Unique Optical Properties of Methylammonium Lead Iodide Nanocrystals Below the Bulk Tetragonal-Orthorhombic Phase Transition

Methylammonium (MA) and formamidinium (FA) lead halides are widely studied for their potential as low-cost, high-performance optoelectronic materials. Here, we present measurements of visible and IR absorption, steady state, and time-resolved photoluminescence from 300 K to cryogenic temperatures. Whereas FAPbI₃ nanocrystals (NCs) are found to behave in a very similar manner to reported bulk behavior, colloidal nanocrystals of MAPbI₃ show a departure from the low-temperature optical behavior of the bulk material. Using photoluminescence, visible, and infrared absorption measurements, we demonstrate that unlike single crystals and polycrystalline films NCs of MAPbI₃ do not undergo optical changes associated with the bulk tetragonal-to-orthorhombic phase transition, which occurs near 160 K. We find no evidence of frozen organic cation rotation to as low as 80 K or altered exciton binding energy to as low as 3 K in MAPbI₃ NCs. Similar results are obtained in MAPbI₃ NCs ranging from 20 to over 100 nm and in morphologies including cubes and plates. Colloidal MAPbI₃ NCs therefore offer a window into the properties of the solar-relevant, room-temperature phase of MAPbI₃ at temperatures inaccessible with single crystals or polycrystalline samples. Exploiting this phenomenon, these measurements reveal the existence of an optically passive photoexcited state close to the band edge and persistent slow Auger recombination at low temperature

Transient Negative Optical Nonlinearity of Indium Oxide Nanorod Arrays in the Full-Visible Range

Dynamic control of the optical response of materials at visible wavelengths is key to future metamaterials and photonic integrated circuits. Materials such as transparent conducting oxides have attracted significant attention due to their large optical nonlinearity under resonant optical pumping condition. However, optical nonlinearities of TCOs are positive in sign and are mostly in the ε-near-zero to metallic range where materials can become lossy. Here we demonstrate large amplitude, negative optical nonlinearity (Δ<i>n</i> from −0.05 to −0.09) of indium oxide nanorod arrays in the full-visible range where the material is transparent. We experimentally quantify and theoretically calculate the optical nonlinearity, which arises from a strong modification of interband optical transitions. The approach toward negative optical nonlinearity can be generalized to other transparent semiconducting oxides and opens door to reconfigurable, subwavelength optical components

Large Transient Optical Modulation of Epsilon-Near-Zero Colloidal Nanocrystals

Epsilon-near-zero materials may be synthesized as colloidal nanocrystals which display large magnitude subpicosecond switching of infrared localized surface plasmon resonances. Such nanocrystals offer a solution-processable, scalable source of tunable metamaterials compatible with arbitrary substrates. Under intraband excitation, these nanocrystals display a red-shift of the plasmon feature arising from the low electron heat capacities and conduction band nonparabolicity of the oxide. Under interband pumping, they show in an ultrafast blueshift of the plasmon resonance due to transient increases in the carrier density. Combined with their high-quality factor, large changes in relative transmittance (+86%) and index of refraction (+85%) at modest control fluences (<5 mJ/cm²) suggest that these materials offer great promise for all-optical switching, wavefront engineering, and beam steering operating at terahertz switching frequencies

Ultrafast Silicon Photonics with Visible to Mid-Infrared Pumping of Silicon Nanocrystals

Dynamic optical control of infrared (IR) transparency and refractive index is achieved using boron-doped silicon nanocrystals excited with mid-IR optical pulses. Unlike previous silicon-based optical switches, large changes in transmittance are achieved without a fabricated structure by exploiting strong light coupling of the localized surface plasmon resonance (LSPR) produced from free holes of p-type silicon nanocrystals. The choice of optical excitation wavelength allows for selectivity between hole heating and carrier generation through intraband or interband photoexcitation, respectively. Mid-IR optical pumping heats the free holes of p-Si nanocrystals to effective temperatures greater than 3500 K. Increases of the hole effective mass at high effective hole temperatures lead to a subpicosecond change of the dielectric function, resulting in a redshift of the LSPR, modulating mid-IR transmission by as much as 27%, and increasing the index of refraction by more than 0.1 in the mid-IR. Low hole heat capacity dictates subpicosecond hole cooling, substantially faster than carrier recombination, and negligible heating of the Si lattice, permitting mid-IR optical switching at terahertz repetition frequencies. Further, the energetic distribution of holes at high effective temperatures partially reverses the Burstein–Moss effect, permitting the modulation of transmittance at telecommunications wavelengths. The results presented here show that doped silicon, particularly in micro- or nanostructures, is a promising dynamic metamaterial for ultrafast IR photonics

