Abstract
The number of excited electrons in a semiconductor system is constrained by the number of absorbed photons, which sets a fundamental limit on the efficiency of processes such as lasing, optical amplification and energy conversion. Here we show that one can break this population limit by integrating semiconductors into plasmonic resonators, which are a type of metal cavity that can generate numerous energetic electrons when optically excited. Specifically, we have fabricated large-area silver (Ag) nano-gratings which are then coupled with semiconductors, including quantum dots (QDs) and 2D semiconductor monolayers. These semiconductors are doped with a high density of hot electrons generated due to the plasmon injection from Ag-gratings at extremely low levels of pump power, which is studied using angle-resolved steady-state, femtosecond transient absorption and transient gate photoluminescence spectroscopies. This hot electron generation and injection efficiency are quantified using mathematical calculations and theoretical modelling. Our results demonstrate the feasibility of attaining a high population with low excitation, offering practical opportunities for zero-threshold lasing and energy conversion devices.
Transient doping of plasmonic hot electrons into cadmium selenide (CdSe)/cadmium sulphide (CdS) core/shell QDs by incorporating them into Ag-gratings is studied. This doping causes an accumulation of carriers in the QD’s conduction band, resulting in more excited electrons than absorbed photons in single QD, as evidenced through a high-energy photoluminescence signal. Furthermore, we also report the plasmonic charging of tungsten disulphide (WS2) monolayers by coupling them with Ag-gratings. This charging creates a high density of excited electrons in the conduction band of WS2 monolayers, causing trion formation, which is observed as a broad pump-induced absorption with an additional signal at energy lower than the primary exciton in femtosecond transient absorption spectra. Our calculations confirmed that such a high energy emission signal from QDs and trion signal in WS2 monolayers are impossible to observe only due to Purcell’s enhancement at very low excitation fluences used in these experiments. Therefore, our findings identify a new cavity-emitter interaction pathway, initiating opportunities for both fundamental studies and practical applications in laser, photovoltaics, and photocatalysis.
The application of these plasmonically coupled semiconductors, namely QDs/Ag-gratings and WS2/Ag-gratings, as energy conversion systems is investigated through photoelectrochemical (PEC) experiments. These PEC experiments reveal enhanced hydrogen production through photocatalytic water splitting, which is observed as an increase in photocurrent. Furthermore, through experiments at optimised excitation conditions, we have made a valuable attempt to find out unambiguous evidence that this enhancement is a consequence of plasmon resonances excited on Ag-gratings. These findings are a significant contribution to the knowledge for the fabrication of solar-to-hydrogen conversion devices.