Laser printing of hybrid nanocomposite chemiresistors - NANO-SENS

Results

The aim of this project is to create hybrid nanocomposites which combine the advantages of carbon nanostructures (CNS) and SnO2, i.e. good electronic conduction at room temperature while maintaining an excellent molecular recognition capability for the fabrication of improved solid-state sensors with very high sensitivity.

In order to achieve the goals of this project, dynamic release layer assisted laser-induced forward transfer (DRL-LIFT) is applied for the fabrication of hybrid nanocomposite-based sensors.

The DRL-LIFT setup already available in the Materials Group at PSI contains a XeCl excimer laser (308 nm, operated at 1 Hz) that is controlled by a shutter. The laser beam intensity is controlled with a motorized attenuator plate. A homogeneous part of the beam is selected with a square/round mask and imaged by a lens onto the back side of the donor substrate. The donor is positioned perpendicular to the laser beam. The laser beam impinges on the film to be transferred through the fused silica substrate. LIFT is realized by positioning a receiver substrate in front of the donor film to collect the ejected material. The donor-receiver system is mounted on a motorized x, y, z translation stage. The whole system is controlled with a computer running LabVIEW software which allows creating a matrix of points for each sample.

The receiver substrates are either rigid or flexible substrates, i.e. glass, fused silica, kapton, etc., with patterned metallic electrodes (chemiresistors sensors).

Different types of donors are prepared for the LIFT experiments: SnO2, carbon nanotubes (CNT) (ultra-purified, shall be further noted with H), CNT (with catalyst impurities, shall be further noted with F), SnO2+CNT(H), SnO2+CNT(F), carbon nanowalls (CNW), and SnO2:CNW.

  1. The SnO2 donors are prepared by spin coating from a SnO2 nanoparticle solution which contains purified water and Triton X-100 as surfactant. Uniform, polycrystalline thin films (200 nm thickness) with average crystallite size of 5 – 10 nm are obtained.
  2. Carbon nano-wall donors are prepared by radiofrequency plasma-beam-enhanced chemical vapor deposition (RF-PECVD). The CNW films consist of a dense array of micron-sized flakes with layered graphite-like structures, dominating vertical orientation and chaotic lateral displacement with hundreds of nm mean spacing. The thickness of each flake varies from several graphene layers to tens of nanometers → term nanowall. A columnar morphology, with quite well-separated columns of 100 nm average diameter and sharp edges are observed by scanning electron microscopy. The thickness of the donors based on CNWs is between 4 and 7 micrometres.
  3. Another donor is prepared from SnO2 NPs that cover the walls of the CNWs. The presence of Sn in the hybrid material composition is confirmed by EDX analysis.
  4. Carbon nanotube (H) donors are prepared by spin coating and drop casting from a solution of CNT(H), water and Triton X-100. These CNTs are highly purified SWCNT. Non-compact thin films are obtained by spin coating, while by drop casting dense films with a thickness of 1 micrometre are attained. Raman microscopy has been used to investigate the donor films before and after LIFT. A common technique for evaluating the degree of defects/disorder in CNT via Raman spectroscopy is to compare the ratio of intensities between the defect-induced D band and the graphitic G band. Raman investigation shows a high crystallinity of the donor (ID/IG ratio: ~ 0.1).

After LIFT, the following observations can be drawn:

  1. SnO2 donors: well defined pixels on glass and silicon and flexible (Kapton) substrates can be transferred only when using an intermediate DRL layer. No debris and pixels with uniform coverage are observed by optical microscopy.
  2. CNW donors: Using a laser fluence of 650 mJ/cm2, well defined pixels (300 x 300 µm2) with good adhesion to Kapton are transferred. Raman investigations performed on the pixels reveal that the G peak intensity (graphitic structure) is lower than the D peak intensity (the disordered structure) (ID/IG ratio: ~ 1.8) showing a low crystallinity of the CNWs.
  3. SnO2:CNW: Using a low laser fluence (380 mJ/cm2 - 420 mJ/cm2) only incomplete pixels are transferred. Increasing the laser fluence to 500 mJ/cm2, well defined pixels (300 x 300 µm2) with good adhesion to glass and Kapton are obtained.
  4. In the case of CNT(H) donor films, a good transfer is only possible with a DRL interlayer. Different donor layers are prepared by drop casting and spin coating. Optical microscopy investigations reveal that non-uniform pixels surrounded by debris are obtained from a donor prepared by drop casting. In contrast, in the case of CNT(H) prepared by spin coating, non-uniform but well-defined pixels are observed. A lower laser fluence (100 mJ/cm2) is necessary for LIFT-ing CNTs prepared by spin coating than by drop casting (550 mJ/cm2). The Raman investigation performed on the transferred pixels confirms that there are no structural changes with respect to the donor structure.
  5. SnO2+CNT(H) with a ratio of 1:2: Transmission electron microscopy investigations of the donor films reveals SnO2 nanoparticles with sizes of 5-10 nm and bundled CNTs. The thinnest CNT has a diameter smaller than 1 nm. Only incomplete pixels can be transferred by LIFT and DRL-LIFT. Lower laser fluences for the transfer are needed (100 mJ/cm2) by using a DRL.
  6. CNT(F) donors are prepared by drop casting and matrix assisted pulsed laser evaporation (MAPLE) (a solution of CNT(F) and chloroform). Non-uniform thin donor films with thicknesses of ~1 micron are obtained by drop casting; uniform thin films with thicknesses of 150 nm and a roughness around 80 nm are grown by MAPLE. CNTs with different sizes (the thinnest 15-20 nm) and Fe impurities (catalyst) are detected (as seen in the TEM images). Raman investigations reveal high disorder (ID/IG=0.67). Well defined, fully covered pixels on glass and Kapton, without debris around the pixels, are obtained at 100 mJ/cm2 laser fluence.

The response of the LIFT-ed sensors is carried out in a controlled atmosphere, by adding analytes (ammonia NH3 and ethanol) in different concentrations. Resistance measurements are acquired by a computer controlled (LabView) setup using a Keithley 2400 sourcemeter and Keithley 2000 multimeter respectively. Pixels (300 x 300 µm2) from the materials described above are printed by LIFT on borax glass with Pt electrodes.

The sensors based on SnO2 have high ammonia sensitivity when operated at 250 degrees C. The sensors prepared from SnO2+CNT(H) are operated at room temperature and show comparable sensitivities to the tin oxide sensors operated at higher temperatures.

For the sensors based on CNT(F) and CNW, no change in resistance is detected in the presence of different analytes.