Photoexcitation Dynamics and Long-Lived Excitons in Strain-Engineered Transition Metal Dichalcogenides.

Strain-engineering in 2D transition metal dichalcogenide (TMD) semiconductors has garnered intense research interest in tailoring the optical properties via strain-induced modifications of the electronic bands in TMDs, while its impact on the exciton dynamics remains less understood. To address this, an extensive study of transient optical absorption (TA) of both W- and Mo-based single-crystalline monolayer TMDs grown by a recently developed laser-assisted evaporation method is performed. All spectral features of the monolayers as grown on fused silica substrates exhibit appreciable redshifts relating to the existence of strain due to growth conditions. Moreover, these systems exhibit a dramatic slowing down of exciton dynamics (100s of picoseconds to few nanoseconds) with an increase in carrier densities, which strongly contrasts with the monolayers in their freestanding form as well as in comparison with more traditionally grown TMDs. The observations are related to the modifications of the electronic bands as expected from the strain and associated population of the intervalley dark excitons that can now interplay with intravalley excitations. These findings are consistent across both the Mo- and W-based TMD families, providing key information about the influence of the growth conditions on the nature of optical excitations and fostering emerging optoelectronic applications of monolayer TMDs.

Gentle Materials Need Gentle Fabrication: Encapsulation of Perovskites by Gas-Phase Alumina Deposition.

Metal halide perovskites have attracted tremendous attention as promising materials for future-generation optoelectronic devices. Despite their outstanding optical and transport properties, the lack of environmental and operational stability remains a major practical challenge. One of the promising stabilization avenues is metal oxide encapsulation via atomic layer deposition (ALD); however, the unavoidable reaction of metal precursors with the perovskite surface in conventional ALD leads to degradation and restructuring of the perovskites' surfaces. This Perspective highlights the development of a modified gas-phase ALD technique for alumina encapsulation that not only prevents perovskites' degradation but also significantly improves their optical properties and air stability. The correlation between precise atomic interactions at the perovskite-metal oxide interface with the dramatically enhanced optical properties is supported by density functional theory calculations, which also underlines the widespread applicability of this gentle technique for a variety of perovskite nanostructures unbarring potential opportunities offered by combination of these approaches.

Interface Matters: Enhanced Photoluminescence and Long-Term Stability of Zero-Dimensional Cesium Lead Bromide Nanocrystals via Gas-Phase Aluminum Oxide Encapsulation.

Cesium lead halide perovskite nanocrystals (PNCs), while possessing facile chemical synthesis routes and high photoluminescence (PL) properties, are still challenged by issues of instability and degradation. Although atomic layer deposition (ALD) of metal oxides has been one of the common encapsulation approaches for longer term stability, its application inevitably resulted in severe loss of emission efficiency and at times partial loss of structural integrity of perovskites, creating a bottleneck in its practical viability. We demonstrate a nondestructive modified gas-phase technique with codeposition of both precursors trimethylaluminum and water to dramatically enhance the PL emission in zero-dimensional (0D) Cs4PbBr6 PNCs via alumina encapsulation. X-ray photoelectron spectroscopy analysis of Cs4PbBr6 films reveals the alumina deposition to be accompanied by elemental composition changes, particularly by the reduction of the excessive cesium content. Ab initio density functional theory simulations further unfold that the presence of excess Cs on the surface of PNCs leads to decomposition of structural [PbBr6]4- octahedra in the 0D perovskite lattice, which can be prevented in the presence of added hydroxyl groups. Our study thus unveils the pivotal role of the PNC surface composition and treatment in the process of its interaction with metal oxide precursors to control the PL properties as well as the stability of PNCs, providing an unprecedented way to use the conventional ALD technique for their successful integration into optoelectronic and photonic devices with improved properties.

Dynamical Interconversion between Excitons and Geminate Charge Pairs in Two-Dimensional Perovskite Layers Described by the Onsager-Braun Model.

Time-resolved photoluminescence (PL) and femtosecond transient absorption (TA) spectroscopy are employed to study the photoexcitation dynamics in a highly emissive two-dimensional perovskite compound (en)4Pb2Br9·3Br with the ethylene diammonium (en) spacer. We find that while the PL kinetics is substantially T-dependent over the whole range of studied temperatures T ∼ 77-350 K, the PL quantum yield remains remarkably nearly T-independent up to T ∼ 280-290 K, appreciably decreasing only at higher temperatures. Considerable differences are also revealed between the TA spectra and the responses to the excitation power at low and at room temperatures. Numerical solutions of Onsager-Braun-type kinetic-diffusion equations illustrate that the salient features of the experimental observations are consistent with the picture of a T-dependent dynamic interplay between tightly bound emissive excitons and larger-size, loosely bound, nonemissive geminate charge pairs arising already at earlier relaxation times. The geminate pairs play the role of "reservoir" states providing a delayed feeding into the emitting excitons, thus giving rise to the longer-time PL decay components and accounting for a stable PL output at lower temperatures. At higher temperatures, the propensity for thermal dissociation of excitons and bound pairs increases, leading subsequently to the precipitous decrease of the PL.

