A two-dimensional (2D) porous layer can make an ideal membrane for separation of chemical mixtures because its infinitesimal thickness promises ultimate permeation. Graphene--with great mechanical strength, chemical stability, and inherent impermeability--offers a unique 2D system with which to realize this membrane and study the mass transport, if perforated precisely. We report highly efficient mass transfer across physically perforated double-layer graphene, having up to a few million pores with narrowly distributed diameters between less than 10 nanometers and 1 micrometer. The measured transport rates are in agreement with predictions of 2D transport theories. Attributed to its atomic thicknesses, these porous graphene membranes show permeances of gas, liquid, and water vapor far in excess of those shown by finite-thickness membranes, highlighting the ultimate permeation these 2D membranes can provide.

Colloidal nanocrystals (NCs) of APbX3-type lead halide perovskites [A = Cs+, CH3NH3+ (methylammonium or MA+) or CH(NH2)2+ (formamidinium or FA+); X = Cl–, Br–, I–] have recently emerged as highly versatile photonic sources for applications ranging from simple photoluminescence down-conversion (e.g., for display backlighting) to light-emitting diodes. From the perspective of spectral coverage, a formidable challenge facing the use of these materials is how to obtain stable emissions in the red and infrared spectral regions covered by the iodide-based compositions. So far, red-emissive CsPbI3 NCs have been shown to suffer from a delayed phase transformation into a nonluminescent, wide-band-gap 1D polymorph, and MAPbI3 exhibits very limited chemical durability. In this work, we report a facile colloidal synthesis method for obtaining FAPbI3 and FA-doped CsPbI3 NCs that are uniform in size (10–15 nm) and nearly cubic in shape and exhibit drastically higher robustness than their MA- or Cs-only cousins with similar sizes and morphologies. Detailed structural analysis indicated that the FAPbI3 NCs had a cubic crystal structure, while the FA0.1Cs0.9PbI3 NCs had a 3D orthorhombic structure that was isostructural to the structure of CsPbBr3 NCs. Bright photoluminescence (PL) with high quantum yield (QY > 70%) spanning red (690 nm, FA0.1Cs0.9PbI3 NCs) and near-infrared (near-IR, ca. 780 nm, FAPbI3 NCs) regions was sustained for several months or more in both the colloidal state and in films. The peak PL wavelengths can be fine-tuned by using postsynthetic cation- and anion-exchange reactions. Amplified spontaneous emissions with low thresholds of 28 and 7.5 μJ cm–2 were obtained from the films deposited from FA0.1Cs0.9PbI3 and FAPbI3 NCs, respectively. Furthermore, light-emitting diodes with a high external quantum efficiency of 2.3% were obtained by using FAPbI3 NCs.

(K.A.). elusive. The use of template molecules to unambiguously dictate the diameter and chirality of the resulting nanotube 8,13-16 holds great promise in this regard, but has hitherto had only limited practical success 7,17,18 . Here we show that this bottom-up strategy can produce targeted nanotubes: we convert molecular precursors into ultrashort singly capped (6,6) 'armchair' nanotube seeds using surface-catalysed cyclodehydrogenation on a Pt(111) surface, and then elongate these during a subsequent growth phase to produce single-chirality and essentially defect-free SWCNTs with lengths up to a few hundred nanometres. We expect that our onsurface synthesis approach will provide a route to nanotube-based materials with highly Fig. 1) was designed and synthesized by multi-step organic synthesis to tackle this challenge (for details, see Methods). Upon intramolecular CDH it affords seed S1, an ultra-short singly capped (6,6) SWCNT bearing a carbon nanotube segment. The selective growth of (6,6) SWCNTs is illustrated in Fig. 1 and combines two steps: (1) formation of seed S1, and (2) subsequent epitaxial elongation. The first step is realized by depositing precursor P1 on a Pt (111) surface followed by annealing to 770 K under ultrahigh vacuum conditions to induce the surfacecatalysed CDH reaction ( Fig. 2a, b). The second step, epitaxial elongation, is achieved by the incorporation of carbon atoms originating from the surface-catalysed decomposition of a carbon feedstock gas ( Fig. 3a-c). indicates that the different topographic features observed for the adsorbed precursors can be attributed to the different adsorption geometries. Importantly, the stereoisomerism does not affect the CDH process, since all chiral centres will disappear during intramolecular cyclization. Over the last two decades, single-walled carbon nanotubes (SWCNTsAlthough P1 is designed to yield seed S1, the conformational flexibility of the peripheral biphenyl groups leads partially to undesired adsorption geometries. In contrast to the stereoisomers discussed above, these molecules will follow a different CDH pathway, ending in the formation of undesired buckybowls (Extended Data Fig. 2). A statistical analysis of more than 100 precursor monomers observed by STM revealed that more than 50% adopt the desired configurations (Extended Data Fig. 1). Most importantly, the condensation products of precursor molecules exhibiting 'wrong' conformations cannot act as seeds for the subsequent CNT growth process via epitaxial elongation, and thus will not affect the selectivity of SWCNT formation.Surface-catalysed CDH of precursors (P1) into seeds (S1) is induced by annealing at 770 K for 10 min. STM images (Fig. 2d) show that the originally quasi-planar three-fold symmetric molecules transform into dome-shaped species with a prominent increase in apparent height from 2 to 4.5 Å (Fig. 2f). Additional proof of successful dehydrogenation of P1 into S1 derives from the good agreement of high-resolution STM images and simulations of the frontier molecu...

