Symposium NM5 : Nanomembrane Materials—From Fabrication to Application

Recent advances in soft electronics have attracted great attention due in large to the potential applications in personalized, bio-integrated healthcare devices. The mechanical mismatch between conventional electronic/optoelectronic devices and soft human tissues causes many challenges, such as the low signal to noise ratio of biosensors because of the incomplete integration of rigid devices with the body, inflammations and excessive immune responses of implanted stiff devices originated from frictions and foreign nature to biotic systems, and the huge discomfort and consequent stress to users in wearing/implanting these devices. Ultraflexible and stretchable electronic devices utilize the low system modulus and the intrinsic system-level softness to solve these issues. Here, we describe our unique strategies in the synthesis and functionalization of nanoscale two dimensional materials, their seamless assembly and integration, and corresponding device designs toward wearable devices. These wearable systems combine recent breakthroughs in unconventional soft electronics to address unsolved issues in the clinical medicine.

Single-crystal silicon (c-Si) has been considered as one of the most important materials in modern industry. Its extraordinary properties such as high carrier mobility, reliability, stability and reproducibility have played critical role in amazing advancement of electronic industries. In addition, via intense research for half-century, the infrastructure in Si industry has been well-established and the advanced techniques, such as doping process using ion-implantation have been developed, which are not available for the other advanced electronic materials such as organic-, oxide-, carbon- and two-dimensional materials.[1-4] However, its mechanical and optical drawbacks, such as brittleness and opacity keep the c-Si away from the novel electronics such as flexible and transparent electronics.
Here, we report the ultra-thin c-Si (UT-Si) whose thickness varies from 1.3 nm to 100 nm. As the brittle and opaque graphite becomes flexible and transparent graphene, the UT-Si with thickness of 7 nm becomes extremely flexible with bending radius of 500 nm, easy to deform with 10⁵ lower bending stiffness than that of wafer and transparent with transmittance of ~80%. In addition, in this regime of thickness from 1.3 nm to 100 nm, we observed a significant change in the intrinsic properties of c-Si, such as band gap and piezoresistivity. Such extraordinary mechanical and optical properties enable the UT-Si to be integrated in the flexible and transparent electronics while we can take advantages of the c-Si, such as good electrical mobility, stability, reproducibility, doping controllability as well as well-established infrastructure. We believe that the UT-Si can offer novel way to breakthrough in flexible electronics where the research has been focused on the advanced materials such as oxide-, polymer-, carbon and 2D layered materials.

[1] T. Someya, T. Sekitani, S. Iba, Y. Kato, H. Kawaguchi and T. Sakurai, Proc. Natl Acad. Sci. USA, 101(27), p.9966-9970 (2004).
[2] K. Nomura, H. Ohta, A. Takagi, T. Kamiya, M. Hirano and H. Hosono, Nature 432, p.488-492 (2004).
[3] S. Jang, H. Jang, Y. Lee, D. Suh, S. Baik, B. H. Hong and J. –H. Ahn, Nanotechnology 21, p.425201 (2010).
[4] H. –Y. Chang, S. Yang, J. Lee, L. Tao, W. –S. Hwang, D. Jena, N. Lu and D. Akinwande, ACS Nano, 7(6), p.5446-5452 (2013).

Assembly of 3D micro/nanostructures in advanced functional materials has important implications across broad areas of technology. Existing approaches are compatible, however, only with narrow classes of materials and/or 3D geometries. This paper introduces ideas for a form of Kirigami that allows precise, mechanically driven assembly of 3D mesostructures of diverse materials from 2D micro/nanomembranes with strategically designed geometries and patterns of cuts. Theoretical and experimental studies demonstrate applicability of the methods across length scales from macro to nano, in materials ranging from monocrystalline silicon to plastic, with levels of topographical complexity that significantly exceed those that can be achieved using other approaches. A broad set of examples includes 3D silicon mesostructures and hybrid nanomembrane–nanoribbon systems, including heterogeneous combinations with polymers and metals, with critical dimensions that range from 100 nm to 30 mm. A 3D mechanically tunable optical transmission window provides an application example of this Kirigami process, enabled by theoretically guided design.

