Enhanced conductivity of sol-gel silica cladding for efficient poling in electro-optic polymer/TiO2 vertical slot waveguide modulators.

We report the enhanced conductivity of sol-gel silica under-cladding for efficient poling of electro-optic (EO) polymer in a hybrid EO polymer/TiO2 vertical slot waveguide modulator. The electrical volume conductivity of sol-gel silica cladding increases approximately 30 times when the calcining time of the cladding layer is critically reduced to 45 minutes, which increases the in-device EO coefficient of the 600-nm-thick EO polymer film in modulators and reduces the lower halfwave voltage (Vπ) of the modulators. The lowest driving voltage (Vπ) of the TiO2 slot waveguide modulator is 2.0 V for an electrode length (Le) of 10 mm and wavelength of 1550 nm (VπLe = 2.0 V·cm) for the low-index guest-host EO polymer SEO125. The optical propagation loss is reduced to 7 dB/cm.

Active self-assembly of piezoelectric biomolecular films via synergistic nanoconfinement and in-situ poling.

Piezoelectric biomaterials have attracted great attention owing to the recent recognition of the impact of piezoelectricity on biological systems and their potential applications in implantable sensors, actuators, and energy harvesters. However, their practical use is hindered by the weak piezoelectric effect caused by the random polarization of biomaterials and the challenges of large-scale alignment of domains. Here, we present an active self-assembly strategy to tailor piezoelectric biomaterial thin films. The nanoconfinement-induced homogeneous nucleation overcomes the interfacial dependency and allows the electric field applied in-situ to align crystal grains across the entire film. The ß-glycine films exhibit an enhanced piezoelectric strain coefficient of 11.2 pm V-1 and an exceptional piezoelectric voltage coefficient of 252 × 10-3 Vm N-1. Of particular significance is that the nanoconfinement effect greatly improves the thermostability before melting (192 °C). This finding offers a generally applicable strategy for constructing high-performance large-sized piezoelectric bio-organic materials for biological and medical microdevices.

Silicon-polymer hybrid slot waveguide ring-resonator modulator.

We demonstrate a ring-resonator modulator based on a silicon-polymer hybrid slot waveguide with a tunability of 12.7 pm/V at RF speeds and a bandwidth of 1 GHz, for optical wavelengths near 1550 nm. Our slot waveguides were fabricated with 193 nm optical lithography, as opposed to the electron beam lithography used for previous results. The tunability is comparable to some of the best ring-based modulators making use of the plasma dispersion effect. The speed is likely limited only by resistance in the strip-loading section, and it should be possible to realize significant improvement with improved processing.

Electro-optic modulator with exceptional power-size performance enabled by transparent conducting electrodes.

An EO phase modulator having transparent conducting oxide electrodes and an inverted rib waveguide structure is demonstrated. This new modulator geometry employs an EO polymer having an in-device r33 = 60pm/V. The measured half-wave voltage Vpi of these devices ranges from 5.3V to 11.2V for 3.8 and 1.5 mm long devices, respectively. The lowest VpiL figure-of-merit corresponds to 0.6V-cm (7.2mW-cm(2) of power length product) in a dual-drive configuration. The trade-off between Vpi, insertion loss and modulation bandwidth is systematically analyzed. An optimized high-speed structure is proposed, with numerical simulation showing that this new structure and an in-device r33 = 150pm/V, can achieve Vpi = 0.5V in a 5mm long active length with dual drive operation. The insertion loss is targeted at 6dB, and a 3dB optical modulation bandwidth can reach > 40GHz.

Assembly of nanomaterials through highly ordered self-assembled monolayers and peptide-organic hybrid conjugates as templates.

