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The electronic structure of few-layer MoS2 is studied by in situ and operando spectroelectrochemistry in conditions relevant to its use as an electrocatalyst. We show that electron injection into the conduction band is coupled with a redshift of the exciton resonance, the magnitude of which depends on the number of vertical MoS2 layers. In addition, the applied electric field/electronic doping imparts uniaxial tensile strain evidenced by broadening Raman signals, indicating that under conditions of electrocatalysis, the system is structurally different from equilibrium. We demonstrate that field/carrier induced changes to the electronic structure of MoS2 alter the band edge positions which changes the fundamental thermodynamic driving force for charge transfer. This property is a function of the applied potential, an effect unique to 2D semiconductors. The dynamic band edge potentials change the relevant interfacial energetics for charge transfer and have strong implications for the mechanistic understanding of (photo)catalytic fuel-forming reactions using two-dimensional systems.

By understanding how the environmental composition impacts the optoelectronic properties of transition metal dichalcogenide monolayers, we demonstrate that simple photoluminescence (PL) measurements of tungsten disulfide (WS2) monolayers can differentiate relative humidity environments. In this paper, we examine the PL and photoconductivity of chemical vapor deposition grown WS2 monolayers under three carefully controlled environments: inert gas (N2), dry air (O2 in N2), and humid nitrogen (H2O vapor in N2). The WS2 PL is measured as a function of 532 nm laser power and exposure time and can be decomposed into the exciton, trion, and lower energy state(s) contributions. Under continuous illumination in either O2 or H2O vapor environment, we find dramatic (and reversible) increases in PL intensity relative to the PL in an inert environment. The PL bathochromically shifts in an O2 environment and is dominated by increased trion emission and diminished exciton emission. In contrast, the WS2 PL increase in a H2O environment results from an overall increase in emission from all spectral components where the exciton contribution dominates. The drastic increases in PL are anticorrelated with corresponding decreases in photoconductivity, as measured by time-resolved microwave conductivity. The results suggest that both O2 and H2O react photochemically with the WS2 monolayer surface, modifying the optoelectronic properties, but do so via distinct pathways. Thus, we use these optoelectronic differences to differentiate the amount of humidity in the air, which we show with 0%, 40%, and 80% relative humidity environments. This deeper understanding of how ambient conditions impact WS2 monolayers enables novel humidity sensors as well as a better understanding of the correlation between TMDC surface chemistry, light emission, and photoconductivity. Moreover, these WS2 measurements highlight the importance of considering the impact of the local environment on reported results.

Photoinduced generation of mobile charge carriers is the fundamental process underlying many applications, such as solar energy harvesting, solar fuel production, and efficient photodetectors. Monolayer transition-metal dichalcogenides (TMDCs) are an attractive model system for studying photoinduced carrier generation mechanisms in low-dimensional materials because they possess strong direct band gap absorption, large exciton binding energies, and are only a few atoms thick. While a number of studies have observed charge generation in neat TMDCs for photoexcitation at, above, or even below the optical band gap, the role of nonlinear processes (resulting from high photon fluences), defect states, excess charges, and layer interactions remains unclear. In this study, we introduce steady-state microwave conductivity (SSMC) spectroscopy for measuring charge generation action spectra in a model WS

Layered transition metal dichalcogenides (TMDs) represent a diverse, emerging source of two-dimensional (2D) nanostructures with broad application in optoelectronics and energy. In particular, tungsten disulfide (WS2) is an efficient visible light absorber with relatively high carrier mobilities and catalytic activity towards hydrogen evolution. While explanation of the quantum confinement and excitonic effects governing TMD optoelectronic properties has progressed in recent years, less is known about the ultra-fast photoresponse and carrier dynamics following light excitation. This work utilizes transient absorption spectroscopy, with pump tunability and broadband visible probing, to monitor the carrier dynamics of both CVD-grown monolayer and solution exfoliated WS2. Picosecond-scale features include simultaneous bleaching of excitonic states and a red-shifted absorption spectrum attributed to bandgap renormalization, while free carriers, defect trapped carriers, and recombination signatures are apparent at increasing pico- to nanosecond lifetimes. Features associated with excitons, trions, and photo-excited carriers exhibit strong dependence on local environmental factors. Moisture, oxygen, chemical dopants, and dielectric environment strongly affect the strength and decay lifetimes of these photo-excited species. These results highlight the importance of understanding and controlling these local environmental factors to the rational design and implementation of 2D TMDs optoelectronic device platforms.