Membrane proteins (MPs) are the biologically derived high-performance nanomaterials that act as gatekeepers for biomembranes. They mediate matter transport, information processing, and energy conversion across the nanoscale membrane boundaries, and show great potential for bio-nanoengineering in synthetic systems. However, a critical challenge exists on how to design and control the properties of synthetic membranes to reconstitute and support MP functions in a robust and scalable manner. Our studies on three different MPs with different structural and functional complexity, i.e., proteorhodopsin (a light-driven proton pump), bacterial reaction center (a light-driven electron charge generator), and bovine rhodopsin (a canonical G-protein coupled receptor), respectively, reveal a broadly applicable charge-interaction-directed reconstitution (CIDR) paradigm that induces spontaneous reconstitution of MPs into a series of well-defined amphiphilic block copolymer membranes. I will discuss our structural and functional assays of the reconstituted MPs, which suggest that proteorhodopsin-mediated proton pumping kinetics depends critically on the membrane moduli and the structural flexibility at the protein-membrane interfaces (ACS Nano, 8: 537-545 (2014)), while reaction center-mediated electron charge separation appears insensitive to that (JPC Lett, 5: 787-791 (2014)). Membrane surface chemistry, on the other hand, plays a key role on activating bovine rhodopsin for its interaction with the G protein transducin (Angewandte Chemie 55(2), 588-592 (2016)). I will also discuss our recent progress on controlling the 3D and 2D assembly of proteopolymer membrane arrays amenable for various biotechnological assays and engineering applications, including the development of polymer nanodiscs (PNDs) that support functional MPs. These results suggest that versatile synthetic nanomembrane designs exist to optimize the stability and performance of MPs in synthetic systems.

Silicon nanomembrane based nanophotonic devices provide novel applications not only on silicon but also on a myriad of unconventional substrates such as glass, III-V compounds and PC boards. It will greatly enhance applications in communications and various sensing applications in rigid and conformable surfaces of various military and civilian platforms. In this presentation, we will present intra-chip and inter-chip optical interconnects using silicon subwavelength gratings. Unlike electrical interconnects, optical interconnects provides the possibility of having three dimensional interconnection layers with two dimensional geometry with very low crossing loss (0.02 dB/node experimentally confirmed). 2D optical beam steering with very large steering angle is demonstrated. Further applications using defect engineered photonic crystal waveguide (PCW) based slow light devices provide us with an ultra-sensitive biosensing platform for any biomarker detection as shown in Fig.1 . Early lung and breast cancer detection results will be presented also with high sensitivity without sacrificing specificity.

Free-standing ultra-thin silicon membranes have been instrumental understand confinement effects, modifications in dispersion relations and phonon lifetime impacting thermal processes [1-5]. We will present a comprehensive study of thermal transport in free-standing Si membrane-based two-dimensional phononic crystals . We will compare holey membranes with membranes having a periodic array of pillars on it and their respective characteristics from a phononics perspective. We will discuss the mechanisms at play presented in the works of other authors and compared them to ours towards a better understanding of nano scale thermal transport and possible information processing phononic components.

[1] A. Shchepetov et al., Appl. Phys. Lett. 102 192108 (2013).
[2] J. Cuffe et al., Nano Lett., 12 3569 (2012).
[3] J. Cuffe et al., Phys. Rev. Lett. 110 095503 (2013).
[4] J. A. Johnson et al., Phys. Rev. Letts. 110 025901 (2013).
[5] S. Neogi et al., under review, ACS Nano 9 3820 (2015).
[6] B. Graczykowski et al., Phys. Rev. B 91 075414 (2015) and to be published