Delayed Photoluminescence and Modified Blinking Statistics in Alumina-Encapsulated Zero-Dimensional Inorganic Perovskite Nanocrystals.

We demonstrate enhancement of the photoluminescence (PL) properties of individual zero-dimensional (0D) Cs4PbBr6 perovskite nanocrystals (PNCs) upon encapsulation by alumina using an appropriately modified atomic layer deposition method. In addition to the increased PL intensity and improved long-term stability of encapsulated PNCs, our single-particle studies reveal substantial changes in the PL blinking statistics and the persistent appearance of the long-lived, "delayed" PL components. The blinking patterns exhibit a modification from the fast switching between fluorescent ON and OFF states found in bare PNCs to a behavior with longer ON states and more isolated OFF states in alumina-encapsulated PNCs. Controlled exposure of 0D nanocrystals to moisture suggests that the observed PL lifetime changes may be related to water-induced "reservoir" states that allow for longer-lived charge storage with subsequent back-feeding into the emissive states. Viable encapsulation of PNCs with metal oxides that can preserve and even enhance their PL properties can be utilized in the fabrication of extended structures on their basis for optoelectronic and photonic applications.

Guiding and binding of cavity photons with patterned two-dimensional semiconductors.

Strong optical responses of two-dimensional (2D) semiconductors like transition metal dichalcogenides (TMDs) draw substantial attention for prospective applications in optoelectronics and photonics. Here, we propose a potentially attractive application avenue via embedding patterns of 2D semiconductors (shaped, e.g., as strips or disks) in planar optical microcavities to engineer photonic modes in the dissipation-free spectral range below the optical gap. While the cavity confines electromagnetic fields to its interior, the high in-plane polarizability of 2D materials causes the appearance of the cavity modes that are bound to the patterned pieces in the lateral directions along the cavity. A TMD strip would then act to guide such bound cavity photons, while a pair of neighboring strips could operate similar to coupled photonic waveguides. Our calculations relying on experimentally measured TMD optical suspectibilities, explicitly demonstrate this type of behavior accompanied by photonic binding energies on the order of 10 meV and micron-scale spatial extents. They indicate that patterned 2D semiconductor structures employed within microcavities could represent a new material platform to enable various functionalities of integrated photonics.

Biexciton and trion energy transfer from CdSe/CdS giant nanocrystals to Si substrates.

Observation of energy transfer (ET) from multiexcitonic (MX) complexes in nanocrystal quantum dots (NQDs) has been severely restricted due to efficient nonradiative Auger recombination leading to very low MX emission quantum yields. Here we employed "giant" CdSe/CdS NQDs with suppressed Auger recombination to study ET of biexcitons (BX) and charged excitons (trions) into Si substrate. Photoluminescence (PL) measurements of (sub)monolayers of gNQDs controllably assembled on various interacting surfaces and augmented by single gNQD's imaging reveal appearance of BX spectral signatures and progressive acceleration of PL lifetimes of all excitonic species on Si substrates. From statistical analysis of a large number of PL lifetime traces, representative exciton, trion and BX ET efficiencies are measured as ∼75%, 55% and 45% respectively. Detailed analysis of the MX's radiative rates demonstrate the crucial role of the radiative (waveguide) ET in maintaining high overall transfer efficiency despite the prevalent Auger recombination. Our observations point towards practical utilization of MX-bearing nanocrystals in future optoelectronics architectures.

Propagation and absorption of light in planar dielectric waveguides with two-dimensional semiconductors.

Strong optical responses of atomically thin two-dimensional (2D) semiconductors make them attractive candidates for integration into various photonic and optoelectronic structures. We discuss some fundamental effects of such integration into planar dielectric waveguides by demonstrating that a substantial modification of the spectrum of waveguide modes can occur due to high in-plane polarizability of 2D layers. Our calculations illustrate both the conceptual possibilities associated with sharp excitonic resonances as well as the results obtained with the experimentally assessed polarizability of monolayer MoS2 over a broad spectral range. We point out that waveguide structures could also enable the tunable absorption by 2D semiconductors of the light that propagates along them, a modality quite different from the traditional light harvesting geometry.

Energy transfer from colloidal nanocrystals to strongly absorbing perovskites.

