Nonvolatile Modulation of Bi2O2Se/Pb(Zr,Ti)O3 Heteroepitaxy

The pursuit of high-performance electronic devices has driven the research focus toward 2D semiconductors with high electron mobility and suitable band gaps. Previous studies have demonstrated that quasi-2D Bi2O2Se (BOSe) has remarkable physical properties and is a promising candidate for further exploration. Building upon this foundation, the present work introduces a novel concept for achieving nonvolatile and reversible control of BOSe's electronic properties. The approach involves the epitaxial integration of a ferroelectric PbZr0.2Ti0.8O3 (PZT) layer to modify BOSe's band alignment. Within the BOSe/PZT heteroepitaxy, through two opposite ferroelectric polarization states of the PZT layer, we can tune the Fermi level in the BOSe layer. Consequently, this controlled modulation of the electronic structure provides a pathway to manipulate the electrical properties of the BOSe layer and the corresponding devices.

Hysteresis-Free Contact Doping for High-Performance Two-Dimensional Electronics

Contact doping is considered crucial for reducing the contact resistance of two-dimensional (2D) transistors. However, a process for achieving robust contact doping for 2D electronics is lacking. Here, we developed a two-step doping method for effectively doping 2D materials through a defect-repairing process. The method achieves strong and hysteresis-free doping and is suitable for use with the most widely used transition-metal dichalcogenides. Through our method, we achieved a record-high sheet conductance (0.16 mS·sq^-1 without gating) of monolayer MoS₂ and a high mobility and carrier concentration (4.1 × 10¹³ cm^-2). We employed our robust method for the successful contact doping of a monolayer MoS₂ Au-contact device, obtaining a contact resistance as low as 1.2 kΩ·μm. Our method represents an effective means of fabricating high-performance 2D transistors.

Directly Visualizing Photoinduced Renormalized Momentum-Forbidden Electronic Quantum States in an Atomically Thin Semiconductor

Resolving the momentum degree of freedom of photoexcited charge carriers and exploring the excited-state physics in the hexagonal Brillouin zone of atomically thin semiconductors have recently attracted great interest for optoelectronic technologies. We demonstrate a combination of light-modulated scanning tunneling microscopy and the quasiparticle interference (QPI) technique to offer a directly accessible approach to reveal and quantify the unexplored momentum-forbidden electronic quantum states in transition metal dichalcogenide (TMD) monolayers. Our QPI results affirm the large spin-splitting energy at the spin-valley-coupled Q valleys in the conduction band (CB) of a tungsten disulfide monolayer. Furthermore, we also quantify the photoexcited carrier density-dependent band renormalization at the Q valleys. Our findings directly highlight the importance of the excited-state distribution at the Q valley in the band renormalization in TMDs and support the critical role of the CB Q valley in engineering the quantum electronic valley degree of freedom in TMD devices.

Internal Built-In Electric Fields at Organic-Inorganic Interfaces of Two-Dimensional Ruddlesden-Popper Perovskite Single Crystals

Two-dimensional (2D) organic-inorganic hybrid Ruddlesden-Popper perovskites (OIRPPs), which consist of naturally formed "multiple quantum well (MQW)-like" structure, have received considerable interest in optoelectronic applications, owing to their outstanding optical properties and tailorable functionalities. While the quantum-confined electrons and holes at an MQW structure are under an applied electric field, the tilt of the energy bands may cause a significant influence on their optical properties. This work demonstrates the presence of internal built-in electric fields (BIEFs) at the as-synthesized 2D OIRPP single crystals. Spontaneous Franz-Keldysh oscillations, which usually act as the fingerprint to account for the presence of BIEFs in the MQW-like structures, are observed at 2D OIRPPs by the highly sensitive differential technique of modulated thermoreflectance spectroscopy. The strength of BIEFs at 2D OIRPP single crystals reduces with increased n values due to the increased width of the quantum well. The origin of the presence of BIEFs at 2D OIRPPs is further unveiled by atomically resolved scanning tunneling microscopy on their electronic band structures at organic-inorganic interfaces. Unlike the conventional III-V MQW semiconductors with the BIEFs, which are dominated by the spatial concentration gradients at heterointerfaces, the presence of BIEFs at the 2D OIRPP single crystals is attributed to the molecular dipoles within organic spacers pointing to the organic-inorganic interfaces. The discovery of internal BIEFs at the 2D OIRPPs may provide deep insight into understanding the fundamental optical properties for the future design of large-area and low-cost perovskite optoelectronic devices.