Nanoporous materials have attracted a great deal of interest in research fields such as energy conversion, drug delivery, and medical diagnostics due to their large internal surface area and tunable pore size distributions. Porous silicon (PSi), a nanostructured material formed by electrochemical etching of silicon substrates, has been considered as a favorable material for constructing low-cost optical biosensors due to the easy manipulation of its pore sizes, optical properties, and surface chemistries. One of the challenges facing PSi biosensors is the infiltration difficulty of target analytes through nanoscale pore openings. Due to the high aspect ratio of the nanopores in these structures, the diffusive transport into each individual pore can be as slow as a few molecules per second for molecules whose size approaches that of the pore opening. In order to overcome inefficient mass transport, we demonstrate an open-ended PSi membrane that allows analytes to flow through the pores in microfluidic-based assays and interact more favorably with the inner pore surfaces. Our experimental results agree with finite element method simulations and show that flow-through biosensing using the PSi membranes enables a 6-fold improvement in sensor response time compared to closed-ended, flow-over PSi sensors when detecting high molecular weight analyte (e.g., streptavidin 52.8 kDa). For small analytes, such as linker molecules, little to no sensor performance improvement is observed as the closed-ended PSi films do not suffer significant mass transport challenges with these molecules. We further quantitatively show how control over the flow velocity of the analyte delivered to PSi membrane sensors affects the sensor response time and total volume of analtye consumed. The experimental and simulation results indicate that the flow-through scheme enables more reasonable response times (i.e., several minutes) for the detection of dilute analytes (i.e., nM to μM) and reduces the volume of solution required for analysis (i.e., μL). The PSi membrane fabrication is compatible with integration in on-chip sensor arrays such that multiplexed detection is enabled. We will conclude our presentation by discussing design modifications that support simple colorimetric detection using a smart phone camera, leading to a portable, low-cost, and highly effective diagnostic solution.

Light emission from germanium (Ge), as a CMOS compatible material, would make it a powerful addition to Group IV electronics and optoelectronics. The introduction of sufficient tensile strain in very thin sheets (nanomembranes) of single-crystal Ge has recently allowed enhancement of light emission and wavelength tuning via modification of the band structure to achieve a direct gap. [1]

It is, however, difficult to observe a large amount of luminescence via the recombination of charges in the conduction band and the highest-energy-level valence subband (cΓ-LH), due to polarization effects. We have developed a special pressure cell that can stretch the Ge nanomembrane (GeNM) biaxially while collecting the emission from cΓ-LH recombination efficiently. Strong emission in the mid-infrared is observed, in good agreement with simulations.

A 2D photonic crystal is integrated with a GeNM in oder to overcome the polarization effect and enhance the luminescence from it. Using electron beam lithography an array of holes, which is embedded in Poly(methyl methacrylate) (PMMA), is fabricated on the Ge template layer of Ge-on-insulator (GOI), then silicon (Si) pillars are formed on this GeNM through a deposition and lift-off procedure. The Si pillars array is released along with the GeNM and transferred to a polyimide (PI) film that can be clamped onto the top of the pressure cell, which can then be expanded to provide biaxial stress. Emission features in the near-infrared and mid-infrared can be observed; these signals are strongly depended on the design of the pillar array, as expected.

The design of the new pressure cell together with the 2D photonic crystal provides a unique capability for investigating the light emission behavior of strained Ge and exploring potential optoelectronic applications.

[1] J.R. Sanchez-Perez et al. Proc. Natl. Acad. Sci. U. S. A., 108 (2011) 18893-18898.

Supported by NSF and AFOSR.

Abstract: Microwave flexible electronics that can perform over 10 GHz of operation frequency, based on various transferrable semiconductor nanomembranes will be reviewed. In addition, highly sensitive flexible phototransistor enabled by the flip transferring of the finished devices and organic/inorganic heterostructured photosensors will be discussed. The demonstrations showed the great potential to further develop advanced flexible components and systems.

Single-crystal semiconductor nanomembranes that can be released from various source wafers such as Si, SiGe, and III-V are mechanically very flexible and exhibit outstanding electronic properties that are equivalent to those of their bulk counterparts [1]-[2]. These thin, flexible single-crystal materials can furthermore be placed, via transfer printing techniques, onto flexible polymer substrate, thus creating the opportunity to realize high performance flexible electronics and optoelectronics. In this talk, we will review some of the research accomplishments we made over the last few years on this emerging field, including SiNM, strained SiNM, III-V NM based microwave flexible transistors [3]-[6]. Moreover, the advanced design and fabrication process for highly sensitive flexible phototransistors and photosensors will be discussed [7]-[8]. Finally, some of preliminary advanced flexible system and process will also be reviewed.
The work was supported by AFOSR under grant FA9550-09-1-0482.

