Inducing and Understanding Pseudocapacitive Behavior in an Electrophoretically Deposited Lithium Iron Phosphate Li-Metal Battery as an Electrochemical Test Platform.

Our study has effectively employed electrophoretic deposition (EPD) using AC voltage to develop a lithium iron phosphate (LFP) Li-ion battery featuring pseudocapacitive properties and improved high C-rate performance. This method has significantly improved the battery's specific capacity, achieving an impressive 100 mAhg-1 at a 5 C discharge rate, which showcases its superior high-rate capability. Additionally, the battery displayed excellent reversibility during its performance cycles. These results confirm the EPD method's efficacy in improving LFP electrode performance, yielding notable improvements in cycling stability and high-rate capability. The enhanced capacity and high-rate performance of the electrophoretic-deposited LFP electrodes are largely due to the fast kinetics facilitated by pseudocapacitive behavior-induced charge storage and a high Li-ion diffusion constant measured in our EPD-deposited LFP electrodes. This underscores the practicality of our approach and the development of a fundamental test platform to investigate the pseudocapacitive charge storage mechanism in electrodes with typical battery-like behavior.

Synergistic Enhancement of Electron Dynamics and Optical Properties in Zeolitic Imidazolate Framework-8-Derived Zinc Oxide via Surface Plasmon Resonance Effects of Silver Nanoparticles under UV Irradiation.

This study investigates the surface plasmon resonance (SPR)-induced UV photoresponse of zinc oxide (ZnO) derived from zeolitic imidazolate framework-8 (ZIF-8) to assess the influence of silver nanoparticles (Ag NPs) on the photoresponse behavior of metal-organic framework (MOF)-derived ZnO. The initial synthesis involved a thermal treatment in air to convert ZIF-8 into ZnO. We noted enhanced optical absorption both in the UV and visible spectra with the deposition of Ag NPs onto the ZIF-8-derived ZnO. Additionally, the presence of Ag NPs in the ZnO resulted in a substantial increase in current, even without any light exposure. This increase is attributed to the transfer of electrons from the Ag NPs to the ZnO. Photocurrent measurements under UV illumination revealed that the photocurrent with Ag NPs was significantly higher-by two orders of magnitude-compared with that without Ag NPs. This demonstrates that SPR-induced absorption markedly boosted the photocurrent, although the current rise and decay time constants remained comparable to those observed with ZnO alone. Although Ag NPs contribute electrons to ZnO, creating a "pre-doping" effect that heightens baseline conductivity (even in the absence of light), this does not necessarily alter the recombination dynamics of the photogenerated carriers, as indicated by the similar rise and decay time constants. The electron transfer from Ag to ZnO increases the density of charge carriers but does not significantly influence their recombination.

Facile Strategy for Modulating the Nanoporous Structure of Ultrathin π-Conjugated Polymer Films for High-Performance Gas Sensors.

Conventional conjugated polymer (CP) films based on organic field-effect transistors (OFETs) tend to limit the performance of gas sensors owing to restricted analyte diffusion and limited interactions with the charge carriers that accumulate in the first few monolayers of the CP film in contact with the dielectric layer. Herein, a facile strategy is presented for modulating the morphology and charge-transport properties of nanoporous CP films using shearing-assisted phase separation of polymer blends for fabricating OFET-based chemical sensors. This approach enables the formation of nanoporous films with pore size and thickness in the ranges of 90-550 and 7-27 nm, respectively, which can be controlled simply by varying the shear rate. The resulting OFET sensors exhibit excellent sensing performance when exposed to NH3 gas, demonstrating a high responsivity (≈70.7%) at 10 ppm and good selectivity toward NH3 over various organic solvent vapors. After a comprehensive analysis of the morphology and electrical properties of the CP films, it is concluded that morphological features, such as film thickness and surface area, affect the sensing performance of nanoporous-film-based OFET sensors more significantly compared to the charge-transport characteristics of the films.

Band to Band Tunneling at the Zinc Oxide (ZnO) and Lead Selenide (PbSe) Quantum Dot Contact; Interfacial Charge Transfer at a ZnO/PbSe/ZnO Probe Device.