Atomically Resolved Quantum-Confined Electronic Structures at Organic-Inorganic Interfaces of Two-Dimensional Ruddlesden-Popper Halide Perovskites

This work demonstrates the direct visualization of atomically resolved quantum-confined electronic structures at organic-inorganic heterointerfaces of two-dimensional (2D) organic-inorganic hybrid Ruddlesden-Popper perovskites (RPPs); this is accomplished with scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) by using solvent engineering to prepare perpendicularly oriented 2D RPPs. Atomically resolved band mapping images across the organic-inorganic interfaces of 2D RPPs yield typical quantum-well-like type-I heterojunction band alignment with band gaps depending on the thicknesses or n values of the inorganic perovskite slabs. The presence of edge states within the band gap due to organic cation vacancies is also observed. In addition, real-space visualization of atomic-scale structural phase transition behavior and changes in local electronic band structures are obtained simultaneously. Our results provide an unequivocal observation and explanation of the quantum-confined electronic structures formed at organic-inorganic interfaces of 2D RPPs.

Photodriven Dipole Reordering: Key to Carrier Separation in Metalorganic Halide Perovskites

Photodriven dipole reordering of the intercalated organic molecules in halide perovskites has been suggested to be a critical degree of freedom, potentially affecting physical properties, device performance, and stability of hybrid perovskite-based optoelectronic devices. However, thus far a direct atomically resolved dipole mapping under device operation condition, that is, illumination, is lacking. Here, we map simultaneously the molecule dipole orientation pattern and the electrostatic potential with atomic resolution using photoexcited cross-sectional scanning tunneling microscopy and spectroscopy. Our experimental observations demonstrate that a photodriven molecule dipole reordering, initiated by a photoexcited separation of electron-hole pairs in spatially displaced orbitals, leads to a fundamental reshaping of the potential landscape in halide perovskites, creating separate one-dimensional transport channels for holes and electrons. We anticipate that analogous light-induced polarization order transitions occur in bulk and are at the origin of the extraordinary efficiencies of organometal halide perovskite-based solar cells as well as could reconcile apparently contradictory materials' properties.

Atomically Resolved Electronic States and Correlated Magnetic Order at Termination Engineered Complex Oxide Heterointerfaces

We map electronic states, band gaps, and interface-bound charges at termination-engineered BiFeO₃/La_0.7Sr_0.3MnO₃ interfaces using atomically resolved cross-sectional scanning tunneling microscopy. We identify a delicate interplay of different correlated physical effects and relate these to the ferroelectric and magnetic interface properties tuned by engineering the atomic layer stacking sequence at the interfaces. This study highlights the importance of a direct atomically resolved access to electronic interface states for understanding the intriguing interface properties in complex oxides.

Spatially Resolved Imaging on Photocarrier Generations and Band Alignments at Perovskite/PbI2 Heterointerfaces of Perovskite Solar Cells by Light-Modulated Scanning Tunneling Microscopy

The presence of the PbI₂ passivation layers at perovskite crystal grains has been found to considerably affect the charge carrier transport behaviors and device performance of perovskite solar cells. This work demonstrates the application of a novel light-modulated scanning tunneling microscopy (LM-STM) technique to reveal the interfacial electronic structures at the heterointerfaces between CH₃NH₃PbI₃ perovskite crystals and PbI₂ passivation layers of individual perovskite grains under light illumination. Most importantly, this technique enabled the first observation of spatially resolved mapping images of photoinduced interfacial band bending of valence bands and conduction bands and the photogenerated electron and hole carriers at the heterointerfaces of perovskite crystal grains. By systematically exploring the interfacial electronic structures of individual perovskite grains, enhanced charge separation and reduced back recombination were observed when an optimal design of interfacial PbI₂ passivation layers consisting of a thickness less than 20 nm at perovskite crystal grains was applied.

A Metal-Insulator Transition of the Buried MnO2 Monolayer in Complex Oxide Heterostructure

A novel artificially created MnO₂ monolayer system is demonstrated in atomically controlled epitaxial perovskite heterostructures. With careful design of different electrostatic boundary conditions, a magnetic transition as well as a metal-insulator transition of the MnO₂ monolayer is unveiled, providing a fundamental understanding of dimensionality-confined strongly correlated electron systems and a direction to design new electronic devices.

