Rapid industrialization of organic light-emitting devices for flat-panel displays and solid-state lighting makes a deep understanding of device physics more desirable than ever. Developing reliable experimental techniques to measure fundamental physical properties such as exciton diffusion lengths is a vital part of developing device physics. In this paper, we present a study of exciton diffusion and long-range energy transfer in working organic light-emitting devices, and a study of the interplay between these two tangled processes through both experimental probes and simulations. With the inclusion of multiple factors including long-range energy transfer, exciton boundary conditions, and the finite width of the exciton generation zone, we quantify exciton migration based on emission characteristics from rubrene sensing layers placed in working organic light-emitting devices. This comprehensive analysis is found to be essential to accurately measuring exciton diffusion length, and in the present case the measured singlet-exciton diffusion length in the archetype material $4^{\prime }$-bis(carbazol-9-yl)biphenyl is $4.3\pm 0.3\mathrm{nm}$ with a corresponding diffusivity of $(2.6\pm 0.3)\times {10}^{-4}{\mathrm{cm}}^2/\mathrm{s}$.

The reactivity of the anatase ${\mathrm{TiO}}_2$ (211) surface is systematically studied by ab initio calculations of the surface energy and water-adsorption energy. We find that anatase (211) has a high surface energy of $0.97\mathrm{J}/{\mathrm{m}}^2$, close to that of the (001) surface, and the unsaturated fourfold-coordinated ${\mathrm{Ti}}_4$ atom is more reactive than the unsaturated fivefold-coordinated ${\mathrm{Ti}}_5$ atom. Accordingly, for water adsorption on the (211) surface, a dissociative form is favored on ${\mathrm{Ti}}_4$ sites, with a large adsorption energy $\mathrm{\Delta }{\mathrm{H}}_{\mathrm{H},\mathrm{OH}}\sim 1.28\mathrm{eV}$, while a nondissociative molecular form is favored on ${\mathrm{Ti}}_5$ sites, with a smaller adsorption energy $\mathrm{\Delta }{\mathrm{H}}_{{\mathrm{H}}_2\mathrm{O}}\sim 0.78\mathrm{eV}$. Such distinct surface properties lead to a mixed dissociative and molecular adsorption configuration when the coverage is increased from $1/3$ to 1 monolayer. These results suggest that, similar to the (001) surface, the anatase (211) surface exhibits high reactivity and may be useful in catalytic and photocatalytic applications as well.

We report the in-orbit operation of a photon-pair source aboard a 1.65-kg nanosatellite and demonstrate pair generation and polarization correlation under space conditions. The in-orbit photon correlations exhibit a contrast of $97\%\pm 2\%$, matching ground-based tests. This pathfinding mission overcomes the challenge of demonstrating in-orbit performance for the components of future entangled-photon experiments.

Topological insulators first observed in electronic systems have inspired many analogues in photonic and phononic crystals in which remarkable one-way propagation edge states are supported by topologically nontrivial band gaps. Such band gaps can be achieved by breaking the time-reversal symmetry to lift the degeneracy associated with Dirac cones at the corners of the Brillouin zone. Here, we report on our construction of a phononic crystal exhibiting a Dirac-like cone in the Brillouin zone center. We demonstrate that simultaneously breaking the time-reversal symmetry and altering the geometric size of the unit cell result in a topological transition that we verify by the Chern number calculation and edge-mode analysis. We develop a complete model based on the tight binding to uncover the physical mechanisms of the topological transition. Both the model and numerical simulations show that the topology of the band gap is tunable by varying both the velocity field and the geometric size; such tunability may dramatically enrich the design and use of acoustic topological insulators.