Integration of colloidal nanocrystal quantum dots (NQDs) with strongly absorbing semiconductors offers the possibility of developing optoelectronic and photonic devices with new functionalities. We examine the process of energy transfer (ET) from photoactive CdSe/ZnS core/shell NQDs into lead-halide perovskite polycrystalline films as a function of distance from the perovskite surface using time-resolved photoluminescence (TRPL) spectroscopy. We demonstrate near-field electromagnetic coupling between vastly dissimilar excitation in two materials that can reach an efficiency of 99% at room temperature. Our experimental results, combined with electrodynamics modeling, reveal the leading role of non-radiative ET at close distances, augmented by the waveguide emission coupling and light reabsorption at separations >10 nm. These results open the way to combining materials with different dimensionalities to achieve novel nanoscale architectures with improved photovoltaic and light emitting functionalities.

Order of magnitude enhancement of monolayer MoS2 photoluminescence due to near-field energy influx from nanocrystal films.

Two-dimensional transition metal dichalcogenides (TMDCs) like MoS2 are promising candidates for various optoelectronic applications. The typical photoluminescence (PL) of monolayer MoS2 is however known to suffer very low quantum yields. We demonstrate a 10-fold increase of MoS2 excitonic PL enabled by nonradiative energy transfer (NRET) from adjacent nanocrystal quantum dot (NQD) films. The understanding of this effect is facilitated by our application of transient absorption (TA) spectroscopy to monitor the energy influx into the monolayer MoS2 in the process of ET from photoexcited CdSe/ZnS nanocrystals. In contrast to PL spectroscopy, TA can detect even non-emissive excitons, and we register an order of magnitude enhancement of the MoS2 excitonic TA signatures in hybrids with NQDs. The appearance of ET-induced nanosecond-scale kinetics in TA features is consistent with PL dynamics of energy-accepting MoS2 and PL quenching data of the energy-donating NQDs. The observed enhancement is attributed to the reduction of recombination losses for excitons gradually transferred into MoS2 under quasi-resonant conditions as compared with their direct photoproduction. The TA and PL data clearly illustrate the efficacy of MoS2 and likely other TMDC materials as energy acceptors and the possibility of their practical utilization in NRET-coupled hybrid nanostructures.

Hybrid light sensor based on ultrathin Si nanomembranes sensitized with CdSe/ZnS colloidal nanocrystal quantum dots.

We report the observation of a large enhancement of the wavelength-dependent photocurrent in ultrathin silicon nanomembranes (SiNM) decorated with colloidal CdSe/ZnS nanocrystal quantum dots (NQDs). Back-gated, field-effect transistor structures based on 75 nm-thick SiNMs are functionalized with self-assembled monolayers (SAMs) preventing surface oxidation and minimizing the surface defect densities. NQDs are drop cast on the active region of the device and the photocurrent is measured as a function of the excitation wavelength across the NQD absorption region. Photocurrent enhancement on the order of several hundred nA's is observed for NQD/SAM/SiNM devices compared to reference SAM/SiNM structures, with the device peak response closely correlated to the NQD absorption peak. We propose light-induced gating of the surface electrostatic potential and forward self-biasing of the FET channel as the two key mechanisms leading to the large photocurrent increase. Our findings open the possibility of employing silicon-nanocrystal hybrid structures for light sensing applications.

Increasing the efficiency of thermoacoustic carbon nanotube sound projectors.

Carbon nanotubes (CNTs) can generate smooth-spectra sound emission over a wide frequency range (1-10(5) Hz) by means of thermoacoustics (TA). However, in the low frequencies f, where the need for large area sound projectors is high, the sound generation efficiency η of open CNT sheets is low, since η [proportionality] f(2). Together with this problem, the nanoscale thickness of CNT sheets, their high sensitivity to the environment and the high surface temperatures useful for TA sound generation are other drawbacks, which we address here by protective encapsulation of free-standing CNT sheets in inert gases. We provide an extensive experimental study of such closed systems for different thermodynamic regimes and rationalize our observations within a basic theoretical framework. The observed sound pressure levels for encapsulated argon filled TA transducers (130 dB in air and 200 dB underwater in the near field at 5 cm distance, and 100 and 170 dB in the far field at 1 m distance) are Q times higher than those for open systems, where Q is the resonant quality factor of the thin enclosure plates. Moreover, the sound generation efficiency of the encapsulated system increases toward low frequencies (η [proportionality] 1/f(2)). Another method to increase η in the low frequency region is by modulation of the applied high frequency carrier current with a low frequency resonant envelope. This approach enables sound generation at the frequency of the applied current without the need for additional energy-consuming biasing. The acoustical and geometrical parameters providing further increases in efficiency and transduction performance for resonant systems are discussed.

