Co-reporter:Xi Chen, Cheng Zhang, Fan Yang, Gaofeng Liang, Qiaochu Li, and L. Jay Guo
ACS Nano October 24, 2017 Volume 11(Issue 10) pp:9863-9863
Publication Date(Web):October 2, 2017
DOI:10.1021/acsnano.7b03584
In this work, a special hyperbolic metamaterial (HMM) metamaterial is investigated for plasmonic lithography of period reduction patterns. It is a type II HMM (ϵ∥ < 0 and ϵ⊥ > 0) whose tangential component of the permittivity ϵ∥ is close to zero. Due to the high anisotropy of the type II epsilon-near-zero (ENZ) HMM, only one plasmonic mode can propagate horizontally with low loss in a waveguide system with ENZ HMM as its core. This work takes the advantage of a type II ENZ HMM composed of aluminum/aluminum oxide films and the associated unusual mode to expose a photoresist layer in a specially designed lithography system. Periodic patterns with a half pitch of 58.3 nm were achieved due to the interference of third-order diffracted light of the grating. The lines were 1/6 of the mask with a period of 700 nm and ∼1/7 of the wavelength of the incident light. Moreover, the theoretical analyses performed are widely applicable to structures made of different materials such as silver as well as systems working at deep ultraviolet wavelengths including 193, 248, and 365 nm.Keywords: epsilon near zero; hyperbolic metamaterial; interference; nanomanufacturing; spatial filtering; UV lithography;
Co-reporter:Cheng Zhang;Nathaniel Kinsey;Long Chen;Chengang Ji;Mingjie Xu;Marcello Ferrera;Xiaoqing Pan;Vladimir M. Shalaev;Alexra Boltasseva and
Advanced Materials 2017 Volume 29(Issue 19) pp:
Publication Date(Web):2017/05/01
DOI:10.1002/adma.201605177
The field of nanophotonics has ushered in a new paradigm of light manipulation by enabling deep subdiffraction confinement assisted by metallic nanostructures. However, a key limitation which has stunted a full development of high-performance nanophotonic devices is the typical large losses associated with the constituent metals. Although silver has long been known as the highest quality plasmonic material for visible and near infrared applications, its usage has been limited due to practical issues of continuous thin film formation, stability, adhesion, and surface roughness. Recently, a solution is proposed to the above issues by doping a proper amount of aluminum during silver deposition. In this work, the potential of doped silver for nanophotonic applications is presented by demonstrating several high-performance key nanophotonic devices. First, long-range surface plasmon polariton waveguides show propagation distances of a few centimeters. Second, hyperbolic metamaterials consisting of ultrathin Al-doped Ag films are attained having a homogeneous and low-loss response, and supporting a broad range of high-k modes. Finally, transparent conductors based on Al-doped Ag possess both a high and flat transmittance over the visible and near-IR range.
Co-reporter:Taehwa Lee
Advanced Optical Materials 2017 Volume 5(Issue 2) pp:
Publication Date(Web):2017/01/01
DOI:10.1002/adom.201600421
Photoacoustic (PA) conversion of metal film absorbers is known to be inefficient because of their low thermal expansion and high optical reflection, as compared to polymeric materials containing light absorbing fillers. Here, highly efficient PA conversion is demonstrated in metal films. By using a metal film absorber sandwiched by transparent polymer layers, PA conversion is significantly enhanced, which is even comparable to in the highest reported in the CNT-polymer composites. Such enhancement is accomplished by ultrathin metal film (10 nm) capable of facilitating heat transfer to the adjacent polymers having high thermal expansion coefficient. This thin metal layer also allows integration of a photonic resonance cavity, effectively compensating the potential absorption loss of the thin metal. This strategy allows for easy spatial PA signal patterns and high conversion efficiency, which not only can be implemented for deep tissue PA imaging of implants or tools, but also provides a guideline for designing photoacoustic transmitters and contrast agents.
Co-reporter:Chengang Ji;Kyu-Tae Lee;Ting Xu;Jing Zhou;Hui Joon Park
Advanced Optical Materials 2017 Volume 5(Issue 20) pp:
Publication Date(Web):2017/10/01
DOI:10.1002/adom.201700368
AbstractRecent advances in fabrication and processing methods have spurred many breakthroughs in the field of nanostructures that provide novel ways of manipulating light interaction on a well controllable manner, thereby enabling a wide variety of innovative applications. Structural colors have shown great promise as an alternative for existing colorant-based filters due to their noticeable advantages, which open up diverse potential applications such as energy-efficient displays, ultrahigh-resolution imaging, ultrahigh-sensitivity biosensors, and building-integrated photovoltaics. Broadband perfect absorbers, which exploit extraordinary optical phenomena at subwavelength scale, have also received increasing attention due to their capability of improving efficiency and performance characteristics of various applications including thermoelectrics, invisibility, solar-thermal-energy harvesting, and imaging. This review highlights some recent progress in these two related fields. The structural colors based on optical resonances in thin-film structures, guided-mode resonances in slab waveguide gratings, and surface plasmon resonances in plasmonic nanoresonators are described. Representative achievements associated with the broadband perfect absorbers, which include schemes employing highly absorbing media, multi-cavity resonances, and broadband impedance matching are investigated.
