In hybrid systems composed of metallic nanoantennas and semiconductor quantum dots (QDs), exciton-plasmon coupling plays a central role in controlling the emission intensity and dynamics of the QDs. Here, we investigate how plasmon decay into hot electrons modulates this coupling through electronic modification of the QD environment. The plasmonic platform consists of closely packed periodic arrays of elongated Au nanoantennas that support plasmonic supercells. Owing to their periodicity, these supercells, formed via near-field coupling along the long axes or through hybrid plasmonic-photonic edge coupling, support plasmonic hot-spots and surface lattice resonances (SLRs), supporting enhanced hot-electron generation. The arrays are coated with a thin Si interlayer followed by an ultrathin InP/ZnS QD film, providing both dielectric coupling and electronic pathways for charge transfer. We show that hot electrons injected across the Au/Si interface can charge the environment of QDs, resulting in a polarization-dependent blue shift of the emission of QDs, accompanied by enhancement of their emission lifetime. These results highlight the key roles of SLRs and plasmonic hot spots in enabling hot-electron-mediated control of QD emission.

Abstract In a recent study to probe the effect of the pinning efficiency of BaZrO 3 (BZO) nanorods in BZO-doped YBa 2 Cu 3 O 7-x (BZO/YBCO) nanocomposite films, Ca diffusion from two Ca 0.3 Y 0.7 Ba 2 Cu 3 O 7-x spacers that form multilayers through alternative stacking with three BZO/YBCO layers was found to significantly enhance the pinning by approximately five folds at high magnetic fields (B) up to 9.0 T. This raises a question on the role of Ca diffused into the multilayer BZO/YBCO nanocomposite films. In order to answer this question, this work investigates the Ca 0.3 Y 0.7 Ba 2 Cu 3 O 7-x films of variable thickness in the range of 33-190 nm to understand whether the carrier over-doping induced by Ca substitution of Y would lead to enhanced pinning. In addition, the effect of the thicknesses of the constituent YBCO and Ca 0.3 Y 0.7 Ba 2 Cu 3 O 7-x layers was also studied. Furthermore, the amount of Ca in Ca 0.3 Y 0.7 Ba 2 Cu 3 O 7-x spacers may be controlled by varying their thickness ranging from 1 nm to 10 nm. Our result suggests that the benefit of over-doping via Ca/Y substitution is minimal on pinning. In addition, the amount of Ca in the Ca 0.3 Y 0.7 Ba 2 Cu 3 O 7-x spacers indeed affects the Ca diffusion and hence pinning enhancement dramatically, which reduces as the spacer thickness is below 5 nm threshold. Above this threshold, the Ca diffusion is highly effective through large BZO/YBCO thicknesses up to 330 nm (total film thickness ~ 1 µm) and significantly enhanced pinning has been obtained in multilayer BZO/YBCO nanocomposites. For example, the I c exceeds 650 A per cm-width at 20 K and 9.0 T when B//c, which is only ~13% lower than that at B//ab due to the intrinsic pinning.

Improved pinning efficiency of c-axis-aligned BaZrO3 nanorods (BZO-NRs) in BZO/YBa2Cu3O7 nanocomposite thin films was obtained recently using a multilayer (ML) approach, in which diffusion of Ca ions from two thin (Ca0.3Y0.7)BCO spacers (10 nm in thickness) sandwiched with three BZO/YBa2Cu3O7 layers. The subsequent Ca (30% larger)/Cu substitution at the Cu-O planes of YBCO was found energetically preferable by inducing c-axis elongation of the YBa2Cu3O7 lattice near the BZO-NRs/YBa2Cu3O7 interface to enable a coherent interface via reducing the BZO/YBCO lattice mismatch from originally 7.7% to 1.4%, leading to significantly enhanced J(c) (B) and F-p in thin ML films of 150 nm in thickness. This work investigates whether improved pinning could be achievable in thicker BZO-NRs/YBa2Cu3O7 ML films with the thickness increased to 1000 nm. Interestingly, similar pinning enhancement has been observed in thick BZO-NRs/YBa2Cu3O7 ML films across a wide temperature range of 20-80 K. In particular, the thicker BZO-NRs/YBa2Cu3O7 ML films outperform their thinner counterparts in both higher value and less anisotropy of J(c) (B). At 1000 nm thickness, I-c (30 K, 9T) reaches up to similar to 680 A/cm-width with a variation of similar to 85% over the entire angular range of B field orientations. This result illustrates the critical role of Ca diffusion at the BZO-NRs/YBCO interface for improving pinning efficiency of BZO-NRs in a wide range of temperatures and B fields.

