#TeddersRecommends

The excitonic relaxation dynamics of perovskite adsorbed on mesoporous thin films of Al₂O₃ and NiO upon excitation at 450 nm were investigated with femtosecond optical gating of photoluminescence (PL) via up-conversion. The temporal profiles of emission observed in spectral region 670–810 nm were described satisfactorily with a composite consecutive kinetic model and three transient components representing one hot and two cold excitonic relaxations. All observed relaxation dynamics depend on the emission wavelength, showing a systematic time–amplitude correlation for all three components. When the NiO film was employed, we observed an extent of relaxation proceeding through the non-emissive surface state larger than through the direct electronic relaxation channel, which quenches the PL intensity more effectively than on the Al₂O₃ film. We conclude that perovskite is an effective hole carrier in a p-type electrode for NiO-based perovskite solar cells showing great performance.

Femtosecond optical gating (FOG) technique was employed to investigate the excitonic relaxation mechanism in a NiO-supported p-type perovskite. The effect of photoluminescence quenching and the excellent photovoltaic performance were rationalized based on this measurement.

A simple and efficient inkjet printing technology is developed for molybdenum disulfide (MoS₂), one of the most attractive two-dimensional layered materials. The technology effectively addresses critical issues associated with normal MoS₂ liquid dispersions (such as incompatible rheology, low concentration, and solvent toxicity), and hence can directly and reliably write uniform patterns of high-quality (5–7 nm thick) MoS₂ nanosheets at a resolution of tens of micrometers. The technology efficiency facilitates the integration of printed MoS₂ patterns with other components (such as electrodes), and hence allows fabricating various functional devices, including thin film transistors, photoluminescence patterns, and photodetectors, in a simple, massive and cost-effective manner while retains the unique properties of MoS₂. The technology has great potential in a variety of applications, such as photonics, optoelectronics, sensors, and energy storage.

An efficient inkjet printing technology is developed for direct and reliable writing of uniform patterns of high-quality MoS₂ nanosheets at a resolution of tens of micro­meters. It provides a facile method for massive and cost-effective production of a variety of electronic and photonic devices, such as thin film transistors, photoluminescence patterns, and photodetectors.

Van der Waals growth of GaAs on silicon using a two-dimensional layered material, graphene, as a lattice mismatch/thermal expansion coefficient mismatch relieving buffer layer is presented. Two-dimensional growth of GaAs thin films on graphene is a potential route towards heteroepitaxial integration of GaAs on silicon in the developing field of silicon photonics. Hetero-layered GaAs is deposited by molecular beam epitaxy on graphene/silicon at growth temperatures ranging from 350 °C to 600 °C under a constant arsenic flux. Samples are characterized by plan-view scanning electron microscopy, atomic force microscopy, Raman microscopy, and X-ray diffraction. The low energy of the graphene surface and the GaAs/graphene interface is overcome through an optimized growth technique to obtain an atomically smooth low­ temperature GaAs nucleation layer. However, the low adsorption and migration energies of gallium and arsenic atoms on graphene result in cluster-growth mode during crystallization of GaAs films at an elevated temperature. In this paper, we present the first example of an ultrasmooth morphology for GaAs films with a strong (111) oriented fiber-texture on graphene/silicon using quasi van der Waals epitaxy, making it a remarkable step towards an eventual demonstration of the epitaxial growth of GaAs by this approach for heterogeneous integration.

Van der Waals growth of GaAs on silicon using graphene as a lattice mismatch/thermal expansion coefficient mismatch relieving vdW buffer layer is reported. The growth of GaAs thin films on graphene is a potential route towards heteroepitaxial integration of GaAs on silicon. An ultrasmooth morphology for GaAs films with a strong (111) oriented fiber-texture on graphene/Si using quasi van der Waals epitaxy is presented.

