Passive thin film circuits of nanomaterials, inkjet printed on flexible temperature sensitive substrates

Abstract

The objective of my work is threefold, firstly, to prepare functional ink dispersions of nanomaterials at high concentration with a well-defined narrow size distribution; secondly, to pattern low resistance, semiconducting and dielectric traces on flexible temperature sensitive substrates based on drop-on-demand inkjet printing without thermal annealing at high temperature; and thirdly, to realize vertically stacked heterostructures and passive planar antenna structures for application in RFID devices.

Ink dispersions were studied using a rotational rheometer and a tensiometer to determine the dynamic viscosity and surface tension, and further characterized by optical absorption spectroscopy and transmission electron microscopy (TEM). Inkjet printed traces on coated PET were characterized by means of scanning electron microscopy (SEM), Raman spectroscopy, contact profilometry and film profile analysis using an optical flatbed scanner. I-V measurements were performed using a source meter, RC circuits were investigated by electrochemical impedance spectroscopy and the impedance of RL‖C circuits was interpreted using a vector network analyser.

Initially, exfoliation of graphite in a matching solvent to give dispersed pristine graphene was used as the conductive ink for laying down electrical traces. The printing procedure using a piezeoelectric inkjet printer with 16 jetting nozzles (? ≈ 22 μm) was very sensitive to both the lateral flake size and dispersion concentration with repeated tests showing optimized values of ~ 170 nm and ~ 1.6 mg/ml respectively. This resulted in stable printing with minimum inkjet nozzle congestion. However, visible striations (aka swathe edges) running parallel the printhead raster direction impaired the quality and uniformity of the printed conductive features. Inkjet printed traces with thickness above 160 nm (N = 8 print passes) displayed thickness-independent conductivity σ of 3000 S m-1 (resistivity ρ: 3.3 x 10-4 Ω∙m). Below this thickness, percolation effects dominated. Also electrical test results showed the sheet resistance of printed graphene lines was 370 Ω/□ for 45 print passes. The resultant resistivity was too high to produce electrically functional graphene networks for application in passive antenna circuits operating at 13.56 MHz.

Light incident on a semiconducting nanomaterial with energy sufficient to promote carriers across the band gap into the conduction band results in photon absorption. MoS2 is nominally an n-type semiconductor having a sizeable bandgap, and as such exhibits photoconductivity on illumination. Solution exfoliated graphene and MoS2 nanosheets have been formed into interdigitated thin film arrays by inkjet deposition onto Teslin?. The MoS2 active channel on top of graphene electrodes formed a sandwich structure with Ohmic contact. To explore the photoconductivity of the device, the I?V response to illumination with a laser (532 nm, 2.3 eV) was measured. A tenfold increase in photocurrent was observed, compared to the dark conductance current, for an incident intensity of 640 mW cm-2. This suggests that such devices are viable for use as low-end photodetectors.

In moving beyond graphene laden ink to create conductive traces, the fabrication of patterned silver nanowire networks on flexible substrates was demonstrated, using piezoelectric inkjet printing as the deposition method. The as-delivered nanowires were shortened by sonication induced scission to a mean length of 2.2 ?m to prevent clogging of the jetting nozzles. Fusing of the overlapping nanowires by thermal treatment to increase the conductivity of the inter-wire junctions translated into a reduction of the sheet resistance. In this way, the resultant networks were reasonably uniform and had good electrical properties, displaying sheet resistances below 18 Ω/□ for film thickness above 130 nm. The DC conductance of the networks increased with connectivity, from the onset of percolation at ~ 1 x 103 S m-1 for very thin films (9 print passes), rising sharply up to a saturation point of 4.4 x 105 S m-1 for thicker films after 20 print passes. However, ageing of the silver nanowire networks was observed, resulting most likely from damage to the surface passivation layer of the nanowires during the sonication process.

In both experiments, evaporation of the solvent vehicle was a challenge, with bleeding leading to non-uniformity of the networks at the edges of the print patterns and reduced electrical performance.

In a further experimental step, electrostatic capacitors were fabricated by inkjet printing and spray-casting, combining conducting graphene and dielectric hexagonal boron nitride (h-BN) to create a hybrid multilayer structure (a sandwich structure of 3 layers). Impedance spectroscopy showed this sandwich structure to perform as a capacitor with a series resistance. Devices showed areal capacitance ranging from 0.24 to 1.1 nF/cm2 with an average series resistance of ~ 120 kΩ. This study provides the foundation to realize all-inkjet printed capacitor stacks for frequency tuning of RL‖C circuits and to regulate the system Q-factor.

To resolve the printing problems of ink concentration, nozzle density, solvent evaporation and flooding, and the formation of swathe edges, the inkjet printing technique shifted to thermal actuation using a Newtonian fluid as carrier.

Aqueous dispersions of silver platelets were prepared by diluting silver flakes with deionized water at a concentration of 15 mg/ml. A standard Canon inkjet photo printer having a printhead cartridge housing a total of 3,072 drop generators with an aperture of 16 ?m, a droplet volume per nozzle of 2 picoliters, and depositing up to 24,000 drops per second, was used for the deposition process.

Printed patterns exhibited bulk conductivity of ~2.66 x 106 S m-1 at a line thickness of ~180 nm, sheet resistance as low as 0.53 Ω/□ after 25 print passes, with low thermal treatment at 50 ?C for 24 hours to remove the deionized water. Thickness data as a function of the number of print passes indicated that the silver ink at 15 mg/ml concentration was depositing an average line thickness of 21.88 nm per print pass.These results pave the way to create passive electronic functionalities on low temperature flexible substrates.

An application in dual interface (DIF) smartcards (ID-1 format) is presented in which card-size planar coupling frame antennas for passive near-field communication at 13.56 MHz are printed by means of thermal inkjet deposition using silver platelet-based ink of high concentration. The contactless interface is realized by patterning a slit extending from a perimeter edge of the smartcard body to a rectangular opening in the silver printed layer to accommodate a pick-up transponder chip module with contact pads on its obverse side and a laser etched module antenna on its reverse side. When the DIF smartcard is interrogated by a reader generating an electromagnetic field, the inkjet- printed coupling frame concentrates surface eddy current density around the slit and opening, to facilitate inductive coupling with the pick-up transponder chip module. The experimental focus is on RF inductive field mapping of printed coupling frames with chip modules, and vector acquisition to extract impedance data. This work encompasses the fabrication and characterization of the first inkjet printed single loop coupling frame on a low temperature substrate for application in dual interface transaction cards. On a laboratory scale, an inkjet system is a versatile tool for the fabrication of printed electronics on a target substrate, but for industrial production, the boundary conditions of ink concentration, substrate selection, processing temperature, solvent evaporation rate, drying temperature and printing speed present a challenge for fast and efficient production. To transcend to a rational industrial process, other deposition techniques and supplementary technologies need to be explored which is beyond the scope of this thesis.