## 4 Patterning towards Devices: Conventional and Unconventional Methods
### Direct-write techniques
#### 4.3.2 Ink-jet printing  
A typical direct-write technique is ink-jet printing, which relays on the direct, local and accurate deposition of minute quantities of solutions, dispersions and melts from an inkjet print-head or nozzle on substrates ranging from silicon, glass, quartz and polymer to ''paper-like'' flexible materials. Such a technique does not involve heat treatment and full substrate surface exposure to a source and thus it allows the processing of fragile and sensitive substrates. This technique is the most suited method for fabricating complex structures such as multiscale and multilevel features on non-planar or even curved surfaces, by using an \"add-on\" approach. Moreover, it enables a convenient placement of microscale features, even onto pre-patterned substrates. Microscale patterns can be simply designed and fabricated in relatively arbitrary 2D and (quasi-) 3D shapes, with a low material consumption, avoiding waste, which represents a crucial issue when high value and expensive materials are used. Despite all these advantages, the resolution of the inkjet process is limited to 20-50 \(\upmu\)m [169]. Stable and reliable micropatterning by inkjet printing is typically limited by (i) clogging of nozzle, (ii) formation of satellite drops and filaments, and (iii) alteration of structure shape by ink accumulation at the feature edges due to the \"'coffee-staining\" effect [170]. These effects are conditioned by the combined characteristics of ink, nozzle and substrate surface. For instance, nozzle clogging typically occurs when the ink used is composed of suspensions or when the fluid characteristics, such as viscosity, change during the deposition. Satellites and filaments form when the fluid has a high viscosity and a very low surface tension, or when the liquid is heterogeneous on a spatial scale on the order of the sizes of the fluid jet diameter. In addition, this occurrence can be a consequence of rapid and inhomogeneous solvent evaporation [170]. To limit such phenomena, the main approaches adopted so far rely on (i) filtering the fluid or operating in a particle controlled environment, (ii) using a single solvent-based ink formed of water and alcoholic solutions or high boiling point solvents (_i.e._ anisole, dioxane, xylene, ethylacetate, acetophenone, _etc._), (iii) dispensing two solvent-based ink solutions, in which one solvent has a high boiling point [170] (iv) substrate heating [171] and (v) depositing inside spatially confined hydrophobic barriers, that is, barriers made by plasma surface treatment. In recent years, several papers have reportedthat the accurate study of the inkjet printing conditions in an appropriate experimental setup allows for fabricating inks based on complex functional NP composites in which the outstanding properties of the nano-objects are transferred to the final microstructures. Well-defined patterns of dots 120 \(\upmu\)m in diameter formed of water soluble luminescent CdTe NCs embedded in poly(vinylalcohol) (PVA) have been inkjet printed [172]. In this work, ethylene glycol (2 vol %) is added to the aqueous solution of CdTe NCs to avoid the \"coffee-stain\" ring formation leading to uniform dots. The PVA matrix prevents the aggregation of the CdTe NCs. The optimal PVA/CdTe NCs ratios in terms of the maximum available PL intensity has been determined for different sized CdTe NCs and combinatorial libraries on luminescent CdTe NC functionalized PVA composites with variable NC sizes have been inkjet printed. [172] Air stable OFETs have been fabricated including highly resolved and highly electrically conductive Au lines inkjet printed and laser sintered at low temperature under ambient pressure [173]. Monolayer-protected Au NPs 1-3 nm in size suspended in alpha-terpineol as carrier medium have been inkjet printed on flexible active substrates of a carboxylate-functionalized polythiophene, namely poly(3-hexylthiophene) (P3 HT), which possesses high air stability.