Co-reporter:Jing Zhou;Xi Chen
Advanced Materials 2016 Volume 28( Issue 15) pp:3017-3023
Publication Date(Web):
DOI:10.1002/adma.201505451
Co-reporter:Bushra Bari, Jinhwan Lee, Taehee Jang, Phillip Won, Seung Hwan Ko, Khalid Alamgir, Muhammad Arshad and L. Jay Guo
Journal of Materials Chemistry A 2016 vol. 4(Issue 29) pp:11365-11371
Publication Date(Web):05 Jul 2016
DOI:10.1039/C6TA03308C
Solution-processed silver nanowire (AgNW) random mesh is a strong contender to commercial indium tin oxide (ITO); however, its performance is limited due to large contact resistance between nanowires and post-processing treatments. As an alternative, long nanowires can decrease the number of contact points and contact resistance. Here, a simple modified hydrothermal method for the synthesis of very-long silver nanowires (AgNWs) and their use in a high quality transparent conducting electrode without post-processing has been developed. Well dispersed very-long and thin silver nanowires are synthesized by using glucose as a reducing agent and silver chloride as a silver source. The lengths of the wires are in the range of 200 to 500 μm with an average diameter of 45–65 nm. To the best of our knowledge, this is the first report on long nanowires having a thin diameter with greater than 200 microns length. As compared to other transparent conductors and nanowire networks, this AgNW network shows a higher percolative figure of merit (FoM, Π) with low haze. A flexible touch screen using the AgNW network is also demonstrated which has shown good performance even on a bendable surface.
Co-reporter:Cheng Zhang, Harish Subbaraman, Qiaochu Li, Zeyu Pan, Jong G. Ok, Tao Ling, Chi-Jui Chung, Xingyu Zhang, Xiaohui Lin, Ray T. Chen and L. Jay Guo
Journal of Materials Chemistry A 2016 vol. 4(Issue 23) pp:5133-5153
Publication Date(Web):18 Apr 2016
DOI:10.1039/C6TC01237J
In order to manufacture large-scale photonic devices of various dimensions at a low cost, a number of patterning techniques have been developed. Nanoimprint lithography is among the most promising given its unique advantages, such as high resolution, fast processing speed, high throughput, compatibility with diverse materials, and low cost. This review covers various aspects of nanoimprint lithography, including its operational principles, material requirements, and different ways of implementation. Nanoimprint lithography facilitates numerous high-performance and low-cost photonic elements, including optical interconnects, sensors, solar cells, and metamaterials. In addition, other related patterning techniques, together with their utilization for photonic device fabrication and their integration with nanoimprint lithography, are briefly discussed.
Co-reporter:Xi Chen, Fan Yang, Cheng Zhang, Jing Zhou, and L. Jay Guo
ACS Nano 2016 Volume 10(Issue 4) pp:4039
Publication Date(Web):April 13, 2016
DOI:10.1021/acsnano.5b06137
Plasmonic lithography, which utilizes subwavelength confinement of surface plasmon polartion (SPP) waves, has the capability of breaking the diffraction limit and delivering high resolution. However, all previously reported results suffer from critical issues, such as shallow pattern depth and pattern nonuniformity even over small exposure areas, which limit the application of the technology. In this work, periodic patterns with high aspect ratios and a half-pitch of about 1/6 of the wavelength were achieved with pattern uniformity in square centimeter areas. This was accomplished by designing a special mask and photoresist (PR) system to select a single high spatial frequency mode and incorporating the PR into a waveguide configuration to ensure uniform light exposure over the entire depth of the photoresist layer. In addition to the experimental progress toward large-scale applications of plasmonic interference lithography, the general criteria of designing such an exposure system is also discussed, which can be used for nanoscale fabrication in this fashion for various applications with different requirements for wavelength, pitch, aspect ratio, and structure.Keywords: interference; nanomanufacturing; next-generation lithography; optical waveguide; plasmonics; spatial filtering; UV lithography
Co-reporter:Chenying Yang, Chengang Ji, Weidong Shen, Kyu-Tae Lee, Yueguang Zhang, Xu Liu, and L. Jay Guo
ACS Photonics 2016 Volume 3(Issue 4) pp:
Publication Date(Web):March 3, 2016
DOI:10.1021/acsphotonics.5b00689
We present a novel scheme for ultrabroadband and omnidirectional perfect absorbers with compact multilayer film structure. Our proposed device shows an average absorption of ∼98% over a wide range of wavelengths ranging from 400 to 2000 nm. The ultrabroadband characteristics are achieved with strongly overlapped optical resonances by designing a tandem structure composed of three absorptive materials, while the overall structure features a graded refractive index profile to obtain a wideband antireflection property. In addition to the high efficiency and ultrabroadband absorption, our perfect absorbers exhibit a great angular tolerance up to 60°, which is attributed to not only relatively broad resonances but also negligible propagation phase shifts in ultrathin highly absorbing layers. Lastly, we explore the effect of the number of semiconductor–metal stacks on the performance of the absorber. The presented approach can have tremendous potential for various applications, such as solar–thermal energy harvesting, thermoelectrics, detection, and imaging.
Co-reporter:Dewei Zhao;Cheng Zhang;Hyunsoo Kim
Advanced Energy Materials 2015 Volume 5( Issue 17) pp:
Publication Date(Web):
DOI:10.1002/aenm.201500768
A efficient indium tin oxide (ITO)-free transparent electrode based on an improved Ag film is designed by introducing small amount of Al during co-deposition, producing ultrathin and smooth Ag film with low loss. A transparent electrode as thin as 4 nm is achieved by depositing the film on top of Ta2O5 layer, and organic solar cells based on such ultrathin electrode are built, producing power conversion efficiency over 7%. The device efficiency can be optimized by simply tuning Ta2O5 layer thickness external to the organic photovoltaic (OPV) structure to create an optical cavity resonance inside the photoactive layer. Therefore Ta2O5/Al-doped Ag films function as a high-performance electrode with high transparency, low resistance, improved photon management capability and mechanical flexibility.
