Surface Phonon Polariton-Mediated Near-Field Radiative Heat Transfer at Cryogenic Temperatures.

Recent experiments, at room temperature, have shown that near-field radiative heat transfer (NFRHT) via surface phonon polaritons (SPhPs) exceeds the blackbody limit by several orders of magnitude. Yet, SPhP-mediated NFRHT at cryogenic temperatures remains experimentally unexplored. Here, we probe thermal transport in nanoscale gaps between a silica sphere and a planar silica surface from 77-300 K. These experiments reveal that cryogenic NFRHT has strong contributions from SPhPs and does not follow the T^{3} temperature (T) dependence of far-field thermal radiation. Our modeling based on fluctuational electrodynamics shows that the temperature dependence of NFRHT can be related to the confinement of heat transfer to two narrow frequency ranges and is well accounted for by a simple analytical model. These advances enable detailed NFRHT studies at cryogenic temperatures that are relevant to thermal management and solid-state cooling applications.

Probing the Limits to Near-Field Heat Transfer Enhancements in Phonon-Polaritonic Materials.

Near-field radiative heat transfer (NFRHT) arises between objects separated by nanoscale gaps and leads to dramatic enhancements in heat transfer rates compared to the far-field. Recent experiments have provided first insights into these enhancements, especially using silicon dioxide (SiO2) surfaces, which support surface phonon polaritons (SPhP). Yet, theoretical analysis suggests that SPhPs in SiO2 occur at frequencies far higher than optimal. Here, we first show theoretically that SPhP-mediated NFRHT, at room temperature, can be 5-fold larger than that of SiO2, for materials that support SPhPs closer to an optimal frequency of 67 meV. Next, we experimentally demonstrate that MgF2 and Al2O3 closely approach this limit. Specifically, we demonstrate that near-field thermal conductance between MgF2 plates separated by 50 nm approaches within nearly 50% of the global SPhP bound. These findings lay the foundation for exploring the limits to radiative heat transfer rates at the nanoscale.

Anisotropic Plasmonic Gold Nanorod-Indocyanine Green@Reduced Graphene Oxide-Doxorubicin Nanohybrids for Image-Guided Enhanced Tumor Theranostics.

The unique physicochemical and localized surface plasmon resonance assets of gold nanorods (GNRs) have offered combined cancer treatments with real-time diagnosis by integrating diverse theragnostic modalities into a single nanoplatform. In this work, a unique multifunctional nanohybrid material based on GNRs was designed for in vitro and in vivo tumor imaging along with synergistic and combinatorial therapy of tumor. The hybrid material with size less than 100 nm was achieved by embedding indocyanine green (ICG) on mesoporous silica-coated GNRs with further wrapping of reduced graphene oxide (rGO) and then attached with doxorubicin (DOX) and polyethylene glycol. The nanohybrid unveiled noteworthy stability and competently protected the embedded ICG from further aggregation, photobleaching, and nucleophilic attack by encapsulation of GNRs-ICG with rGO. Such combination of GNRs-ICG with rGO and DOX served as a real-time near-infrared (NIR) contrast imaging agent for cancer diagnosis. The hybrid material exhibits high NIR absorption property along with three destined capabilities, such as, nanozymatic activity, photothermal activity, and an excellent drug carrier for drug delivery. The integrated properties of the nanohybrid were then utilized for the triple mode of combined therapeutics of tumor cells, through synergistic catalytic therapy and chemotherapy with combinatorial photothermal therapy to achieve the maximum cancer killing efficiency. It is assumed that the assimilated multimodal imaging and therapeutic capability in single nanoparticle platform is advantageous for future practical applications in cancer diagnosis, therapy, and molecular imaging.

Quantitative Mapping of Unmodulated Temperature Fields with Nanometer Resolution.

Quantitative mapping of temperature fields with nanometric resolution is critical in various areas of scientific research and emerging technology, such as nanoelectronics, surface chemistry, plasmonic devices, and quantum systems. A key challenge in achieving quantitative thermal imaging with scanning thermal microscopy (SThM) is the lack of knowledge of the tip-sample thermal resistance (RTS), which varies with local topography and is critical for quantifying the sample temperature. Recent advances in SThM have enabled simultaneous quantification of RTS and topography in situations where the temperature field is modulated enabling quantitative thermometry even when topographical features cause significant variations in RTS. However, such an approach is not applicable to situations where the temperature modulation of the device is not readily possible. Here we show, using custom-fabricated scanning thermal probes (STPs) with a sharp tip (radius ∼25 nm) and an integrated heater/thermometer, that one can quantitatively map unmodulated temperature fields, in a single scan, with ∼7 nm spatial resolution and ∼50 mK temperature resolution in a bandwidth of 1 Hz. This is accomplished by introducing a modulated heat input to the STP and measuring the AC and DC responses of the probe's temperature which allow for simultaneous mapping of the tip-sample thermal resistance and sample surface temperature. The approach presented here─contact resistance resolved scanning thermal microscopy (CR-SThM)─can greatly facilitate temperature mapping of a variety of microdevices under practical operating conditions.