Dopant Diffusion and Activation in Silicon Nanowires Fabricated by ex Situ Doping: A Correlative Study via Atom-Probe Tomography and Scanning Tunneling Spectroscopy

Dopants play a critical role in modulating the electric properties of semiconducting materials, ranging from bulk to nanoscale semiconductors, nanowires, and quantum dots. The application of traditional doping methods developed for bulk materials involves additional considerations for nanoscale semiconductors because of the influence of surfaces and stochastic fluctuations, which may become significant at the nanometer-scale level. Monolayer doping is an ex situ doping method that permits the post growth doping of nanowires. Herein, using atom-probe tomography (APT) with subnanometer spatial resolution and atomic-ppm detection limit, we study the distributions of boron and phosphorus in ex situ doped silicon nanowires with accurate control. A highly phosphorus doped outer region and a uniformly boron doped interior are observed, which are not predicted by criteria based on bulk silicon. These phenomena are explained by fast interfacial diffusion of phosphorus and enhanced bulk diffusion of boron, respectively. The APT results are compared with scanning tunneling spectroscopy data, which yields information concerning the electrically active dopants. Overall, comparing the information obtained by the two methods permits us to evaluate the diffusivities of each different dopant type at the nanowire oxide, interface, and core regions. The combined data sets permit us to evaluate the electrical activation and compensation of the dopants in different regions of the nanowires and understand the details that lead to the sharp p-i-n junctions formed across the nanowire for the ex situ doping process.

Control of the Metal-Insulator Transition at Complex Oxide Heterointerfaces through Visible Light

The coupling of the localized surface plasmon resonance of Au nanoparticles is utilized to deliver a visible-light stimulus to control conduction at the LaAlO3 /SrTiO3 interface. A giant photoresponse and the controllable metal-insulator transition are characterized at this heterointerface. This study paves a new route to optical control of the functionality at the heterointerfaces.

Precisely Controlled Ultrastrong Photoinduced Doping at Graphene-Heterostructures Assisted by Trap-State-Mediated Charge Transfer

Ultrastrong and precisely controllable n-type photoinduced doping at a graphene/TiOx heterostructure as a result of trap-state-mediated charge transfer is demonstrated, which is much higher than any other reported photodoping techniques. Based on the strong light-matter interactions at the graphene/TiOx heterostructure, precisely controlled photoinduced bandgap opening of a bilayer graphene device is demonstrated.

Conduction control at ferroic domain walls via external stimuli

Intriguing functionalities at nano-sized domain walls have recently spawned a new paradigm for developing novel nanoelectronics due to versatile characteristics. In this study, we explore a new scenario to modulate the local conduction of ferroic domain walls. Three controlling parameters, i.e., external electrical field, magnetic field and light, are introduced to the 90° domain walls (90° DWs) of BiFeO₃. Electrical modulation is realized by electrical transport, where the mobility of 90° DWs can be altered by gating voltage. We further use the ferromagnetic/antiferromagnetic coupling to reveal the inherent magnetism at the DWs. With an established magnetic nature, magnetotransport has been conducted to introduce magnetic controlling parameter, where a giant positive magnetoresistance change can be observed up to 200%. In addition, light modulated conduction, a core factor for multifunctional applications, is successfully demonstrated (current enhancement by a factor of 2 with 11 W white lamp). These results offer new insights to discover the tunability of domain wall nanoelectronics.

Parallel p-n junctions across nanowires by one-step ex situ doping

The bottom-up synthesis of nanoscale building blocks is a versatile approach for the formation of a vast array of materials with controlled structures and compositions. This approach is one of the main driving forces for the immense progress in materials science and nanotechnology witnessed over the past few decades. Despite the overwhelming advances in the bottom-up synthesis of nanoscale building blocks and the fine control of accessible compositions and structures, certain aspects are still lacking. In particular, the transformation of symmetric nanostructures to asymmetric nanostructures by highly controlled processes while preserving the modified structural orientation still poses a significant challenge. We present a one-step ex situ doping process for the transformation of undoped silicon nanowires (i-Si NWs) to p-type/n-type (p-n) parallel p-n junction configuration across NWs. The vertical p-n junctions were measured by scanning tunneling microscopy (STM) in concert with scanning tunneling spectroscopy (STS), termed STM/S, to obtain the spatial electronic properties of the junction formed across the NWs. Additionally, the parallel p-n junction configuration was characterized by off-axis electron holography in a transmission electron microscope to provide an independent verification of junction formation. The doping process was simulated to elucidate the doping mechanisms involved in the one-step p-i-n junction formation.

Adjusting to a seizure-free "new normal" life following discontinuation of antiepileptic drugs during adolescence

This qualitative study sought to understand how children in adolescence adjust to their newly acquired normal life without epilepsy, following discontinuation of antiepileptic drugs during this dynamic period of growth and development. Three major themes with subthemes were identified: 1) setting the body and mind free; 2) engaging in self-regulation; and 3) protection by significant others. A sense of relief from constraints related to treatment schedules, special diets, and avoiding seizure-provoking activities was expressed by all participants. Freedom from side effects of the antiepileptic drugs improved life at home and school. Most of the participants said that they were not worried about seizure recurrence but would use caution against a possible relapse. Family members also must adjust to a new lifestyle. Medical staff needs to provide support and adequate care to adolescents during their period of identity adjustment following antiepileptic drug discontinuation.