Co-reporter:Moon Kyu Kwak, Jong G. Ok, Sung Ho Lee and L. Jay Guo
Materials Horizons 2015 vol. 2(Issue 1) pp:86-90
Publication Date(Web):30 Sep 2014
DOI:10.1039/C4MH00159A
The fabrication of a large area mold is one of the most difficult challenges in the roll to roll nanoimprint field and eliminating the visible seams on a large area mold is even more difficult. We present a visually tolerable tiling (VTT) method to make large area micro/nano-patterns without visible seams and their applications. In this study, with small area micro/nano-pattern molds, the VTT method could produce a scaled up mold with the same features as the original mold without any visible seam lines. Also, a fabricated large size mold was used in the roll to roll imprinting process as a flexible mold. By using the VTT method, large size metal wire grid polarizers and micro-prism sheets were fabricated and their potential was confirmed as feasible applications.
Co-reporter:Kyu-Tae Lee, Masanori Fukuda, Suneel Joglekar and L. Jay Guo
Journal of Materials Chemistry A 2015 vol. 3(Issue 21) pp:5377-5382
Publication Date(Web):27 Apr 2015
DOI:10.1039/C5TC00622H
Multi-functional solar cells that can create semitransparency or desired colors have recently garnered much attention due to their potential in incorporating aesthetic functionalities into building envelopes, such as windows, facades, and walls, allowing large surfaces of the buildings to be utilized for light-harvesting. Here, we present optical microcavity-embedded decorative perovskite solar cells producing distinctive transmissive colors. Any individual semitransparent colors can be created by strong interference effects in the microcavity system, which are easily tuned by changing the thickness of the optical spacer layer of the microcavity. Our colored cell devices show a power conversion efficiency of up to ∼4% with high internal quantum efficiency that is attributed to greatly minimized charge carrier recombination. This approach could bring us one step closer towards energy-saving ultra-thin color display devices and power generating panels for building decoration.
Co-reporter:Hyoung Won Baac, Jong G. Ok, Taehwa Lee and L. Jay Guo
Nanoscale 2015 vol. 7(Issue 34) pp:14460-14468
Publication Date(Web):20 Jul 2015
DOI:10.1039/C5NR03769G
We demonstrate nano-structural characteristics of carbon nanotube (CNT)–polydimethylsiloxane (PDMS) composite films that can be used as highly efficient and robust ultrasound transmitters for diagnostic and therapeutic applications. An inherent architecture of the nano-composite provides unique thermal, optical, and mechanical properties that are accommodated not just for efficient energy conversion but also for extraordinary robustness against pulsed laser ablation. First, we explain a thermoacoustic transfer mechanism within the nano-composite. CNT morphologies are examined to determine a suitable arrangement for heat transfer to the surrounding PDMS. Next, we introduce an approach to enhance optical extinction of the composite films, which uses shadowed deposition of a thin Au layer through an as-grown CNT network. Finally, the transmitter robustness is quantified in terms of laser-induced damage threshold. This reveals that the CNT–PDMS films can withstand an order-of-magnitude higher optical fluence (and extinction) than a Cr film used as a reference. Such robustness is crucial to increase the maximum-available optical energy for optoacoustic excitation and pressure generation. All of these structure-originated characteristics manifest the CNT–PDMS composite films as excellent optoacoustic transmitters for high-amplitude and high-frequency ultrasound generation.
Co-reporter:Kyu-Tae Lee;Sungyong Seo
Advanced Optical Materials 2015 Volume 3( Issue 3) pp:347-352
Publication Date(Web):
DOI:10.1002/adom.201400533
Co-reporter:Hui Joon Park, L. Jay Guo
Chinese Chemical Letters 2015 Volume 26(Issue 4) pp:419-425
Publication Date(Web):April 2015
DOI:10.1016/j.cclet.2015.02.001
In this article, the optical enhancement effects of plasmonic nanostructures on OPV cells were reviewed as an effective way to resolve the mismatch problems between the short exciton diffusion length in organic semiconductors (around 10 nm) and the large thickness required to fully absorb sunlight (e.g. hundreds of nanometers). Especially, the performances of OPVs with plasmonic nanoparticles in photoactive and buffer layers and with periodic nanostructures were investigated. Furthermore, nanoimprint lithography-based nanofabrication processes that can easily control the dimension and uniformity of structures for large-area and uniform plasmonic nanostructures were demonstrated.In this article, the optical enhancement effects of plasmonic nanostructures on OPV cells were reviewed as an effective way to resolve the mismatch problems between the short exciton diffusion length in organic semiconductors (around 10 nm) and the large thickness required to fully absorb sunlight (e.g. hundreds of nanometers).
Co-reporter:Sung-Liang Chen, L. Jay Guo, Xueding Wang
Photoacoustics 2015 Volume 3(Issue 4) pp:143-150
Publication Date(Web):December 2015
DOI:10.1016/j.pacs.2015.11.001
Three-dimensional photoacoustic microscopy (PAM) has gained considerable attention within the biomedical imaging community during the past decade. Detecting laser-induced photoacoustic waves by optical sensing techniques facilitates the idea of all-optical PAM (AOPAM), which is of particular interest as it provides unique advantages for achieving high spatial resolution using miniaturized embodiments of the imaging system. The review presents the technology aspects of optical-sensing techniques for ultrasound detection, such as those based on optical resonators, as well as system developments of all-optical photoacoustic systems including PAM, photoacoustic endoscopy, and multi-modality microscopy. The progress of different AOPAM systems and their representative applications are summarized.