Nanohybrids based on van der Waals (vdW) heterostructures of two dimensional (2D) atomic materials have recently emerged as a unique scheme for designing high‐performance quantum sensors. This work explores vdW nanohybrids for photodetection, which consist of graphene decorated with intermingled transition‐metal dichalcogenide (TMDC) nanodiscs (TMDC‐NDs) obtained using wafer‐size, layer‐by‐layer growth. The obtained TMDC‐NDs/graphene nanohybrids take advantage of strong quantum confinement in graphene for high charge mobility and hence high photoconductive gain, and localized surface plasmonic resonance (LSPR) enabled on the TMDC‐NDs for enhanced light absorption. Since the LSPR depends on the nanostructure's size and density, intermingled TMDC‐NDs of different kinds of TMDCs, such as WS 2 (W) and MoS 2 (M), have been found to allow small‐size, high‐concentration TMDC‐NDs to be achieved for high photoresponse. Remarkably, high photoresponsivity up to 31 A/W (550 nm wavelength and 20 µW cm −2 light intensity) has been obtained on the WMW‐NDs/graphene nanohybrids photodetectors made using three consecutive coatings of WS 2 (1st and 3rd coating) and MoS 2 (2nd coating), which is considerably higher by a factor of ≈4 than that of the counterparts MoS 2 ‐ND/graphene or WS 2 ‐NDs/graphene devices. This result provides a facile approach to control the size and concentration of the TMDC‐NDs for high‐performance, low‐cost optoelectronic device applications.

As electronic devices continue to scale down from the current sub-5 nm range, atomic-scale control of defects becomes increasingly crucial to suppressing their impact on the physical properties of the devices. Memristors present an excellent example of a nonlinear and dynamic device with high speed and endurance required for electronic applications ranging from neuromorphic computing to nonvolatile memories. Herein we investigate the impact of atomic defects in sub-2 nm thick MgO/Al2O3 atomic layer stack (ALS) memristors that use an M1 (switching layer)/M2 (oxygen vacancy reservoir layer) bilayer structure grown using in vacuo atomic layer deposition (iALD). Intriguingly, we revealed a direct correlation of the atomic defects in the M2 layer with the memristor dynamic behavior using combined analysis of in situ scanning tunneling spectroscopy (iSTS) on the M2 layer and ex situ characterization on the memristors. Specifically, incomplete coverage of the first ALD atomic layer of M2 on the electrode yields defects at the M2/electrode interface. Despite the monotonic increase of ALD coverage, by almost 3-fold from ∼30% to >90%, at completion of the M2 layer of ∼0.7 nm in thickness, the impact of the defects on the M2/electrode interface has been found to be detrimental to both memristor switching speed and endurance. Guided by atomistic simulation, we addressed the issue of interface defects via tuning of the Al surface hydroxylation to increase the first atomic layer ALD coverage to ∼75%, leading to improved memristor switching speed and endurance by several orders of magnitude. These findings shed light on the correlation between the atomic defects and the dynamic behavior of sub-2 nm memristors and the importance of minimizing the atomic defects in memristors for future electronic applications.

Precise control over the shapes and sizes of metallic nanoantennas (meta-atoms) in plasmonic metasurfaces has made them promising candidates for a wide range of applications, from optical devices to encryption and information processing. In this article, we investigate the coherent control of depolarization in the in-plane scattering of metasurfaces composed of unit cells ranging from flat Au nanoantennas to regions densely packed with randomly shaped and sized Au nanoislands. We demonstrate that when the nanoisland regions interact with light, they evolve into "coherent" domains through surface lattice resonances, forming coherently activated metasurfaces. Our analysis of the Stokes parameters of in-plane scattering reveals that such metasurfaces exhibit strong scattering with distinct depolarization characteristics, including the formation of zero degree-of-polarization states. We show that as the nanoislands grow larger and coalesce into highly flat nanoantennas, significant intensity and depolarization redistribution occur, fundamentally altering the in-plane scattering behavior and polarization properties. The results highlight the profound application of polarization analysis for tracing details of coherent processes and the potential of metasurfaces that leverage coherent engineering of plasmonic modes.

