Double Charged Surface Layers in Lead Halide Perovskite Crystals.

Understanding defect chemistry, particularly ion migration, and its significant effect on the surface's optical and electronic properties is one of the major challenges impeding the development of hybrid perovskite-based devices. Here, using both experimental and theoretical approaches, we demonstrated that the surface layers of the perovskite crystals may acquire a high concentration of positively charged vacancies with the complementary negatively charged halide ions pushed to the surface. This charge separation near the surface generates an electric field that can induce an increase of optical band gap in the surface layers relative to the bulk. We found that the charge separation, electric field, and the amplitude of shift in the bandgap strongly depend on the halides and organic moieties of perovskite crystals. Our findings reveal the peculiarity of surface effects that are currently limiting the applications of perovskite crystals and more importantly explain their origins, thus enabling viable surface passivation strategies to remediate them.

Band mapping across a pn-junction in a nanorod by scanning tunneling microscopy.

We map band-edges across a pn-junction that was formed in a nanorod. We form a single junction between p-type Cu2S and n-type CdS through a controlled cationic exchange process of CdS nanorods. We characterize nanorods of the individual materials and the single junction in a nanorod with an ultrahigh vacuum scanning tunneling microscope (UHV-STM) at 77 K. From scanning tunneling spectroscopy and correspondingly the density of states (DOS) spectra, we determine the conduction and valence band-edges at different points across the junction and the individual nanorods. We could map the band-diagram of nanorod-junctions to bring out the salient features of a diode, such as p- and n-sections, band-bending, depletion region, albeit interestingly in the nanoscale.

Magnetic moment assisted layer-by-layer film formation of a Prussian Blue analog.

We formed magnetic moment assisted layer-by-layer (LbL) films of a Prussian Blue analogue (PB). We applied an external magnetic field to each monolayer of PB to orient the magnetic moment of the compound perpendicular to the substrate. Aligned moments or orientation of the magnetic compounds themselves were immobilized in each monolayer, so that the moments could augment formation of the subsequent monolayers of LbL adsorption process. We hence could form multilayered LbL films of PB molecules with their magnetic moments oriented perpendicular to the substrate. We also formed LbL films of the compound with their moments oriented parallel to the substrate and facing one particular direction. We have measured conductivity and dielectric constant of the two types of films and compared the parameters with that of conventional LbL films deposited without orienting magnetic moments of the molecules.

Layer-by-layer electrostatic assembly with a control over orientation of molecules: anisotropy of electrical conductivity and dielectric properties.

While forming layer-by-layer (LbL) electrostatic assembly of a magnetic organic molecule, namely, nickel phthalocyanine (NiPc), we apply a magnetic field. The field orients the magnetic moment of the molecules on a monolayer along the direction of magnetic field. Such an orientation of the molecules is then electrostatically immobilized with a monolayer of a polycation. By repeating the dipping cycle, we form LbL films with planar NiPc molecules facing a particular direction. With NiPc's moment perpendicular to the molecular plane, two types of LbL films were formed: (a) NiPc's molecular plane parallel to the substrate (moment is perpendicular) and (b) molecules perpendicular to the substrate and facing one particular direction, the direction of magnetic field. Such films, with the molecules lying either (a) parallel or (b) perpendicular to the substrate, provide unique systems to study anisotropy of optical, dielectric, and electrical characteristics in these planar organic molecules. The latter film responds to the polarization of incident beam in electronic absorption spectroscopy. Here we show methods to obtain an orientation of molecules in LbL films and study anisotropy of dielectric constant and conductivity of the molecules in ultrathin films.

Layer-by-layer electrostatic-assembly: magnetic-field assisted ordering of organic molecules.

During the layer-by-layer (LbL) electrostatic assembly process, we orient organic molecules (nickel phthalocyanine) by an external magnetic field. Orientation of the magnetic moment of the molecules in a monolayer is immobilized by depositing a monolayer of a suitable polycation. Due to the orientation of magnetic moments, the electrostatic adsorption process becomes enhanced in subsequent layers. By cycling the deposition sequence layer after layer, we have achieved highly ordered and closely packed LbL films of the molecules with their magnetic moments oriented perpendicular to the substrate. Nonmagnetic copper phthalocyanine expectedly showed neither a magnetic-field assisted alignment nor an enhanced adsorption in LbL film deposition process.