Co-reporter:Hongseok Youn, Taehwa Lee and L. Jay Guo
Energy & Environmental Science 2014 vol. 7(Issue 8) pp:2764-2770
Publication Date(Web):20 May 2014
DOI:10.1039/C4EE01073F
To produce practical large area polymer solar cells (PSCs), it is highly desirable that the Ag (silver) top electrodes be made by a printing process rather than by vacuum evaporation. However, directly printing electrodes using highly conductive metal inks, such as organometallic and nanoparticle inks, has risks which can cause the infiltration and contamination of the underlying polymer layers during the printing and annealing processes. Moreover, the metal inks usually require high sintering temperatures to achieve high-conductivity electrodes. To overcome these limitations, we introduce a multi-layer roll transferring (MRT) approach, in which a high performance solution processed Ag electrode is prepared separately from the rest of the organic layers, and the device is completed by a final transferring process. By optimizing the processing conditions of the reductive organometallic Ag solution, the resulting metal electrode has an excellent resistivity (3.4 μΩ cm−1) and a morphology comparable to that of a thermally evaporated silver film. The performances of the devices fabricated by the MRT process were comparable to those of metal evaporated devices. Furthermore we achieved fully solution processed devices fabricated by integrating the roll-to-roll coating of the polymer cathode, polymer semiconductor and charge extraction layer and the MRT process.
Co-reporter:Cheng Zhang;Dewei Zhao;Deen Gu;Hyunsoo Kim;Tao Ling;Yi-Kuei Ryan Wu
Advanced Materials 2014 Volume 26( Issue 32) pp:5696-5701
Publication Date(Web):
DOI:10.1002/adma.201306091
Co-reporter:Kyu-Tae Lee;Sungyong Seo;Jae Yong Lee
Advanced Materials 2014 Volume 26( Issue 36) pp:6324-6328
Publication Date(Web):
DOI:10.1002/adma.201402117
Co-reporter:Jiayu Wan, Alex F. Kaplan, Jia Zheng, Xiaogang Han, Yuchen Chen, Nicholas J. Weadock, Nicholas Faenza, Steven Lacey, Teng Li, Jay Guo and Liangbing Hu
Journal of Materials Chemistry A 2014 vol. 2(Issue 17) pp:6051-6057
Publication Date(Web):11 Oct 2013
DOI:10.1039/C3TA13546B
One-dimensional (1-D) nanostructures such as nanowires and nanotubes have been widely explored for anodes with high specific capacity in Li-ion batteries, which effectively release the mechanical stress to avoid structure pulverization. However, 1-D nanostructures typically have a high surface area, which leads to a large irreversible capacity in the first cycle due to a solid electrolyte interface (SEI) formation. Two dimensional (2-D) nanowalls can address the same challenges as 1-D nanostructures, with a much lower surface area. For the first time, we demonstrated a 2-D nanowall structure with silicon for Li-ion batteries. Excellent performance for the first Coulombic efficiency (CE) has been achieved. Such a 2-D nanowall structure can also be applied in other devices with improved performance where nanostructures are needed but a high surface area is problematic.
Co-reporter:Jong G. Ok, Ashwin Panday, Taehwa Lee and L. Jay Guo
Nanoscale 2014 vol. 6(Issue 24) pp:14636-14642
Publication Date(Web):15 Oct 2014
DOI:10.1039/C4NR05567E
We present a versatile and simple methodology for continuous and scalable 2D micro/nano-structure fabrication via sequential 1D patterning strokes enabled by dynamic nano-inscribing (DNI) and vibrational indentation patterning (VIP) as well as a ‘single-stroke’ 2D patterning using a DNI tool in VIP.
Co-reporter:Jinbao Guo;Chad M. Huard;Yang Yang;Young Jae Shin;Kyu-Tae Lee
Advanced Optical Materials 2014 Volume 2( Issue 5) pp:435-441
Publication Date(Web):
DOI:10.1002/adom.201300525
Co-reporter:Jae Yong Lee, Taehwa Lee, Hui Joon Park, L. Jay Guo
Organic Electronics 2014 Volume 15(Issue 11) pp:2710-2714
Publication Date(Web):November 2014
DOI:10.1016/j.orgel.2014.08.005
•We have applied ultra-thin Alq3 for efficient cathode contact in solar cells.•For BHJ organic cells, Alq3 added device shows 12.3% enhanced performance.•The contact resistance of the Alq3 added cathode is reduced.•The energy band simulation at the cathode interface proves Voc increase.•Ultra-thin Alq3 added cathode improves a-Si/organic hybrid cell performance.We report a new finding that ultra-thin Tris(8-hydroxyquinoline) aluminum (Alq3) improves an Al cathode interface for photovoltaic (PV) with inorganic amorphous silicon (a-Si) as well as organic bulk heterojunction (BHJ) photoactive layers. Contact resistance characterization is used to investigate the effect of the added Alq3. The experimental results show that the inserted Alq3 is observed to reduce the contact resistance at the cathode interface. Supported by our numerical analysis, the enhanced cathode interface by Alq3 provides better Ohmic contact, thereby increasing Voc. The overall power efficiency is enhanced accordingly benefited by the Alq3 added cathode regardless of photoactive layers.
