Flexible electronic circuits are generally composed of conductors and elastomers. There are some materials for conductor. They can be traditional metal films and conductive silver pastes, transparent conductive oxide inks, new nanocrystal inks such as metal nanowires, graphene, carbon nanotubes, and fully flexible liquid metals. Let's talk about nanocrystal inks.

Nanocrystalline inks

Nanocrystalline ink is made by dispersing nano-conductive materials in solvents. It can be used for flexible circuits by printing, spin coating and spraying. There are no need for high temperature and vacuum operation. It saves raw materials.

Nano-conductive materials can be transparent conductive oxides (TCOs), metal nanowires (NW), graphite, carbon nanotubes (CNTs), etc.. They can be dissolved in low-cost solutes to make conductive inks in a simple way.

Nanocrystalline materials are very small in size, so you can make very small circuits, and even some optical circuits and devices with transparency requirements.

1. Transparent conductive oxide ink

It is hard to control electrode circuit when preparing indium tin oxide (ITO) conductive film on a flexible substrate through the existing vacuum sputtering process. The yield is generally less than 30%. Therefore, there is an urgent need to develop alternative processes for thin film sputtering deposition and alternative electrodes for traditional ITO conductive films.

This kind of ink is based on common wide-bandgap metal oxide materials, such as ITO, fluorine-doped titanium oxide (FTO) and aluminum-doped zinc oxide (AZO). It has good optical transparency and conductivity. The ink can print smooth, crack-free, highly transparent and highly conductive films.

2. Metal nanowire ink

Random nanowire network has high transparency and good conductivity. It is widely used in flexible and stretchable circuits of wearable electronics, robotic skin, implantable medical devices, flexible and stretchable displays, and OLEDs. Metal nanowire ink has wild application due to simple production methods.

Silver nanowires have good conductivity and flexibility. They are ideal materials for elastic conductors. Chemical reduction is one of the typical methods for synthesizing silver nanowire ink. The silver nanowires obtained by this way can be dispersed in various solvents such as water, ethanol, and isopropanol.

Copper nanowires are also common metal nanowires. However, copper nanowires are easily oxidized, so the conductivity is unstable.

3. Graphene ink

Graphene is a two-dimensional crystal formed by tightly stacked carbon atoms. It has the features of ultra-thinness, ultra-lightness, ultra-high strength, high electrical and thermal conductivity, light transmittance, and stable structure. These advantages make graphene powerful in printed electronic products.

There are a lot of studies on making graphene into conductive ink for inkjet printing. By printing graphene on a substrate through inkjet printing, it can be made into flexible transparent conductive films, sensors, supercapacitors, radio frequency antennas and other electronic devices.

High conductivity, good stability and nanosheet structure determine that graphene can be used as a high-quality conductive filler in conductive inks. Graphene ink has effectively solved the problem of low conductivity of traditional carbon-based ink. It is easily compatible with printers in terms of production formula.

4. Carbon nanotube ink

Carbon nanotube can be considered as tubular carbon-based nanostructure formed by curling up a single layer of graphene. It has unique advantages such as high carrier mobility, good conductivity and mechanical flexibility. It is one of the important materials used in flexible electronic devices.

Generally speaking, carbon nanotubes tend to agglomerate, but through chemical modification or doping with solubilizing additives such as surfactants, cellulose or conductive polymers, the dispersibility in conventional solvents can be greatly improved, thereby making conductive inks.