Optical properties and thermal relaxation dynamics of resonantly excited plasmons are important in applications for optoelectronics, biomedicine, energy, and catalysis. Geometric optics of polydimethylsiloxane (PDMS) thin films containing uniformly or asymmetrically distributed polydisperse reduced AuNPs or uniformly distributed monodisperse solution-synthesized AuNPs were recently evaluated using a compact linear algebraic sum. Algebraic calculation of geometric transmission, reflection, and attenuation for AuNP-PDMS films provides a simple, workable alternative to effective medium approximations, computationally expensive methods, and fitting of experimental data. This approach allows for the summative optical responses of a sequence of 2D elements comprising a 3D assembly to be analyzed. Thin PDMS films containing 3-7 micron layers of reduced AuNPs were fabricated with a novel diffusive-reduction synthesis technique. Rapid diffusive reduction of AuNPs into asymmetric PDMS thin films provided superior photothermal response relative to thicker films with AuNPs reduced throughout, with a photon-to-heat conversion of up to 3000°C/watt which represents 3-230-fold increase over previous AuNP-functionalized systems. Later work showed that introduction of AuNPs into PDMS enhanced thermoplasmonic dissipation coincident with internal reflection of incident resonant irradiation. Measured thermal emission and dynamics of AuNP-PDMS thin films exceeded emission and dynamics attributable by finite element analysis to Mie absorption, Fourier heat conduction, Rayleigh convection, and Stefan-Boltzmann radiation. Refractive-index matching experiments and measured temperature profiles indicated AuNP-containing thin films internally reflected light and dissipated power transverse to the film surface. Enhanced thermoplasmonic dissipation from metal-polymer nanocomposite thin films could affect opto- and bio-electronic implementation of these systems.

Thermal relaxation dynamics of resonantly excited plasmons is important in optoelectronic, medical, and catalytic applications. This work shows introducing gold nanoparticles (AuNPs) into thin polydimethylsiloxane (PDMS) films enhanced thermoplasmonic dissipation coincident with internal reflection of incident resonant irradiation. Measured thermal emission and dynamics of AuNP–PDMS thin films exceeded emission and dynamics attributable by finite element analysis to Mie absorption, Fourier heat conduction, Rayleigh convection, and Stefan–Boltzmann radiation. Refractive-index matching experiments and measured temperature profiles indicated AuNP-containing thin films internally reflected light and dissipated power transverse to the film surface. Enhanced thermoplasmonic dissipation from metal–polymer nanocomposite thin films could affect opto- and bioelectronic implementation of these systems.

Hyper Rayleigh scattering (HRS) was used to measure the second-order nonlinear susceptibility, 𝝌(2), for liquid exfoliated WS2 monolayers. To the best of our knowledge, it is the first reported application of the HRS technique to assess the bulk-like 𝝌(2) of a two-dimensional (2D) material. The concentration-dependent HRS signal indicated a 4.90±0.30×10−25 esu first hyperpolarizability for 42 nm WS2 monolayers under 1064 nm laser irradiation using para-nitroaniline as an external reference. The corresponding value of 𝝌(2)𝒙𝒙𝒙 was calculated to be 460±28 pm V−1. This was within 46% of independent density functional theory predictions. Agreement with theory was improved over related microscopy-based approaches. These results support the use of HRS to evaluate 2D materials for nonlinear frequency mixing applications.

Polymer films containing plasmonic nanostructures are of increasing interest for development of responsive energy, sensing, and therapeutic systems. A series of novel gold nanoparticle (AuNP)-polydimethylsiloxane (PDMS) films were fabricated to elucidate enhanced optical extinction from diffractive and scattering induced internal reflection. AuNPs with dramatically different scattering-to-absorption ratios were compared at variable interparticle separations to differentiate light trapping from optical diffraction and Mie scattering. Description of interfacial optical and thermal effects due to these interrelated contributions has progressed beyond Mie theory, Beer’s law, effective media, and conventional heat transfer descriptions. Thermal dissipation rates in AuNP-PDMS with this interfacial optical reflection was enhanced relative to films containing heterogeneous AuNPs and a developed thermal dissipation description. This heuristic, which accounts for contributions of both internal and external thermal dissipations, has been shown to accurately predict thermal dissipation rates from AuNP-containing insulating and conductive substrates in both two and three-dimensional systems. Enhanced thermal response rates could enable design and adaptive control of thermoplasmonic materials for a variety of implementations.