Co-reporter:Alex F. Kaplan, Jason A. Gilbert, Rachel Trabert, Thomas H. Zurbuchen, and L. Jay Guo
ACS Photonics 2014 Volume 1(Issue 7) pp:554
Publication Date(Web):June 11, 2014
DOI:10.1021/ph500018b
The integration of compact, sturdy, and lightweight sensors into satellites for measurement of low-density plasmas has been hindered by their inherent sensitivity to extreme UV photons. Silicon nanograting structures that reject extreme UV light while allowing particles to pass through have improved on previous gold gratings in terms of stability and ease of fabrication, but comparable UV rejection was not attained since Si acts as a plasmonic material in the extreme UV. In order to alter the plasmonic properties of the structure, atomic layer deposition technology was utilized during fabrication, and extreme UV transmission comparable to previous results was achieved. This work reports the fabrication and measurement of coated, free-standing Si nanogratings created over a large area using nanoimprint lithography. With reduction in defect density, these structures could outperform current gold gratings as UV filters in space-based mass spectrometry systems.Keywords: mass spectrometry; nanogratings; nanoimprint; plasmonics; polarizer
Co-reporter:Taehee Jang, Hongseok Youn, Young Jae Shin, and L. Jay Guo
ACS Photonics 2014 Volume 1(Issue 3) pp:279
Publication Date(Web):February 19, 2014
DOI:10.1021/ph400172u
We present an optically transparent, flexible, and polarization-independent broadband microwave absorber. It is designed to possess two spectrally overlapped resonances of a bow-tie array, which originates from the fundamental resonance mode and the coupling between the neighboring units. An Al wire gird is used to construct the bow-tie array to induce high ohmic loss and broaden the bandwidth of the resonances. As a result, the combined resonances lead to more than 90% total absorption covering a wide frequency range from 5.8 to 12.2 GHz. The transparent and flexible properties provide more flexibility for absorber applications. The optical transmittance of the whole structure is more than 62%.Keywords: absorber; broadband; flexible; microwave; transparent
Co-reporter:Deen Gu, Cheng Zhang, Yi-Kuei Wu, and L. Jay Guo
ACS Nano 2014 Volume 8(Issue 10) pp:10343
Publication Date(Web):September 11, 2014
DOI:10.1021/nn503577c
Rough surface and poor stability of ultrathin Ag films limit their applications in nanophotonic and optoelectronic devices. Here, we report an approach for fabricating ultrasmooth and thermally stable Ag-based thin films on SiO2/Si substrates by Al-doping. The effect of Al-doping on the surface morphology and stability of ultrathin Ag films at room temperature and elevated temperature was investigated. The 15 nm Al-doped Ag films with an Al atomic concentration of 4% have a root-mean-square roughness as low as 0.4 nm. The smooth surface morphology is maintained even after 300 °C annealing in N2. Al-doping enhances the nuclei density of films. Moreover, a capping layer spontaneously formed over the Al-doped Ag films restrains the surface diffusion and mass transportation of Ag atoms. Therefore, Al-doping induces ultrathin Ag films with highly stable and ultrasmooth surface morphology.Keywords: Ag films; Al-doping; capping layer; RMS roughness; thermal stability; ultrasmoothness;
Co-reporter:Cheng Zhang, Tao Ling, Sung-Liang Chen, and L. Jay Guo
ACS Photonics 2014 Volume 1(Issue 11) pp:1093
Publication Date(Web):October 20, 2014
DOI:10.1021/ph500159g
We demonstrate an ultrasonic detector with unprecedented broad bandwidth and high sensitivity, based on an imprinted polymer optical microring. It has an acoustic response of up to 350 MHz at −3 dB and noise-limited detectable pressure as low as 105 Pa in this frequency range. Application of such a detector in photoacoustic imaging leads to improved axial resolving ability compared with using the conventional ultrasound detector, and sub-3 μm axial resolution is achieved, which is more than a 2-fold improvement with respect to the reported record. The device’s miniaturized cavity height guarantees its broadband response, and at the same time, its high optical quality factor ensures the detection sensitivity. Our work suggests that the polymer-based miniature microring resonator works as a high-performance ultrasound detector and has potential for acquiring volumetric photoacoustic images with cellular/subcellular resolution in three dimensions.Keywords: axial resolution; broadband ultrasound detector; microring resonator; photoacoustic imaging
Co-reporter:Jing Zhou, Alexander F. Kaplan, Long Chen, and L. Jay Guo
ACS Photonics 2014 Volume 1(Issue 7) pp:618
Publication Date(Web):June 25, 2014
DOI:10.1021/ph5001007
A broadband absorber based on a tapered, alternating metal–dielectric multilayered structure was realized in the visible and IR range. The structure is fabricated by using a versatile method easily scalable to large areas by taking advantage of the line width reduction that occurs naturally in masked evaporation processes. The multilayered structure can be treated as a hyperbolic metamaterial (HMM), and the tapered structure can be regarded as a HMM waveguide with varying width. Light couples into the tapered structure most strongly at the mode cutoff position in the waveguide due to the hyperbolic dispersion and the phase matching condition. Resonant cavities are formed between the top of the tapered structure and the cutoff level, producing strong absorption peaks. These resonances are closely spaced spectrally due to the high k modes in HMM. Finally a broad absorption band is formed due to the broadening of the resonances from an array of coupled HMM tapered structures.Keywords: broadband absorption; hyperbolic metamaterial; IR; self-mask deposition; visible
Co-reporter:Kyu-Tae Lee, Jae Yong Lee, Sungyong Seo and L Jay Guo
Light: Science & Applications 2014 3(10) pp:e215
Publication Date(Web):2014-10-01
DOI:10.1038/lsa.2014.96
Most current solar panels are fabricated via complex processes using expensive semiconductor materials, and they are rigid and heavy with a dull, black appearance. As a result of their non-aesthetic appearance and weight, they are primarily installed on rooftops to minimize their negative impact on building appearance. The large surfaces and interiors of modern buildings are not efficiently utilized for potential electric power generation. Here, we introduce dual-function solar cells based on ultrathin dopant-free amorphous silicon embedded in an optical cavity that not only efficiently extract the photogenerated carriers but also display distinctive colors with the desired angle-insensitive appearances. Light-energy-harvesting colored signage is demonstrated. Furthermore, a cascaded photovoltaics scheme based on tunable spectrum splitting can be employed to increase power efficiency by absorbing a broader band of light energy. This study pioneers a new approach to architecturally compatible and decorative thin-film photovoltaics.
