Muhammad Qazi

Trace gas detection using nanostructured graphite layers

Nanostructured graphite (NG) has been investigated as a sensing material using a highly sensitive potentiometric detection technique. NO2 concentration down to 60ppb was detected in ambient conditions using NG functionalization layer. Simultaneous current and surface work function (SWF) change transients measured using NG functionalization layer reveal much shorter response time for the later, which is attributed to its dependence solely on surface molecular adsorption. The gradient of SWF with respect to the current transient was found to be independent of gaseous concentration and fraction of preoccupied surface states.

Gas sensing using electrostatic force potentiometry

A highly sensitive potentiometric technique generally applicable for detection of gases utilizing adsorption-induced changes in surface work function is demonstrated. This technique is applied to sense hydrogen based on work function change of a Pt thin film. The surface work function changes of Pt upon exposure to pure and 1000ppm hydrogen were found to be ∼900 and ∼270mV, respectively. These work function changes are much higher than corresponding changes in the Schottky barrier height in Pt-semiconductor based amperometric sensor devices for similar hydrogen concentration. Using this technique, detection down to 8ppm hydrogen concentration is demonstrated.

NO2 Detection Using Microcantilever Based Potentiometry

A highly sensitive and novel sensor platform for gases and volatile chemicals using microcantilever based potentiometry is reported. A resonant cantilever is used to detect the changes in surface work functions of functionalized substrates caused by adsorption of target gas molecules. Surface work function (SWF) changes were measured for different functionalization layers made of transition metal oxide thin films with the flow of NO2. The rate of change in SWF for In2O3 and SnO2 were found to be ∼80 and ∼100 μV/sec, respectively, for 70 ppm NO2. A sensitivity of 64 μV/sec for SWF change was also found for 70 ppm NO2 concentration for isolated clusters of ZnO nanowires, indicating that this technique is applicable even for nano-clusters of sensing materials where amperometric detection is impossible due to material discontinuity. NO2 detection as low as 400 ppb was possible using highly insulating In2O3 and SnO2 thin films (resistivity > 1 TΩ/□. Two different forms of nano scale graphite were compared with the transition oxide based functionalization layer for sensing sub-ppm NO2 sensing. It was observed that nanostructured graphite (NG) shows much higher sensitivity and lower response time than transition metal oxides.

NO2 detection by adsorption induced work function changes in In2O3 thin films

A potentiometric sensor platform has been used for NO2 detection at room temperature by measuring the adsorption induced surface work function changes in In2O3 thin films deposited on Si using a solution based process. The highly resistive films were unsuitable for amperometric detection of NO2; however, significant work function changes of ∼30mV were measured for ∼50s of exposure to 70ppm NO2, and detection down to 600ppb (parts per 109) was possible. A model for the transient response was developed, which satisfactorily fits the experimental data. Acceleration of the desorption transient under ultraviolet illumination has also been investigated.

III-V Nitride based piezoresistive microcantilever for sensing applications

III-V Nitride based microcantilevers, with AlGaN/GaN heterostructure field effect transistor as the piezoresistive deflection transducer, have been investigated under steady state, transient, ac, and UV illuminated conditions and compared to theoretical calculations. The steady state transverse gauge factor (GFt) was found to be much larger than theoretical estimates and increased regularly with more negative gate bias. Transient GFt demonstrated opposite sign but similar gate bias dependence and was measured as high as ∼860. Measurements under ac biasing conditions and UV illumination resulted in a lower GFt of ∼13, which agrees with theoretical calculations owing to elimination of charge trapping effects.

Growth direction modulation and diameter-dependent mobility in InN nanowires

Diameter-dependent electrical properties of InN nanowires (NWs) grown by chemical vapor deposition have been investigated. The NWs exhibited interesting properties of coplanar deflection at specific angles, either spontaneously, or when induced by other NWs or lithographically patterned barriers. InN NW-based back-gated field effect transistors (FETs) showed excellent gate control and drain current saturation behaviors. Both NW conductance and carrier mobility calculated from the FET characteristics were found to increase regularly with a decrease in NW diameter. The observed mobility and conductivity variations have been modeled by considering NW surface and core conduction paths.

Two-dimensional signatures for molecular identification

Simultaneous measurements of the conductance and surface work function (SWF) changes on nanostructured graphite layers have been performed to detect several gaseous analyte molecules. It has been observed that the gradient of the SWF versus conductance response plotted for specific analyte molecules is constant irrespective of their concentration or fractional occupancy of surface adsorption sites. The SWF and conductance changes have been found to be uncorrelated for different analyte molecules, resulting in unique gradients that can be used as two-dimensional signatures for molecular identification.

High frequency dynamic bending response of piezoresistive GaN microcantilevers

Static and dynamic ac responses of piezoresistive GaN microcantilevers, with integrated AlGaN/GaN heterostructure field effect transistors as highly sensitive deflection transducers, have been investigated. Very high gauge factor exceeding 3500 was exhibited by the microcantilevers, with quality factor determined from electronically transduced ac response exceeding 200 in air and 4500 at low pressure. The gauge factor reduced at resonance frequency of the cantilevers, possibly due to reduced charge exchange with surface donor and trap states. Ultrasonic waves generated in air by a piezochip, and in the Si substrate through photoacoustic effect, could be detected by the cantilevers with high sensitivity.

Molecular Adsorption Behavior of Epitaxial Graphene Grown on 6H-SiC Faces

Epitaxial graphene layers grown on 6H-SiC faces were investigated for molecular adsorption by electron withdrawing NO2 and electron donating NH3. From amperometric measurements performed on these samples, we observed that epitaxial graphene grown on C-face SiC mostly behaved as a p-type sensing layer in contrast to the Si-face graphene, which behaved as n-type. Potentiometric sensing experiments performed reveal that epitaxial graphene on both C- and Si-faces have similar charge transfer mechanism with respect to a specific adsorbent gas.

Surface electronic property of SiC correlated with NO2 adsorption

Correlations between surface electronic properties of SiC and NO2 adsorption were investigated using electrostatic force potentiometry. It was observed that surface work function (SWF) of both 6H and 3C–SiC changes significantly with NO2 adsorption. Measurements on semi-insulating 6H–SiC revealed that the Si face has higher sensitivity toward NO2 molecules than C face producing more change in SWF due to NO2 adsorption, which can be related with the difference in their surface free energies. For an n+-doped 6H–SiC, the SWF of the C face was found to increase much more than the Si face, showing correspondingly higher NO2 sensitivity. Upon exposure to superbandgap ultraviolet (UV) illumination, the surface band bending of both the faces was found to increase for undoped 6H–SiC, which resulted in enhanced sensitivity to NO2 adsorption. Measurements on doped SiC also supported similar correlations, although the surface band bending initially decreased under UV illumination. Our results indicate that adsorption...