Graphene Quantum Rings Doped PEDOT:PSS Based Composite Layer for Efficient Performance of Optoelectronic Devices

The method of synthesis has a profound influence on the properties of carbon nanostructures endowed with quantum confinement. Here, we report synthesis method of the graphene quantum rings (GQRs) from pitch carbon fiber by a thermal hydrolysis process and their application in optoelectronic devices. Synthesized GQRs with excitation-dependent photoluminescence and excellent electrical conducting properties have a work function of −4.16 eV suitable for optoelectronic applications. The electrical properties of the GQRs are studied by metal–semiconductor–metal (MSM), field-effect transistor (FET), and hybrid (inorganic and organic) solar cell devices comprising the GQRs as the p-type channel and hole transporting material, respectively. The current–voltage (<i>I</i>–<i>V</i>) characteristic from a diode-type MSM device shows a conductance of 1.5 mA at 3.5 V in the dark. Under an illumination of 1.5 AM the current increases to 5.28 mA at 3.5 V, showing that these GQRs are very good candidates for photodiodes and other optoelectronic properties. The GQRs doped poly­(3,4-ethylene­dioxy­thiophene):polystyrene­sulfonate (PEDOT:PSS) composite layer served as an efficient hole extraction layer in hybrid solar cells, which shows excellent power conversion efficiency as maximum as 7.4%. Such performances of the devices are attributed to the unique electrical properties and hole extraction applicability of the GQRs. These results demonstrate the great potential of GQRs for future electronic and other optoelectronic devices

Lithography-Free Broadband Ultrathin-Film Absorbers with Gap-Plasmon Resonance for Organic Photovoltaics

Strategies to confine electromagnetic field within ultrathin film emerge as essential technologies for applications from thin-film solar cells to imaging and sensing devices. We demonstrate a lithography-free, low-cost, large-scale method to realize broadband ultrathin-film metal-dielectric-metal (MDM) absorbers, by exploiting gap-plasmon resonances for strongly confined electromagnetic field. A two-steps method, first organizing Au nanoparticles via thermal dewetting and then transferring the nanoparticles to a spacer–reflector substrate, is used to achieve broader absorption bandwidth by manipulating geometric shapes of the top metallic layer into hemiellipsoids. A fast-deposited nominal Au film, instead of a conventional slow one, is employed in the Ostwald ripening process to attain hemiellipsoidal nanoparticles. A polymer supported transferring step allows a wider range of dewetting temperature to manipulate the nanoparticles’ shape. By incorporating circularity with ImageJ software, the geometries of hemiellipsoidal nanoparticles are quantitatively characterized. Controlling the top geometry of MDM structure from hemisphere to hemiellipsoid increases the average absorption at 500–900 nm from 23.1% to 43.5% in the ultrathin film and full width at half-maximum from 132 nm to 324 nm, which is consistently explained by finite-difference time-domain simulation. The structural advantages of our scheme are easily applicable to thin-film photovoltaic devices because metal electrodes can act as metal reflectors and semiconductor layers as dielectric spacers

Structural Origin of the Band Gap Anomaly of Quaternary Alloy Cd_<i>x</i>Zn_1–<i>x</i>S_<i>y</i>Se_1–<i>y</i> Nanowires, Nanobelts, and Nanosheets in the Visible Spectrum