Co-reporter:Hui Joon Park, Hyunsoo Kim, Jae Yong Lee, Taehwa Lee and L. Jay Guo
Energy & Environmental Science 2013 vol. 6(Issue 7) pp:2203-2210
Publication Date(Web):10 May 2013
DOI:10.1039/C3EE24410E
In polymer photovoltaic (PV) cell, it is desirable to use a relatively thick polymer semiconductor film in order to maximize the light absorption, and to achieve better controllability and reproducibility of the film in manufacturing processes. However, the low fill factor due to restricted charge transport and extraction at large film thickness serially limits the performance of the polymer PV cell. In this work, we investigate the factors that can impact the device performances as film thickness is increased. We also introduce ways to help alleviate these problems in thick BHJ PVs. Our measurement results, based on the space-charge limited-current (SCLC) model and the photo-induced carrier extraction by linearly increasing voltage (photo-CELIV) method, show that the thicker BHJ devices have relatively low electron mobility compared with hole mobility, which directly correlates with high contact resistance at the top cathode interface that prevents efficient transport of photo-generated electrons. Specifically, we found that the newly introduced ESSENCIAL fabrication process helps improve the blend donor and acceptor domain morphologies; and adding an ultrathin C60 layer at the cathode interface helps improve the surface morphology and significantly reduce the contact resistance. The effects of the added thin C60 layer on PV cells were further studied by examining several important diode characteristics. Our results proved that this layer not only decreases the contact resistance at the cathode but also improves the hole-blocking, thereby providing significantly suppressed recombination at the cathode interface. Consequently, the fabricated PV devices optimized in morphology and interface show significantly improved internal quantum efficiency (IQE) as compared with the thermally annealed conventional PV cells, leading to 5.11% PCE from a P3HT:PCBM blend system. The modifications to the fabrication of BHJ PV cells described in this work allow for photoactive layers to be hundreds of nanometers thick for efficient light absorption and better controllability.
Co-reporter:Jong G. Ok;Moon Kyu Kwak;Chad M. Huard;Hong Seok Youn
Advanced Materials 2013 Volume 25( Issue 45) pp:6554-6561
Publication Date(Web):
DOI:10.1002/adma.201303514
Co-reporter:Hui Joon Park;Jae Yong Lee;Taehwa Lee
Advanced Energy Materials 2013 Volume 3( Issue 9) pp:1135-1142
Publication Date(Web):
DOI:10.1002/aenm.201300245
Co-reporter:Se Hyun Ahn;Jong G. Ok;Moon Kyu Kwak;Kyu-Tae Lee;Jae Yong Lee
Advanced Functional Materials 2013 Volume 23( Issue 37) pp:4739-4744
Publication Date(Web):
DOI:10.1002/adfm.201300293
Abstract
A template-free, high-throughput patterning technique named vibrational indentation-driven patterning (VIP), which achieves continuous, period-tunable fabrication of micro/nanometer-scale grating structures, is reported. In VIP, a tilted edge of a hard material vertically vibrating at high frequency makes periodic indentations onto a moving substrate of any material softer than the tool, thereby continuously creating grating patterns at high speed. By modulating the tool vibration frequency, substrate feeding rate, and the tool tilting angle, the period-variable chirped gratings and angle-tunable blazed gratings can be easily achieved; they can be utilized in various optoelectronics and photonics applications. As an example, an infrared polarizer directly fabricated from the VIP-created blazed grating is demonstrated.
Co-reporter:Jong G. Ok, Se Hyun Ahn, Moon Kyu Kwak and L. Jay Guo
Journal of Materials Chemistry A 2013 vol. 1(Issue 46) pp:7681-7691
Publication Date(Web):18 Jul 2013
DOI:10.1039/C3TC30908H
This feature article provides an overview of several mechanical-based micro- and nanopatterning technologies that can achieve continuous and high-throughput fabrication of various sub-wavelength structures without resorting to the conventional optical lithography technique. These include a template-based and versatile roll-to-roll nanoimprint lithography technique, cost-effective dynamic mould sweeping patterning as well as mould-free patterning methods. Examples of demonstrated and potential applications in optoelectronics and photonics are also discussed.
Co-reporter:Andrew E. Hollowell
Advanced Optical Materials 2013 Volume 1( Issue 4) pp:343-348
Publication Date(Web):
DOI:10.1002/adom.201300024
The integration of optical components directly into contact lens materials permits the realization of advanced display technologies and integrated medical instruments. A process is developed for the fabrication of aluminium nanowire grid polarizers encapsulated in a flexible, gas permeable, and biocompatible material for integration onto rigid, gas-permeable contact lenses. Gas permeability values an order of magnitude higher than the minimum to prevent eye damage are measured while maintaining strong polarization performance. Further, the methods developed in this work can readily be applied to various optical and electronic technologies, allowing standard fabrication processes on rigid substrates and subsequent transfer to flexible, gas permeable, and biocompatible materials.
Co-reporter:Young Jae Shin;Yi-Kuei Wu;Kyu-Tae Lee;Jong G. Ok
Advanced Optical Materials 2013 Volume 1( Issue 11) pp:863-868
Publication Date(Web):
DOI:10.1002/adom.201300276
A wire grid polarizer (WGP) is fabricated by using a simplified approach with a combination of nanoimprint lithography (NIL) and angled metal deposition. The period of the imprinted polymer nanograting used in this study is 180 nm. The WGP is formed by two consecutive angled aluminium evaporation processes, essentially halving the period for the aluminium (Al) nanograting. The fabricated WGP shows good optical properties for polarized light. More importantly, because of the slight reduction of the period of the nanograting as compared with previous results, the viewing angle, which is one of the most important characters in display equipment, is extended greatly. Encapsulation of the WGP using poly(methyl methacrylate) is conducted for practical application of the WGP. The thickness of the coating is controlled to be less than 1 μm, to prevent the degradation of the optical properties of the WGP.
