Hiding in plain sight: The prevalence and impact of trions and Fermi polarons in transient absorption spectroscopy experiments of 2D semiconductors.

Transient absorption (TA) spectroscopy is one of the most popular experimental methods to measure the excited state lifetimes and charge carrier recombination mechanisms in two dimensional (2D) semiconductors. This fundamental information is essential for designing and optimizing the next generation of ultrathin and lightweight 2D semiconductor-based optoelectronic devices. However, the interpretation of TA spectroscopy data varies across the community.

Characterizing the Ligand Shell Morphology of PEG-Coated ZnO Nanocrystals Using FRET Spectroscopy.

Poly(ethylene glycol) (PEG) ligands can inhibit proteins and other biomolecules from adhering to underlying surfaces, making them excellent surface ligands for nanocrystal (NC)-based drug carriers. Quantifying the PEG ligand shell morphology is important because its structure determines the permeability of biomolecules through the shell to the NC surface. However, few in situ analytical tools can reveal whether the PEG ligands form either an impenetrable barrier or a porous coating surrounding the NC.

Toward Imaging Defect-Mediated Energy Transfer between Single Nanocrystal Donors and Single Molecule Acceptors.

Defect-mediated energy transfer is an energy transfer process between midgap electronic states in a semiconductor nanocrystal (NC) and molecular acceptors, such as fluorescent dye molecules. Super-resolution fluorescence microscopy represents an exciting technique for pinpointing the nanoscale positions of lattice defect sites in, for example, a micrometer-sized particle or thin film sample by spatially resolving the location of the acceptor dye molecules with nanometer resolution. Toward this goal, our group performed ensemble-level, time-resolved fluorescence spectroscopy measurements of ZnO NC/Alexafluor 555 (A555) mixtures and calculated that the emissive defect sites are located, on average, 0.

Hot carrier extraction from 2D semiconductor photoelectrodes.

Hot carrier-based energy conversion systems could double the efficiency of conventional solar energy technology or drive photochemical reactions that would not be possible using fully thermalized, "cool" carriers, but current strategies require expensive multijunction architectures. Using an unprecedented combination of photoelectrochemical and in situ transient absorption spectroscopy measurements, we demonstrate ultrafast (<50 fs) hot exciton and free carrier extraction under applied bias in a proof-of-concept photoelectrochemical solar cell made from earth-abundant and potentially inexpensive monolayer (ML) MoS. Our approach facilitates ultrathin 7 Å charge transport distances over 1 cm areas by intimately coupling ML-MoS to an electron-selective solid contact and a hole-selective electrolyte contact.

Single Nanoflake Photoelectrochemistry Reveals Intrananoflake Doping Heterogeneity That Explains Ensemble-Level Photoelectrochemical Behavior.

Transition metal dichalcogenide (TMD) nanoflake thin films are attractive electrode materials for photoelectrochemical (PEC) solar energy conversion and sensing applications, but their photocurrent quantum yields are generally lower than those of bulk TMD electrodes. The poor PEC performance has been primarily attributed to enhanced charge carrier recombination at exposed defect and edge sites introduced by the exfoliation process. Here, a single nanoflake PEC approach reveals how an alternative effect, doping heterogeneity, limits ensemble-level PEC performance.

Local Substrate Heterogeneity Influences Electrochemical Activity of TEM Grid-Supported Battery Particles.

Understanding how particle size and morphology influence ion insertion dynamics is critical for a wide range of electrochemical applications including energy storage and electrochromic smart windows. One strategy to reveal such structure-property relationships is to perform transmission electron microscopy (TEM) of nanoparticles that have been cycled on TEM grid electrodes. One drawback of this approach is that images of some particles are correlated with the electrochemical response of the entire TEM grid electrode.

Ensemble-level energy transfer measurements can reveal the spatial distribution of defect sites in semiconductor nanocrystals.

Energy transfer measurements are widely used to measure the distance between donors and acceptors in heterogeneous environments. In nanocrystal (NC)-molecule donor-acceptor systems, NC defects can participate in electronic energy transfer (EnT) in a defect-mediated EnT process. Here, we explore whether ensemble-level spectroscopy measurements can quantify the distance between the donor defect sites in the NC and acceptor molecules.

Influence of the Substrate on the Optical and Photo-electrochemical Properties of Monolayer MoS.

Substrates influence the electrical and optical properties of monolayer (ML) MoS in field-effect transistors and photodetectors. Photoluminescence (PL) and Raman spectroscopy measurements have shown that conducting substrates can vary the doping concentration and influence exciton decay channels in ML-MoS. Doping and exciton decay dynamics are expected to play a major role in the efficiency of light-driven chemical reactions, but it is unclear to what extent these factors contribute to the photo(electro)catalytic properties of ML-MoS.

