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As a result of the depletion of conventional oil reserves and the unprecedented growth in world energy demand, increasing attention has focused on the extraction and utilization of unconventional o...

Metal–insulator transition materials such as VO2 have garnered much attention in the field of neuromorphic devices because of their nonlinear behavior and orders of magnitude scale property changes...

Transformations between different atomic configurations of a material oftentimes bring about dramatic changes in functional properties as a result of the simultaneous alteration of both atomistic a...

The manuscript presents a detailed analysis of embodied energy and carbon footprint reduction enabled by microalloying of steel, thereby providing a rich global perspective of the (outsized) role of chemical elements added in trace concentrations on the overall footprint of the construction industry. As such, the manuscript addresses an important and timely topic at the intersection of materials criticality, structural performance, life cycle assessment, and policy interventions. The United Nations estimates that the worldwide energy consumption of buildings accounts for 30—40% of global energy production, underlining the importance of the judicious selection of construction materials. Much effort has focused on the use of high-strength low-alloy steels in reinforcement bars whose economy of materials use is predicated upon improved yield strengths in comparison to low-carbon steels. While microalloying is known to allow for reduced steel consumption, a sustainability analysis in terms of embodied energy and CO 2 has not thus far been performed. Here we calculate the impact of supplanting lower grade reinforcement bars with higher strength vanadium microalloyed steels on embodied energy and carbon footprint. We find that the increased strength of vanadium microalloyed steel translates into substantial material savings over mild steel thus reducing the total global fossil carbon footprint by as much as 0.385%. A more granular analysis pegs savings for China and the European Union at 1.01 and 0.19%, respectively, of their respective emissions.

Transformations between different atomic configurations of a material oftentimes bring about dramatic changes in functional properties as a result of the simultaneous alteration of both atomistic and electronic structure. Transformation barriers between polytypes can be tuned through compositional modification, generally in an immutable manner. Continuous, stimulus-driven modulation of phase stabilities remains a significant challenge. Utilizing the metal-insulator transition of VO

Fenestration elements that enable spectrally selective dynamic modulation of the near-infrared region of the electromagnetic spectrum are of great interest as a means of decreasing the energy consumption of buildings by adjusting solar heat gain in response to external temperature. The binary vanadium oxide VO2 exhibits a near-room-temperature insulator–metal electronic transition accompanied by a dramatic modulation of the near-infrared transmittance. The low-temperature insulating phase is infrared transparent but blocks infrared transmission upon metallization. There is considerable interest in harnessing the thermochromic modulation afforded by VO2 in nanocomposite thin films. However, to prepare a viable thermochromic film, the visible-light transmittance must be maintained as high as possible while maximizing thermochromic modulation in the near-infrared region of the electromagnetic spectrum, which necessitates the development of high-crystalline-quality VO2 nanocrystals of the optimal particle siz...

Materials exhibiting pronounced metal–insulator transitions such as VO2 have acquired great importance as potential computing vectors and electromagnetic cloaking elements given the large accompanying reversible modulation of properties such as electrical conductance and optical transmittance. As a first-order phase transition, considerable phase coexistence and hysteresis is typically observed between the heating insulator → metal and cooling metal → insulator transformations of VO2. Here, we illustrate that substitutional incorporation of tungsten greatly modifies the hysteresis of VO2, both increasing the hysteresis as well as introducing a distinctive kinetic asymmetry wherein the heating symmetry-raising transition is observed to happen much faster as compared to the cooling symmetry-lowering transition, which shows a pronounced rate dependence of the transition temperature. This observed kinetic asymmetry upon tungsten doping is attributed to the introduction of phase boundaries resulting from stabi...

Buildings consume an inordinate amount of energy, accounting for 30–40% of worldwide energy consumption. A major portion of solar radiation is transmitted directly to building interiors through windows, skylights, and glazed doors where the resulting solar heat gain necessitates increased use of air conditioning. Current technologies aimed at addressing this problem suffer from major drawbacks, including a reduction in the transmission of visible light, thereby resulting in increased use of artificial lighting. Since currently used coatings are temperature-invariant in terms of their solar heat gain modulation, they are unable to offset cold-weather heating costs that would otherwise have resulted from solar heat gain. There is considerable interest in the development of plastic fenestration elements that can dynamically modulate solar heat gain based on the external climate and are retrofittable onto existing structures. The metal–insulator transition of VO2 is accompanied by a pronounced modulation of n...

The thermally driven orders-of-magnitude modulation of resistance and optical transmittance observed in VO2 makes it an archetypal first-order phase transition material and underpins functional applications in logic and memory circuitry, electromagnetic cloaking, ballistic modulation, and thermochromic glazing to provide just a few representative examples. VO2 can be reversibly switched from an insulating to a metallic state at an equilibrium transition temperature of 67 °C. Tuning the phase diagram of VO2 to bring the transition temperature closer to room temperature has been a longstanding objective and one that has tremendous practical relevance. Substitutional incorporation of dopants has been the most common strategy for modulating the metal—insulator transition temperature but requires that the dopants be incorporated during synthesis. Here we demonstrate a novel postsynthetic diffusive annealing approach for incorporating interstitial B dopants within VO2. The postsynthetic method allows for the tr...