Nanohybrids of graphene and colloidal semiconductor quantum dots (QDs/Gr) provide a promising quantum sensing scheme for photodetection. Despite exciting progress made in QDs/Gr photodetectors in broadband from ultraviolet to short-wave infrared, the device performance is limited in middle-wave infrared (MWIR) detection. A fundamental question arises as to whether the thermal noise-induced dark current and hence poor signal-to-noise ratio in conventional uncooled MWIR photodetectors persist in QDs/Gr nanohybrids. Herein, we investigated noise, responsivity (R*), and specific detectivity (D*) in HgTe QDs/Gr nanohybrids, revealing that the noise and R* are decoupled in nanohybrids and each can be optimized independently toward its theoretical limit. Specifically, the noise in the QDs/Gr nanohybrids is dominated by that of graphene with a negligible effect from the dark current in HgTe QDs and can be optimized to its intrinsic limit by removing charge doping of adsorbed polar molecules on graphene. Furthermore, the R* is proportional to the photoconductive gain enabled by the strong quantum confinement in QDs and Gr. Achieving high gain in the MWIR spectrum, however, is challenging and requires elimination of charge traps primarily from the surface states of the narrow-bandgap semiconductor HgTe QDs. Using grain-rotation-induced grain-coalescence growth of single-layer and core/shell HgTe QDs, we show the that HgTe QDs surface states caused by Te deficiency can be dramatically suppressed, resulting in high gain up to 4.0 × 107 in the MWIR spectrum. The optimized noise and R* have led to high uncooled MWIR D* up to 2.4 × 1011 Jones, making nanohybrids promising to surpass the fundamental dark-current limit in conventional photodetectors.

High critical current (I-c) in high magnetic fields (B) with minimal variations with respect to the orientation of the B field is demanded by many applications such as high-field magnets for fusion systems. Motivated by this, this work studies 6 vol. % BaZrO3/YBa2Cu3O7 (BZO/YBCO) multilayer nanocomposite films by stacking two 10 nm thick Ca0.3Y0.7Ba2Cu3O7 (CaY-123) spacers with three BZO/YBCO layers of thickness varied from 50 to 330 nm to make the total film thickness of 150-1000 nm. The Ca diffusion from the spacers into BZO/YBCO was shown to dramatically enhance pinning efficiency of c-axis aligned BZO nanorods, which yields high and almost thickness independent critical current density (J(c)) in the BZO/YBCO multilayer nanocomposite films. Remarkably, enhanced Jc was observed in these multilayer samples at a wide temperature range of 20-80 K and magnetic fields up to 9.0 T. In particular, the thicker BZO/YBCO multilayer films outperform their thinner counterparts in both higher value and less anisotropy of Jc at lower temperatures and higher fields. At 20 K and 9.0 T, Ic is up to 654 A/cm-width at B//c in the 6% multilayer (1000 nm) sample, which is close to 753 A/cm-width at B//ab due to the intrinsic pinning. This result illustrates the critical role of the Ca cation diffusion into the YBCO lattice in achieving high and isotropic pinning in thick BZO/YBCO multilayer films.

We present a six-step cascade that converts 1,3-distyrylbenzenes (bis-stilbenes) into nonsymmetric pyrenes in 40-60% yields. This sequence merges photochemical steps, E,Z-alkene isomerization, a 6 pi photochemical electrocyclization (Mallory photocyclization); the new bay region cyclization, with two radical iodine-mediated aromatization steps; and an optional aryl migration. This work illustrates how the inherent challenges of engineering excited state reactivity can be addressed by logical design. An unusual aspect of this cascade is that the same photochemical process (the Mallory reaction) is first promoted and then blocked in different stages within a photochemical cascade. The use of blocking groups is the key feature that makes simple bis-stilbenes suitable substrates for directed double cyclization. While the first stilbene subunit undergoes a classic Mallory photocyclization to form a phenanthrene intermediate, the next ring-forming step is diverted from the conventional Mallory path into a photocyclization of the remaining alkene at the phenanthrene's bay region. Although earlier literature suggested that this reaction is unfavorable, we achieved this diversion via incorporation of blocking groups to prevent the Mallory photocyclization. The two photocyclizations are assisted by the relief of the excited state antiaromaticity. Reaction selectivity is controlled by substituent effects and the interplay between photochemical and radical reactivity. Furthermore, the introduction of donor substituents at the pendant styrene group can further extend this photochemical cascade through a radical 1,2-aryl migration. Rich photophysical and supramolecular properties of the newly substituted pyrenes illustrate the role of systematic variations in the structure of this classic chromophore for excited state engineering.