Ferroelectric control of the conduction at the LaAlO₃/SrTiO₃ heterointerface

Modulation of band bending at a complex oxide heterointerface by a ferroelectric layer is demonstrated. The as-grown polarization (Pup ) leads to charge depletion and consequently low conduction. Switching the polarization direction (Pdown ) results in charge accumulation and enhances the conduction at the interface. The metal-insulator transition at a conducting polar/nonpolar oxide heterointerface can be controlled by ferroelectric doping.

Atomic-scale interfacial band mapping across vertically phased-separated polymer/fullerene hybrid solar cells

Using cross-sectional scanning tunneling microscope (XSTM) with samples cleaved in situ in an ultrahigh vacuum chamber, this study demonstrates the direct visualization of high-resolution interfacial band mapping images across the film thickness in an optimized bulk heterojunction polymer solar cell consisting of nanoscale phase segregated blends of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM). We were able to achieve the direct observation of the interfacial band alignments at the donor (P3HT)-acceptor (PCBM) interfaces and at the interfaces between the photoactive P3HT:PCBM blends and the poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) anode modification layer with an atomic-scale spatial resolution. The unique advantage of using XSTM to characterize polymer/fullerene bulk heterojunction solar cells allows us to explore simultaneously the quantitative link between the vertical morphologies and their corresponding local electronic properties. This provides an atomic insight of interfacial band alignments between the two opposite electrodes, which will be crucial for improving the efficiencies of the charge generation, transport, and collection and the corresponding device performance of polymer solar cells.

Comparison of bilateral pulmonary arterial level and diameter in transposition of the great arteries

In normal anatomy, the left pulmonary artery (LPA) is usually situated higher than the right pulmonary artery (RPA); however, transposition of the great arteries (TGA), the LPA is not always situated higher than the RPA. This study was performed to clarify the relative position of the RPA and the LPA in transposition of the great arteries (TGA) as well as the implications. We reviewed 101 angiograms of patients with TGA (age 4.1 ± 1.2 months). The width of the RPA, the LPA, and the pulmonary trunk (PT) were measured just before their first branch in the frontal view. They were classified into four groups according to the ratio between the RPA and the PT (RPA/PT). The initial courses of the LPA and the RPA were compared and defined according to their height in the frontal view, and the preferential flow (or not) to the RPA was recorded. The equation of hydrodynamics was applied to evaluate the bifurcation angle. Both PAs were the same size in all cases. Forty-eight patients (47.5 %) had a RPA/PT diameter ratio < 0.49. The LPA coursed higher than the RPA in the majority of cases (81 [80.2 %]); in a minority of cases the LPA and RPA were at the same level (6 [5.9 %]); and in some cases the RPA coursed higher than the LPA (14 [13.9 %]). Patients with a high degree of PA hypoplasia tended to have both PAs at the same level or a higher-positioned RPA. Autopsy (1 of 3 cases) showed a posterior ridge against the bronchus in the higher RPA. Hydrodynamic calculation showed that the greater the angle between the RPA/PT, the greater the preferential flow. Preferential flow to the RPA in TGA did not necessarily result in LPA hypoplasia before its first branch. Higher RPA position relative to the LPA was associated with greater flow in it against the posterior bronchus. This situation was more prevalent in patients with severe PA hypoplasia.

Mapping band alignment across complex oxide heterointerfaces

In this study, direct observation of the evolution of electronic structures across complex oxide interfaces has been revealed in the LaAlO(3)/SrTiO(3) model system using cross-sectional scanning tunneling microscopy and spectroscopy. The conduction and valence band structures across the LaAlO(3)/SrTiO(3) interface are spatially resolved at the atomic level by measuring the local density of states. This study directly maps out the electronic reconstructions and a built-in electric field in the polar LaAlO(3) layer. Results also clearly reveal the band bending and the notched band structure in the SrTiO(3) adjacent to the interface.

Local conduction at the BiFeO(3)-CoFe(2)O(4) tubular oxide interface

In strongly correlated oxides, heterointerfaces, manipulating the interaction, frustration, and discontinuity of lattice, charge, orbital, and spin degrees of freedom, generate new possibilities for next generation devices. In this study, existing oxide heterostructures are examined and local conduction at the BiFeO(3)-CoFe(2)O(4) vertical interface is found. In such hetero-nanostructures the interface cannot only be the medium for the coupling between phases, but also a new state of the matter. This study demonstrates a novel concept on for oxide interface design and opens an alternative pathway for the exploration of diverse functionalities in complex oxide interfaces.

