Ultra-High Selective Gas Sensors: novel approaches and future developments Martin W. G. Hoffmann 1,3,5 , Joan Daniel Prades 3 , Leonhard Mayrhofer 2 , Francisco Hernan- dez-Ramirez 3,4 , Tommi T. Järvi 2 , Michael Moseler 2 , Andreas Waag 1,5 , and Hao Shen 1,5 1 Institute of Semiconductor Technology, Braunschweig University of Technology, D-38106, Braunschweig, Germany. Phone: +49 (0) 531 391- 3773 Email: a.waag@tu-bs.de 2 Fraunhofer Institute for Mechanics of Materials IWM D-79108, Freiburg, Germany 3 Department of Electronics, University of Barcelona, E-08028 Barcelona, Spain 4 Department of Advanced Materials for Energy Applications, Catalonia Institute for Energy Research (IREC), E-08930 Bar- celona, Spain 5 Laboratory of Nanometrology, Braunschweig University of Technology Abstract Purely inorganic gas sensors are generally facing the problem of a low selectivity. Systematically overcoming this problem would put sensor technology on a whole new level of performance. Organic–inorganic hybrid gas sensors are potentially offering outstanding perfor- mance in terms of selectivity and sensitivity towards single gas species. The enormous variety of organic functionalities enables novel flexibility of active sensor surfaces compared to commonly used pure inorganic materials, but on the other hand the hybrid combina- tion of different materials goes along with a substantial increase in system complexity. Even today a sensor de- sign with predictable selectivity and sensitivity is not yet possible. In this work, a strategy for the development of highly selective gas sensors is proposed and demon- strated. As an example, an ultra-selective NO 2 sensor is realized based on self-assembled monolayer (SAM)-modified semiconductor nanowires (NWs). The crucial chemical and electronic parameters for an effec- tive interaction between the sensor and different gas species are identified using density functional theory simulations. The theoretical findings are consistent with the experimentally observed extraordinary selectivity and sensitivity of the amine-terminated SnO 2 NW to- wards NO 2 . The energetic position of the SAM–gas frontier orbitals with respect to the NW Fermi level is the key to ensure or impede an efficient charge transfer between the NW and the gas. As this condition strongly depends on the gas species and the sensor system, these insights into the charge transfer mechanisms can have a substantial impact on the development of highly selec- tive hybrid gas sensors. 1. Introduction The modern anthropogenic environmental hazards, like toxic gases, are implicated in a range of impacts on human health. The selective detection of a certain predefined gas species is the most critical requirement in different fields like pollution and food control, health care, security or in- dustrial process control. However, predictive strategies towards the development of highly selective nanostructured gas sensors are still missing [1]. Organo-functionalized low dimensional materials could already show improved characteristics [2] in this field compared to commonly used purely inorganic materials or heterostructures that usually suffer from unspecific surface interactions with the target gases. Here, we present a sensor system composed of semiconductor nanowire (NW) sur- faces with defined organic self-assembled monolayers (SAMs) in order to accomplish exclusive chemical and electronic conditions for the selective detection of a single gas species. We demonstrate that SnO 2 NWs modified with amine terminated SAMs show both extraordinary selectivi- ty and sensitivity towards NO2 at room temperature. This system can not only serve as a novel efficient and selective NO 2 sensor, but also as a model system for the theoretical reconstruction of crucial sensor-target interactions. Our simulations reveal that an energy level alignment of the SAM-gas system with the Fermi level of the SAM–NW system is the key to understand and achieve high detection selectivity and are consistent with our experimentally ob- served results. The here reported results show a convincing potential for the development of theoretically designed se- lective gas sensors, with flexible organic surface design and predictable response [3]. 3. Results and discussion To develop a strategy for coherent experimental and theoretical gas sensor design, an optimal organic function- alization of SnO 2 NWs for selective NO 2 detection was first evaluated experimentally. The identified system then served as starting point to build up a theoretical NW–SAM–gas model and define critical parameters for selective sensing interactions. Amines, due to their electron donating character, were chosen as functional units to achieve strong surface–gas interactions with the electron affine NO 2 target. Amine SAMs showed a very high sensi- tivity and selectivity towards a low NO2 concentration (400 ppb; Figure 2), whereas only small or mostly no response was observed for higher concentrations of the other gases (SO 2 , NO, NH 3 , ethanol, CO, and CO 2 ; concentrations be- tween 2 ppm and 5%). Among all amines, the en-APTAS 1 functionalization with a primary and secondary group uni- Extended Abstracts of the 2014 International Conference on Solid State Devices and Materials, Tsukuba, 2014, - 576 - D-1-6 pp576-577