We report the features of an ion source based on two-color photoionization of a laser-cooled cesium beam outsourced from a pyramidal magneto-optical trap. The ion source operates in continuous or pulsed mode. At acceleration voltages below 300 V, it delivers some ten ions per bunch with a relative energy spread $\mathrm{\Delta }{U}_{\mathrm{rms}}/U\simeq 0.032$, as measured through the retarding field-energy-analyzer approach. Space-charge effects are negligible thanks to the low ion density attained in the interaction volume. The performances of the ion beam in a configuration using focused laser beams are extrapolated on the basis of the experimental results. Calculations demonstrate that our low-energy and low-current ion beam can be attractive for the development of emerging technologies requiring the delivery of a small amount of charge, down to the single-ion level and its eventual focusing in the 10-nm range.

Placing a nanomechanical object in the evanescent near field of a high-$Q$ optical microcavity gives access to strong gradient forces and quantum-limited displacement readout, offering an attractive platform for both precision sensing technology and basic quantum optics research. Robustly implementing this platform is challenging, however, as it requires integrating optically smooth surfaces separated by $\lesssim \lambda /10$. Here we describe an exceptionally high-cooperativity, single-chip optonanomechanical transducer based on a high-stress ${\mathrm{Si}}_3{\mathrm{N}}_4$ nanobeam monolithically integrated into the evanescent near field of ${\mathrm{SiO}}_2$ microdisk cavity. Employing a vertical integration technique based on planarized sacrificial layers, we realize beam-disk gaps as little as 25 nm while maintaining mechanical $Qf>{10}^{12}\mathrm{Hz}$ and intrinsic optical $Q\sim {10}^7$. The combination of low loss, small gap, and parallel-plane geometry results in radio-frequency flexural modes with vacuum optomechanical coupling rates of 100 kHz, single-photon cooperativities in excess of unity, and large zero-point frequency (displacement) noise amplitudes of $10\mathrm{kHz}(\mathrm{fm})/\surd \mathrm{Hz}$. In conjunction with the high power-handling capacity of ${\mathrm{SiO}}_2$ and low extraneous substrate noise, the transducer performs particularly well as a sensor, with recent deployment in a 4-K cryostat realizing a displacement imprecision 40 dB below that at the standard quantum limit (SQL) and an imprecision-backaction product $<5\hslash$ [Wilson et al., Nature (London) 524, 325 (2015)]. In this report, we provide a comprehensive description of device design, fabrication, and characterization, with an emphasis on extending Heisenberg-limited readout to room temperature. Towards this end, we describe a room-temperature experiment in which a displacement imprecision 32 dB below that at the SQL and an imprecision-backaction product $<60\hslash$ is achieved. Our results extend the outlook for measurement-based quantum control of nanomechanical oscillators and suggest an alternative platform for functionally integrated “hybrid” quantum optomechanics.

The resistance of doped single grain boundaries (GBs) in copper is calculated from first principles and systematically compared to its pure single GB equivalent. As a first step, a state-of-the-art ab initio method is used to calculate the resistivity of doped bulk copper for 16 doping elements at concentration 1 at. %. Results are in qualitatively excellent and quantitatively reasonable agreement with the corresponding experimental data, and allow us to determine Ag, Zn, Mg, Pd, Al, and In as best candidates for GB doping. These atoms have a minimal impact on the bulk resistivity, while they also conform to a set of established criteria for alloying with copper. Then, the specific resistivity of six twin GBs is determined for these elements over a wide spectrum of doping concentrations for the submonolayer and the monolayer GB complexions. Reduced resistivity is observed for Zn, Mg, Al, In, and other elements in two high-$\mathrm{\Sigma }$ GBs, and is qualitatively related to the segregation enthalpy as well as to a low number of empty states around the Fermi energy in the boundary plane region of the GB. The results indicate the possibility for a reduced net resistivity in copper interconnects by GB doping.

