A micro-system based on glass-nanoporous silicon for optical sensing of organic solvent vapor

We present a recent experimental study on the application of nanoporous silicon (np-Si) to an optical vapor sensor. We fabricated the micro-system based on a glass-nanoporous silicon layer on a p(+)-type silicon wafer. To check the selectivity and sensitivity of the np-Si layer to organic vapors, we prepared three types of np-Si layer samples--a single layer, distributed Bragg reflector (DBR) layer, and microcavity layer--and investigated its reflectance spectra upon exposure to different concentrations of various organic vapors. When the np-Si layer samples were exposed to the organic vapors, a red-shift occurred in the reflectance spectrum, and we determined that this red-shift can be attributed to the changes in the refractive index induced by the capillary condensation of the organic vapor within the pores of the np-Si layer. The np-Si layer samples showed excellent sensing ability to different types and concentrations of organic vapors. After removing the organic vapors, the reflectance spectrum immediately returned to its original state.     

Application of nanoporous silicon for a metal-semiconductor-metal visible light photodetector

In this paper we present a study on the application of nanoporous silicon to an optoelectronic device called a nanoporous silicon metal-semiconductor-metal (MSM) visible light photodetector. This device was fabricated on a nanoporous silicon layer which was formed by electrochemical etching of a silicon wafer in a hydrofluoric acid solution under various anodization conditions such as the resistivity of the silicon wafer, current density, concentration of the hydrofluoric acid solution and anodization time. The structure of this device has two square Al/nanoporous silicon Schottky-barrier junctions on the silicon substrate and the electrode spacing is 500 microm. The experiment will study photoresponse and the response time of a nanoporous silicon MSM photodetector which was fabricated on the various porosity of a nanoporous silicon layer. It is found that when devices are fabricated on a higher porosity nanoporous silicon layer, the photoresponse of the device will expand toward the short-wavelength and the bandwidth of the spectrum response will cover visible light. In addition, it is found that the response time of the device decreases.     

Low-concentration NO/sub 2/ detection with an adsorption porous silicon FET

Adsorption porous silicon FET (APSFET) is a porous silicon (PS)-based device constituted of a FET structure with a porous adsorbing layer between drain and source. Adsorbed gas molecules in the porous layer induce an inverted channel in the crystalline silicon under the PS itself. The mobile charge per unit area in the channel depends on the molecular gas concentrations in the sensing layer so that adsorbed gas molecules play a role similar to the charge on the gate of a FET. In this work, NO/sub 2/ detection by using the APSFET is demonstrated for the first time. NO/sub 2/ concentration as low as 100 ppb was detected. Devices with both as-grown and oxidized PS layers were fabricated and compared in order to investigate the effect of a low-temperature thermal oxidation on the electrical performances of the sensor. Nonoxidized sensors show a high sensitivity only for fresh devices, which reduces with the aging of the sample. Oxidation of the PS layer improves the electrical performance of sensors, in terms of stability, recovery time, and interference with the relative humidity level, keeping the high sensitivity to nitrogen dioxide.     

Application of ZnO single-crystal wire grown by the thermal evaporation method as a chemical gas sensor for hydrogen sulfide

A zinc oxide single-crystal wire was synthesized for application as a gas-sensing material for hydrogen sulfide, and its gas-sensing properties were investigated in this study. The gas sensor consisted of a ZnO thin film as the buffer layer and a ZnO single-crystal wire. The ZnO thin film was deposited over a patterning silicon substrate with a gold electrode by the CFR method. The ZnO single-crystal wire was synthesized over the ZnO thin film using zinc and activated carbon as the precursor for the thermal evaporation method at 800 degrees C. The electrical properties of the gas sensors that were prepared for the growth of ZnO single-crystal wire varied with the amount of zinc contained in the precursor. The charged current on the gas sensors increased with the increasing amount of zinc in the precursor. It was concluded that the charged current on the gas sensors was related to ZnO single-crystal wire growth on the silicon substrate area between the two electrodes. The charged current on the gas sensor was enhanced when the ZnO single-crystal wire was exposed to a H2S stream. The experimental results obtained in this study confirmed that a ZnO single-crystal wire can be used as a gas sensor for H2S.     

Polypyrrole-functionalized porous silicon for gas sensing applications

Electrodeposition of polypyrrole film on porous silicon surface was used to improve its photoluminescence properties for sensing of chemical species in gas phase. Photoluminescence quenching sensor response was measured for water and a homological set of linear alcohols in gas phase. We observed modified photoluminescence quenching response from polypyrrole-modified porous silicon as compared with as-prepared porous silicon. For as-prepared porous silicon samples, the dependence of photoluminescence quenching on analyte concentration revealed Stern–Volmer behavior. Concentration dependence of photoluminescence quenching response of polypyrrole-modified samples for water and methanol exhibited Stern–Volmer behavior as well, for C₂–C₆ linear alcohols a period of photoluminescence enhancement on the concentration dependence onset followed by photoluminescence quenching at higher concentrations was observed. The interval of photoluminescence enhancement response from polypyrrole-modified samples was continuously decreasing with the length of alcohol chain. Dramatic reduction of sensor photoluminescence response to lower alcohols was attributed to strong interaction with polypyrrole surface layer and suppressed analyte penetration into porous matrix. Operational stability of polypyrrole-modified porous silicon was improved as compared to as-prepared porous silicon.     