Probing Charge Carrier Transport and Recombination Pathways in Monolayer MoS/WS Heterojunction Photoelectrodes.

Monolayer heterojunctions such as MoS/WS are attractive for solar energy conversion applications because the interfacial electric field spatially separates charge carriers in less than 100 fs. Photoelectrochemical cells represent an intriguing platform to collect the spatially separated carriers. However, the recombination, transport, and interfacial charge transfer processes that take place following the ultrafast charge separation step have not been investigated.

Single nanoparticle photoelectrochemistry: What is next?

Semiconductor photoelectrochemistry is a fascinating field that deals with the chemistry and physics of photodriven reactions at solid/liquid interfaces. The interdisciplinary field attracts (electro)chemists, materials scientists, spectroscopists, and theorists to study fundamental and applied problems such as carrier dynamics at illuminated electrode/electrolyte interfaces and solar energy conversion to electricity or chemical fuels. In the pursuit of practical photoelectrochemical energy conversion systems, researchers are exploring inexpensive, solution-processed semiconductor nanomaterials as light absorbers.

High-Throughput Single-Nanoparticle-Level Imaging of Electrochemical Ion Insertion Reactions.

Nanoparticle electrodes are attractive for electrochemical energy storage applications because their nanoscale dimensions decrease ion transport distances and generally increase ion insertion/extraction efficiency. However, nanoparticles vary in size, shape, defect density, and surface composition, which warrants their investigation at the single-nanoparticle level. Here we demonstrate a nondestructive high-throughput electro-optical imaging approach to quantitatively measure electrochemical ion insertion reactions at the single-nanoparticle level.

Influence of single-nanoparticle electrochromic dynamics on the durability and speed of smart windows.

Nanomaterials have tremendous potential to increase electrochromic smart window efficiency, speed, and durability. However, nanoparticles vary in size, shape, and surface defects, and it is unknown how nanoparticle heterogeneity contributes to particle-dependent electrochromic properties. Here, we use single-nanoparticle-level electro-optical imaging to measure structure-function relationships in electrochromic tungsten oxide nanorods.

Laser Annealing Improves the Photoelectrochemical Activity of Ultrathin MoSe Photoelectrodes.

Understanding light-matter interactions in transition-metal dichalcogenides (TMDs) is critical for optoelectronic device applications. Several studies have shown that high intensity light irradiation can tune the optical and physical properties of pristine TMDs. The enhancement in optoelectronic properties has been attributed to a so-called laser annealing effect that heals chalcogen vacancies.

Efficient Ultrathin Liquid Junction Photovoltaics Based on Transition Metal Dichalcogenides.

Ultrathin photovoltaics made of MoS or WSe have the potential to convert solar energy to electricity with high efficiency because all photogenerated carriers are produced at a charge-collecting interface. However, solid-state monolayer photovoltaic devices typically require that charge carriers travel parallel to, instead of perpendicular to, the three atom-thick material toward charge-collecting contacts. Parallel charge transport across long distances decreases energy conversion efficiency.

Quantifying Photocurrent Loss of a Single Particle-Particle Interface in Nanostructured Photoelectrodes.

Particle-particle interfaces are ubiquitous in nanostructured photoelectrodes and photovoltaics, which are important devices for solar energy conversion. These interfaces are expected to cause performance losses in these devices, but how much loss they would incur is poorly defined. Here we use a subparticle photoelectrochemical current measurement, in combination with specific photoelectrode configurations, to quantify the current losses from single particle-particle interfaces formed between individual TiO nanorods operating as photoanodes in aqueous electrolytes.

Role of Photogenerated Iodine on the Energy-Conversion Properties of MoSe Nanoflake Liquid Junction Photovoltaics.

Transition metal dichalcogenides (TMDs) such as MoSe and WSe are efficient materials for converting solar energy to electrical energy in photoelectrochemical photovoltaic cells. One limiting factor of these liquid junction solar cells is that photogenerated oxidation products accumulate on the electrode surface and decrease the photocurrent efficiency. However, it is unclear where the reaction products accumulate on the electrode surface and how they impact the local photoelectrochemical response.

Bimetallic Effect of Single Nanocatalysts Visualized by Super-Resolution Catalysis Imaging.

Compared with their monometallic counterparts, bimetallic nanoparticles often show enhanced catalytic activity associated with the bimetallic interface. Direct quantitation of catalytic activity at the bimetallic interface is important for understanding the enhancement mechanism, but challenging experimentally. Here using single-molecule super-resolution catalysis imaging in correlation with electron microscopy, we report the first quantitative visualization of enhanced bimetallic activity within single bimetallic nanoparticles.