Magic numbers of atoms in surface-supported planar clusters

Surface-supported planar clusters can sprout active research and create numerous applications in the realm of nanotechnology. Exploitation of these clusters will be more extended if their properties on a supported substrate are thoroughly apprehended, and if they can be fabricated in a controllable way. Here we report finding the magic numbers in two-dimensional Ag clusters grown on Pb quantum islands. We demonstrate, with the images and energy spectra of atomic precision, the transition from electronic origin to a geometric one within the same system. Applying the magic nature, we can also produce a large array of planar clusters with well-defined sizes and shapes.

Self-organized growth of nanopucks on Pb quantum islands

Electronic Moirè patterns found on lead (Pb) quantum islands can serve as a template to grow self-organized cluster (nanopucks) arrays of various materials. These patterns can be divided into fcc- and hcp-stacked areas, which exhibit different binding strengths to the deposited adatoms. For Ag adatoms, the binding energy can differ substantially and the confined nucleation thus occurs in the fcc sites. Both the size distribution and spatial arrangement of the Ag nanopucks are analyzed and found to be commensurate with the characteristics of the template island, which exhibits a bilayer oscillatory behavior.

Outbreaks of nosocomial bloodstream infections associated with multiresistant Klebsiella pneumoniae in a pediatric intensive care unit

BACKGROUND

Between June and October 1997, and during April 1998, a cluster of nosocomial bloodstream infections (BSIs) associated with Klebsiella pneumoniae was observed in 8 premature neonates from 1 pediatric intensive care unit (TPICU) in a 4000-bed medical center in northern Taiwan. An investigation was conducted to identify the possible reservoirs and mode of transmission.

METHODS

Epidemiologic surveillance and infection control interventions were executed. The environment was checked by submitting several swab samples for microbiological studies. The antibiograms and results from 2 molecular typing methods (pulsed-field gel electrophoresis and infrequent-restriction site polymerase chain reaction) of all bacteremic and environmental isolates of K. pneumoniae were compared.

RESULTS

Totally 39 K. pneumoniae isolates, including 9 from bacteremia, 26 from the environment, and 4 controls, were analyzed. One major pattern was found in 21 isolates, which included 8 bacteremic isolates with identical antibiograms, a single isolate from rectal swab screening, 2 of 8 isolates from hand cultures of medical staff, and 10 of 17 isolates from swabs of sinks in the TPICU. All 21 isolates illustrated identical antibiograms, while the other 18 isolates shared 4 antibiograms and 15 unique patterns.

CONCLUSIONS

The nosocomial BSIs appeared to be an outbreak induced by 1 multiresistant K. pneumoniae strain. The sinks may have acted as reservoirs for this outbreak strain. During washing, splattered water droplets containing the bacterial particles may have contaminated the hands of medical personnel and were then further transmitted to patients.

Molecular investigation of two clusters of hospital-acquired bacteraemia caused by multi-resistant Klebsiella pneumoniae using pulsed-field gel electrophoresis and in frequent restriction site PCR. Infection Control Group

Two molecular typing methods, DNA macrorestriction analysis with XbaI resolved by pulsed-field gel electrophoresis (PFGE) and infrequent restriction site PCR (IRS-PCR) assay with adapters designed for XbaI and HhaI restriction sites, were used to investigate two clusters of hospital-acquired bacteraemia associated with multi-resistant Klebsiella pneumoniae which occurred in a paediatric intensive care unit (PICU). A total of 56 K. pneumoniae isolates were analysed. These included 10 bacteraemic isolates from eight patients, 26 isolates obtained during an epidemiological survey, and 20 epidemiologically non-related isolates incorporated as controls. One major pattern was demonstrated in 22 of the 56 isolates analysed. These included nine of the 10 bacteraemic isolates, a single rectal isolate, two hand culture isolates and 10 sink isolates. All of these 22 isolates illustrated identical antibiograms, whilst the other 34 isolates shared six antibiograms and 31 unique patterns by either PFGE or IRS-PCR assay. The two clusters of bacteraemia appeared to be outbreaks induced by the same strain of K. pneumoniae which may have utilized sinks as reservoirs and been transmitted through the hands of medical personnel to patients. IRS-PCR demonstrates concordant results with PFGE analysis in studying the genetic relationships among K. pneumoniae isolates, and serves as an excellent epidemiological tool for this bacterium.