Hybrid optical-electrochemical electronic nose system based on Zn-porphyrin and multi-walled carbon nanotube composite

In this work, we have enhanced the capability of an e-nose system based on combined optical and electrochemical transduction within a single gas sensor array. The optical part of this e-nose is based on detection of the absorption changes of light emitted from eight light emitting diodes (LEDs) as measured by a CMOS photo-detector. The electrochemical part works by measuring the change in electrical resistivity of the sensing materials upon contact with the sample vapor. Zinc-5,10,15,20-tetra-phenyl-21H,23H-porphyrin (ZnTPP) and multi-walled carbon nanotube (MWCNT) composite was used as the sensing materials based on its good optoelectronic properties. This sensing layer was characterized by UV-Vis spectroscopy and atomic force microscope and tested with various VOC vapors. Density functional theory (DFT) calculations were performed to investigate the electronic properties and interaction energies between ZnTPP and analyte molecules. It can be clearly seen that this hybrid optical-electrochemical electronic nose system can classify the vapor of different volatile organic compounds.     

1D and 2D Field Effect Transistors in Gas Sensing: A Comprehensive Review

Rapid progress in the synthesis and fundamental understanding of 1D and 2D materials have solicited the incorporation of these nanomaterials into sensor architectures, especially field effect transistors (FETs), for the monitoring of gas and vapor in environmental, food quality, and healthcare applications. Yet, several challenges have remained unaddressed toward the fabrication of 1D and 2D FET gas sensors for real-field applications, which are related to properties, synthesis, and integration of 1D and 2D materials into the transistor architecture. This review paper encompasses the whole assortment of 1D—i.e., metal oxide semiconductors (MOXs), silicon nanowires (SiNWs), carbon nanotubes (CNTs)—and 2D—i.e., graphene, transition metal dichalcogenides (TMD), phosphorene—materials used in FET gas sensors, critically dissecting how the material synthesis, surface functionalization, and transistor fabrication impact on electrical versus sensing properties of these devices. Eventually, pros and cons of 1D and 2D FETs for gas and vapor sensing applications are discussed, pointing out weakness and highlighting future directions.     

Accelerated resistive humidity sensing properties of silicon nanoporous pillar array

Silicon nanoporous pillar array (Si-NPA), with micro/nanometer composite structure, was prepared by hydrothermally etching single crystal silicon. Resistive humidity sensors were fabricated through evaporating coplanar interdigital aluminium electrodes on Si-NPA and the humidity sensing properties were tested. It was shown that with relative humidity changing from 11.3% to 94.6%, a resistance device response over one order of magnitude with response time less than 1 s was achieved at frequency of 1 kHz. This extraordinary property was mainly attributed to the unique morphology of Si-NPA, i.e., the regular pillar array provided an effective pathway for vapor transportation and the nanoporous structure of the pillars greatly enlarged the sensing areas.     

Impact of Device Technology Processes on the Surface Properties and Biocompatibility of Group III Nitride Based Sensors

In this work we have investigated the impact of typical device processing steps on the surface properties (roughness, chemical composition, contact angle to water) of group III‐nitride based chemical sensors with emphasis on the electrical performance of the sensor and the biocompatibility. Basic sensing device is an AlGaN/GaN high electron mobility transistor. The widely distributed mammalian cell cultures HEK 293FT and CHO‐K1 served as biological model systems. The processing of the devices had only little influence on the cell growth onto the sensor. In all cases it was superior to silicon surfaces. Fluorine dry etching smoothes the surface and forms an oxide, which improves the electrical properties of the AlGaN/GaN sensor. In contrast, autoclave treatment enhances the carbon contamination with negative impact on the sensor properties and increased the contact angle to water. For all other treatments the contact angle recaptures a value of about 50 ± 5° after exposure to air or water droplets for some hours due to the contamination by hydrocarbons.     

