The fourth contribution of the thesis is the development of ZnO NW mats and ZnO single-wire nanowire Schottky field-effect transistors (SBFETs) using MCP technology. 45 4.1 Comparison of ZnO NW-µMD and ZnO NW-µBD gas sensors with other reported sensors.
Introduction
Printed Electronics: Basics and Types
- Inkjet printing
- Gravure printing
- Screen printing
- Other printing techniques
A pressure wave is introduced into the ink as the constrictor bends in the chamber and then reverses, pushing the ink past the nozzle plate. Once the ink transfer process is complete, the individual drops of ink released from the gravure cells are spread onto the substrate and dried to form the final pattern.
Micro-Cantilever Printing (MCP)
The ink flows from the reservoir to the micro-power point through the micro-channel. This technology is similar to inkjet printing technology, but its main advantage over inkjet is that it can print features much closer to the ink particle size due to its low-volume loading technique, thereby offering feature sizes in a much wider range from 0.5 to 1000 µm.
Motivation
Since MCP uses dot-by-dot printing, it is a slow technique, but can be used in pen drawing mode for increased speed and custom control of feature shape and size. Furthermore, MCP can be performed at room temperature and vacuum-free conditions, is mask-free, environmentally friendly, and can print various types of inks (metallic, organic, inorganic semiconductors, polymers, polar and non-polar liquids, biological samples such as proteins, nucleic acids and so on) directly to the target location on the substrate with a printing resolution below 2 µm.
Problem Definition
Fabrication of Back-to-Back Schottky Microdiodes Using MCP-Based AgNP Film and ZnO Mutilple Nanowires for Gas Sensing Applications. Fabrication and Analytical Model Development of MCP-Based ZnO Nanowire(s)-Based FETs for NO2 Detection.
Thesis Outline
The electrical response of the printed diodes has been verified with an analytical model based on the theory of thermionic emission. The drain current is modeled with the help of the surface potential and the effective charge of the nanowire channel.
Materials and Methods
- Selection of particle size of the sample material
- Cleaning of SPTs and substrates
- Preparation of PANI-ES ink suitable for printing
- Solvent selection
- Concentration of the material within the inks
- Printing environment
- Selection of substrate
- Post-processing
- Characterisation of printed features
This requires a lower concentration of the dispersant to avoid clogging the pores of the printhead. Electrical resistance of the monolithic resistors was measured via Keithley 4200A-SCS Parametric Analyzer [61] linked to DC Probe Stations by Lakeshore cryotronics (Model: CPX) and EverBeing (Model: BD6).
Results and Discussion
- Concentration of PANI-ES ink suitable for printing
- Selection of substrate
- Solvent evaporation rate from substrate
- Ink-substrate adhesion via contact angle measurement
- Printing of PANI-ES micro-arrays on various substrates
- Printing of continuous lines of silver over various substrates
- Printing of monolithic resistors using AgNP conducting ink on glass substrate . 24
- Effect of thickness of printed devices
Contact angle measurements help measure the adhesive response of the ink to a given substrate when dropped from the printhead. Therefore, it can be concluded that a humidity range of 46–57% was suitable for AgNP ink to maintain a fine flow of the ink to the micro-cantilever tip without spill-over.
Mathematical model for current in printed resistors
Summary
37] used poly-4-vinylphenol (PVP) ink as dielectric layer and AgNP ink as conductor to fabricate inductors and capacitors using IPT. All of the above reports emphasized the fabrication of a capacitor, inductor, or diode, but none of the reports went into detail about the fabrication of a printed resistor. There are few reports discussing the various aspects related to the manufacturing of printed resistors, but these printed resistors have dimensions in 'mm' range, focus more on device application and lack in-depth analysis for various physical and chemical parameters involved are involved in the manufacturing process.
SOP, DIPSOP and SDP based AgNP PMRs
Comparison between SOP, DIPSOP and SDP
However, SOP can only be performed if the SPT micro-cantilever channel is properly cleaned and the ink meets certain physico-chemical prerequisites. DIPSOP can be done with almost any ink that can be pulled out of the SPT micro-cantilever. In addition, SDP has the least chances of contamination as it ensures that only micro-cantilevers of SPT touch the surface of the substrate.
Results and Discussions
- Effect of annealing on PMRs and Tip scratch effect (TSE)
- Methods for print-thickness control
- Analysis of uniformity of thickness for PMRs
- Electrical characterisation of AgNP printed micro-resistors
- Comparison between proposed and SMD Resistors
Multiple AFM scans are performed on PMR with different scan ranges such as (50 X 50) µm, (20 X 20) µm and (5 X 5) µm and the smallest value of the roughness parameter is noted. However, by optimizing the physical and chemical properties of the paint and its interaction with the substrate and some other parameters, surface roughness can be minimized. This difference is observed possibly due to the higher surface roughness of PMRs resulting in the surface distribution of charge carriers and the subsequent resistance change [75, 76].
