No abstract

Medical X-ray imaging procedures require digital flat detectors operating at low doses to reduce radiation health risks. Solution-processed organic-inorganic hybrid perovskites have characteristics that make them good candidates for the photoconductive layer of such sensitive detectors. However, such detectors have not yet been built on thin-film transistor arrays because it has been difficult to prepare thick perovskite films (more than a few hundred micrometres) over large areas (a detector is typically 50 centimetres by 50 centimetres). We report here an all-solution-based (in contrast to conventional vacuum processing) synthetic route to producing printable polycrystalline perovskites with sharply faceted large grains having morphologies and optoelectronic properties comparable to those of single crystals. High sensitivities of up to 11 microcoulombs per air KERMA of milligray per square centimetre (μC mGy cm) are achieved under irradiation with a 100-kilovolt bremsstrahlung source, which are at least one order of magnitude higher than the sensitivities achieved with currently used amorphous selenium or thallium-doped cesium iodide detectors. We demonstrate X-ray imaging in a conventional thin-film transistor substrate by embedding an 830-micrometre-thick perovskite film and an additional two interlayers of polymer/perovskite composites to provide conformal interfaces between perovskite films and electrodes that control dark currents and temporal charge carrier transportation. Such an all-solution-based perovskite detector could enable low-dose X-ray imaging, and could also be used in photoconductive devices for radiation imaging, sensing and energy harvesting.

A fully sealed field-emission display 4.5 in. in size has been fabricated using single-wall carbon nanotube ͑CNT͒-organic binders. The fabricated displays were fully scalable at low temperature, below 415°C, and CNTs were vertically aligned using paste squeeze and surface rubbing techniques. The turn-on fields of 1 V/m and field emission current of 1.5 mA at 3 V/m (J ϭ90 A/cm 2) were observed. Brightness of 1800 cd/m 2 at 3.7 V/m was observed on the entire area of a 4.5 in. panel from the green phosphor-indium-tin-oxide glass. The fluctuation of the current was found to be about 7% over a 4.5 in. cathode area.

We have studied the field-screening effect provoked by the proximity of neighboring tubes by changing the tube height of highly ordered carbon nanotubes fabricated on porous anodic aluminum oxide templates. The field emission was critically affected by the tube height that protruded from the surface. The field emission was optimal when the tube height was similar to the intertube distance. The intertube distance to the tube height for maximum field emission is about one half the intertube distance predicted by Nilsson et al. [Appl. Phys. Lett. 76, 2071 (2000)].

chemical tunability, structural flexibility, and ease of exfoliation. [6] There is recent intensifying interest in an emergent class of 2D nanosheets constructed by downsizing 3D metal-organic frameworks (MOFs), [3a,7] which are inorganic-organic (hybrid) structures possessing an enormous physicochemical [8] and structural versatility. [9] Furthermore, the nanoscale porosity of MOFs could function as a vessel to "host" a variety of functional "guest" molecules, [10] imparting a unique combination of properties through intimate host-guest interactions. [11] Yet, preparation of a functionalized MOF as 2D nanosheets exemplifying a tunable host-guest sensing response is uncommon in literature, unlike its traditional counterparts. [12] In this work, we present a simple supramolecular self-assembly strategy to accomplish concomitant 2D nanosheet synthesis and functionalization of a porous MOF system, and demonstrate its efficacy for application as a tunable optochemical sensor. We leverage our recently elucidated supramolecular "high-concentration reactions" (HCR) approach, [13] to realize one-pot synthesis of functionalized MOF nanosheets at ambient conditions; the basic concept is illustrated in Scheme 1. Here we describe a representative study employing 1,4-benzenedicarboxylic acid (BDC) as the organic linker, because of its strong propensity to construct an extended chemical network upon coordination with metal centers; for instance, here we utilized divalent Zn 2+ . Triethylamine base (NEt 3 ) featuring a flexible tripodal geometry was used to trigger fast activation (deprotonation) of the BDC linkers. [14] It is striking to see that, a white gel-like fibrous soft matter was immediately obtained at room temperature (Figure 1a), arising from the HCR between Zn 2+ and BDC 2− , augmented by the NEt 3 + cations. We observed a discernible twostage material transformation via optical microscopy ( Figure 1b): initially witnessing development of highly oriented fibers, prior to formation of a visually shiny phase prevalent on the fiber surfaces. The gel fiber diameter was found to be ≈1-10s µm by scanning electron microscopy (SEM), see Figure 1c. Intriguingly, SEM revealed those supramolecular fibers are, in fact, constituting densely packed crystalline nanosheets (Figure 1d), thus confirming the (shiny) faceted appearance detected under optical microscopy ( Figure 1b). To establish the detailed 2D morphologies, we examined nanosheets harvested from the supramolecular gels using transmission electron micro scopy (TEM) and SEM (Figure 1e,f), as well as by atomic force microscopy (AFM) with which we have estimated the nominal thickness of 2D nanosheets are contemporary materials with exceptional physical and functional properties, derived from a broad class of low-dimensional solids containing atomically thin structures, [1] exfoliated 2D frameworks, [2] and molecular membranes.[3] Considerable efforts are being devoted to 2D graphene-related materials [4] to yield improved mechanical, electronic, and optical modulat...