Near-field thermophotovoltaics for efficient heat to electricity conversion at high power density.

Thermophotovoltaic approaches that take advantage of near-field evanescent modes are being actively explored due to their potential for high-power density and high-efficiency energy conversion. However, progress towards functional near-field thermophotovoltaic devices has been limited by challenges in creating thermally robust planar emitters and photovoltaic cells designed for near-field thermal radiation. Here, we demonstrate record power densities of ~5 kW/m2 at an efficiency of 6.8%, where the efficiency of the system is defined as the ratio of the electrical power output of the PV cell to the radiative heat transfer from the emitter to the PV cell. This was accomplished by developing novel emitter devices that can sustain temperatures as high as 1270 K and positioning them into the near-field (<100 nm) of custom-fabricated InGaAs-based thin film photovoltaic cells. In addition to demonstrating efficient heat-to-electricity conversion at high power density, we report the performance of thermophotovoltaic devices across a range of emitter temperatures (~800 K-1270 K) and gap sizes (70 nm-7 µm). The methods and insights achieved in this work represent a critical step towards understanding the fundamental principles of harvesting thermal energy in the near-field.

Microwatt-Resolution Calorimeter for Studying the Reaction Thermodynamics of Nanomaterials at High Temperature and Pressure.

Calorimetry of reactions involving nanomaterials is of great current interest, but requires high-resolution heat flow measurements and long-term thermal stability. Such studies are especially challenging at elevated reaction pressures and temperatures. Here, we present an instrument for measuring the enthalpy of reactions between gas-phase reactants and milligram scale nanomaterial samples. This instrument can resolve the net change in the amount of gas-phase reactants due to surface reactions in an operating range from room temperature to 300 °C and reaction pressures of 10 mbar to 30 bar. The calorimetric resolution is shown to be <3 µW/√Hz, with a long-term stability <4 µW/hour. The performance of the instrument is demonstrated via a set of experiments involving H2 absorption on Pd nanoparticles at various pressures and temperatures. For this specific reaction, we obtained a mass balance resolution of 0.1 µmol/√Hz. Results from these experiments are in good agreement with past studies establishing the feasibility of performing high resolution calorimetry on milligram scale nanomaterials, which can be employed in future studies probing catalysis, phase transformations, and thermochemical energy storage.

Towards efficient and stable perovskite solar cells employing non-hygroscopic F4-TCNQ doped TFB as the hole-transporting material.

Designing an efficient and stable hole transport layer (HTL) material is one of the essential ways to improve the performance of organic-inorganic perovskite solar cells (PSCs). Herein, for the first time, an efficient model of a hole transport material (HTM) is demonstrated by optimized doping of a conjugated polymer TFB (poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4'-(N-(4-sec-butylphenyl)diphenylamine)]) with a non-hygroscopic p-type dopant F4-TCNQ (2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane) for high-efficiency PSCs. The PSC with the F4-TCNQ doped TFB exhibits the best power conversion efficiency (PCE) of 17.46%, which surpasses that of the reference devices, i.e., 16.64 (LiTFSI + TBP-doped Spiro-OMeTAD as the HTM) and 11.01% (LiTFSI + TBP-doped TFB as the HTM). F4-TCNQ doped TFB was believed to favor efficient charge and energy transfer between the perovskite and the hole transport layer and to reduce charge recombination as evidenced by steady-state photoluminescence (PL) and time-resolved photoluminescence (TRPL) analysis. Moreover, the hydrophobic nature of F4-TCNQ contributed to enhancing the stability of the device under ambient conditions with a RH of 45%. The device reported herein retained ca. 80% of its initial efficiency after 10 days, significantly superior to both LiTFSI + TBP-doped Spiro-OMeTAD (ca. 30%) and LiTFSI + TBP-doped TFB (ca. 10%) based counterparts. This simple yet novel strategy paves the way for demonstrating a promising route for a wide range of highly efficient solar cells and other photovoltaic applications.

