Quantum-Confined Semiconductors

We prepared Pickering emulsions with semiconductor quantum dots (QDs). Amphiphilic lead sulfide (PbS) QDs are prepared by constructing Janus-ligand shells comprised of lipophilic oleic acid and hydrophilic 4- (2,2-dicyanovinyl)cinnamic acid ligands. Upon homogenizing, the QDs with Janus-ligand shells self-assemble at the water-dichloromethane interface forming stable Pickering emulsions.

Research Details

• PbS QD with Janus-ligand shells were fabricated.
• Pickering emulsions were characterized.
• Enhanced photocatalytic activity was measured.

Significance and Impact

Pickering emulsions have potential applications in a number of industries, e.g., biomedicine, food, catalysis, petroleum, and cosmetics, etc. QDs can endow Pickering emulsions with multiple interesting characteristics, such as strong photoluminescence, charge transfer to substrates in the different phases for photocatalytic reactions.

Partner

Colorado School of Mines

We found that Janus-ligand shells form more readily on smaller QDs than on bigger QDs likely because the ligand density is smaller on the larger QDs reducing the strength of the ligand-ligand coupling that drives Janus-ligand shell formation. This study broadens our understanding of how to construct Janus and patchy ligand shell morphologies on small QDs.

Research Details

• PbS QD with Janus-ligand shells were fabricated as function of size.
• 2D nuclear magnetic resonance  was used the characterize the Janus-ligand shells.  

Significance and Impact

Asymmetry is an essential property of any functional system. Janus nanoparticles (JNPs) are nanostructures that have an asymmetric distribution of one of its compositions, but yet still retain physically symmetric shape and have been intensely studied for a range of applications. Very few studies show how to form JNPs with semiconductor nanocrystals or QDs.

Partner

University of Colorado Boulder

We discovered semiconducting molybdenum disulfide (MoS₂) monolayers are heavily n-type doped or phase converted to the 1T/1T’ (metallic/semi-metallic) phase with n-butyl lithium immersion treatments. These doped or phase converted monolayers are stabilized by adding surface functional groups.

Research Details

• Using n-butyl lithium treatments, we heavily n-type doped MoS₂ monolayers with shorter immersion times (10–120 min) or convert MoS₂ to the 1T/1T’ phase with longer immersion times (6–24 h).
• The n-butyl lithium treatments plus surface functionalization p-(CH3CH2)2NPh-MoS₂ (Et2N-MoS2) maintain heavily n-type doped 2H phase or the 1T/1T’ phase across the monolayer for over 2 weeks.
• MoS₂ monolayers are characterized with various confocal Raman, photoluminescence, absorption, X-ray photoelectron spectroscopy, scanning Kelvin probe microscopy, scanning electrochemical microscopy, and four-point probe sheet resistance measurements.

Significance and Impact

Transition metal dichalcogenide monolayers could be useful in a variety of applications, such as gas sensors, catalysis, field-effect transistors, etc. Our results provide a simple surface functionalization technique that will allow these monolayers to be incorporated into air-exposed devices.

Partners

Queens College

City University of New York

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

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.

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