[1] J. A. Rogers et al., Nature, vol. 477, pp. 45-53, 2011.
[2] K. Zhang et al., Journal of Physics D: Applied Physics, vol. 45, pp. 143001, 2012.
[3] H. Zhou et al., Scientific reports, vol. 3, pp. 1291. 2013.
[4] G. Qin et al., Applied Physics Letters, vol. 106, pp. 043504, 2015.
[5] Y. Jung et al., Nature Communication, vol. 6, pp. 7170, 2015.
[6] J.-H. Seo et al., Scientific Reports, vol. 6, pp. 24771, 2016.
[7] J.-H. Seo et al., Advanced Functional Materials, vol. 23, pp. 3365-3365, 2013.
[8] J.-H. Seo et al., Advanced Optical Materials, vol. 4, pp. 120-125, 2016.

We review surface-normal Fano resonance photonic crystal membrane photonic devices based on heterogeneously integrated silicon and InP crystalline semiconductor nanomembranes. Devices to be reviewed include two types of photonic crystal surface emitting membrane lasers on SOI and on bulk silicon substrates, with potentials for on-chip integrated 3D silicon photonics and flexible optoelectronics.

Light emitters that are wavelength-tunable over the 2.1-2.5-µm spectral region have applications in chemical and biological sensing, as well as spectroscopy and secure free-space optical communications. The introduction of biaxial strain to the Ge crystal lattice modifies the band structure from indirect-band-gap to direct,[1] offering the possibility of wavelength tunable light emitting diodes in a CMOS compatible material. Precise control over the strain state would allow tuning of the emission wavelength.
Although germanium-on-insulator (GOI) has been commercially available, there are distinct advantages in using III-V substrates for creating GOI structures. These advantages are based both on the low quality of commercial GOI and on the possibility of generating many new NM-based materials architectures and electronic/optical properties by combining Ge with Group III-V materials. In particular, the near perfect lattice match between GaAs and Ge allows the direct growth of low-defect-density heterostructures. Furthermore, the availability of equally well-lattice-matched sacrificial buried alloy layers and selective wet chemical etches can allow fabrication of high-quality and freestanding crystalline composite NMs with unexpected new properties. Growth on III-V materials also allows a wide range of Ge thicknesses, as critical-thickness constraints are avoided because strain is externally introduced after growth and transfer to a new host.
We present here initial results on MOCVD growth of Ge films on GaAs, and release/ transfer methods to generate Ge NM’s, which we transfer to new host substrates. The ability to stretch these NMs mechanically, after transfer to a flexible substrate, combined with the integration of Ge with III-V materials, offers both the opportunity to introduce strain in more complex NM heterostructures, and a route to generating hybrid III-V NM materials.

[1] J.R. Sanchez-Perez et al. Proc. Natl. Acad. Sci. U. S. A., 108 (2011) 18893-18898.

Supported by MRSEC and DOE

Novel semiconducting laser sources are always of interest for the budding field of integrated photonics especially those emitting in the near-IR (~1.5μm) and geared towards telecommunications applications. Our membrane laser devices are crafted from InGaAsP epitaxially grown multiple quantum wells on top of an InP substrate. These lasers operate on the principle of bound states in the continuum (BIC). The BIC lasers are fabricated using standard nanofabrication techniques. The structure is fabricated using electron-beam lithography and reactive ion etching (RIE) to define the cylindrical resonators followed by a wet etching step to remove the substrate in order to create the membrane. The release of the membrane has been optimized for best performance. These devices are vertical emitting lasers that have low lasing thresholds (i.e. power efficient).