Single-atom catalysts have the advantage of high chemical efficiency, which requires atomic-scale control during catalyst formation. In order to address this challenge, this work explores the synthesis of single-atom platinum (SA-Pt) catalysts using atomic-layer deposition (ALD) on vertical graphene (VG), in which a large number of graphene edges serve as energetically favorable nucleation sites for SA-Pt, as predicted by density functional theory calculations. Interestingly, SA-Pt has been achieved on VGs at low ALD cycle numbers of up to 60. With a further increase in the number of ALD cycles, an increasing number of Pt clusters with diameters <2 nm and Pt nanoparticles (NPs) with diameters >2 nm become dominant (nano-Pt @VG). This is in contrast to the observation of predominantly nano-Pt on other carbon nanostructures, such as carbon nanotubes and monolayer graphene, under the same ALD growth conditions, indicating that the edge states on VG indeed play a critical role in facilitating the formation of SA-Pt. Profound differences are revealed in a comparative study on H2 sensing. SA-Pt exhibits both a higher sensitivity and faster response than its nano-Pt counterpart by more than an order of magnitude, illustrating the high catalytic efficiency of SA-Pt and its potential for gas sensing and a variety of other catalytic applications.

Photodetectors based on colloidal quantum dots (QD)/graphene nanohybrids are quantum sensors due to strong quantum confinement in both QD and graphene. The optoelectronic properties of QD/graphene nanohybrids are affected by the quantum physics that predicts a high photoconductive gain and hence photoresponsivity (R*) depending on the pixel length (L) as R*proportional to L-2. Experimental confirmation of the effect of the pixel geometric parameters on the optoelectronic properties of the QD/graphene photodetector is therefore important to elucidate the underlying quantum physics. Motivated by this, an array of PbS QDs/graphene nanohybrid photodetectors are designed with variable QD/graphene pixel length L and width (W) in the range of 10-150 mu m for a study of R*, noise, and specific detectivity (D*) in a broad spectrum of 400-1500 nm. Intriguingly, R* exhibits a monotonic decreasing trend of 1/L2 while being independent of W, confirming experimentally the theoretical prediction. Interestingly, this geometric effect on the photoresponsivity seems to be partially compensated by that in noise, leading to D* independent of L and W at wavelengths in the ultraviolet-visible-near infrared range. This result sheds light on the quantum physics underlying the optoelectronic process in QD/graphene nanohybrids, which is important to the design of high-quality QD/graphene photodetectors and imaging systems. A study on the effect of channel geometry on quantum dot/graphene nanohybrid photodetector noise and figures of merit. The results indicate that responsivity and noise are geometry-dependent and precisely cancel out to achieve a geometry-independent detectivity. image

One-dimensional artificial pinning centers (1D-APCs) in YBa2Cu3O7-x nanocomposite films provide strong collective pinning at magnetic field B//c-axis. In this work, we reveal a 1D-APC/YBa2Cu3O7-x interface is preferred for high pinning efficiency of individual 1D-APCs including BaHfO3 and BaZrO3. The coherent 1D-APC/YBa2Cu3O7-x interface may be obtained via either growth of the nanocomposite films at optimal condition or Ca-diffusion to dynamically reduce the interface strain during the nanocomposite film growth. Interestingly, the high pinning efficiency of the 1D-APCs with coherent interfaces with YBCO not only lead to a high critical current density (J(c)) in magnetic fields up to 9.0 T at H//c-axis but also enhanced J(c) over a larger angular range when H is away from H//c-axis up to theta = 60-80 degree than that in the case the interface is defective. This result suggests the importance of understanding and engineering the APC/YBCO interface for optimal pinning in nanocomposite films.

C-axis aligned BaZrO3 (BZO) nanorods formed via strain-mediated self-assembly in BZO-doped YaBa2Cu3O7-x (BZO/YBCO) nanocomposite films can provide strong pinning to the quantized magnetic vortices. While the strain initiated from the BZO/YBCO lattice mismatch plays a critical role in nucleation and evolution of the BZO nanorods, it also leads to a highly defective BZO/YBCO interface and hence reduced pinning efficiency of BZO nanorods. This work reports a recent study in probing the effect of BZO/YBCO interface on the pinning efficiency of the BZO nanorods as the interface is repaired dynamically during the BZO nanorod growth using Ca doping. Within the BZO doping range of 2-8 vol.%, significantly enhanced pinning efficiency of the BZO nanorods have been observed. A peak enhancement up to five-fold of critical current density at 9.0 T and 65-77 K has been obtained in the 6 vol.% BZO/YBCO nanocomposites after the interface repair. This result not only illustrates the critical importance of the BZO/YBCO interface in the pinning efficiency, but also provides a facile scheme to achieve such an interface to restore the pristine pinning efficiency of the BZO nanorods.