Co-reporter:Jong G. Ok;Hui Joon Park;Moon Kyu Kwak;Carlos A. Pina-Hernez;Se Hyun Ahn
Advanced Materials 2011 Volume 23( Issue 38) pp:4444-4448
Publication Date(Web):
DOI:10.1002/adma.201102199
Co-reporter:Carlos Pina-Hernandez, Peng-Fei Fu, and L. Jay Guo
ACS Nano 2011 Volume 5(Issue 2) pp:923
Publication Date(Web):January 6, 2011
DOI:10.1021/nn102127z
We propose a simple and robust scheme for a precise and controlled fabrication of ultrasmall structures through the direct size modification (either reduction or increment) of functional nanoimprinted silsesquioxane (SSQ) patterns. The size modification of nanopatterned SSQ polymer features was achieved according to two different independent approaches. In the first approach, feature size was reduced by a simple heat-induced mass loss mechanism; in the second approach structure size increment was achieved by building multiple polymeric layers on top of imprinted patterns. The fabricated arrays follow the shape contour of the patterned structures so the original imprinted profile is preserved. The engineered capabilities were applied to produce high resolution stamps for nanoimprinting. These approaches free the need for sophisticated nanofabrication techniques and expensive facilities required for nanopatterning.Keywords (keywords): functional pattern; nanofabrication; nanoimprinting; silsesquioxane
Co-reporter:Hui Joon Park;Myung-Gyu Kang;Se Hyun Ahn
Advanced Materials 2010 Volume 22( Issue 35) pp:E247-E253
Publication Date(Web):
DOI:10.1002/adma.201000250
Co-reporter:Myung-Gyu Kang;Ting Xu;Hui Joon Park;Xiangang Luo
Advanced Materials 2010 Volume 22( Issue 39) pp:4378-4383
Publication Date(Web):
DOI:10.1002/adma.201001395
Co-reporter:Carlos Pina-Hernandez, L. Jay Guo and Peng-Fei Fu
ACS Nano 2010 Volume 4(Issue 8) pp:4776
Publication Date(Web):July 15, 2010
DOI:10.1021/nn100478a
Epoxysilsesquioxane (SSQ)-based materials have been developed as patterning layers for large-area and high-resolution nanoimprinting. The SSQ polymers, poly(methyl-co-3-glycidoxypropyl) silsesquioxanes (TMeTEp), poly(phenyl-co-3-glycidoxypropyl) silsesquioxanes (TPhTEp), and poly(phenyl-co-3-glycidoxypropyl-co-perfluorooctyl) silsesquioxanes (TPhTEpTFluo), were precisely designed and synthesized by incorporating the necessary functional groups onto the SSQ backbone. The materials possess a variety of characteristics desirable for NIL, such as great coatability, high modulus, good mold release, and excellent dry etch resistance. In particular, the presence of epoxy functional groups allows the resists to be solidified within seconds under UV exposure at room temperature, and the presence of the fluoroalkyl groups in the SSQ resins greatly facilitate mold release after the imprint process. In addition, the absence of metal in the resins makes the materials highly compatible with applications involving Si CMOS integrated circuits fabrication.Keywords: flexible substrate; nanoimprint lithography; patterning; roll-to-roll
Co-reporter:Hui Joon Park, Myung-Gyu Kang and L. Jay Guo
ACS Nano 2009 Volume 3(Issue 9) pp:2601
Publication Date(Web):August 26, 2009
DOI:10.1021/nn900701p
We developed simple fabrication methods to effectively transfer the block copolymer nanopatterns to a substrate material. High aspect ratio, sub-20 nm nanopillar and nanohole structures are successfully fabricated in a SiO2 layer in large area format, and the versatile utilities of these nanostructures as nanoimprint molds are studied. Nanoimprint lithography using these molds makes it possible to easily replicate densely packed block copolymer nanotemplate patterns on arbitrary substrates in a short processing time by using a large variety of polymer materials, including functional materials such as conjugated polymers. In addition, the PDMS soft stamps with both nanohole and nanopillar pattern polarities, which are useful tools for soft lithography and transparent template applications, are also successfully fabricated using the pillar- and hole-type SiO2 molds. These soft stamps provide an effective way to fabricate controllable as well as reproducible plasmonic metal nanostructures with tunable surface plasmon resonances.Keywords: block copolymer; conjugated polymer; nanoimprint lithography; plasmonic nanostructures; self-assembly; soft lithography
Co-reporter:Bron D. Lucas;Jin-Sung Kim;Christine Chin
Advanced Materials 2008 Volume 20( Issue 6) pp:1129-1134
Publication Date(Web):
DOI:10.1002/adma.200700225
Co-reporter:L. J. Guo
Advanced Materials 2007 Volume 19(Issue 4) pp:495-513
Publication Date(Web):25 JAN 2007
DOI:10.1002/adma.200600882
Nanoimprint lithography (NIL) is a nonconventional lithographic technique for high-throughput patterning of polymer nanostructures at great precision and at low costs. Unlike traditional lithographic approaches, which achieve pattern definition through the use of photons or electrons to modify the chemical and physical properties of the resist, NIL relies on direct mechanical deformation of the resist material and can therefore achieve resolutions beyond the limitations set by light diffraction or beam scattering that are encountered in conventional techniques. This Review covers the basic principles of nanoimprinting, with an emphasis on the requirements on materials for the imprinting mold, surface properties, and resist materials for successful and reliable nanostructure replication.