Synthesis and processing techniques have now been established for obtaining high quality monodisperse nanocrystals of various metallic and semiconducting materials, fullerenes of distinct properties, single- and multi-wall carbon nanotubes, polymeric dendrimers with tailored functionalities, as well as other nanophase constructs. The next key step towards novel applications of nanostructured materials concerns their positioning, arrangement, and connection into functional networks without mutual aggregation. In this review, we highlight the recent progress of using anthracene- and pyrene-based self-assembling molecules with tunable energetic (pi-pi interactions, hydrogen bonding, dipole-dipole interactions) and variable geometries to create stable, highly ordered, and rigid self-assembled monolayer (SAM) templates with adjustable superlattices on crystalline substrates. Based on aromatic SAM templates, stable and highly ordered self-assembled structures of optoelectronically active C60 have been obtained and shown to exhibit desirable electrical and optoelectronic properties, such as nonlinear transporting effect for molecular electronics and efficient photocurrent generation for mimicking photosynthesis in nature. By using genetically engineered polypeptides with surface recognition for specific inorganics, selective integration of nanoparticles onto aromatic SAM templates have also been realized. Through a combination of spatially confined surface chemical reaction and microcontact printing, sub-micron arrays of peptide-organic hybrid conjugates were successfully generated to serve as templates to achieve the patterned assembly of nanoparticles.

Improved Ambient-Stable Perovskite Solar Cells Enabled by a Hybrid Polymeric Electron-Transporting Layer.

In this work, an efficient inverted perovskite solar cell with decent ambient stability is successfully demonstrated by employing an n-type polymer, poly{[N,N'-bis(2-octyldodecyl)-1,4,5,8-naphthalene diimide-2,6-diyl]-alt-5,5'-(2,2'-bithiophene)} (N2200), as the electron-transporting layer (ETL). The device performance can be further enhanced from a power conversion efficiency (PCE) of 15 to 16.8 % by tailoring the electronic properties of N2200 with a polymeric additive, poly[9,9-bis(6'-(N,N-diethylamino)propyl)-fluorene-alt-9,9-bis(3-ethyl(oxetane-3-ethyloxy)-hexyl) fluorene] (PFN-Ox). More importantly, the device derived from this hybrid ETL can maintain good ambient stability inherent from the pristine N2200 ETL, for which 60-70 % of initial PCE can be retained after being stored in air with 10-20 % humidity for 45 days.

pH-dependent, thermosensitive polymeric nanocarriers for drug delivery to solid tumors.

Polymeric micelles are promising carriers for anti-cancer agents due to their small size, ease of assembly, and versatility for functionalization. A current challenge in the use of polymeric micelles is the sensitive balance that must be achieved between stability during prolonged blood circulation and release of active drug at the tumor site. Stimuli-responsive materials provide a mechanism for triggered drug release in the acidic tumor and intracellular microenvironments. In this work, we synthesized a series of dual pH- and temperature-responsive block copolymers containing a poly(ε-caprolactone) (PCL) hydrophobic block with a poly(triethylene glycol) block that were copolymerized with an amino acid-functionalized monomer. The block copolymers formed micellar structures in aqueous solutions. An optimized polymer that was functionalized with 6-aminocaproic acid (ACA) possessed pH-sensitive phase transitions at mildly acidic pH and body temperature. Doxorubicin-loaded micelles formed from these polymers were stable at blood pH (~7.4) and showed increased drug release at acidic pH. In addition, these micelles displayed more potent anti-cancer activity than free doxorubicin when tested in a tumor xenograft model in mice.

Efficient poling of electro-optic polymers in thin films and silicon slot waveguides by detachable pyroelectric crystals.

Pyroelectric crystals are used as a conformal and detachable electric field source to efficiently pole electro-optic (E-O) polymers in both parallel-plate (transverse) and in-plane (quasi-longitudinal) configurations. Large Pockels coefficients in poled thin films and high tunability of resonance wavelength shift in hybrid polymer silicon slot waveguide ring-resonator modulators have been achieved using this method.

High-optical-quality blends of anionic polymethine salts and polycarbonate with enhanced third-order non-linearities for silicon-organic hybrid devices.

A series of anionic polymethine dyes with different aromatic counterions are prepared to improve their compatibility as guests in an amorphous polycarbonate host. When they are used as the cladding material for silicon hybrid slot waveguides, four-wave mixing wavelength conversion and two-photon absorption-based optical-power modulation are observed. Such guest-host materials may be attractive candidates for all-optical signal-processing applications.

Surface characterization of polythiophene:fullerene blends on different electrodes using near edge X-ray absorption fine structure.