The understanding of charge transport through films of semiconductor nanocrystals (NCs) is fundamental for most applications envisaged for these materials, e.g., light-emitting diodes, solar cells, and thin-film field-effect transistors (FETs). In this work, we show that three-dimensional film-thickness-dependent percolation effects taking place above the percolation threshold strongly affect the charge transport in NC films and greatly determine the performance of NC devices such as NC FETs. We use thin films of Si NCs with a wide range of thicknesses controllable by spray coating of NC inks to thoroughly investigate the electronic properties and charge transport in thin NC films. We find a steep (superlinear) increase of the electrical conductivity with increasing film thickness, which is not observed in bulk semiconductor thin films with bandlike charge transport. We explain this increase by an exponentially increasing number of charge percolation paths in a system dominated by hopping charge transport. Thin-film NC FETs reveal thickness-independent field-effect mobilities and threshold voltages, whereas on:off current ratios decrease quickly with increasing film thickness. We show that the steep enhancement of electrical conductivity with increasing film thickness provided by three-dimensional percolation effects is, in fact, responsible for the dramatic degradation of NC FET performance observed with increasing film thickness. Our work demonstrates that the performance of NC FETs is much more critically sensitive to film thickness than in conventional FET-based bulk semiconductor materials.

A set of seven $\mathrm{InAs}/\mathrm{In}(\mathrm{As},\mathrm{Sb})$ type-II superlattices (T2SLs) are designed to have specific band-gap energies between 290 meV ($4.3\mu \mathrm{m}$) and 135 meV ($9.2\mu \mathrm{m}$) in order to study the effects of the T2SL band-gap energy on the minority-carrier lifetime. A temperature-dependent optical pump-probe technique is used to measure the carrier lifetimes, and the effect of a midgap defect level on the carrier-recombination dynamics is reported. The Shockley-Read-Hall (SRH) defect state is found to be at energy of approximately $-250\pm 12\mathrm{meV}$ relative to the valence-band edge of bulk GaSb for the entire set of T2SL structures, even though the T2SL valence-band edge shifts by 155 meV on the same scale. These results indicate that the SRH defect state in $\mathrm{InAs}/\mathrm{In}(\mathrm{As},\mathrm{Sb})$ T2SLs is singular and is nearly independent of the exact position of the T2SL band-gap or band-edge energies. They also suggest the possibility of engineering the T2SL structure such that the SRH state is removed completely from the band gap, a result that should significantly increase the minority-carrier lifetime.

This paper reports the formation of highly localized Mie resonances on a closed metasurface encapsulating a rigid core and the realization of directional sensing at deep-subwavelength scale (diameter $\sim \lambda /8$) with the proposed physical model. Based on modal-expansion and mode-matching methods, it is theoretically shown that the extremely anisotropic metasurface shell can support varied orders of Mie resonances around the rigid core. We further experimentally demonstrate that the Mie resonance with a dipolelike profile is strongly excited under the illumination of a plane wave at low frequencies, enabling the sensitive directional sensing due to the intensified and azimuthally dependent pressure field.

A key challenge in cadmium-telluride (CdTe) semiconductors is obtaining stable and high hole density. Group-I elements substituting Cd can form acceptors but easily self-compensate and diffuse quickly. For example, CdTe photovoltaics have relied on copper as a dopant, but this creates stability problems and hole density that has not exceeded ${10}^{15}{\mathrm{cm}}^{-3}$. If hole density can be increased beyond ${10}^{16}{\mathrm{cm}}^{-3}$, CdTe solar technology can exceed multicrystalline silicon performance and provide levelized costs of electricity below conventional energy sources. Group-V elements substituting Te offer a solution, but they are very difficult to incorporate. Using time-of-flight secondary-ion mass spectrometry, we examine bulk and grain-boundary diffusion of phosphorus (P) in CdTe in Cd-rich conditions. We find that in addition to slow bulk diffusion and fast grain-boundary diffusion, there is a critical fast bulk-diffusion component that enables deep P incorporation in CdTe. Detailed first-principle calculations indicate the slow bulk-diffusion component is caused by substitutional P diffusion through the Te sublattice, whereas the fast bulk-diffusion component is caused by P diffusing through interstitial lattice sites following the combination of a kick-out step and two rotation steps. The latter is limited in magnitude by high formation energy, but is sufficient to manipulate P incorporation. In addition to an increased physical understanding, these results open up experimental possibilities for group-V doping in CdTe applications.