Visible to near-infrared sensitization of silicon substrates via energy transfer from proximal nanocrystals: further insights for hybrid photovoltaics.

We provide a unified spectroscopic evidence of efficient energy transfer (ET) from optically excited colloidal nanocrystal quantum dots (NQDs) into Si substrates in a broad range of wavelengths: from visible (545 nm) to near-infrared (800 nm). Chemical grafting of nanocrystals on hydrogenated Si surfaces is achieved via amine-modified carboxy-alkyl chain linkers, thus ensuring complete surface passivation and accurate NQD positioning. Time-resolved photoluminescence (PL) has been measured for a set of CdSe/ZnS and CdSeTe/ZnS NQDs of various sizes and compositions grafted on Si and SiO2 substrates. The measured acceleration of the PL decays on Si substrates is in good agreement with theoretical expectations based on the frequency-dependent dielectric properties of Si and NQD-Si separation distances. A comparative analysis reveals separate contributions to ET coming from the nonradiative (NRET) and radiative (RET) channels: NRET is a dominant mechanism for proximal NQDs in the middle of the visible range and becomes comparable with RET toward near-infrared wavelengths. The broad range over which the ET efficiency is estimated to be at the level of ∼90% further supports the concept that hybrid nanocrystal/silicon thin-film photovoltaic devices could efficiently harvest solar energy across the entire spectrum of wavelengths.

Efficient radiative and nonradiative energy transfer from proximal CdSe/ZnS nanocrystals into silicon nanomembranes.

We demonstrate efficient excitonic sensitization of crystalline Si nanomembranes via combined effects of radiative (RET) and nonradiative (NRET) energy transfer from a proximal monolayer of colloidal semiconductor nanocrystals. Ultrathin, 25-300 nm Si films are prepared on top of insulating SiO(2) substrates and grafted with a monolayer of CdSe/ZnS nanocrystals via carboxy-alkyl chain linkers. The wet chemical preparation ensures that Si surfaces are fully passivated with a negligible number of nonradiative surface state defects and that the separation between nanocrystals and Si is tightly controlled. Time-resolved photoluminescence measurements combined with theoretical modeling allow us to quantify individual contributions from RET and NRET. Overall efficiency of ET into Si is estimated to exceed 85% for a short distance of about 4 nm from nanocrystals to the Si surface. Effective and longer-range radiative coupling of nanocrystal's emission to waveguiding modes of Si films is clearly revealed. This demonstration supports the feasibility of an advanced thin-film hybrid solar cell concept that relies on energy transfer between strong light absorbers and adjacent high-mobility Si layers.

Mirage effect from thermally modulated transparent carbon nanotube sheets.

The single-beam mirage effect, also known as photothermal deflection, is studied using a free-standing, highly aligned carbon nanotube aerogel sheet as the heat source. The extremely low thermal capacitance and high heat transfer ability of these transparent forest-drawn carbon nanotube sheets enables high frequency modulation of sheet temperature over an enormous temperature range, thereby providing a sharp, rapidly changing gradient of refractive index in the surrounding liquid or gas. The advantages of temperature modulation using carbon nanotube sheets are multiple: in inert gases the temperature can reach > 2500 K; the obtained frequency range for photothermal modulation is ~100 kHz in gases and over 100 Hz in high refractive index liquids; and the heat source is transparent for optical and acoustical waves. Unlike for conventional heat sources for photothermal deflection, the intensity and phase of the thermally modulated beam component linearly depends upon the beam-to-sheet separation over a wide range of distances. This aspect enables convenient measurements of accurate values for thermal diffusivity and the temperature dependence of refractive index for both liquids and gases. The remarkable performance of nanotube sheets suggests possible applications as photo-deflectors and for switchable invisibility cloaks, and provides useful insights into their use as thermoacoustic projectors and sonar. Visibility cloaking is demonstrated in a liquid.

Giant-stroke, superelastic carbon nanotube aerogel muscles.

Improved electrically powered artificial muscles are needed for generating force, moving objects, and accomplishing work. Carbon nanotube aerogel sheets are the sole component of new artificial muscles that provide giant elongations and elongation rates of 220% and (3.7 x 10(4))% per second, respectively, at operating temperatures from 80 to 1900 kelvin. These solid-state-fabricated sheets are enthalpic rubbers having gaslike density and specific strength in one direction higher than those of steel plate. Actuation decreases nanotube aerogel density and can be permanently frozen for such device applications as transparent electrodes. Poisson's ratios reach 15, a factor of 30 higher than for conventional rubbers. These giant Poisson's ratios explain the observed opposite sign of width and length actuation and result in rare properties: negative linear compressibility and stretch densification.