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Laser processing ups efficiency of optoelectronic devices

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Scientists at the US Naval Research Laboratory (NRL) have developed a versatile laser processing technique capable of reducing defects in next-generation optical materials, enabling their optical quality to be improved.

The new technique, reported in ACS Applied Materials & Interfacescould enable the miniaturisation of light emitting diodes and other optical elements.

Using a laser, the NRL scientists are able to significantly improve the optical properties of monolayer molybdenum disulphide (MoS2) – a direct gap semiconductor – with high spatial resolution.

Their process produces a 100-fold increase in the material’s optical emission efficiency in the areas ‘written’ with the laser beam.

‘From a chemistry standpoint, we have discovered a new photocatalytic reaction using laser light and water molecules, which is new and exciting,’ said Dr Saujan Sivaram, lead author of the study. ‘From a general perspective, this work enables the integration of high quality, optically active, atomically thin material in a variety of applications, such as electronics, electro-catalysts, memory, and quantum computing applications.’

According to Sivaram, atomically thin layers of transition metal dichalcogenides (TMDs), such as MoS2, are promising components for flexible devices, solar cells, and optoelectronic sensors due to their high optical absorption and direct band gap.

‘These semiconducting materials are particularly advantageous in applications where weight and flexibility are a premium,’ he said. ‘Unfortunately, their optical properties are often highly variable and non-uniform, making it critical to improve and control the optical properties of these TMD materials to realise reliable high efficiency devices.

‘Defects are often detrimental to the ability of these monolayer semiconductors to emit light. These defects act as non-radiative trap states, producing heat instead of light, therefore, removing or passivating these defects is an important step towards high efficiency optoelectronic devices.’

In a traditional LED, approximately 90 per cent of the device is a heat sink to improve cooling. Removing defects from monolayer semiconductors will therefore enable smaller devices that consume less power to be made – for example distributed sensors and low-power electronics with a longer operational lifetime.

The researchers demonstrated that water molecules passivate the MoS2 only when exposed to laser light with an energy above the band gap of the TMD. The result is an increase in photoluminescence with no spectral shift.

Treated regions maintain a strong light emission compared to the untreated regions that exhibit much a weaker emission. This suggest that the laser light drives a chemical reaction between the ambient gas molecules and the MoS2.

‘This is a remarkable achievement,’ said Dr Berend Jonker, senior scientist and principal investigator. ‘The results of this study pave the way for the use of TMD materials critical to the success of optoelectronic devices and relevant to the Department of Defense mission.’

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