Memristive devices are the precursors to high density nanoscale memories and the building blocks for neuromorphic computing. In this work, a unique room temperature synthesized perovskite oxide (amorphous SrTiO₃: a-STO) thin film platform with engineered oxygen deficiencies is shown to realize high performance and scalable metal-oxide-metal (MIM) memristive arrays demonstrating excellent uniformity of the key resistive switching parameters. a-STO memristors exhibit nonvolatile bipolar resistive switching with significantly high (10³–10⁴) switching ratios, good endurance (>10⁶I–V sweep cycles), and retention with less than 1% change in resistance over repeated 10⁵ s long READ cycles. Nano-contact studies utilizing in situ electrical nanoindentation technique reveal nanoionics driven switching processes that rely on isolatedly controllable nano-switches uniformly distributed over the device area. Furthermore, in situ electrical nanoindentation studies on ultrathin a-STO/metal stacks highlight the impact of mechanical stress on the modulation of non-linear ionic transport mechanisms in perovskite oxides while confirming the ultimate scalability of these devices. These results highlight the promise of amorphous perovskite memristors for high performance CMOS/CMOL compatible memristive systems.

High performance CMOS/CMOL compatible memristive arrays based on amorphous SrTiO₃ thin films with engineered oxygen deficiencies are presented. Isolated nano-switches are found to be responsible for the excellent switching performance of a-STO memory cells. Nanoscale electromechanical investigations highlight the assistive role of mechanical stress in nanoionics based conduction and resistive switching in a-STO devices and confirm their ultimate scalability.

Engineered metal-dielectric-metal nanostructures with broadband absorbing properties in the visible spectral range are fabricated by combining the plasmonic resonances of different noble metal nanostructures. Silver nanocubes and gold nanogratings couple to each other using a dielectric polymer spacer with controllable thickness, resulting in a large multiplicative enhancement of absorption properties across a broad spectral range. Narrow, long nanogrooves in a gold film are first fabricated using electron beam lithography, after which a polymer spacer layer with a controllable thickness ranging from 4 to 12 nm is assembled by spin-assisted layer-by-layer assembly. Finally, silver nanocubes with different surface coverages ranging from 12% to 22% are deposited using the Langmuir–Blodgett technique. The individual plasmon resonances of these different nanostructures are located at significantly different optical frequencies and are tuned in this study to allow a significant increase of light absorbance of the original gratings to an average value of 84% across the broad wavelength range of 450–850 nm.

An efficient broadband absorber in the visible wavelength range is constructed from the individual plasmonic resonances of gold nanogratings and silver nanocubes separated from one another by a dielectric spacer. An average 84% absorption in the 450–850 nm wavelength range is ultimately achieved for this unique design.

Tin sulfide (SnS), as a promising absorber material in thin-film photovoltaic devices, is described. Here, it is confirmed that SnS evaporates congruently, which provides facile composition control akin to cadmium telluride. A SnS heterojunction solar cell is demons trated, which has a power conversion efficiency of 3.88% (certified), and an empirical loss analysis is presented to guide further performance improvements.

Molybdenum disulfide (MoS₂) is a layered semiconducting material with a tunable bandgap that is promising for the next generation nanoelectronics as a substitute for graphene or silicon. Despite recent progress, the synthesis of high-quality and highly uniform MoS₂ on a large scale is still a challenge. In this work, a temperature-dependent synthesis study of large-area MoS₂ by direct sulfurization of evaporated Mo thin films on SiO₂ is presented. A variety of physical characterization techniques is employed to investigate the structural quality of the material. The film quality is shown to be similar to geological MoS₂, if synthesized at sufficiently high temperatures (1050 °C). In addition, a highly uniform growth of trilayer MoS₂ with an unprecedented uniformity of ±0.07 nm over a large area (> 10 cm²) is achieved. These films are used to fabricate field-effect transistors following a straightforward wafer-scale UV lithography process. The intrinsic field-effect mobility is estimated to be about cm² V^–1 s^–1 and compared to previous studies. These results represent a significant step towards application of MoS₂ in nanoelectronics and sensing.

