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Strong and stable printing: Researchers formulate recipe for Black Phosphorus ink

Black phosphorus crystal. Image credit: Smart-elements.com

Guohua Hu, a fourth-year PhD student at St John’s is the lead author of new study which has successfully developed a formula for turning a new graphene-like material called black phosphorus (BP) into ink. The ink, which is compatible with conventional inkjet printing techniques, enables the high-speed and ultralow-cost mass production of next-generation laser and optoelectronic devices for use in industry and scientific research. 

For the first time, a team of scientists at the Cambridge Graphene Centre have developed a formula which transforms crystals of a two-dimensional nanomaterial similar to graphene into a functional ink. The study, reported in Nature Communications, has produced a unique ink that can be used for the high-speed, low-cost printing of BP-based laser and optoelectronic devices.

BP is a new material with properties that make it particularly attractive for use in laser and optoelectronic devices. It is a semiconductor with a direct band gap that is responsive to a wide range of the electromagnetic spectrum. This means that electrons can be stimulated using visible light as well as infrared.

However, the development of applications for BP has been blighted by its delicate disposition. BP is very sensitive to oxygen and moisture and will oxidise quickly in the ambient environment. Previous attempts to use it have been made under nitrogen, which is a serious impediment to cost effectivity.

Now, researchers have come up with a new formula for BP-based ink that solution-processes BP crystals into nanoflakes a few atoms thick. The flakes are then dispersed into a mixture of alcohols, a recipe which, along with the rapid speed at which the ink dries, ensures the stability of BP against oxidation. With the addition of a light coat of protective material, the resulting devices are stable and durable, surpassing what previous studies of BP have been able to achieve and opening up exciting new prospects for printed optoelectronics.

The ink can now be used for the low-cost manufacture of devices such as nonlinear photonic devices and photodetectors such as light sensors. Printed BP can be inserted easily into laser cavities to act as an optical shutter converting a continuous beam of laser radiation into a repetitive series of bursts of light, lasting 10 quadrillionths of a second.  These ultra-fast lasers – similar to those used in laser eye surgery – have numerous industrial and medical applications such as machining, drilling, imaging and sensing.  BP ink can also be printed in combination with existing silicon technologies, such as silicon based photodetectors, to enhance their performance and extend their operational wave length.

The high-quality printing also allows for the BP ink to be precisely and uniformly printed onto a variety of surfaces such as silicon wafer, glass, plastic and paper. The ability to print on soft substrates offers the potential for the manufacture of flexible devices such as wearable electronics.

The capacity for wafer-scale device printing that integrates thousands of devices on a wafer is a unique benefit to the new ink formulation strategy and offers the potential for industrial-scale applications for BP and other graphene-like materials.

Guohua Hu lead author of the paper and fourth-year PhD student at St John’s said: “The formulation represents a significant scientific and technical achievement in terms of using BP for future applications. The functional ink, containing very small “flakes” of BP, allows us to print on a wide variety of substrates, producing devices with excellent performances that remain stable for a prolonged period.”

Black phosphorus ink formulation for inkjet printing of optoelectronics and photonics is available via: https://www.nature.com/articles/s41467-017-00358-1