Epitaxial Atomic Layer Deposition of Sn-Doped Indium Oxide

Coherently strained, epitaxial Sn-doped In₂O₃ (ITO) thin films were fabricated at temperatures as low as 250 °C using atomic layer deposition (ALD) on (001)-, (011)-, and (111)-oriented single-crystal Y-stabilized ZrO₂ (YSZ) substrates. Resultant films possess cube-on-cube epitaxial relationships with the underlying YSZ substrates and are smooth, highly conductive, and optically transparent. This epitaxial ALD approach is favorable compared to many conventional growth techniques as it is a large-scale synthesis method that does not necessitate the use of high temperatures or ultrahigh vacuum. These films may prove valuable as a conductive growth template in areas where high-quality crystalline thin film substrates are important, such as solar energy materials, light-emitting diodes, or wide bandgap semiconductors. Furthermore, we discuss the applicability of this ALD system as an excellent model system for the study of ALD surface chemistry, nucleation, and film growth

Gigahertz Acoustic Vibrations of Elastically Anisotropic Indium–Tin-Oxide Nanorod Arrays

Active control of light is important for photonic integrated circuits, optical switches, and telecommunications. Coupling light with acoustic vibrations in nanoscale optical resonators offers optical modulation capabilities with high bandwidth and small footprint. Instead of using noble metals, here we introduce indium–tin-oxide nanorod arrays (ITO-NRAs) as the operating media and demonstrate optical modulation covering the visible spectral range (from 360 to 700 nm) with ∼20 GHz bandwidth through the excitation of coherent acoustic vibrations in ITO-NRAs. This broadband modulation results from the collective optical diffraction by the dielectric ITO-NRAs, and a high differential transmission modulation up to 10% is achieved through efficient near-infrared, on-plasmon-resonance pumping. By combining the frequency signatures of the vibrational modes with finite-element simulations, we further determine the anisotropic elastic constants for single-crystalline ITO, which are not known for the bulk phase. This technique to determine elastic constants using coherent acoustic vibrations of uniform nanostructures can be generalized to the study of other inorganic materials

System Optimization for Fischer–Tropsch Liquid Fuels Production via Solar Hybridized Dual Fluidized Bed Gasification of Solid Fuels

A new configuration of solar hybridized dual fluidized bed (DFB) gasification process is proposed with char separation for the production of Fischer–Tropsch (FT) liquid fuels from solid fuels of biomass and/or coal. The addition of carbon capture with sequestration and FT reactor tail-gas recycle configurations is also assessed. The studied FT liquid fuels production systems are simulated by using a pseudodynamic model incorporating a year long, hourly averaged solar insolation time-series. For the case with a solar multiple (i.e., the heliostat field area relative to that required to meet the demand of the DFB gasifier at the point of peak solar thermal output) of 2.64 and bed material storage capacity of 16 h, the calculated annual solar share of the solar hybridized coal-to-liquids system can be increased from 12.2 to 20.3% by the addition of the char separation for a char gasification conversion of 80%. To achieve the well-to-wheel greenhouse gas emissions for FT liquid fuels parity with diesel derived from mineral crude oil, a calculated biomass fraction of 58% is required for the nonsolar coal case, also with a char gasification conversion of 80%. This fraction can be reduced to 30% by carbon capture and sequestration and further reduced to 17% by the integration of solar energy, based on a solar multiple of 2.64 and bed material storage capacity of 16 h. This reduction is significant given that biomass is much more expensive than coal. However, because of the higher content of light hydrocarbons content in the syngas produced with the studied biomass gasification, the specific FT liquids output per unit feedstock of the system decreases with an increase in the biomass fraction. As the biomass fraction is increased from 0 to 100%, this specific output is decreased from 59.6 to 48.3% but can be increased to 71.5 and 70.9%, respectively, by incorporating tail-gas recycle

