C. Baratto

Adsorption effects of NO2 at ppm level on visible photoluminescence response of SnO2 nanobelts

The visible photoluminescence (PL) of tin oxide nanobelts is quenched by nitrogen dioxide at ppm level in a fast (time scale order of seconds) and reversible way. Besides, the response seems highly selective toward humidity and other polluting species, such as CO and NH3. We believe that adsorbed gaseous species that create surface states can quench PL by creating competitive nonradiative paths. A comparison between conductometric and PL response suggests that the two responses are ascribable to different adsorption processes.

NO2 monitoring at room temperature by a porous silicon gas sensor

Abstract A study on reactivity of p + porous silicon layers (PSL) to different gas atmosphere has been carried out. Substrate doping was 5–15 mΩ cm and 0.5 Ω cm, porosity ranged from 30 to 75% and the thickness of the porous layers was 20–30 μm. Three different processes to insure good electrical contact are proposed and discussed. PSL were kept at constant bias and current variations due to interaction with different concentrations of NO 2 were monitored at constant relative humidity (R.H.). Measurements were performed at room temperature (R.T.) and at atmospheric pressure. Concentrations as low as 1 ppm were tested, but the high sensitivity of the sensor makes possible to test lower values. The recovery time of the sensor is of the order of one minute. Response to interfering gases (methanol, humidity, CO, CH 4 , NO, NO 2 ) has been examined also. In-situ FTIR spectroscopy in NO 2 atmosphere shows a fully reversible free-carrier detrapping in the IR region, confirming the validity of the models proposed in the recent past for electrical conduction in mesoporous silicon.

Single crystal ZnO nanowires as optical and conductometric chemical sensor

ZnO nanowires were deposited using the vapour phase technique. The nanowires cover the substrate and appear uniform in morphology with lengths of up to several micrometres and uniform lateral size of the order of tenths of a nanometre. The electrical and optical properties of ZnO nanowires have been characterized in the presence of nitrogen oxide. Electrical measurements highlight a remarkable response even at low operating temperatures, with detection limits lower than 200?ppb and an optimal operating temperature of 100?C, while the ZnO nanowire photoluminescence is reversibly quenched by the introduction of a few ppm of NO2 even at room temperature.

Front-side micromachined porous silicon nitrogen dioxide gas sensor

Abstract In this paper a new C-MOS compatible fabrication process is presented for a nitrogen dioxide porous silicon (PS) sensor. Electrically insulated PS sensing layers and even free-standing PS membranes have been obtained by front side electrochemical micromachining, reducing in this way electrical leakage towards the crystalline substrate. The electrical behavior of the device in a controlled environment was measured by means of a volt–amperometric technique at constant bias. A huge variation of the current was detected at RT for NO 2 concentrations as low as 200 ppb (Δ G / G =111). The interference of humidity, ethanol, methanol, CO and ozone is also discussed.

Gas detection with a porous silicon based sensor

Abstract Porous silicon (PS) layers with 60% porosity and 80 μm thick were prepared from n-type silicon wafer. We present the sensitivity of PS photoluminescence to 250 ppm of carbon monoxide. Besides the variation of conductivity of the device due to presence of organic vapors such as chloroform, methanol, ethanol and toluene have been carried out.

Metal Oxide Nanowire and Thin-Film-Based Gas Sensors for Chemical Warfare Simulants Detection

This work concerns with metal oxide (MOX) gas sensors based on nanowires and thin films. We focus on chemical warfare agents (CWAs) detection to compare these materials from the functional point-of-view. We work with different chemicals including simulants for Sarin nerve agents, vescicant gases, cyanide agents, and analytes such as ethanol, acetone, ammonia, and carbon monoxide that can be produced by everyday activities causing false alarms. Explorative data analysis has been used to demonstrate the different sensing performances of nanowires and thin films. Within the chosen application, our analysis reveal that the introduction of nanowires inside the array composed by thin films can improve its sensing capability. Cyanide simulants have been detected at concentrations close to 1 ppm, lower than the Immediately Dangerous for Life and Health (IDLH) value of the respective warfare agent. Higher sensitivity has been obtained to simulants for Sarin and vescicant gases, which have been detected at concentrations close or even lower than 100 ppb. Results demonstrate the suitability of the proposed array to selectively detect CWA simulants with respect to some compounds produced by everyday activities.

Gold-catalysed porous silicon for NOx sensing

Abstract Porous silicon (PS), obtained by electrochemical anodization of an n-type silicon wafer, was catalysed by sputtering gold onto the surface (4, 8, 15 and 40-nm nominal thickness). Investigation by Rutherford backscattering spectroscopy (RBS) and by electron microscopy showed that gold did not form a continuous layer, but rather formed clusters penetrating into the pores of PS by about 1 μm. A variation of the sample conductivity in the presence of a few parts per million of NO2 and NO was recorded at room temperature. We demonstrated that, as a result of Au catalysation, PS is suitable for sensing nitrogen oxides with negligible influence by interfering gases such as CO, CH4 or methanol. Indeed, we found that humidity appreciably affected the response.

Ozone adsorption on carbon nanotubes: Ab initio calculations and experiments

The electrical response to O3 of 150-nm-thick carbon nanotube (CNT) thin films prepared by radio frequency-plasma enhanced chemical vapor deposition has been investigated at different operating temperatures starting from the room temperature. The interaction between ozone molecules and a carbon nanotube film is studied by means of first-principles calculations. Experiments show that CNT films are responsive to O3 with a decrease of the resistance similar to that observed for NO2. Our theoretical results suggest the interaction to be pretty strong, as shown by a relatively short equilibrium molecule-tube distance, as well as by an appreciable binding energy and charge transfer from the tube to the adsorbed molecule. The analysis of the density of states shows that a peak in proximity of the nanotube Fermi level is induced by the ozone adsorption. This effect enhances the p-type character of the nanotube and, therefore, the conductivity of the whole film increases, in excellent consistency with the experime...

Room-temperature gas sensing based on visible photoluminescence properties of metal oxide nanobelts

The gas-sensitive light emission properties of structurally uniform and single-crystal tin dioxide and zinc oxide nanobelts are reported. Strong visible gas-sensitive photoluminescence (PL) emission was observed and the influence of the environment was studied by comparing measurements obtained in the presence of air and a low concentration of NO2 as a function of temperature from 300 to 500 K. Visible photoluminescence was reversibly quenched by nitrogen dioxide at the 5 ppm level at room temperature. Preliminary results on time-resolved PL properties of tin dioxide nanobelts are also shown.

Monitoring penetration of ethanol in a porous silicon microcavity by photoluminescence interferometry

A photoluminescent porous silicon microcavity is exposed to saturated vapor of ethanol. The ethanol substitutes the air inside the pores giving rise to a progressive monotonic redshift of the interference pattern of the photoluminescence spectrum. On the other hand, the photoluminescence intensity of the cavity peak oscillates in time. Both effects can be explained in terms of a very simple model based on the progressive change of the effective refractive index of single layers of the cavity. The change is due to the difference between the index of refraction of air and ethanol. The result suggests that a porous silicon microcavity can be a tool to study the dynamics of gas penetration into porous silicon since it allows a monitoring of the depth reached by the ethanol at any given time.