Magnetic-field-assisted layer-by-layer electrostatic assembly of ferromagnetic nanoparticles.

We report that layer-by-layer (LbL) electrostatic assembly of Fe(3)O(4) nanoparticles can be supplemented by orienting magnetic domains of the nanoparticles. With the oriented domains of ionic-capped nanoparticles, both magnetic and electrostatic forces of attraction become operative during the LbL deposition process. The magnetic-field-assisted LbL adsorption process has been evidenced by increased electronic absorbance of the films. While atomic force microscopy studies rule out formation of multiple layers during a single adsorption process, magnetic force microscopy images evidence oriented domains in the LbL films. The results show a novel route for LbL deposition of ferromagnetic nanoparticles with oriented magnetic domains in the thin films.

Single crystal hybrid perovskite field-effect transistors.

The fields of photovoltaics, photodetection and light emission have seen tremendous activity in recent years with the advent of hybrid organic-inorganic perovskites. Yet, there have been far fewer reports of perovskite-based field-effect transistors. The lateral and interfacial transport requirements of transistors make them particularly vulnerable to surface contamination and defects rife in polycrystalline films and bulk single crystals. Here, we demonstrate a spatially-confined inverse temperature crystallization strategy which synthesizes micrometre-thin single crystals of methylammonium lead halide perovskites MAPbX3 (X = Cl, Br, I) with sub-nanometer surface roughness and very low surface contamination. These benefit the integration of MAPbX3 crystals into ambipolar transistors and yield record, room-temperature field-effect mobility up to 4.7 and 1.5 cm2 V-1 s-1 in p and n channel devices respectively, with 104 to 105 on-off ratio and low turn-on voltages. This work paves the way for integrating hybrid perovskite crystals into printed, flexible and transparent electronics.

Functional Two-Dimensional Coordination Polymeric Layer as a Charge Barrier in Li-S Batteries.

Ultrathin two-dimensional (2D) polymeric layers are capable of separating gases and molecules based on the reported size exclusion mechanism. What is equally important but missing today is an exploration of the 2D layers with charge functionality, which enables applications using the charge exclusion principle. This work demonstrates a simple and scalable method of synthesizing a free-standing 2D coordination polymer Zn2(benzimidazolate)2(OH)2 at the air-water interface. The hydroxyl (-OH) groups are stoichiometrically coordinated and implement electrostatic charges in the 2D structures, providing powerful functionality as a charge barrier. Electrochemical performance of the Li-S battery shows that the Zn2(benzimidazolate)2(OH)2 coordination polymer layers efficiently mitigate the polysulfide shuttling effects and largely enhance the battery capacity and cycle performance. The synthesis of the proposed coordination polymeric layers is simple, scalable, cost saving, and promising for practical use in batteries.

Bromination of Graphene: A New Route to Making High Performance Transparent Conducting Electrodes with Low Optical Losses.

The unique optical and electrical properties of graphene have triggered great interest in its application as a transparent conducting electrode material and significant effort has been invested in achieving high conductivity while maintaining high transparency. Doping of graphene has been a popular route for reducing its sheet resistance, but this has typically come at a significant loss in optical transmittance. We demonstrate doping of few layers graphene (FLG) with bromine as a means of enhancing the conductivity via intercalation without major optical losses. Our results demonstrate the encapsulation of bromine within the FLG, leading to air-stable transparent conducting electrodes with 5-fold improvement of sheet resistance reaching ∼180 Ω/□ at the cost of only 2-3% loss of optical transmittance. The remarkably low trade-off in optical transparency leads to the highest enhancements in the figure of merit reported thus far for FLG. Furthermore, we tune the work function by up to 0.3 eV by tuning the bromine content. These results should help pave the way for further development of graphene as a potential substitute to transparent conducting polymers and metal oxides used in optoelectronics, photovoltaics, and beyond.