We propose and demonstrate a physical platform for the realization of integrated photonic memories based on laser-written waveguides in rare-earth-doped crystals. Using femtosecond-laser micromachining, we fabricate waveguides in ${\mathrm{Pr}}^{3+}:{\mathrm{Y}}_2{\mathrm{SiO}}_5$ crystal. We demonstrate that the waveguide inscription does not affect the coherence properties of the material and that the light confinement in the waveguide increases the interaction with the active ions by a factor of 6. We also demonstrate that analogous to the bulk crystals, we can operate the optical pumping protocols necessary to prepare the population in atomic-frequency combs that we use to demonstrate light storage in excited and spin states of the Praseodymium ions. Our results represent a realization of laser-written waveguides in a ${\mathrm{Pr}}^{3+}:{\mathrm{Y}}_2{\mathrm{SiO}}_5$ crystal and an implementation of an integrated on-demand spin-wave optical memory. They open perspectives for integrated quantum memories.

Amorphous fluorinated plastic can be produced with a refractive index similar to that of water, a condition that makes it essentially invisible when immersed in aqueous solutions. Because of this property, even a small amount of adsorbed molecules on the plastic-water interface provides a detectable optical signal. We investigate two distinct substrates made of this material, characterized by different interface areas: a prism and a microporous membrane. We demonstrate that both substrates enable the label-free detection of molecular compounds in water even without any surface functionalization. The adsorption of molecules on the planar surface of the prism provides an increase of optical reflectivity, whereas the adsorption on the internal surface of the microporous membrane yields an increase of scattered light. Despite the different mechanisms, we find a similar optical response upon adsorption. We confirm this result by a theoretical model accounting for both reflection and scattering. We investigate the spontaneous adsorption process for different kinds of molecules: surfactants with different charges, a protein (lysozyme), and a constituent of gasoline (hexane). The measured equilibrium and kinetic constants for adsorption differ by orders of magnitudes among the different classes of molecules. By suitable analytical models, accounting for the effects of mass limitation and transport, we find a simple and general scaling of the adsorption parameters with the molecular size.

We present optically and electrically tunable conductance modifications of a site-controlled quantum-dot memristor. The conductance of the device is tuned by electron localization on a quantum dot. The control of the conductance with voltage and low-power light pulses enables applications in neuromorphic and arithmetic computing. As in neural networks, applying pre- and postsynaptic voltage pulses to the memristor allows us to increase (potentiation) or decrease (depression) the conductance by tuning the time difference between the electrical pulses. Exploiting state-dependent thresholds for potentiation and depression, we are able to demonstrate a memory-dependent induction of learning. The discharging of the quantum dot can further be induced by low-power light pulses in the nanowatt range. In combination with the state-dependent threshold voltage for discharging, this enables applications as generic building blocks to perform arithmetic operations in bases ranging from binary to decimal with low-power optical excitation. Our findings allow the realization of optoelectronic memristor-based synapses in artificial neural networks with a memory-dependent induction of learning and enhanced functionality by performing arithmetic operations.

We investigate magnetoresistance in spin valves involving CoPd-contacted carbon nanotubes. Both the temperature and bias-voltage dependence clearly indicate tunneling magnetoresistance as the origin. We show that this effect is significantly affected by the tunnel-barrier strength, which appears to be one reason for the variation between devices previously detected in similar structures. Modeling the data by means of the scattering matrix approach, we find a nontrivial dependence of the magnetoresistance on the barrier strength. Furthermore, an analysis of the spin precession observed in a nonlocal Hanle measurement yields a spin lifetime of ${\tau }_s=1.1\mathrm{ns}$, a value comparable with those found in silicon- or graphene-based spin-valve devices.