The perovskite cesium lead bromide (CsPbBr3) has emerged as an attractive thermally and chemically robust alternative to hybrid lead perovskite halides and analogues of this material show excellent tunability of exciton binding energies, high absorption cross-sections, and intense photoluminescence. Dimensional reduction, particularly in proximity of the Bohr exciton radius, allows for substantial tunability of the photophysical properties of this material as a result of quantum confinement. The use of surface passivating ligands, particularly alkylammonium cations, has been developed as a means of inducing directional growth and facilitates dimensional confinement of the obtained perovskite nanocrystals. Here, we demonstrate that the crystalline order of the ligand-shell assembly, as dictated by the length of the alkyl chains, the degree of branching, the reaction temperature, and ligand concentration, strongly influences the extent of dimensional confinement attainable for the perovskite nanoplatelets. The spatial extent of the ligand shell and the degree of ordering of ligand molecules greatly impact the diffusion and addition of monomeric species. The interplay between enthalpic stabilization from crystalline packing and entropic loss from loss of configurational degrees of freedom provides substantial opportunity to tune the parameter space as a function of ligand structure and reaction variables. Mechanistic understanding of thermodynamic and kinetic regimes provides a means to rationally optimize synthetic parameters to obtained desired dimensionality and thus allows for control over nanocrystal thickness in precise increments.

Upconversion of low-energy photons theoretically allows the creation of single-junction solar cells with efficiency far above the Shockley-Queisser (SQ) limit. However, the net efficiency gains that can be realized depend critically on details of the upconversion process employed. We define three important metrics of the performance of an upconversion material: upconversion quantum efficiency (UQE), photon energy sacrifice (PES), and absorption bandwidth (AB). We analyze the performance of existing upconversion materials relative to both these metrics and existing computational models of a single-junction photovoltaic (PV) cell backed by an upconverter. Guided by the results of this analysis, we develop a design for new solid state upconversion nanostructures that suppresses the dominant energy loss pathways and can enable substantial improvements in overall solar energy harvesting. We describe and model the performance of a specific realization of this design that uses an InAs quantum dot (QD) and a graded InAlBiAs layer to suppress both radiative and nonradiative loss pathways. We show that this design can be tailored to maximize upconversion efficiency and can enable a practical upconversion-backed PV system to exceed the SQ limit.

We synthesized quantum dot (QD) heterostructures via the N,N′-dicyclohexylcarbodiimide-mediated formation of amide bonds between capping ligands on CdS QDs and CdSe QDs. Products of ligand-exchange and coupling reactions were characterized by FTIR, 1H NMR, transmission electron micrscopy, and electronic absorption and emission spectroscopy. This cross-linking strategy yields exclusively heterostructures and prohibits the undesired formation of homostructures consisting of a single type of QD. The ground-state absorption spectra of the presynthesized colloidal QDs were unperturbed upon incorporation into heterostructures. Photoexcited CdS QDs transferred holes to molecularly tethered CdSe QDs, as evidenced by significant dynamic quenching of the trap-state emission from CdS QDs and the rapid ( 10–5 s) transient absorption band in the visible region. These spectral signatures were absent for mixed dispersions of noninteracting CdS and CdSe QDs. Our results reveal t...

Although methods for harvesting subbandgap solar photons have been demonstrated, present approaches still face substantial challenges. We evaluate carrier escape mechanisms in an InAs/GaAs quantum dot (QD) intermediate band photovoltaic (PV) device using photocurrent measurements under subbandgap illumination. We show that subbandgap photons can generate photocurrent through a two-photon absorption process, but that carrier trapping and retrapping limit the overall photocurrent regardless of whether the dominant carrier escape mechanism is optical, tunneling, or thermal. We introduce a new design for an InAs QD-based nanostructured material that can efficiently upconvert two low-energy photons into one high-energy photon. Efficiency is enhanced by intentionally sacrificing a small amount of photon energy to minimize radiative and nonradiative loss. Upconversion PV devices based on this approach separate the absorption of subbandgap photons from the current-harvesting junction, circumventing the carrier-trapping problems.