Co-reporter:X. Cheng;L. J. Guo;P.-F. Fu
Advanced Materials 2005 Volume 17(Issue 11) pp:
Publication Date(Web):24 MAY 2005
DOI:10.1002/adma.200401192
A new UV-curable liquid resist based on cationic polymerization of silicone epoxies has been developed for UV-assisted nanoimprint lithography. Uniform films with thicknesses ranging from below 50 nm to over 1 μm can be easily spin-coated using a suitable undercoating layer on a substrate. Patterns with feature sizes ranging from tens of micrometers to 20 nm (see Figure) are imprinted at room temperature with a pressure of less than 0.1 MPa.
Co-reporter:Bushra Bari, Jinhwan Lee, Taehee Jang, Phillip Won, Seung Hwan Ko, Khalid Alamgir, Muhammad Arshad and L. Jay Guo
Journal of Materials Chemistry A 2016 - vol. 4(Issue 29) pp:NaN11371-11371
Publication Date(Web):2016/07/05
DOI:10.1039/C6TA03308C
Solution-processed silver nanowire (AgNW) random mesh is a strong contender to commercial indium tin oxide (ITO); however, its performance is limited due to large contact resistance between nanowires and post-processing treatments. As an alternative, long nanowires can decrease the number of contact points and contact resistance. Here, a simple modified hydrothermal method for the synthesis of very-long silver nanowires (AgNWs) and their use in a high quality transparent conducting electrode without post-processing has been developed. Well dispersed very-long and thin silver nanowires are synthesized by using glucose as a reducing agent and silver chloride as a silver source. The lengths of the wires are in the range of 200 to 500 μm with an average diameter of 45–65 nm. To the best of our knowledge, this is the first report on long nanowires having a thin diameter with greater than 200 microns length. As compared to other transparent conductors and nanowire networks, this AgNW network shows a higher percolative figure of merit (FoM, Π) with low haze. A flexible touch screen using the AgNW network is also demonstrated which has shown good performance even on a bendable surface.
Co-reporter:Kyu-Tae Lee, Masanori Fukuda, Suneel Joglekar and L. Jay Guo
Journal of Materials Chemistry A 2015 - vol. 3(Issue 21) pp:NaN5382-5382
Publication Date(Web):2015/04/27
DOI:10.1039/C5TC00622H
Multi-functional solar cells that can create semitransparency or desired colors have recently garnered much attention due to their potential in incorporating aesthetic functionalities into building envelopes, such as windows, facades, and walls, allowing large surfaces of the buildings to be utilized for light-harvesting. Here, we present optical microcavity-embedded decorative perovskite solar cells producing distinctive transmissive colors. Any individual semitransparent colors can be created by strong interference effects in the microcavity system, which are easily tuned by changing the thickness of the optical spacer layer of the microcavity. Our colored cell devices show a power conversion efficiency of up to ∼4% with high internal quantum efficiency that is attributed to greatly minimized charge carrier recombination. This approach could bring us one step closer towards energy-saving ultra-thin color display devices and power generating panels for building decoration.
Co-reporter:Jong G. Ok, Se Hyun Ahn, Moon Kyu Kwak and L. Jay Guo
Journal of Materials Chemistry A 2013 - vol. 1(Issue 46) pp:NaN7691-7691
Publication Date(Web):2013/07/18
DOI:10.1039/C3TC30908H
This feature article provides an overview of several mechanical-based micro- and nanopatterning technologies that can achieve continuous and high-throughput fabrication of various sub-wavelength structures without resorting to the conventional optical lithography technique. These include a template-based and versatile roll-to-roll nanoimprint lithography technique, cost-effective dynamic mould sweeping patterning as well as mould-free patterning methods. Examples of demonstrated and potential applications in optoelectronics and photonics are also discussed.
Co-reporter:Cheng Zhang, Harish Subbaraman, Qiaochu Li, Zeyu Pan, Jong G. Ok, Tao Ling, Chi-Jui Chung, Xingyu Zhang, Xiaohui Lin, Ray T. Chen and L. Jay Guo
Journal of Materials Chemistry A 2016 - vol. 4(Issue 23) pp:NaN5153-5153
Publication Date(Web):2016/04/18
DOI:10.1039/C6TC01237J
In order to manufacture large-scale photonic devices of various dimensions at a low cost, a number of patterning techniques have been developed. Nanoimprint lithography is among the most promising given its unique advantages, such as high resolution, fast processing speed, high throughput, compatibility with diverse materials, and low cost. This review covers various aspects of nanoimprint lithography, including its operational principles, material requirements, and different ways of implementation. Nanoimprint lithography facilitates numerous high-performance and low-cost photonic elements, including optical interconnects, sensors, solar cells, and metamaterials. In addition, other related patterning techniques, together with their utilization for photonic device fabrication and their integration with nanoimprint lithography, are briefly discussed.
Co-reporter:Jiayu Wan, Alex F. Kaplan, Jia Zheng, Xiaogang Han, Yuchen Chen, Nicholas J. Weadock, Nicholas Faenza, Steven Lacey, Teng Li, Jay Guo and Liangbing Hu
Journal of Materials Chemistry A 2014 - vol. 2(Issue 17) pp:NaN6057-6057
Publication Date(Web):2013/10/11
DOI:10.1039/C3TA13546B
One-dimensional (1-D) nanostructures such as nanowires and nanotubes have been widely explored for anodes with high specific capacity in Li-ion batteries, which effectively release the mechanical stress to avoid structure pulverization. However, 1-D nanostructures typically have a high surface area, which leads to a large irreversible capacity in the first cycle due to a solid electrolyte interface (SEI) formation. Two dimensional (2-D) nanowalls can address the same challenges as 1-D nanostructures, with a much lower surface area. For the first time, we demonstrated a 2-D nanowall structure with silicon for Li-ion batteries. Excellent performance for the first Coulombic efficiency (CE) has been achieved. Such a 2-D nanowall structure can also be applied in other devices with improved performance where nanostructures are needed but a high surface area is problematic.