Oxidation State Discrimination in the Atomic Layer Deposition of Vanadium Oxides

We describe the use of a vanadium 3+ precursor for atomic layer deposition (ALD) of thin films that span the common oxidation states of vanadium oxides. Self-limiting surface synthesis of V₂O₃, VO₂, and V₂O₅ are realized through four distinct reaction mechanisms accessed via judicious choice of oxygen ALD partners. <i>In situ</i> quartz crystal microbalance and quadrupole mass spectrometry were used to study the reaction mechanism of the vanadium precursor with O₃, H₂O₂, H₂O/O₂, and H₂O₂/H₂. A clear distinction between nonoxidative protic ligand exchange and metal oxidation is demonstrated through sequential surface reactions with different nonmetal precursors. This synergistic effect provides greater control of the resultant metal species in the film, as well as reactive surface species during growth. In an extension of this approach, we introduce oxidation state control through reducing equivalents of H₂ gas. When H₂ is dosed after H₂O₂ during growth, amorphous films of VO₂ are deposited that are readily crystallized with a low temperature anneal. These VO₂ films show a temperature dependent Raman spectroscopy response in the expected range and consistent with the well-known phase-change behavior of VO₂

Cross-Linkable Molecular Hole-Transporting Semiconductor for Solid-State Dye-Sensitized Solar Cells

In this study, we investigate the use of a cross-linkable organosilane semiconductor, 4,4′-bis­[(<i>p</i>-trichlorosilylpropylphenyl)­phenylamino]­biphenyl (TPDSi₂), as a hole-transporting material (HTM) for solid-state dye-sensitized solar cells (ssDSSCs) using the standard amphiphilic Z907 dye which is compatible with organic HTM deposition. The properties and performance of the resulting cells are then compared and contrasted with the ones based on poly­(3-hexylthiophene) (P3HT), a conventional polymeric HTM, but with rather limited pore-filling capacity. When processed under N₂, TPDSi₂ exhibits excellent infiltration into the mesoporous TiO₂ layer and thus enables the fabrication of relatively thick devices (∼5 μm) for efficient photon harvesting. When exposed to ambient atmosphere (RH_amb ∼ 20%), TPDSi₂ readily undergoes cross-linking to afford a rigid, thermally stable hole-transporting layer. In addition, the effect of <i>tert</i>-butylpyridine (TBP) and lithium bis­(trifluoromethylsulfonyl)­imide salt (Li-TFSI) additives on the electrochemical properties of these HTMs is studied via a combination of cyclic voltammetry (CV) and ultraviolet photoemission spectroscopy (UPS) measurements. The results demonstrate that the additives significantly enhance the space charge limited current (SCLC) mobilities for both the P3HT and TPDSi₂ HTMs and induce a shift in the TPDSi₂ Fermi level, likely a p-doping effect. These combined effects of improved charge transport characteristics for the TPDSi₂ devices enhance the power conversion efficiency (PCE) by more than 2-fold for ssDSSCs

Solvent-Mediated Crystallization of CH₃NH₃SnI₃ Films for Heterojunction Depleted Perovskite Solar Cells

Organo-lead halide perovskite solar cells have gained enormous significance and have now achieved power conversion efficiencies of ∼20%. However, the potential toxicity of lead in these systems raises environmental concerns for widespread deployment. Here we investigate solvent effects on the crystallization of the lead-free methylammonium tin triiodide (CH₃NH₃SnI₃) perovskite films in a solution growth process. Highly uniform, pinhole-free perovskite films are obtained from a dimethyl sulfoxide (DMSO) solution via a transitional SnI₂·3DMSO intermediate phase. This high-quality perovskite film enables the realization of heterojunction depleted solar cells based on mesoporous TiO₂ layer but in the absence of any hole-transporting material with an unprecedented photocurrent up to 21 mA cm^–2. Charge extraction and transient photovoltage decay measurements reveal high carrier densities in the CH₃NH₃SnI₃ perovskite device which are one order of magnitude larger than CH₃NH₃PbI₃-based devices but with comparable recombination lifetimes in both devices. The relatively high background dark carrier density of the Sn-based perovskite is responsible for the lower photovoltaic efficiency in comparison to the Pb-based analogues. These results provide important progress toward achieving improved perovskite morphology control in realizing solution-processed highly efficient lead-free perovskite solar cells