Ultrathin (sub-2 nm) Al _2 O _3 /MgO memristors were recently developed using an in vacuo atomic layer deposition (ALD) process that minimizes unintended defects and prevents undesirable leakage current. These memristors provide a unique platform that allows oxygen vacancies (V _O ) to be inserted into the memristor with atomic precision and study how this affects the formation and rupture of conductive filaments (CFs) during memristive switching. Herein, we present a systematic study on three sets of ultrathin Al _2 O _3 /MgO memristors with V _O -doping via modular MgO atomic layer insertion into an otherwise pristine insulating Al _2 O _3 atomic layer stack (ALS) using an in vacuo ALD. At a fixed memristor thickness of 17 Al _2 O _3 /MgO atomic layers (∼1.9 nm), the properties of the memristors were found to be affected by the number and stacking pattern of the MgO atomic layers in the Al _2 O _3 /MgO ALS. Importantly, the trend of reduced low-state resistance and the increasing appearance of multi-step switches with an increasing number of MgO atomic layers suggests a direct correlation between the dimension and dynamic evolution of the conducting filaments and the V _O concentration and distribution. Understanding such a correlation is critical to an atomic-scale control of the switching behavior of ultrathin memristors.

After theoretical discovery of quantized magnetic vortices in type II superconductors by Abrikosov, which received 2003 Nobel Prize in Physics, vortex pinning has been an important topic of research for high critical current densities in applied magnetic fields desired for a variety of applications in electric and electronic devices and systems. The small vortex core size in high temperature superconductors (HTSs), of a few nanometers, has prompted an intensive research in development of nanoscale artificial pinning centers (APCs) in so-called HTS nanocomposites. Exciting results of much enhanced in-field critical current densities and pinning force densities have been achieved. This talk intends to highlight the progress made recently in HTS nanocomposites towards controllable generation of APCs with desired morphologies, dimension, concentration, and pinning efficiency for targeted applications. The future research in HTS nanocomposites to meet the need of practical applications will also be discussed.

An important research goal in the applications of high temperature superconductor YBa 2 Cu 3 O 7-δ (YBCO) thin films is increasing both the critical current density and also the isotropic nature of the film. YBCO is inherently anisotropic due to its layered perovskite structure. The critical current density of YBCO thin films is enhanced by increasing the flux pinning sites in the film by the addition of insulating nano-phase materials, such as BaZrO 3 (BZO) nanorods, which are also anisotropic in nature. Using a multilayer pulsed laser deposition technique has been shown to produce films with inclusions that are more isotropic in nature. However, the defective BZO nanorod interface, resulting from its lattice mismatch with YBCO, prevents obtaining optimum pinning force. This research explores the effect of Ca doped YBCO space layers in the multilayer composite film, on the BZO nanorod/YBCO interface, over a wide range of conditions of 65–5 K and 0–9T that are suitable for various applications. The interplay of combining these three variables: BaZrO 3 addition to YBCO, multilayer film growth resulting from varying pulsed laser deposition conditions, and employing calcium doped YBCO space layers, and the resulting impact on film microstructures and superconducting properties, will be presented.

Localized surface plasmonic resonance (LSPR) provides a unique scheme for light management and has been demonstrated across a large variety of metallic nanostructures. More recently, non-metallic nanostructures of two-dimensional atomic materials and heterostructures have emerged as a promising, low-cost alternative in order to generate strong LSPR. In this paper, a review of the recent progress made on non-metallic LSPR nanostructures will be provided in comparison with their metallic counterparts. A few applications in optoelectronics and sensors will be highlighted. In addition, the remaining challenges and future perspectives will be discussed.

Whether tetra-tert-butyl-s-indacene (TtB-s-indacene) is a symmetric D2h structure or a bond alternating C2h structure remains a standing puzzle. Agreement between 1H NMR data and computed proton chemical shifts based on minima structures optimized at the M06-2X, B97X-D, and M11 levels confirm a bond localized C2h symmetryconsistent with its antiaromaticity. X-ray structures and computed B3LYP geometries of D2h TtB-s-indacene poorly reproduce experimental NMR data. The limitations and complications of using B3LYP geometries for interpreting the structures and paratropicities of -expanded antiaromatic systems are discussed.