Solution-Processed PEDOT:PSS/MoS2 Nanocomposites as Efficient Hole-Transporting Layers for Organic Solar Cells.

An efficient hole-transporting layer (HTL) based on functionalized two-dimensional (2D) MoS2-poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) composites has been developed for use in organic solar cells (OSCs). Few-layer, oleylamine-functionalized MoS2 (FMoS2) nanosheets were prepared via a simple and cost-effective solution-phase exfoliation method; then, they were blended into PEDOT:PSS, a conducting conjugated polymer, and the resulting hybrid film (PEDOT:PSS/FMoS2) was tested as an HTL for poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) OSCs. The devices using this hybrid film HTL showed power conversion efficiencies up to 3.74%, which is 15.08% higher than that of the reference ones having PEDOT:PSS as HTL. Atomic force microscopy and contact angle measurements confirmed the compatibility of the PEDOT:PSS/FMoS2 surface for active layer deposition on it. The electrical impedance spectroscopy analysis revealed that their use minimized the charge-transfer resistance of the OSCs, consequently improving their performance compared with the reference cells. Thus, the proposed fabrication of such HTLs incorporating 2D nanomaterials could be further expanded as a universal protocol for various high-performance optoelectronic devices.

Synergistic Nanozymetic Activity of Hybrid Gold Bipyramid-Molybdenum Disulfide Core@Shell Nanostructures for Two-Photon Imaging and Anticancer Therapy.

In recent years, the concept of combined therapy using gold hybrid nanomaterials has been broadly adopted to pioneer new anticancer treatments. However, their synergistic anticancer effects have yet to be thoroughly investigated. Herein,a hybrid gold nanobipyramid nanostructure coated with molybdenum disulfide (MoS2) semiconductor (AuNBPs@MoS2) was proposed as a smart nanozyme for anticancer therapy and two-photon bioimaging. The hybrid material showed dramatically enhanced localized surface plasmon resonance property under excitation owing to its anisotropic nature, coupled with the rich electron density in MoS2, resulting in the superior in situ photogeneration of reactive oxidative species (ROS - 1O2, •OH). We demonstrated that the synergistic effect of enhanced photothermal conversion and generation of ROS could increase the anticancer effect of AuNBPs@MoS2. Two-photon luminescence imaging confirmed that AuNBPs@MoS2 was successfully internalized in cancer cells and that simultaneous anticancer treatments based on catalytic and photothermal therapy could be achieved. This study highlighted, for the first time, a novel approach of plasmon-mediated powerful anticancer therapy and imaging via the unprecedented combination of anisotropic AuNBPs and two-dimensional MoS2 material.

Viable stretchable plasmonics based on unidirectional nanoprisms.

Well-defined ordered arrays of plasmonic nanostructures were fabricated on stretchable substrates and tunable plasmon-coupling-based sensing properties were comprehensively demonstrated upon extension and contraction. Regular nanoprism patterns consisting of Ag, Au and Ag/Au bilayers were constructed on the stretchable polydimethylsiloxane substrate. The nanoprisms had the same orientation over the entire substrate (3 × 3 cm2) via metal deposition on a single-crystal microparticle monolayer assembly. The plasmonic sensor based on the Ag/Au bilayer showed a 6-fold enhanced surface enhanced Raman scattering signal under 20% uniaxial extension, whereas a 3-fold increase was observed upon 6% contraction, compared with the Au nanoprism arrays. The sensory behaviors were corroborated by finite-difference time-domain simulation, demonstrating the tunable electromagnetic field enhancement effect via the localized surface plasmon resonance coupling. The advanced flexible plasmonic-coupling-based devices with tunable and quantifiable performance herein suggested are expected to unlock promising potential in practical bio-sensing, biotechnological applications and optical devices.

Plasmonic Periodic Nanodot Arrays via Laser Interference Lithography for Organic Photovoltaic Cells with >10% Efficiency.