We provide a comprehensive understanding of interfacial charge transfer at the lead selenide (PbSe) quantum dot (QD)/zinc oxide (ZnO) interface, proposing band to band tunneling process as a charge transfer mechanism in which initial hopping of carriers from ZnO to PbSe QDs is independent of temperature. Using the transmission line method (TLM) in a ZnO/PbSe/ZnO geometry device, we measured the ZnO/PbSe electrical contact resistance, a measure of charge transfer efficiency. Fabrication of a highly conductive ZnO film through Al doping allows for the formation of ZnO source and drain electrodes, replacing conventional metal electrodes. We found that band to band tunneling at the PbSe QD/ZnO interface governs charge transfer based on temperature-independent PbSe QD/ZnO contact resistance. In contrast, the PbSe QD channel sheet resistance decreased as the temperature increased, indicating thermally activated transport process in the PbSe QD film. These results demonstrate that, at the ZnO/PbSe QD interface, temperature-independent tunneling process initiates carrier injection followed by thermally activated carrier hopping, determining the electrical contact resistance.

Emission Enhancement of Cu-Doped InP Quantum Dots through Double Shelling Scheme.

The doping of transition metal ions, such as Cu+ and Mn2+ into a quantum dot (QD) host is one of the useful strategies in tuning its photoluminescence (PL). This study reports on a two-step synthesis of Cu-doped InP QDs double-shelled with ZnSe inner shell/ZnS outer shell. As a consequence of the double shelling-associated effective surface passivation along with optimal doping concentrations, Cu-doped InP/ZnSe/ZnS (InP:Cu/ZnSe/ZnS) QDs yield single Cu dopant-related emissions with high PL quantum yields of 57-58%. This study further attempted to tune PL of Cu-doped QDs through the variation of InP core size, which was implemented by adopting different types of Zn halide used in core synthesis. As the first application of doped InP QDs as electroluminescent (EL) emitters, two representative InP:Cu/ZnSe/ZnS QDs with different Cu concentrations were then employed as active emitting layers of all-solution-processed, multilayered QD-light-emitting diodes (QLEDs) with the state-of-the-art hybrid combination of organic hole transport layer plus inorganic electron transport layers. The EL performances, such as luminance and efficiencies of the resulting QLEDs with different Cu doping concentrations, were compared and discussed.

The formation of a functional pentacene/CH3NH3PbI3-xClx perovskite interface: optical gating and field-induced charge retention.

We fabricated a functional pentacene/CH3NH3PbI3-xClx perovskite interface where optical gating and field assisted charge retention occur. Using a pentacene/perovskite field effect transistor (FET) test platform, we investigated the interfacial charge transfer associated with optical gating through threshold voltage measurements under illumination. Importantly, bistable electrical conduction in pentacene/perovskite FET devices was achieved as a result of field-induced charge retention at the interface and the origin is discussed to be associated with interfacial charging at the pentacene/perovskite interface. Interfacial contact modification associated with ion migration and other possible effects in the perovskite layer plays a crucial role in forming a functional interface involving organic semiconducting materials.

Understanding charge trapping/detrapping at the zinc oxide (ZnO)/MAPbI3 perovskite interface in the dark and under illumination using a ZnO/perovskite/ZnO test platform.

We fabricated a zinc oxide (ZnO)/methylammonium lead iodide (MAPbI3) perovskite/ZnO field effect transistor (FET) test platform device through which ZnO/perovskite interfacial contact properties can be probed in the dark and under illumination. Using pulsed laser deposition, highly conductive (0.014 Ω cm) ZnO source and drain electrodes were fabricated allowing for the investigation of the interfacial charge transfer properties through current-voltage characteristics of a ZnO/perovskite/ZnO FET. With a bottom-contact FET device, gate voltage dependent current hysteresis in the drain current-gate voltage curves was probed at low temperature to minimize the effect of ion migration on electronic charge transport in the perovskite layer. Under illumination, importantly, ZnO/perovskite electrical contact properties were significantly altered due to electronic energy barrier change at the interface arising from the detrapping of electrons from the ZnO/perovskite interface, resulting in an enhanced dark current and a suppressed photocurrent. The origin of current hysteresis in the ZnO/perovskite/ZnO FET device is discussed relating it to interfacial charging/discharging associated with ultraviolet (UV)-induced oxygen adsorption/desorption. The results presented herein demonstrate that interfacial electronic properties at the donor (perovskite)/acceptor (ZnO) interface can be altered by photoinduced carrier trapping/detrapping, providing insights that UV-induced persistent photoconduction in transition metal oxide electron transport layers including ZnO may be contributing to the current hysteresis observed in the perovskite photovoltaic devices.

Control of Molecular Ordering, Alignment, and Charge Transport in Solution-Processed Conjugated Polymer Thin Films.