Spin-orbit coupling is a manifestation of special relativity. In the reference frame of a moving electron, electric fields transform into magnetic fields, which interact with the electron spin and lift the degeneracy of spin-up and spin-down states. In solid-state systems, the resulting spin-orbit fields are referred to as Dresselhaus or Rashba fields, depending on whether the electric fields originate from bulk or structure inversion asymmetry, respectively. Yet, it remains a challenge to determine the absolute value of both contributions in a single sample. Here we show that both fields can be measured by optically monitoring the angular dependence of the electrons' spin precession on their direction of movement with respect to the crystal lattice. Furthermore, we demonstrate spin resonance induced by the spin-orbit fields. We apply our method to GaAs/InGaAs quantum-well electrons, but it can be used universally to characterise spin-orbit interactions in semiconductors, facilitating the design of spintronic devices.Symmetry-breaking electric fields in semiconductors induce a spin splitting, because electric fields appear to a moving electron as magnetic fields, which interact with the electron spin and couple it with the electron momentum, or wave vector, k. In zinc-blende-type crystals, such as GaAs, the electric fields resulting from the lack of an inversion centre lead to bulk inversion asymmetry (BIA) and to the Dresselhaus term in the Hamiltonian [1]. In the conduction band, its coupling is linear or cubic in k with proportionality constants β and γ, respectively. In heterostructures, additional electric fields are introduced owing to structure inversion asymmetry (SIA), giving rise to the Rashba term [2], which for conduction-band electrons is linear in k with coupling constant α. Both contributions have been extensively studied [3], since a potential use of electron spins in future devices (e.g. a spin transistor [4]) requires precise control of the spin's environment and of the Dresselhaus and Rashba fields [5]. Spin-orbit fields also contribute to spin decoherence [6].In two-dimensional systems, such as quantum wells (QWs), usually α β and γ ≈ 0 [7,8,9,10]. Therefore, measurements of the spin-orbit coupling initially focused on the Rashba term in QWs and concentrated on the study of beatings in Shubnikov-de-Haas oscillations [8,10,11,12,13], whose interpretation, however, is debated [14,15]. More recent experiments include the investigation of antilocalization in magnetotransport [16] or the analysis of photocurrents [17]. In the latter experiment, the ratio α/β could be determined. A gateinduced transition from weak localization to antilocalization allowed the discrimination between Rashba, as well as linear and cubic Dresselhaus contributions to the spinorbit field [18]. Tuning of the Rashba coupling has been achieved by introducing additional electric fields from gates [9,19] or by changing the electron density [20,21].The influence of effective spin-orbit magnetic fields on optical measurements in ...

We investigate the triplet-singlet relaxation in a double quantum dot defined by top gates in an InAs nanowire. In the Pauli spin blockade regime, the leakage current can be mainly attributed to spin relaxation. While at weak and strong interdot coupling relaxation is dominated by two individual mechanisms, the relaxation is strongly reduced at intermediate coupling and finite magnetic field. In addition we observe a characteristic bistability of the spin-nonconserving current as a function of magnetic field. We propose a model where these features are explained by the polarization of nuclear spins enabled by the interplay between hyperfine and spin-orbit mediated relaxation.