Buildings consume an inordinate amount of energy, accounting for 30-40% of worldwide energy consumption. A major portion of solar radiation is transmitted directly to building interiors through windows, skylights, and glazed doors where the resulting solar heat gain necessitates increased use of air conditioning. Current technologies aimed at addressing this problem suffer from major drawbacks, including a reduction in the transmission of visible light, thereby resulting in increased use of artificial lighting. Since currently used coatings are temperature-invariant in terms of their solar heat gain modulation, they are unable to offset cold-weather heating costs that would otherwise have resulted from solar heat gain. There is considerable interest in the development of plastic fenestration elements that can dynamically modulate solar heat gain based on the external climate and are retrofittable onto existing structures. The metal-insulator transition of VO

Photon upconversion is a promising approach to realizing photovoltaics with efficiency beyond the Shockley-Quessier limit by harvesting low-energy photons and converting them to high-energy photons that can be absorbed by a host single-junction solar cell. Existing upconversion materials have limited potential benefit for solar energy harvesting applications because of their narrow absorption bandwidth and low quantum efficiency. We first present numerical simulations of semiconductor nanostructures designed to overcome these limitations. The computational results demonstrate the potential impact of such upconversion materials and identify critical material parameters that must be achieved for practical realization of upconverters that can have a meaningful impact on solar energy harvesting. We then present experimental progress toward realizing this semiconductor upconversion paradigm. We describe the choice of material composition and structure, growth of the nanostructures, and ultrafast optical characterization of the carrier dynamics that result in these structures. We show that upconversion can be achieved in at least one realization of this semiconductor upconversion paradigm. We conclude with a discussion about the prospects for realizing efficient photon upconversion and the constraints that must be addressed to meet this goal.

Solution-processed core/multi-shell semiconductor quantum dots (QDs) could be tailored to facilitate the carrier separation and recombination mechanisms necessary to implement photon upconversion. We have synthesized CdTe cores capped with CdS and then CdSe shells. We present steady-state and time-resolved photoluminescence (PL) spectroscopy measurements of these QDs as a function of varying shell thicknesses. We quantify and report carrier emission energies and lifetimes as a function of shell addition and shell thicknesses. The results provide a framework for understanding how CdS and CdSe shells affect carrier separation and recombination dynamics, and thus inform the design of core/multi-shell QDs for efficient photon upconversion.

We have used steady-state and time-resolved emission measurements to characterize interfacial electron transfer, or electron injection, from CdSe quantum dots (QDs) to molecularly linked TiO2 nanoparticles. Electrons were injected from both band-edge and trap states on relatively fast ( 10–8 s) time scales. The quantum yield of electron injection from trap states decreased as the trap-state distribution was shifted, by varying excitation energy, to lower energies. This effect probably arose from a driving-force dependence of the rate constant for electron injection. In contrast, the quantum yield of electron injection from band-edge states was independent of excitation energy. Our results highlight the key role of trapped carriers in interfacial charge-transfer processes of QDs and the influence of the energies and densities of trap states on the efficiencies of such processes.

Complex core/multi-shell quantum dot (QD) configurations can be used to tailor light absorption and emission properties. We present a core/shell/shell/shell structure designed to implement efficient photon upconversion. We report on the design and characterization of two intermediate products that could progress to implement this design: InP/CdS/CdSe and CdTe/CdS/CdSe. Products were synthesized by adapting established procedures for the colloidal synthesis of nanoparticles in solution. Transmission electron microscopy (TEM) and x-ray diffraction (XRD) were used to determine particle size, crystallinity, and band alignment. The photophyscial properties of the intermediate products were characterized using steady-state absorption and photoluminescence (PL) spectroscopy. The results suggest that the core/multi-shell structure successfully confines an optically-generated hole in the QD core while allowing the electron to escape to the outer shell. This independent control over carrier transfer dynamics provides the tools necessary to implement efficient photon upconversion.

We present a computational model for predicting the net solar energy conversion efficiency of a single-junction solar cell backed by a solid-state photon upconverter. We describe and model the upconversion process using a series of kinetic rate equations to calculate equilibrium populations in a single upconversion nanostructure. We determine the internal upconversion quantum efficiency of the proposed nanostructure as a function of intentionally introduced photon energy sacrifice. We integrate the results of this model with the detailed balance method for estimating solar cell efficiency to determine the optimal nanostructure design to achieve maximum net solar energy conversion efficiency for a host solar cell backed by this upconverter. We show that net efficiencies in excess of 40% could be possible for a wide-bandgap host cell using this upconversion nanostructure.

We investigate the effect of doping on the mechanisms of carrier escape from intermediate states in delta-doped InAs/GaAs intermediate band solar cells. The intermediate states arise from InAs quantum dots embedded in a GaAs p-i-n junction cell. We find that doping the sample increases the number of excited-state carriers participating in a cycle of trapping and carrier escape via thermal, optical, and tunneling mechanisms. However, we find that the efficiency of the optically-driven carrier escape mechanism is independent of doping and remains small.

Intermediate band solar cells (IBSCs) are third-generation photovoltaic (PV) devices that can harvest sub-bandgap photons normally not absorbed in a single-junction solar cell. Despite the large increase in total solar energy conversion efficiency predicted for IBSC devices, substantial challenges remain to realizing these efficiency gains in practical devices. We evaluate carrier escape mechanisms in an InAs/GaAs quantum dot intermediate band p-i-n junction PV device using photocurrent measurements under sub-bandgap illumination. We show that sub-bandgap photons generate photocurrent through a two-photon absorption process, but that carrier trapping and retrapping limit the overall photocurrent. The results identify a key obstacle that must be overcome in order to realize intermediate band devices that outperform single junction photovoltaic cells.