Summary
For ZnO NW-µBD, AgNP contact pads are printed in step (b), and the remaining steps are the same. The main idea behind the development of ZnO NW-µMD is to check the response of ZnO NWs to CO2, CO and NO2 in a simple and fast way on the go. This is probably the first time that micro-cantilever printed AgNP contacts have been reported for the fabrication of gas sensors based on ZnO NW mat/NW bridge-Schottky diodes.
Gas sensing set-up
Contact modification in printed diodes
Further, to connect the Cu tape to the AgNP contact pads, 1 µL of AgNP ink is drop-cast and annealed over the interrupted area between the tape and the contact pads.
Gas sensing procedure
Results and Discussion
- Characterization of fabricated devices
- Electrical characterization of the fabricated devices
- ZnO NW-µMD based sensor response
- ZnO NW-µBD based sensor response
- Gas sensing mechanism
4.7: (a,b,c) AER variation with gas concentration and sensor response of ZnO NW-µMD based gas sensors (d) Transient response of ZnO NW-µMD based NO2 sensor. Similarly, Figure 4.4 shows the FESEM image of the ZnO NW bridge in the channel region and its corresponding EDS spectra, which shows a weight % of 50.1% and 49.9% for Zn and O respectively. Contrary to this, the AER shows a decreasing trend for CO and NO2 indicating that these gases act as reducing agents towards the ZnO NW mats.
Summary
After the ZnO SNWs are properly placed on the substrate, the MCP technique is used to overlap both sides of the selected ZnO SNWs with AgNP metal contacts to fabricate ZnO SNW-based Schottky diodes. MCP-printed ZnO SNW Schottky diodes are electrically characterized and experimental results are validated through an analytical model based on thermionic emission, and subsequent discussions are presented. The second part of this chapter discusses the use of ZnO SNW-based Schottky diodes for the sensing of the two main air pollutants CO2 and CO.
Fabrication of ZnO SNW Schottky diode
Materials used
The optimized ZnO SNWs solution helps to place the NWs at a reasonable distance on the substrate, which helps to target a SNW for printing metal contact pads to complete the device fabrication. AgNP contact pads are printed using MCP technology, which uses a molecular printing system (Make: BioForce Nanosciences) containing a surface patterning tool (SPT) consisting of a silicon micro-cantilever print head. The micro-cantilever has 5-10 µm wide micro-channels through which the ink to be printed flows or is temporarily stored.
AgNP Contact Pad fabrication
Furthermore, the camera focus is readjusted to the tip of the micro-cantilever to prevent spillover (the proximity of the micro-cantilever and the drop can result in a direct hit of the tool over the entire surface on the ink source). The SDP technique is only used for the deposition of metallic contact pads where the micro-cantilever tip is dragged to a shorter length (μm) in such a way that the dragged part of the ink is not separated from the LIR. When the micro-cantilever tip is gradually lifted, the separated volume of the ink is uniformly distributed within a part of the dragged area without touching the LIR due to the longer length and small volume of the ink deposit.
Results and Discussions
Material Characterization for ZnO SNWs
Electrical Characterization for printed Schottky diodes
It is noted that the I-V characteristics are consistent in each run, which validates the repeatability of the printing process. It can be seen from Table 5.2 that the electrical parameters extracted from the I-V characteristics of MCP-based Schottky diodes are similar to those previously reported. However, the MCP technology is in the nascent stage and needs further optimization to improve the electrical parameters of the.
ZnO SNW Schottky diode-based gas sensors
Introduction
However, there are only a few reports on the use of ZnO SNW-based Schottky diodes for gas sensing applications. However, ZnO SNW-based Schottky diodes have not been reported for CO2 or CO sensing so far. In this work, it is shown that ZnO SNW-based Schottky diodes can sense CO2 and CO at RT with higher sensitivity and low response time.
Materials and Methods
Fabrication of ZnO SNW Schottky diode using DI water as dispersion medium 74
It is observed that the addition of acetic acid with dispersion at the same ratio can significantly separate VZN ZnO and form longer (> 10 µm) VZN ZnO as shown in Fig. C.2(a). The addition of a higher concentration of acetic acid to the dispersion (in a ratio of 3:1) results in an improved separation between ZnO SNWs, as shown in Fig. C.2(b). However, further addition of a higher concentration of acetic acid results in complete or partial dissolution of the ZnO SNWs in the acid.