Fast electrical response to volatile organic compounds of 2D Au nanoparticle layers embedded into polymers

The conductivity of polymer–metal nanocomposites close to the percolation threshold is very sensitive to changes in the metal nanoparticle distances. Here the technical feasibility of a novel type of easy to prepare polymer–metal nanocomposite sensor is explored, which shall be able to detect a unique signal for various volatile organic compounds (VOCs) exhibiting a fast and reversible response. The composite consists of a nearly 2-dimensional Au nanoparticle layer near the percolation threshold thermally embedded into a thermoplastic polymer film. The sensoric response is based on the swelling behavior of the polymeric matrix upon exposure to the organic vapor molecules. Different from conventional nanocomposite sensors that require long-range diffusion of the volatile compound into the bulk of the matrix, the electrical response here only requires the penetration of the VOC a few nanometer below the surface thus causing a rapid detection. The degree of swelling depends on the type of polymer and VOC used as well as on the vapor pressure of the VOC leading to a characteristic response of each polymer to a specific VOC. This enables a “fingerprint” detection of different VOCs by an array of different polymer nanocomposite combined into one sensoric device.     

An Ultrasensitive Organic Semiconductor NO2 Sensor Based on Crystalline TIPS‐Pentacene Films

Organic semiconductor gas sensor is one of the promising candidates of room temperature operated gas sensors with high selectivity. However, for a long time the performance of organic semiconductor sensors, especially for the detection of oxidizing gases, is far behind that of the traditional metal oxide gas sensors. Although intensive attempts have been made to address the problem, the performance and the understanding of the sensing mechanism are still far from sufficient. Herein, an ultrasensitive organic semiconductor NO₂ sensor based on 6,13‐bis(triisopropylsilylethynyl)­pentacene (TIPS‐petacene) is reported. The device achieves a sensitivity over 1000%/ppm and fast response/recovery, together with a low limit of detection (LOD) of 20 ppb, all of which reach the level of metal oxide sensors. After a comprehensive analysis on the morphology and electrical properties of the organic films, it is revealed that the ultrahigh performance is largely related to the film charge transport ability, which was less concerned in the studies previously. And the combination of efficient charge transport and low original charge carrier concentration is demonstrated to be an effective access to obtain high performance organic semiconductor gas sensors.     

Gas sensor applications of porous Si layers

The porous silicone (PSi) is a relatively new and promising semiconductor material with special physical and chemical properties which somewhat differ from the properties of single crystal Si. Some of these properties are valuable in the field of gas sensor technology, but a lot of questions arise in connection with its application. Do we really need porous semiconductor material for proper gas sensing function? How can electrical properties of the PSi layer be measured if the electrical contacting is problematic? Is it possible to activate the PSi with catalytic noble metal layers or particles? What about the Fermi-level pinning in the PSi layer? The main target of this article is to seek answers to questions listed above and to give a short, but still comprehensive review of the application of the PSi layers on the field of the gas sensor technology, with special care on electrical output signal giving sensors.     

Nano Chemical Sensors With Polymer-Coated Carbon Nanotubes

A simple sensor platform consisting of an interdigitated electrode (IDE) pattern has been fabricated for sensing gas and organic vapors. Purified single-walled carbon nanotubes (SWNTs) in the form of a network laid on the IDE by solution casting serve as the sensor material. The electrical conductivity of the SWNT network changes reproducibly upon exposure to various gases and vapors. Selectivity to specific gases, for example, chlorine and hydrochloric acid vapor, is demonstrated by coating the SWNTs with polymers such as chlorosulfonated polyethylene and hydroxypropyl cellulose.     

Improving the stability of amorphous silicon ultraviolet sensors

In this paper we present an improved structure of an amorphous silicon/amorphous silicon carbide ultraviolet sensor, previously presented in literature, whose overall performances have been enhanced by growing a very thin layer of chromium silicide film on the top of the sensor. The sensor is a n-type amorphous silicon/intrinsic amorphous silicon/p-type amorphous silicon carbide stacked structure deposited on a glass substrate. The substrate is covered with a chromium film that acts as back metal contact. The top metal contact is a grid shaped chromium/aluminum/chromium metal stack that allows the incident radiation to reach the active p-type layer.The responses of two sets of sensors fabricated with and without the alloy film under ultraviolet radiation have been studied. The role of the very thin chromium silicide layer is to increase the conductivity of the top surface without attenuating the UV radiation absorbed in the device active layer. The increased top-surface conductivity ensures a better collection of the photogenerated carriers and hides the resistivity variation of the underlying p-doped layer under ultraviolet light caused by dopant activation, leading to a stable and linear behavior. Comparing the photocurrent values obtained on sensors with different area and distance between the grid electrodes, we found that the presence of the chromium silicide film extends the charge collection length by a factor of 10, allowing a better device photoresponse.     

