Quantum-Confined Semiconductors
NREL researchers seek to understand and control the photophysical properties of quantum-confined semiconductors through rational design, synthesis, and spectroscopic investigations of nanostructures.
This is done through:
- Using surface-bound ligands to tune photophysics, electronic environment, morphologies, and aggregate formation
- Doping to influence carrier concentrations, quantum dot film conductivities, and energy harvesting
- Creating films and arrays to sustain highly mobile photoexcited charges or excitons and to control cooling and direct carriers across interfaces
- Influencing excited state dynamics with plasmonic nanostructures.
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
Measuring Photoexcited Free Charge Carriers in Mono- to Few-Layer Transition-Metal Dichalcogenides with Steady-State Microwave Conductivity, Journal of Physical Chemistry Letters (2020)
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
University of Colorado Boulder