High-resolution mapping of electronic transport phenomena plays an increasingly important role for the characterization of ferroic domains and their functionality. At present, spatially resolved electronic transport data are commonly gained from local two-point measurements, collected in line-by-line scans with a conducting nanosized probe. Here, we introduce an innovative experimental approach based on low-energy electron microscopy. As a model case, we study polar domains of varying conductance in strained ${\mathrm{SrMnO}}_3$. By a direct comparison with conductive atomic force and electrostatic force microscopy, we reveal that the applied low-energy electron-microscopy experiment can be considered as an inverse $I(V)$ measurement, providing access to the local electronic conductance with nanoscale resolution and short data-acquisition times in the order of $10–{10}^2\mathrm{ms}$. Low-energy electrons thus hold yet unexplored application opportunities as a minimal-invasive probe for local electronic transport phenomena, opening a promising route towards spatially resolved, high-throughput sampling at the nanoscale.

The subpicosecond metal-insulator phase transition in vanadium dioxide (${\mathrm{VO}}_2$) has attracted extensive attention with potential applications in ultrafast Mott transistors, which are based on electric-field-induced phase transition. However, the development of ${\mathrm{VO}}_2$-based transistors lags behind, owing to inefficient and hysteretic gate modulation. Here we report ambipolar doping and strong field effects free of hysteresis in single-crystal ${\mathrm{VO}}_2$ nanowires synthesized via catalyst-free chemical vapor deposition. The ambipolarly doped ${\mathrm{VO}}_2$ nanowires are achieved by controlling the oxygen vacancy density during the synthesis and show strong gate effects because of their relatively low doping level. Both the doping type of the nanowires and the band-bending direction at the metal-insulator domain walls are reversibly switched by electrochemical gating, as revealed by scanning photocurrent microscopy. Furthermore, we eliminate the hysteresis in gate sweep via a hybrid gating method, which combines the merits of liquid-ionic and solid gating. The capability of efficient field effect modulation of ambipolar conduction and band alignment offers opportunities on understanding the phase transition mechanism and enables electronic applications based on ${\mathrm{VO}}_2$.

Long carrier spin lifetimes are a double-edged sword for the prospect of constructing “spintronic” logic devices: Preservation of the logic variable within the transport channel or interconnect is essential to successful completion of the logic operation, but any spins remaining past this event will pollute the environment for subsequent clock cycles. Electric fields can be used to manipulate these spins on a fast time scale by careful interplay of spin-orbit effects, but efficient controlled depolarization can only be completely achieved with amenable materials properties. Taking III-VI monochalcogenide monolayers as an example 2D semiconductor, we use symmetry analysis, perturbation theory, and ensemble calculation to show how this longstanding problem can be solved by suitable manipulation of conduction electrons.

The quality and crystallinity of ultrathin dielectric layers depend crucially on the details of interface formation and chemical stability. Using a combination of photoelectron (XPS) and electron-energy-loss spectroscopy, low-energy electron-diffraction, and transmission electron microscopy (TEM), we show that crystalline epitaxial layers of ${\mathrm{Ba}}_2{\mathrm{SiO}}_4$ can be grown on Si(100) substrates from evaporated Ba in oxygen background atmosphere at $650°\mathrm{C}$. Since the silicate is chemically by far more stable than the oxides of Si and Ba, an atomically sharp interface with no interface oxide is formed, as confirmed by XPS and TEM. However, the interface is rough on the atomic scale. dc and frequency-dependent electrical measurements reveal a relative dielectric constant of 22.8, low hysteresis in $CV$ measurements, and low leakage currents but still fairly high interface trap densities.

The interaction of shear bands with crystalline nanoprecipitates in Cu-Zr-based metallic glasses is investigated by a combination of high-resolution TEM imaging and molecular-dynamics computer simulations. Our results reveal different interaction mechanisms: Shear bands can dissolve precipitates, can wrap around crystalline obstacles, or can be blocked depending on the size and density of the precipitates. If the crystalline phase has a low yield strength, we also observe slip transfer through the precipitate. Based on the computational results and experimental findings, a qualitative mechanism map is proposed that categorizes the various processes as a function of the critical stress for dislocation nucleation, precipitate size, and distance.