Morphology of conjugated polymers is a critical factor that significantly affects intrinsic charge transport characteristics and in turn performance of polymer-based devices. Morphological defects including misaligned crystalline grains and grain boundaries significantly impede efficient charge hopping between transport sites, resulting in degradation of device performance. Therefore, one important challenge is to control morphology of active polymer thin-films for achieving high performance flexible electronic devices. In the past decade, significant progress has been achieved in morphology control of conjugated polymer thin-films using solution-based processing techniques. This review focuses on recent advances in processing strategies that can tune the morphologies and thus impact charge transport properties of conjugated polymer thin films. Of the available processing strategies, polymer solution treatments and film deposition techniques will be mainly highlighted. The correlation between processing conditions, active layer morphologies, and device performance will be also be discussed.

Triangular Black Phosphorus Atomic Layers by Liquid Exfoliation.

Few-layer black phosphorus (BP) is the most promising material among the two-dimensional materials due to its layered structure and the excellent semiconductor properties. Currently, thin BP atomic layers are obtained mostly by mechanical exfoliation of bulk BP, which limits applications in thin-film based electronics due to a scaling process. Here we report highly crystalline few-layer black phosphorus thin films produced by liquid exfoliation. We demonstrate that the liquid-exfoliated BP forms a triangular crystalline structure on SiO2/Si (001) and amorphous carbon. The highly crystalline BP layers are faceted with a preferred orientation of the (010) plane on the sharp edge, which is an energetically most favorable facet according to the density functional theory calculations. Our results can be useful in understanding the triangular BP structure for large-area applications in electronic devices using two-dimensional materials. The sensitivity and selectivity of liquid-exfoliated BP to gas vapor demonstrate great potential for practical applications as sensors.

Upconversion luminescence enhancement in plasmonic architecture with random assembly of metal nanodomes.

We report an experimental study on the highly enhanced upconversion luminescence (UCL) of ß-NaYF4:Yb(3+)/Er(3+) nanocrystals (NCs) in a plasmonic architecture. For the architecture, we designed a thin film device composed of a thin layer of NCs capped with an upper layer of a plasmonic nanodome array (pNDA) and lower substrate of a back reflector (BR). Compared to the UCL intensity observed in a glass reference substrate, the designed plasmonic architecture exhibits distinctively strong luminescence enhanced by up to 800-fold. The intensity considerably exceeds the previously reported luminescence intensity regardless of the excitation power. We elucidated a mechanism explaining the large UCL enhancement, which quantitatively analyzes the combination of plasmonic effects as well as multiple large scattering. More importantly, we provided a detailed analysis of the Ag NDA-derived and BR-assisted plasmonic effects that contribute to an increase in the radiative decay rate and an enhancement of the absorption of incident light. The present study is expected to be beneficial for designing a thin film-based plasmonic structure with a randomized metal nanostructure for high-efficiency photovoltaic devices and infrared detectors.

Production of quasi-2D graphene nanosheets through the solvent exfoliation of pitch-based carbon fiber.

Stable dispersion of quasi-2D graphene sheets with a concentration up to 1.27 mg mL(-1) was prepared by sonication-assisted solvent exfoliation of pitch-based carbon fiber in N-methyl pyrrolidone with the mass yield of 2.32%. Prepared quasi-2D graphene sheets have multi-layered 2D plate-like morphology with rich inclusions of graphitic carbons, a low number of structural defects, and high dispersion stability in aprotic polar solvents, and facilitate the utilization of quasi-2D graphene sheets prepared from pitch-based carbon fiber for various electronic and structural applications. Thin films of quasi-2D graphene sheets prepared by vacuum filtration of the dispersion of quasi-2D graphene sheets demonstrated electrical conductivity up to 1.14 × 10(4) Ω/□ even without thermal treatment, which shows that pitch-based carbon fiber might be useful as the source of graphene-related nanomaterials. Because pitch-based carbon fiber could be prepared from petroleum pitch, a very cheap structural material for the pavement of asphalt roads, our approach might be promising for the mass production of quasi-2D graphene nanomaterials.

Photoinduced Anisotropic Assembly of Conjugated Polymers in Insulating Polymer Blends.