A temperature-dependent synthesis study of large-area MoS₂ by direct sulfurization of evaporated Mo thin films is presented. The resulting film quality is similar to geological MoS₂. An unprecedented uniformity of nm over a large area (>10 cm²) is achieved with trilayer MoS₂. The estimated intrinsic field-effect mobility is approximately 6.5 ± 2.2 cm² V^–1 s^–1.

Two carbazole-based small molecule hole-transport materials (HTMs) are synthesized and investigated in solid-state dye-sensitized solar cells (ssDSCs) and perovskite solar cells (PSCs). The HTM X51-based devices exhibit high power conversion efficiencies (PCEs) of 6.0% and 9.8% in ssDSCs and PSCs, respectively. These results are superior or comparable to those of 5.5% and 10.2%, respectively, obtained for the analogous cells using the state-of-the-art HTM Spiro-OMeTAD.

Semiconductor-sensitized solar cells (SSCs) are emerging as promising devices for achieving efficient and low-cost solar-energy conversion. On page 5337, W.-J. Zhang, C.-S. Lee, and co-workers review the recent advances in ZnOnanostructure-based SSCs highlighting on approaches to improve the energy-conversion efficiency through various surface and interface engineering techniques.

A simple, low temperature solution process for Pb/Sn binary-metal perovskite planar-heterojunction solar cells is demonstrated. Sn inclusion substantially influences the band-gap, crystallization kinetics, and thin-film formation leading to a broadened light absorption and enhanced film coverage on ITO/PEDOT:PSS. As a result, the optimized device shows a PCE exceeding 10%, which is the best result for binary-metal perovskite solar cells so far.

A self-organized hole extraction layer (SOHEL) with high work function (WF) is designed for energy level alignment with the ionization potential level of CH₃NH₃PbI₃. The SOHEL increases the built-in potential, photocurrent, and power conversion efficiency (PCE) of CH₃NH₃PbI₃ perovskite solar cells. Thus, interface engineering of the positive electrode of solution-processed planar heterojunction solar cells using a high-WF SOHEL is a very effective way to achieve high device efficiency (PCE = 11.7% on glass).

Theoretical models predict that a variety of self-assembled structures of closely packed spherical particles may result when they are confined in a cylindrical domain. In the present work we demonstrate for the first time that the polymer-coated nanoparticles confined in the self-assembled cylindrical domains of a block copolymer pack in helical morphology, where we can isolate individual fibers filled with helically arranged nanoparticles. This finding provides unique possibilities for fundamental as well as application-oriented research in similar directions.

Packing makes coated screws: Silver nanoparticles confined in the self-assembled cylindrical domains of polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) pack in an interesting helical morphology. This observation opens up possibilities for the discovery of novel hierarchical structures in such composite systems and also provides new opportunities for the application of such materials in nanotechnology.

An in situ study is reported on the structural evolution in nanocluster films under He⁺ ion irradiation using an advanced helium ion microscope. The films consist of loosely interconnected nanoclusters of magnetite or iron-magnetite (Fe-Fe₃O₄) core-shells. The nanostructure is observed to undergo dramatic changes under ion-beam irradiation, featuring grain growth, phase transition, particle aggregation, and formation of nanowire-like network and nanopores. Studies based on ion irradiation, thermal annealing and electron irradiation have indicated that the major structural evolution is activated by elastic nuclear collisions, while both electronic and thermal processes can play a significant role once the evolution starts. The electrical resistance of the Fe-Fe₃O₄ films measured in situ exhibits a super-exponential decay with dose. The behavior suggests that the nanocluster films possess an intrinsic merit for development of an advanced online monitor for fast neutron radiation with both high detection sensitivity and long-term applicability, which can enhance safety measures in many nuclear operations.

An in situ study on the nanostructural evolution and electrical resistance variation of Fe₃O₄ and Fe-Fe₃O₄ core–shell nanocluster films under ion irradiation is presented. Grain growth, phase transition, particle aggregation, and formation of nanowire-like network with nanopores are observed. The electrical resistance exhibits a super-exponential decay with dose. This type of films may have potential as a sensing material for fast neutron monitoring.