment or pre-production stages is of utmost importance considering the critical requirements of IoT applications in terms of reduced cost, life-time, and available energy. This paper presents a comprehensive model for the power consumption of wireless sensor nodes that accounts for all the energy expenditures at system-level: communications, acquisition and processing. The model is only based on parameters that can be empirically quantified, once the platform (i.e., technology) and the application (i.e., operation conditions) are defined. This results in a new framework for the study and analysis the energy live-cycle within the applications, suitable to determine in advance the specific weight of application parameters and to understand the tolerance margins and trade-offs in the system. Index TermsLow power models, Sensor system networks, networkable sensors, sensor system integration B. Martínez is with

SnO 2 nanocrystals were prepared by injecting a hydrolyzed methanol solution of SnCl 4 into a tetradecene solution of dodecylamine. The resulting materials were annealed at 500 °C, providing 6-8 nm nanocrystals. The latter were used for fabricating NO 2 gas sensing devices, which displayed remarkable electrical responses to as low as 100 ppb NO 2 concentration. The nanocrystals were characterized by conductometric measurements, X-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance (EPR), and cathodoluminescence (CL) spectroscopy. The results, interpreted by means of molecular modeling in the frame of the density functional theory (DFT), indicated that the nanocrystals contain topographically well-defined surface oxygen vacancies. The chemisorption properties of these vacancies, studied by DFT modeling of the NO 2 /SnO 2 interaction, suggested that the in-plane vacancies facilitate the NO 2 adsorption at low operating temperatures, while the bridging vacancies, generated by heat treatment at 500 °C, enhance the charge transfer from the surface to the adsorbate. The behavior of the oxygen vacancies in the adsorption properties revealed a gas response mechanism in oxide nanocrystals more complex than the size dependence alone. In particular, the nanocrystals surface must be characterized by enhanced transducing properties for obtaining relevant gas responses.

Low power consumption and reliable selectivity are the two main requirements for gas sensors to be applicable in mobile devices. [1] These technological platforms, e.g. smart phones or wireless sensor platforms will facilitate personalized detection of environmental and health conditions, and hence becoming the basis of the future core technology of ubiquitous sensing. Even today, health control as well as environmental monitoring is relying on immobile and complex detection systems with very limited availability in space and time. Recent works have shown promising concepts to realize selfpowered gas sensors that are capable of detecting gases without the need of external power sources to Submitted to 2 activate the sensor-gas interaction or to actively generate a read out signal. [2,3] These sensors drastically reduce power consumption compared to conventional semiconductor gas sensors and additionally reduce the required space for integration. All these attempts so far were based on purely nano structured inorganic metal oxide sensor materials that provide a good sensitivity towards different gases due to their high surface-to-volume ratio. However, due to their non-selective sensing mechanism based on oxygen vacancy-gas interactions, these purely inorganic sensors cannot accomplish a meaningful gas selectivity. [4,5] High selectivities towards single gas species have been recently reported via modifying the inorganic surface of nanostructured semiconductors with a defined organic functionality. [6][7][8][9] Theoretical simulations based on ab-initio density functional theory (DFT) for a system composed of SnO2 NWs modified with a defined self assembled monolayer (SAM) elucidated the reason for the high selectivity of such gas sensor: the energetic position of the SAM-gas frontier orbitals with respect to the NW Fermi level have been identified to be the crucial factor to ensure an efficient charge transfer upon gas-SAM binding interactions and thus to sense or discriminate a certain gas species. [7] The high flexibility of organic surface modifications in terms of functional groups as well as their sterical and electronic structure possibly might enable the targeted design of various specific gas sensors. However, all organic surface modified sensor systems so far are based on compact conductometric or field effect transistor (FET) sensor concepts that still require a remarkable amount of energy to generate a sensor signal (e.g. by applying a source-drain current). Up to date, none of the semiconductor based gas sensor systems could accomplish both, the selfpowered/low powered sensor operation and highly selective gas detection within a single and compact device.In this work, we present a semiconductor based gas sensor concept that combines the two substantial requirements of mobile gas sensing in a singular sensor device: self-powered operation combined with high gas selectivity. Beyond the combination of self-powered sensing and high selectivity, also a very high sensitivity could also been demonst...

Gallium nitride (GaN) light-emitting-diode (LED) technology has been the revolution in modern lighting. In the last decade, a huge global market of efficient, long-lasting, and ubiquitous white light sources has developed around the inception of the Nobel-prize-winning blue GaN LEDs. Today, GaN optoelectronics is developing beyond solid-state lighting, leading to new and innovative devices, e.g., for microdisplays, being the core technology for future augmented reality and visualization, as well as point light sources for optical excitation in communications, imaging, and sensing. This explosion of applications is driven by two main directions: the ability to produce very small GaN LEDs (micro-LEDs and nano-LEDs) with high efficiency and across large areas, in combination with the possibility to merge optoelectronic-grade GaN micro-LEDs with silicon microelectronics in a hybrid approach. GaN LED technology is now even spreading into the realm of display technology, which has been occupied by organic LEDs and liquid crystal displays for decades. In this review, the technological transition toward GaN micro- and nanodevices beyond lighting is discussed including an up-to-date overview on the state of the art.

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