Quantum-Confined Semiconductors

NREL researchers seek to control the optical and electronic properties of quantum-confined semiconductors through doping.

This involves coupling of the core electronic structures to surface bound ligands or interactions with plasmonic nanoparticles to direct and transduce energy. We aim to utilize rational synthesis of nanocrystals, including production of asymmetric and multifunctional structures to investigate the unique phenomena offered by quantum confinement.

Research Highlights

Disentangling Oxygen and Water Vapor Effects on Optoelectronic Properties of Monolayer Tungsten Disulfide

We have demonstrated the sensitivity of monolayer tungsten disulfide (WS₂) to its local environment and its potential for detecting relative humidity.

Research Details

• Dry air (O₂ in N₂) plus illumination yields a photoluminescence increase and red shift that is due to increased trion emission, whereas humidified N₂ (H₂O vapor in N₂) plus illumination results in an overall photoluminescence increase that is dominated by exciton emission.
• Time-resolved microwave conductivity shows both O₂ or H₂O vapor environment reduces WS₂ photoconductivity, which is anti-correlated with the photoluminescence .

Significance and Impact

Monolayer WS₂ is typically considered an inert 2D layer. However, we demonstrated that the optoelectronic properties are heavily influenced by the local environment. Therefore, this nanomaterial could be used as a relative humidity sensor via photoluminescence detection.

Partner

The Institute for Basic Science/Sung Kyun Kwan University

Atomically Thin Metal Sulfides

We developed a cation exchange reaction that produces a series of atomically thin (one monolayer) of metal sulfides. We specifically demonstrated atomically thin metal sulfides of PbS, CdS, ZnS, and Co₂S.

Research Details

• Cation exchange reaction proceeds from monolayer Ag₂S at room temperature.
• Optical properties suggest that the electronic structure of the atomically thin metal sulfides are unique.

Significance and Impact

The development of 2D or atomically thin material systems has led to the discovery of unique properties not found in bulk or 3D counterparts. Here we develop the synthesis of a new class of 2D semiconductors.

Measuring Photoexcited Free Charge Carriers in Mono- to Few-Layer Transition-Metal Dichalcogenides with Steady-State Microwave Conductivity

We used steady-state microwave conductivity for measuring charge generation action spectra in WS₂ mono- to few-layer transition metal dichalcogenide systems at solar-equivalent light fluences. Multilayer portions of the sample were shown to be primarily responsible for the observed photoconductivity.

Research Details

• WS₂ monolayers grown by chemical vapor deposition; increased sulfur produces more multi-layers
• Specially designed microwave cavity with exceptionally high sensitivity
• Complementary steady-state and time-resolved measurements for characterizing full mechanism

Significance and Impact

Multilayers are shown to be a primary source for the formation of long-lived free charges in transition metal dichalcogenides, an important finding for solar fuels and photovoltaics.

Steady-state microwave conductivity has the sensitivity to provide mechanistic information on charge carrier generation in mono- to few-layer semiconductors, where the total absorptance of light is less than 3%.

Partners

Institute for Basic Science

University of Colorado Boulder

Warren Wilson College

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