Interest in the optical properties of plasmonic nanoparticles embedded in transparent polymers is expanding due to potential uses in sustainability, biomedicine, and manufacturing. Geometric optics of polydimethylsiloxane (PDMS) thin films containing uniformly or asymmetrically distributed polydisperse reduced gold nanoparticles (AuNPs) or uniformly distributed monodisperse solution synthesized AuNPs were recently evaluated using a compact linear algebraic sum. Algebraic calculation of geometric transmission, reflection, and attenuation for AuNP-PDMS films provides a simple, workable alternative to effective medium approximations, computationally expensive methods, and fitting of experimental data. Generally, transmission and reflection increased with AuNP isotropy and particle density, as displayed on a novel ternary diagram. Irregular AuNP morphology and size distribution caused optical attenuation from polydisperse films to increase in proportion to log 10 increases in gold content, resulting in lower attenuation per gold mass when compared to monodisperse AuNPs. Uniform monodisperse AuNP-PDMS films attenuated light in proportion to gold content, with films attenuating 0.15 fractional units per 0.1 mass-percent AuNPs. Thin layers of concentrated AuNPs attenuated light more efficiently. A 25 micron thick layer of 1.2 mass-percent AuNPs attenuated 0.5 fractional units, the same number as a 130 micron thick 0.6 mass-percent film. Measured optical responses from asymmetric AuNP-PDMS films with an adjacent back-reflector and pairs of uniformly distributed films were predictable within 0.04 units of linear algebraic estimates based on geometric optics. This approach allows for the summative optical responses of a sequence of 2D elements comprising a 3D assembly to be analyzed.

Opto-electronic coupling of plasmonic nano-antennas in the near infrared water window in vitro and in vivo is of growing interest for imaging contrast agents, spectroscopic labels and rulers, biosensing, drug-delivery, and optoplasmonic ablation. Metamaterials composed of nanoplasmonic meta-atoms offer improved figures of merit in many applications across a broader spectral window. Discrete and coupled dipole approximations effectively describe localized and coupled resonance modes in nanoplasmonic metamaterials. From numeric and experimental results have emerged four design principles to guide fabrication and implementation of metamaterials in bio-related devices and systems. Resonance intensity and sensitivity are enhanced by surface-to-mass of meta-atoms and lattice constant. Fano resonant coupling is dependent on meta-atom polarizability and lattice geometry. Internal reflection in plasmonic metaatom- containing polymer films enhances dissipation rate. Dimensions of self-assembled meta-atoms depend on balancing electrochemical and surface forces. Examples of these principles from our lab compare computation with images and spectra from ordered metal-ceramic and polymeric nanocomposite metamaterials for bio/opto theranostic applications. These principles speed design and description of new architectures for nanoplasmonic metamaterials that show promise for bioapplications.

Plasmonic nanoparticles embedded in polymer films enhance optoelectronic properties of photovoltaics, sensors, and interconnects. This work examined optical extinction of polymer films containing randomly dispersed gold nanoparticles (AuNP) with negligible Rayleigh scattering cross-sections at particle separations and film thicknesses less than (sub-) to greater than (super-) the localized surface plasmon resonant (LSPR) wavelength, λ LSPR . Optical extinction followed opposite trends in sub- and superwavelength films on a per nanoparticle basis. In ∼ 70 - nm -thick polyvinylpyrrolidone films containing 16 nm AuNP, measured resonant extinction per particle decreased as particle separation decreased from ∼ 130 to 76 nm, consistent with trends from Maxwell Garnett effective medium theory and coupled dipole approximation. In ∼ 1 - mm -thick polydimethylsiloxane films containing 16-nm AuNP, resonant extinction per particle plateaued at particle separations ≥ λ LSPR , then increased as particle separation radius decreased from ∼ 514 to 408 nm. Contributions from isolated particles, interparticle interactions and heterogeneities in sub- and super- λ LSPR films containing AuNP at sub- λ LSPR separations were examined. Characterizing optoplasmonics of thin polymer films embedded with plasmonic NP supports rational development of optoelectronic, biomedical, and catalytic activity using these nanocomposites.

Optical properties of polymer films embedded with plasmonic nanoparticles (NPs) are important in many implementations. In this work, optical extinction by polydimethylsiloxane (PDMS) films containing gold (Au) NPs was enhanced at resonance compared to AuNPs in suspensions, Beer?Lambert law, or Mie theory by internal reflection due to optical diffraction in 16 nm AuNP?PDMS films and Mie scattering in 76 nm AuNP?PDMS films. Resonant extinction per AuNP for 16 nm AuNPs with negligible resonant Mie scattering was enhanced up to 1.5-fold at interparticle separation (i.e., Wigner?Seitz radii) comparable to incident wavelength. It was attributable to diffraction through apertures formed by overlapping electric fields of adjacent, resonantly excited AuNPs at Wigner?Seitz radii equal to or less than incident wavelengths. Resonant extinction per AuNP for strongly Mie scattering 76 nm AuNPs was enhanced up to 1.3-fold at Wigner?Seitz radii four or more times greater than incident wavelength. Enhanced light trapping from diffraction and/or scattering is relevant to optoelectronic, biomedical, and catalytic activity of substrates embedded with NPs.