In this study, we demonstrate a viable and promising optical engineering technique enabling the development of high-performance plasmonic organic photovoltaic devices. Laser interference lithography was explored to fabricate metal nanodot (MND) arrays with elaborately controlled dot size as well as periodicity, allowing spectral overlap between the absorption range of the active layers and the surface plasmon band of MND arrays. MND arrays with ∼91 nm dot size and ∼202 nm periodicity embedded in a poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) hole transport layer remarkably enhanced the average power conversion efficiency (PCE) from 7.52% up to 10.11%, representing one of the highest PCE and degree of enhancement (∼34.4%) levels compared to the pristine device among plasmonic organic photovoltaics reported to date. The plasmonic enhancement mechanism was investigated by both optical and electrical analyses using finite difference time domain simulation and conductive atomic force microscopy studies.

Reduced graphene oxide wrapped core-shell metal nanowires as promising flexible transparent conductive electrodes with enhanced stability.

Transparent conductive electrodes (TCEs) are widely used in a wide range of optical-electronic devices. Recently, metal nanowires (NWs), e.g. Ag and Cu, have drawn attention as promising flexible materials for TCEs. Although the study of core-shell metal NWs, and the encapsulation/overcoating of the surface of single-metal NWs have separately been an object of focus in the literature, herein for the first time we simultaneously applied both strategies in the fabrication of highly stable Ag-Cu NW-based TCEs by the utilization of Ag nanoparticles covered with reduced graphene oxide (rGO). The incorporation of Ag nanoparticles by galvanic displacement reaction was shown to significantly increase the long term stability of the electrode. Upon comparison with a CuNW reference, our novel rGO/Cu-AgNW-based TCEs unveiled remarkable opto-electrical properties, with a 3-fold sheet resistance decrease (from 29.8 Ω sq-1 to 10.0 Ω sq-1) and an impressive FOM value (139.4). No detrimental effect was noticed in the relatively high transmittance value (T = 77.6% at 550 nm) characteristic of CuNWs. In addition, our rGO/Cu-AgNW-based TCEs exhibited outstanding thermal stability up to 20 days at 80 °C in air, as well as improved mechanical flexibility. The superior performance herein reported compared with both CuNWs and AgNWs, and with a current conventional ITO reference, is believed to highlight the great potential of these novel materials as promising alternatives in optical-electronic devices.

Comprehensive Study on the Controlled Plasmon-Enhanced Photocatalytic Activity of Hybrid Au/ZnO Systems Mediated by Thermoresponsive Polymer Linkers.

Hybrid semiconductor/noble metal nanostructures coupled with responsive polymers were used to probe unique plasmon-mediated photocatalytic properties associated with swelling-shrinking transitions in polymer chains triggered by specific external stimuli. Poly(N-isopropylacrylamide) (PNIPAM) brushes were anchored on Au films by atom transfer radical polymerization and ZnO nanoparticles were immobilized on the PNIPAM layer to explore controlled photocatalytic activity. The plasmon-enhanced photocatalytic activity was dictated by two critical parameters, that is, grafting density and molecular weight of PNIPAM involved in Au film-PNIPAM-ZnO. The effect of the areal density of PNIPAM chains on the temperature-responsive UV light photocatalytic activities showed mutually antagonistic trends at two different temperatures. The performance at high density was higher above a lower critical solution temperature (LCST), that is, under contracted configuration, while the sample with low density showed higher activity below LCST, that is, extended configuration. Among all the cases explored, the UV light activity was highest for the sample with thin PNIPAM layer and high density above LCST. The visible light activity was induced only for thin PNIPAM layer and high density, and it was higher above LCST. The efficiency of photocatalytic decomposition of phenol pollutant was dramatically enhanced from 10% to 55% upon the increase in temperature under visible light illumination.

Enhanced light scattering and trapping effect of Ag nanowire mesh electrode for high efficient flexible organic solar cell.

Ag nanowire (NW) mesh is used as transparent conducting electrode for high efficient flexible organic solar cells (OSCs). The Ag NW mesh electrode facilitates light scattering and trapping, allowing enhancement of light absorption in the active layer. OSCs incorporating Ag NW mesh electrode exhibit maximum power conversion efficiency (PCE) of 4.47%, 25%, higher than that of OSCs with a conventional ITO electrode (3.63%).

Effect of geometric lattice design on optical/electrical properties of transparent silver grid for organic solar cells.