Spectroscopy for the Analysis of Nanoporous Silicon Gas and Humidity Sensors

In this work, a Raman spectroscopic study of nanoporous silicon sensor samples demonstrated its use as a method of gauging the sensor potential via quantitative data it provides on the sensor nanostructure dimensions. This special property of the Raman spectroscopy technique also showed its potential to determine mechanical stability of the samples over 3 months. This work also shows that the Raman spectroscopy technique is sensitive to step changes in relative humidity in all the sensor samples via its measurement of the strain-free crystalline silicon (c-Si) Raman peak. Since the Raman technique is non-destructive and senses remotely on the fragile nanoporous sensor samples it will be the ideal replacement of the presently used electrical capacitance techniques as the primary determination of relative humidity.     

Bioinspired Flexible Volatile Organic Compounds Sensor Based on Dynamic Surface Wrinkling with Dual‐Signal Response

Living systems can respond to external stimuli by dynamic interface changes. Moreover, natural wrinkle structures allow the surface to switch dynamically and reversibly from flat to rough in response to specific stimuli. Artificial wrinkle structures have been developed for applications such as optical devices, mechanical sensors, and microfluidic devices. However, chemical molecule‐triggered flexible sensors based on dynamic surface wrinkling have not been demonstrated. Inspired by human skin wrinkling, herein, a volatile organic compound (VOC)‐responsive flexible sensor with a switchable dual‐signal response (transparency and resistance) is achieved based on a multilayered Ag nanowire (AgNW)/SiO_x/polydimethylsiloxane (PDMS) film. Wrinkle structures can form dynamically in response to VOC vapors (such as ethanol, toluene, acetone, formaldehyde, and methanol) due to the instability of the multilayer induced by their different swelling capabilities. By controlling the modulus of PDMS and the thickness of the SiO_x layer, tunable sensitivities in resistance and transparency of the device are achieved. Additionally, the proximity mechanism of the solubility parameter is proposed, which explains the high selectivity of the device toward ethanol vapor compared with that of other VOCs well. This stimuli‐responsive sensor exhibits the dynamic visual feedback and the quantitative electrical signal, which provide a novel approach for developing smart flexible electronics.     

Research Progress of Organic Field-Effect Transistor Based Chemical Sensors

Due to their high sensitivity and selectivity, chemical sensors have gained significant attention in various fields, including drug security, environmental testing, food safety, and biological medicine. Among them, organic field-effect transistor (OFET) based chemical sensors have emerged as a promising alternative to traditional sensors, exhibiting several advantages such as multi-parameter detection, room temperature operation, miniaturization, flexibility, and portability. This review paper presents recent research progress on OFET-based chemical sensors, highlighting the enhancement of sensor performance, including sensitivity, selectivity, stability, etc. The main improvement programs are improving the internal and external structures of the device, as well as the organic semiconductor layer and dielectric structure. Finally, an outlook on the prospects and challenges of OFET-based chemical sensors is presented.     

Enhanced radiative emission rate and quantum efficiency in coupled silicon nanocrystal-nanostructured gold emitters

We report local-field-enhanced light emission from silicon nanocrystals close to a film of nanoporous gold. We resolve photoluminescence as the gold-Si nanocrystal separation distance is varied between 0 and 20 nm and observe a fourfold luminescence intensity enhancement concomitant with increases in the coupled silicon nanocrystal/nanoporous gold absorbance cross section and radiative decay rate. A detailed analysis of the luminescence data indicated a local-field-enhanced quantum efficiency of 58% for the Si nanocrystals coupled to the nanoporous gold layer.     

Distributed Bragg reflector enhancement of electroluminescence from a silicon nanocrystal light emitting device

We demonstrate distributed Bragg reflector (DBR) enhanced electroluminescence from a silicon nanocrystal-based light emitting device. An a-Si/SiO₂ superlattice containing silicon nanocrystals serves as the intrinsic layer in an n-i-n device that is embedded in a DBR cavity consisting of alternating layers of silicon and silicon dioxide. The entire structure, including DBR, superlattice and contact layers, is deposited by plasma-enhanced chemical vapor deposition. The photoluminescence, electroluminescence (EL) and optical output power are measured and compared to a reference device. The DBR is found to enhance the peak EL intensity by a factor of 25 and the external quantum and power conversion efficiencies by a factor of 2.     

Study on the characteristics of amorphous low-K thin film for solar cells

The most practical solar cells are silicon based crystal silicon solar cells. Phosphorus oxychloride for n+ type doping was diffused on a p+ Si, SiC and poly Si using N2 carrier gas by low pressure chemical vapor deposition. The series resistances on various p type silicon substrates were researched. An n(+)-p+ junction was fabricated by thermal diffusion of phosphorus oxychloride into a p+ Si wafer. For the rear metallization, Al was deposited using screen printing and SiOC film was used instead of SiO2 film as a passivation material for the metal layer. SiOC film was made by the capacitive coupled plasma chemical vapor deposition. When the Fourier transform infrared spectra of SiOC film shows organic properties including a strong peak of the Si-CH3 bond, the efficiency was increased, because of the reduction of the recombination at the back surface.