We study the top surface composition of blends of the conjugated polymer regioregular poly-3-hexylthiophene (P3HT) with the fullerene (6,6)-phenyl-C(61)-butyric acid methyl ester (PCBM), an important model system for organic photovoltaics (OPVs), using near-edge X-ray absorption fine structure spectroscopy (NEXAFS). We compare the ratio of P3HT to PCBM near the air/film interface that results from preparing blend films on two sets of substrates: (1) poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) coated indium tin oxide (ITO) as is commonly used in conventional OPV structures and (2) ZnO substrates that are either unmodified or modified with a C(60)-like self-assembled monolayer, similar to those that have been recently reported in inverted OPV structures. We find that the top surface (the film/air interface) is enriched in P3HT compared to the bulk, regardless of substrate or annealing conditions, indicating that changes in device performance due to substrate modification treatments should be attributed to the buried substrate/film interface and the bulk of the film rather than the exposed film/air interface.

Utilization of micelles formed from poly(ethylene glycol)-block-poly(epsilon-caprolactone) block copolymers as nanocarriers to enable hydrophobic red two-photon absorbing emitters for cells imaging.

A hydrophobic two-photon absorbing (2PA) red emitter (R) was successfully incorporated into micelles formed from two block copolymers, poly(epsilon-caprolactone)-block-poly(ethylene glycol)s, for imaging and toxicity studies. In micelles, the chromophore R exhibits a 2PA cross-section of 400 GM (1 GM = 1 x 10(-50) cm(4) s photon(-1) molecule(-1)) at 820 nm, which is among the highest values reported for red 2PA emitters. The micelles with a cationic amino moiety-containing poly(ethylene glycol) corona showed an enhancement of cell internalization and delivered the dye into the cytoplasmic regions of the mouse macrophage RAW 264.7 cells. In comparison, the dye in micelles with neutral poly(ethylene glycol) as corona could not be delivered into the cells. Cytotoxicity of the micelle-R constructs was studied using a 3-(4,5-dimethyl thiazol-2-yl)-2,5-diphenyltetrazolium bromide assay. More than 90% of the cells were viable after they were stained with the dye-containing micelles at different concentrations (dye concentrations of 2-6 muM and polymer concentrations of 0.05-0.15 mg/mL) for 16 h. This is the first reported application of a hydrophobic 2,1,3-benzothiadiazole-containing 2PA red emitter delivered into the cytoplasm of cells for bioimaging and toxicity assessment.

Single-cell patterning and adhesion on chemically engineered poly(dimethylsiloxane) surface.

We demonstrate a new approach to achieve single cell arrays using chemically modified poly(dimethylsiloxane) (PDMS) substrates. Four different microwell geometries (ranging from 10 to 50 microm in diameter) and interstitial spacing (ranging from 30 to 250 microm) were fabricated using soft lithography. The surface of each microwell was sputtered with 25 nm of gold and functionally engineered with a self-assembled monolayer (SAM) of (10-mercaptomethyl-9-anthyl)(4-aldehydephenyl)acetylene (MMAAPA), a fused-ring aromatic thiolated molecule. Collagen was covalently bound to the SAM of MMAAPA using Schiff base chemistry. Cells were found to be attracted and adherent to the chemically modified microwells. By tuning the structural parameters, microwells with a diameter of 20 microm and interstitial spacing of 250 microm resulted in single cell arrays. By combining soft lithography and surface engineering, a simple methodology produced single cell arrays on biocompatible substrates.

Molecular biomimetics: nanotechnology through biology.

Proteins, through their unique and specific interactions with other macromolecules and inorganics, control structures and functions of all biological hard and soft tissues in organisms. Molecular biomimetics is an emerging field in which hybrid technologies are developed by using the tools of molecular biology and nanotechnology. Taking lessons from biology, polypeptides can now be genetically engineered to specifically bind to selected inorganic compounds for applications in nano- and biotechnology. This review discusses combinatorial biological protocols, that is, bacterial cell surface and phage-display technologies, in the selection of short sequences that have affinity to (noble) metals, semiconducting oxides and other technological compounds. These genetically engineered proteins for inorganics (GEPIs) can be used in the assembly of functional nanostructures. Based on the three fundamental principles of molecular recognition, self-assembly and DNA manipulation, we highlight successful uses of GEPI in nanotechnology.