Sensor preparation and gas sensing conditions
Results and Discussion
- Characterization of ZnO SNW
- Sensing response of U-ZnO SNW Schottky diode based sensor
- Sensing response of M-ZnO SNW Schottky diode based sensor
- Sensitivity calculation
- Effect of humidity and high temperature
- Gas sensing mechanism for U-ZnO SNW based sensor
- Gas sensing mechanism for M-ZnO SNW based sensor
- Room temperature sensing
5.16: (a) AER variation with CO2 concentration and response of U-ZnO SNW diode-based sensor for lower concentration range of CO2(b) AER variation with CO2 concentration and response of U-ZnO SNW diode-based CO sensor (c) I-V response of U-ZnO SNW (d) AER variation with CO2 concentration and response of U-ZnO SNW diode-based sensor for higher concentration range of CO2 (e) AER variation with CO2 concentration and response of M-ZnO SNW diode-based CO2 sensor ( f) AER variation with CO concentration and response of M-ZnO SNW diode-based CO sensor. Field emission transmission electron microscope (FETEM) [Make: JEOL, Model-2100F] images are taken for ZnO SNW dispersed in DI water. The M-ZnO SNW Schottky diode-based sensor response towards CO2 and CO is shown in Figure 5.16(e and f).
Summary
The back-to-back Schottky diodes, fabricated in previous chapters, using ZnO NWM and ZnO SNW, are further modified to field-effect transistors by depositing the native or grown layer of SiO2 on the backside of the SiO2/Si substrate. etch using buffered HF (BHF). The purpose of AgNP layer deposition on the backside of the substrate is to avoid any formation of possible Schottky contact formation between the probing metal and the doped silicon substrate. Furthermore, in the case of specific applications such as sensing, the printed FETs are expected to improve the sensitivity and response of the sensors based on the shift in the threshold voltage after the introduction of the analyte [148].
Electrical characterization of ZnO NWM-SBFET
Electrical characterization of ZnO SNW-SBFET
ZnO NWM-SBFETs as NO 2 sensor
Modeling of ZnO NWFET drain current
- Structure used for Modeling
- Model for surface potential
- Results and Discussion
- Summary
Finally, a surface potential model was developed by solving the 1-D Poisson equation using a bias-dependent charge carrier concentration. Finally, the drain current was modeled using the surface potential and effective charge of the nanowire channel. In this report, nv was calculated using the 1-D equilibrium electron density in the nL nanowire.
Future Directions
The output, transmission, and C-V curves of the SBFETs support n-type conduction in the ZnO SNW channel. The effect of Schottky contacts on the C-V characteristics of SBFETs can be analyzed in detail. This technique is slow but ensures the flow of ink from the microconsole of the seamless tube.
Dispersion Preparation
- Results and Discussion
- Effect of acid addition to ZnO NW dispersion in LST-DI water
- Effect of base addition to ZnO NW dispersion in LST-DI water
- Effect of annealing temperature and annealing rate
It is observed that the number of ZnO NWs over the imaged surface decreases as the concentration of the LST-DI water increases according to the dilution level. ZnO NW dispersion is prepared by adding 2 mg ZnO NW powder to 1200 µl LST-DI water. The ZnO NWs are thermally stable and their morphological properties are retained up to 220◦C, as shown in Figure B.9.
Summary
ZnO NWs show appreciable thermal and structural stability at temperatures of ∼220◦C and immediate heating of the cast dispersion with acetic acid to 120◦C helps to form long, sharp-edged ZnO NWs. single and very separate. To improve the dispersion of ZnO NWs and lower the surface tension of DI water, 0.24 mg of sodium dodecyl sulfate (SDS) is added to 1.2 mL of DI water. Drop casting is performed at higher temperature to immediately evaporate the DI water to avoid its interaction with the AgNP contact pads in order to keep them intact with the SiO2/Si surface and transfer only the ZnO NWs over the canal.
ZnO NW-µBD fabrication
It is noteworthy that several mat-like structures, similar to those shown in Figure C.1(a), are deposited in the channel area. The printed microdots serve as the AgNP contact pads for the device as shown in Figure C.1(d). It is clear from Figure C.1(f) that ZnO NWs are properly connected with AgNP contact pads.
U-ZnO SNW based Schottky diode fabrication
Peng, “Inkjet printed electrodes in thin film transistors,” IEEE Journal of the Electron Devices Society, vol. Ghosh, “Ultrafast and reversible gas sensing properties of ZnO nanowire arrays grown by hydrothermal technique,” The Journal of Physical Chemistry C , vol. Myoung, “Fabrication and characterization of ZnO single nanowire-based hydrogen sensor,” The Journal of Physical Chemistry C , vol.