Patternable Solvent-Processed Thermoplastic Graphite Electrodes.

Since their invention in the 1950s, composite carbon electrodes have been employed in a wide variety of applications, ranging from batteries and fuel cells to chemical sensors, because they are easy to make and pattern at millimeter scales. Despite their widespread use, traditional carbon composite electrodes have substandard electrochemistry relative to metallic and glassy carbon electrodes. As a result, there is a critical need for new composite carbon electrodes that are highly electrochemically active, have universal and easy fabrication into complex geometries, are highly conductive, and are low cost.

Sub-particle reaction and photocurrent mapping to optimize catalyst-modified photoanodes.

The splitting of water photoelectrochemically into hydrogen and oxygen represents a promising technology for converting solar energy to fuel. The main challenge is to ensure that photogenerated holes efficiently oxidize water, which generally requires modification of the photoanode with an oxygen evolution catalyst (OEC) to increase the photocurrent and reduce the onset potential. However, because excess OEC material can hinder light absorption and decrease photoanode performance, its deposition needs to be carefully controlled--yet it is unclear which semiconductor surface sites give optimal improvement if targeted for OEC deposition, and whether sites catalysing water oxidation also contribute to competing charge-carrier recombination with photogenerated electrons.

Size selective photoetching of CdSe quantum dot sensitizers on single-crystal TiO₂.

Cadmium selenide quantum dots covalently attached to and photosensitizing single-crystal TiO2 surfaces are observed to corrode under illumination in aqueous electrolyte containing iodide as a regenerator. Comparison of photocurrent spectra before and after long-term monochromatic illumination indicated that the CdSe QD sensitizers photocorroded and decreased in size until their band gap energy exceeded the excitation energy. This wavelength-dependent photoelectrochemical etching mechanism can be used to tune the size distribution of surface adsorbed QDs and may account for the instability of QD sensitized solar cells that do not employ sulfide-based electrolytes.

Approaches to single-nanoparticle catalysis.

Nanoparticles are among the most important industrial catalysts, with applications ranging from chemical manufacturing to energy conversion and storage. Heterogeneity is a general feature among these nanoparticles, with their individual differences in size, shape, and surface sites leading to variable, particle-specific catalytic activity. Assessing the activity of individual nanoparticles, preferably with subparticle resolution, is thus desired and vital to the development of efficient catalysts.

Photooxidation of chloride by oxide minerals: implications for perchlorate on Mars.

We show that highly oxidizing valence band holes, produced by ultraviolet (UV) illumination of naturally occurring semiconducting minerals, are capable of oxidizing chloride ion to perchlorate in aqueous solutions at higher rates than other known natural perchlorate production processes. Our results support an alternative to atmospheric reactions leading to the formation of high concentrations of perchlorate on Mars.

Interfacial morphology and photoelectrochemistry of conjugated polyelectrolytes adsorbed on single crystal TiO2.

The nanoscale morphology and photoactivity of conjugated polyelectrolytes (CPEs) deposited from different solvents onto single crystal TiO(2) were investigated with atomic force microscopy (AFM) and photocurrent spectroscopy. CPE surface coverages on TiO(2) could be incremenentally increased by adsorbing the CPEs from static solutions. The solvents used for polymer adsorption influenced the surface morpohology of the CPEs on the TiO(2) surface.

Photoelectrochemical characterization of nanocrystalline thin-film Cu₂ZnSnS₄ photocathodes.

Cu₂ZnSnS₄ (CZTS) nanocrystals, synthesized by a hot injection solution method, have been fabricated into thin films by dip-casting onto fluorine doped tin oxide (FTO) substrates. The photoresponse of the CZTS nanocrystal films was evaluated using absorbance measurements along with photoelectrochemical methods in aqueous electrolytes. Photoelectrochemical characterization revealed a p-type photoresponse when the films were illuminated in an aqueous Eu(3+) redox electrolyte.

Multiple exciton collection in a sensitized photovoltaic system.

Multiple exciton generation, the creation of two electron-hole pairs from one high-energy photon, is well established in bulk semiconductors, but assessments of the efficiency of this effect remain controversial in quantum-confined systems like semiconductor nanocrystals. We used a photoelectrochemical system composed of PbS nanocrystals chemically bound to TiO(2) single crystals to demonstrate the collection of photocurrents with quantum yields greater than one electron per photon. The strong electronic coupling and favorable energy level alignment between PbS nanocrystals and bulk TiO(2) facilitate extraction of multiple excitons more quickly than they recombine, as well as collection of hot electrons from higher quantum dot excited states.