Single-crystalline alloy II–VI semiconductor nanostructures have been used as functional materials to propel photonic and optoelectronic device performance in a broad range of the visible spectrum. Their functionality depends on the stable modulation of the direct band gap (<i>E</i>_g), which can be finely tuned by controlling the properties of alloy composition, crystallinity, and morphology. We report on the structural correlation of the optical band gap anomaly of quaternary alloy Cd_<i>x</i>Zn_1–<i>x</i>S_<i>y</i>Se_1–<i>y</i> single-crystalline nanostructures that exhibit different morphologies, such as nanowires (NWs), nanobelts (NBs), and nanosheets (NSs), and cover a wide range of the visible spectrum (<i>E</i>_g = 1.96–2.88 eV). Using pulsed laser deposition, the nanostructures evolve from NWs <i>via</i> NBs to NSs with decreasing growth temperature. The effects of the growth temperature are also reflected in the systematic variation of the composition. The alloy nanostructures firmly maintain single crystallinity of the hexagonal wurtzite and the nanoscale morphology, with no distortion of lattice parameters, satisfying the virtual crystal model. For the optical properties, however, we observed distinct structure-dependent band gap anomalies: the disappearance of bowing for NWs and maximum and slightly reduced bowing for NBs and NSs, respectively. We tried to uncover the underlying mechanism that bridges the structural properties and the optical anomaly using an empirical pseudopotential model calculation of electronic band structures. From the calculations, we found that the optical bowings in NBs and NSs were due to residual strain, by which they are also distinguishable from each other: large for NBs and small for NSs. To explain the origin of the residual strain, we suggest a semiempirical model that considers intrinsic atomic disorder, resulting from the bond length mismatch, combined with the strain relaxation factor as a function of the width-to-thickness ratio of the NBs or NSs. The model agreed well with the observed optical bowing of the alloy nanostructures in which a mechanism for the maximum bowing for NBs is explained. The present systematic study on the structural–optical properties correlation opens a new perspective to understand the morphology- and composition-dependent unique optical properties of II–VI alloy nanostructures as well as a comprehensive strategy to design a facile band gap modulation method of preparing photoconverting and photodetecting materials

Nanoscale Pillar-Enhanced Tribological Surfaces as Antifouling Membranes

We present a nonconventional membrane surface modification approach that utilizes surface topography to manipulate the tribology of foulant accumulation on water desalination membranes via imprinting of submicron titanium dioxide (TiO₂) pillar patterns onto the molecularly structured, flat membrane surface. This versatile approach overcomes the constraint of the conventional approach relying on interfacial polymerization that inevitably leads to the formation of ill-defined surface topography. Compared to the nonpatterned membranes, the patterned membranes showed significantly improved fouling resistance for both organic protein and bacterial foulants. The use of hydrophilic TiO₂ as a pattern material increases the membrane hydrophilicity, imparting improved chemical antifouling resistance to the membrane. Fouling behavior was also interpreted in terms of the topographical effect depending on the relative size of foulants to the pattern dimension. In addition, computational fluid dynamics simulation suggests that the enhanced antifouling of the patterned membrane is attributed to the enhancement in overall and local shear stress at the fluid–TiO₂ pattern interface

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 <i>in situ</i> 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

Competition between Charge Transport and Energy Barrier in Injection-Limited Metal/Quantum Dot Nanocrystal Contacts

Injection-limited contacts in many of electronic devices such as light-emitting diodes (LEDs) and field effect transistors (FETs) are not easily avoided. We demonstrate that charge injection in the injection-limited contact is determined by charge transport properties as well as the charge injection energy barrier due to vacuum energy level alignment. Interestingly, injection-limited contact properties were observed at 5 nm diameter lead sulfide (PbS) quantum dot (QD)/Au contacts for which carrier injection is predicted to be energetically favorable. To probe the effect of charge transport properties on carrier injection, the electrical channel resistance of PbS nanocrystal (NC) FETs was varied through thermal annealing, photoillumination, ligand exchange, surface treatment of the gate dielectric, and use of different sized PbS NCs. Injection current through the PbS/Au contact varied with the FET mobility of PbS NC films consistent with a theoretical prediction where the net injection current is dominated by carrier mobility. This result suggests that the charge transport properties, that is, mobility, of QD NC films should be considered as a means to enhance carrier injection along with the vacuum level energy alignment at the interface between QD NCs and metal electrodes. Photocurrent microscopic images of the PbS/Au contact demonstrate the presence of a built-in potential in a two-dimensionally continuous PbS film near the metal electrodes

Simultaneous Enhancement of Upconversion and Downshifting Luminescence via Plasmonic Structure

We describe a metal nanodisk–insulator–metal (MIM) structure that enhances lanthanide-based upconversion (UC) and downshifting (DS) simultaneously. The structure was fabricated using a nanotransfer printing method that facilitates large-area applications of nanostructures for optoelectronic devices. The proposed MIM structure is a promising way to harness the entire solar spectrum by converting both ultraviolet and near-infrared to visible light concurrently through resonant-mode excitation. The overall photoluminescence enhancements of the UC and DS were 174- and 29-fold, respectively