Low-dose UV irradiation of poly(3-hexylthiophene) (P3HT)-insulating polymer (polystyrene (PS) or polyisobutylene (PIB)) blend solutions led to the formation of highly ordered P3HT nanofibrillar structures in solidified thin films. The P3HT nanofibers were effectively interconnected through P3HT islands phase-separated from insulating polymer regions in blend films comprising a relatively low fraction of P3HT. Films prepared with a P3HT content as low as 5 wt % exhibited excellent macroscopic charge transport characteristics. The impact of PS on P3HT intramolecular and intermolecular interactions was systematically investigated. The presence of PS chains appeared to assist in the UV irradiation process of the blend solutions to facilitate molecular interactions of the semiconductor component, and to enhance P3HT chain interactions during spin coating because of relatively unfavorable P3HT-PS chain interactions. However, P3HT lamellar packing was hindered in the presence of PS chains, because of favorable hydrophobic interactions between the P3HT hexyl substituents and the PS chains. As a result, the lamellar packing d-spacing increased, and the coherence length corresponding to the lamellar packing decreased, as the amount of PS in the blend films increased.

Fabrication of a white electroluminescent device based on bilayered yellow and blue quantum dots.

Until now most work on colloidal quantum dot-light-emitting diodes (QLEDs) has been focused on the improvement of the electroluminescent (EL) performance of monochromatic devices, and multi-colored white QLEDs comprising more than one type of QD emitter have been rarely investigated. To demonstrate a white EL as a result of color mixing between blue and yellow, herein a unique combination of two dissimilar QDs of blue- CdZnS/ZnS plus a yellow-emitting Cu-In-S (CIS)/ZnS is used for the formation of the emitting layer (EML) of a multilayered QLED. First, the QLED consisting of a single EML randomly mixed with two QDs is fabricated, however, its EL is dominated by blue emission with the contribution of yellow emission substantially weaker. Thus, another EML configuration is devised in the form of a QD bilayer with two stacking sequences of CdZnS/ZnS//CIS/ZnS QD and vice versa. The QLED with the former stacking sequence shows an overwhelming contribution of blue EL, similar to the mixed QD EML-based device. Upon applying the oppositely stacked QD bilayer of CIS/ZnS//CdZnS/ZnS, however, a bicolored white EL can be successfully achieved by means of the effective extension of the radiative excitonic recombination zone throughout both QD EML regions. Such QD EML configuration-dependent EL results, which are discussed primarily using the proposed device energy level diagram, strongly suggest that the positional design of individual QD emitters is a critical factor for the realization of multicolored, white emissive devices.

Tailor-made polyamide membranes for water desalination.

Independent control of the extrinsic and intrinsic properties of the polyamide (PA) selective layer is essential for designing thin-film composite (TFC) membranes with performance characteristics required for water purification applications besides seawater desalination. Current commercial TFC membranes fabricated via the well-established interfacial polymerization (IP) approach yield materials that are far from ideal because their layer thickness, surface roughness, polymer chemistry, and network structure cannot be separately tailored. In this work, tailor-made PA-based desalination membranes based on molecular layer-by-layer (mLbL) assembly are presented. The mLbL technique enables the construction of an ultrathin and highly cross-linked PA selective layer in a precisely and independently controlled manner. The mLbL-assembled TFC membranes exhibit significant enhancements in performance compared to their IP-assembled counterparts. A maximum sodium chloride rejection of 98.2% is achieved along with over 2.5 times higher water flux than the IP-assembled counterpart. More importantly, this work demonstrates the broad applicability of mLbL in fabricating a variety of PA-based TFC membranes with nanoscale control of the selective layer thickness and roughness independent of the specific polyamide chemistry.

Boron nitride nanosheets decorated with silver nanoparticles through mussel-inspired chemistry of dopamine.

Boron nitride nanosheet (BNNS) decorated with silver nanoparticles (AgNPs) was successfully synthesized via mussel-inspired chemistry of dopamine. Poly(dopamine)-functionalized BNNS (PDA-BNNS) was prepared by adding dopamine into the aqueous dispersion of hydroxylated BNNS (OH-BNNS) at alkaline condition. AgNPs were decorated on PDA-BNNS through spontaneous reduction of silver cations by catechol moieties of a PDA layer on BNNS, resulting in AgNP-BNNS with good dispersion stability. Incorporation of PDA on BNNS not only played a role as a surface functionalization method of BNNS, but also provided a molecular platform for creating very sophisticated two-dimensional (2D) BNNS-based hybrid nanomaterials such as metal nanoparticle-decorated BNNS.

Anisotropic assembly of conjugated polymer nanocrystallites for enhanced charge transport.