Silver (Ag) grid transparent electrode is one of the most promising transparent conducting electrodes (TCEs) to replace conventional indium tin oxide (ITO). We systematically investigate an effect of geometric lattice modifications on optical and electrical properties of Ag grid electrode. The reference Ag grid with 5 µm width and 100 µm pitch (duty of 0.05) prepared by conventional photo-lithography and lift-off processes shows the sheet resistance of 13.27 Ω/sq, transmittance of 81.1%, and resultant figure of merit (FOM) of 129.05. Three different modified Ag grid electrodes with stripe added-mesh (SAM), triangle-added mesh (TAM), and diagonal-added mesh (DAM) are suggested to improve optical and electrical properties. Although all three of SAM, TAM, and DAM Ag grid electrodes exhibit the lower transmittance values of about 72 - 77%, they showed much decreased sheet resistance of 6 - 8 Ω/sq. As a result, all of the lattice-modified Ag grid electrodes display significant improvement of FOM and the highest value of 171.14 is obtained from DAM Ag grid, which is comparable to that of conventional ITO electrode (175.46). Also, the feasibility of DAM Ag gird electrode for use in organic solar cell is confirmed by finite difference time domain (FDTD) simulations. Unlike a conventional ITO electrode, DAM Ag grid electrode can induce light scattering and trapping due to the diffuse transmission that compensates for the loss in optical transparency, resulting in comparable light absorption in the photo active layer of poly(3-hexylthiophene) (P3HT): [6,6]-phenyl-C61-butyric acid methyl ester (PC60BM). P3HT:PC60BM based OSCs with the DAM Ag grid electrode were fabricated, which also showed the potential for ITO-free transparent electrode.

Does the presence of hypoechoic lesions on transrectal ultrasound suggest a poor prognosis for patients with localized prostate cancer?

PURPOSE: The purpose of this study was to investigate the value of hypoechoic lesions on transrectal ultrasound (TRUS) as a prognostic factor for patients with localized prostate cancer. MATERIALS AND METHODS: The patients consisted of 71 patients with pT2N0M0 disease following radical prostatectomy between 2002 and 2008. The group with hypoechoic lesions was labeled group 1, whereas the group without hypoechoic lesions was labeled group 2. The presence of hypoechoic lesions on preoperative TRUS was analyzed as a prognostic factor along with several parameters, including preoperative factors and pathologic factors. The biochemical progression-free survival (BPFS) rate was compared between the two groups according to the presence of hypoechoic lesions on TRUS. RESULTS: A total of 35 patients had hypoechoic lesions on TRUS, whereas 36 had no hypoechoic lesions. Preoperative baseline characteristics were not significantly different between the two groups. In the univariate analysis, BPFS showed significant differences according to the presence of hypoechoic lesions on TRUS and the preoperative prostate-specific antigen level. The BPFS rates over the first 24 months were 97.0% in group 1 and 97.1% in group 2; however, the difference in the BPFS rate over 48 months significantly widened to 75.3% compared with 91.7%, respectively. Despite this finding, no significant independent prognostic factor for BPFS was found on multivariate analysis in this patient cohort. CONCLUSIONS: The presence of hypoechoic lesions on TRUS may suggest worse prognostic characteristics in pT2 prostate cancer. Further studies involving larger subject populations are needed to corroborate the significance of the presence of hypoechoic lesions as a prognostic factor.

Use of acid-suppressive drugs and risk of pneumonia: a systematic review and meta-analysis.

BACKGROUND: Observational studies and randomized controlled trials have yielded inconsistent findings about the association between the use of acid-suppressive drugs and the risk of pneumonia. We performed a systematic review and meta-analysis to summarize this association. METHODS: We searched three electronic databases (MEDLINE [PubMed], Embase and the Cochrane Library) from inception to Aug. 28, 2009. Two evaluators independently extracted data. Because of heterogeneity, we used random-effects meta-analysis to obtain pooled estimates of effect. RESULTS: We identified 31 studies: five case-control studies, three cohort studies and 23 randomized controlled trials. A meta-analysis of the eight observational studies showed that the overall risk of pneumonia was higher among people using proton pump inhibitors (adjusted odds ratio [OR] 1.27, 95% confidence interval [CI] 1.11-1.46, I(2) 90.5%) and histamine(2) receptor antagonists (adjusted OR 1.22, 95% CI 1.09-1.36, I(2) 0.0%). In the randomized controlled trials, use of histamine(2) receptor antagonists was associated with an elevated risk of hospital-acquired pneumonia (relative risk 1.22, 95% CI 1.01-1.48, I(2) 30.6%). INTERPRETATION: Use of a proton pump inhibitor or histamine(2) receptor antagonist may be associated with an increased risk of both community- and hospital-acquired pneumonia. Given these potential adverse effects, clinicians should use caution in prescribing acid-suppressive drugs for patients at risk.