The anisotropic assembly of P3HT nanocrystallites into longer nanofibrillar structures was demonstrated via sequential UV irradiation after ultrasonication to the pristine polymer solutions. The morphology of resultant films was studied by atomic force microscopy (AFM), and quantitative analysis of intra- and intermolecular ordering of polymer chains was performed by means of static absorption spectroscopy and quantitative modeling. Consequently, the approach to treat the precursor solution enhanced intra- and intermolecular ordering and reduced the incidence of grain boundaries within P3HT films, which contributed to the excellent charge carrier transport characteristics of the corresponding films (µ ≈ 12.0 × 10(-2) cm(2) V(-1) s(-1) for 96% RR P3HT).

Probing surface states in PbS nanocrystal films using pentacene field effect transistors: controlling carrier concentration and charge transport in pentacene.

We used a bilayer field effect transistor (FET) consisting of a thin PbS nanocrystals (NCs) film interfaced with vacuum-deposited pentacene to probe trap states in NCs. We interpret the observed threshold voltage shift in context of charge carrier trapping by PbS NCs and relate the magnitude of the threshold voltage shift to the number of trapped carriers. We explored a series of NC surface ligands to modify the interface between PbS NCs and pentacene and demonstrate the impact of interface chemistry on charge carrier density and the FET mobility in a pentacene FET.

Additive-free hollow-structured Co3O4 nanoparticle Li-ion battery: the origins of irreversible capacity loss.

Origins of the irreversible capacity loss were addressed through probing changes in the electronic and structural properties of hollow-structured Co3O4 nanoparticles (NPs) during lithiation and delithiation using electrochemical Co3O4 transistor devices that function as a Co3O4 Li-ion battery. Additive-free Co3O4 NPs were assembled into a Li-ion battery, allowing us to isolate and explore the effects of the Co and Li2O formation/decomposition conversion reactions on the electrical and structural degradation within Co3O4 NP films. NP films ranging between a single monolayer and multilayered film hundreds of nanometers thick prepared with blade-coating and electrophoretic deposition methods, respectively, were embedded in the transistor devices for in situ conduction measurements as a function of battery cycles. During battery operation, the electronic and structural properties of Co3O4 NP films in the bulk, Co3O4/electrolyte, and Co3O4/current collector interfaces were spatially mapped to address the origin of the initial irreversible capacity loss from the first lithiation process. Further, change in carrier injection/extraction between the current collector and the Co3O4 NPs was explored using a modified electrochemical transistor device with multiple voltage probes along the electrical channel.

Over 40 cd/A efficient green quantum dot electroluminescent device comprising uniquely large-sized quantum dots.

Green CdSe@ZnS quantum dots (QDs) of 9.5 nm size with a composition gradient shell are first prepared by a single-step synthetic approach, and then 12.7 nm CdSe@ZnS/ZnS QDs, the largest among ZnS-shelled visible-emitting QDs available to date, are obtained through the overcoating of an additional 1.6 nm thick ZnS shell. Two QDs of CdSe@ZnS and CdSe@ZnS/ZnS are incorporated into the solution-processed hybrid QD-based light-emitting diode (QLED) structure, where the QD emissive layer (EML) is sandwiched by poly(9-vinlycarbazole) and ZnO nanoparticles as hole and electron-transport layers, respectively. We find that the presence of an additional ZnS shell makes a profound impact on device performances such as luminance and efficiencies. Compared to CdSe@ZnS QD-based devices the efficiencies of CdSe@ZnS/ZnS QD-based devices are overwhelmingly higher, specifically showing unprecedented values of peak current efficiency of 46.4 cd/A and external quantum efficiency of 12.6%. Such excellent results are likely attributable to a unique structure in CdSe@ZnS/ZnS QDs with a relatively thick ZnS outer shell as well as a well-positioned intermediate alloyed shell, enabling the effective suppression of nonradiative energy transfer between closely packed EML QDs and Auger recombination at charged QDs.

Blue inorganic light emitting diode on flexible polyimide substrate using laser lift-off process.

The fabrication process for the blue GaN inorganic light emitting diode (ILED) on flexible polyimide (PI) substrate by laser lift off (LLO) method was demonstrated. The GaN epi-structure was grown on patterned sapphire wafer. GaN samples were temporary bonded with polyimide substrate by flexible silver epoxy. Separation of the whole GaN LED film from GaN/sapphire wafer was accomplished using a single KrF excimer (248 nm) laser pulse directed through the transparent sapphire wafer. Device fabrication was carried out on both rigid silicon and flexible polyimide substrate, and I-V performance for both devices was measured. The optimized LLO process for the whole GaN LED film transfer would be applicable in flexible LED applications without compromising electrical properties.