Due to their highly favorable materials properties such as direct bandgap, appropriate bandgap energy against solar spectrum, and ability to form multiple junctions, epitaxially grown III-V compound semiconductors such as gallium arsenide have provided unmatched performance over silicon in solar energy harvesting. However, their large-scale deployment in terrestrial photovoltaics remains as a daunting challenge mainly due to the high cost of growing device-quality epitaxial materials. In this regard, releasable multilayer epitaxial growth in conjunction with printing-based deterministic materials assemblies represents a promising approach that can overcome this challenge but also create novel engineering designs and device functionalities, each with significant practical values in photovoltaic technologies. This talk will provide an overview of recent advances in materials design, fabrication concept, and nanophotonic light management of multilayer-grown nanomembrane-based GaAs solar cells aiming for high performance, cost-efficient platforms of III-V photovoltaics.

III-nitride nanomembranes (NMs) represent a new embodiment of the nitride compound semiconductors having identical crystalline perfection and optoelectronic efficacy. The III-nitride NMs are naturally compatible with flexible hosts due to the reduced flexural rigidity and can be incorporated into layered stacks with other two-dimensional materials. A major reason to hinder the nitride NM from realizing devices is the ceramic-like chemical inertness of nitride compound semiconductor, making it difficult to etch or to undercut which resulted in the formation of freestanding nanomembrane.
Based on a recent discovery of conductivity selective electrochemical (EC) etching of GaN, we demonstrated GaN NMs with a freestanding thickness from 50 to 500 nm produced from state-of-the-art epitaxial structures. We confirmed that the microstructural, morphological, and optical properties of the separated NM layers were not affected by the membrane fabrication process. The performance of devices including LEDs, MOS transistors, AlGaN heterostructure transistors, and novel photonic devices will be discussed.

Deep understanding of the phonon transport properties in nanoscale systems as well as nanostructured materials is of fundamental importance in realizing high-performance thermoelectric devices for both energy harvesting and solid-state refrigeration. As the modern electronic device dimensions continue to shrink, the effect of nanomembrane structure and interfacial scattering is increasingly dominating the electron and phonon transport. Here we present temperature dependent thermal conductivity measurements of self-assembled and free-standing hybrid multilayer nanomembrane systems: radial and planar Si/SiO₂ superlattices, which are fabricated by a straightforward “roll-up” and “compression” technique ^[1]. Different from conventional nanomembrane transferring techniques, which are used to fabricate single-crystalline nanomembranes layer-by-layer, tubular radial superlattices consisting of mechanically bonded single-crystalline Si nanomembranes are self-assembled by the intrinsic build-in strain in the thin Si film. Ultrathin native SiO₂ layers form on each side of the Si nanomembranes in-situ during the rolling process, resulting in a Si/SiO₂ hybrid multilayer system with well-defined interfaces. The in-plane thermal conductivity of as-rolled Si/SiO₂ tubes with five windings (containing five coaxial nanomembranes in the tubes) is 4 WK^-1m^-1 at 300K, being one fourth of that of single-crystalline Si nanomembranes with an equivalent thickness ^[2] and only slightly higher than that of nano-patterned Si thin-film phononic crystals ^[3]. Detailed structural characterization of the as-rolled and compressed interfaces by high-resolution transmission electron microscopy will be presented, as well as a theoretical model calculating the phonon dispersion relations within the framework of Born-von Karman lattice dynamics. The theoretical results demonstrate that the thermal conductivity of the fabricated Si/SiO₂ hybrid nanomembrane superlattices is to a great extent determined by the phonon processes in the amorphous SiO₂ layers, which is important for any thermoelectric application of Si-based nanomaterials.

Titanates form an interesting family of materials that can be ferroelectric, ferromagnetic, insulating or highly conductive, and can be used in various applications, such as varactors and memory components. Titanates are difficult to grow and usually pulsed laser deposition (PLD) is used to grow the films. Very promising thermoelectric properties and very high figure of merit have been reported on PLD grown thin films [1], and, consequently, titanates are considered as promising materials for high temperature thermoelectrics. Using PLD epitaxial films can be deposited but typically only on small areas. In this work we have deposited 25-150 nm thick Nb doped strontium titanate (STO) films on oxidized 150 mm Si wafers by magnetron sputtering and released the membranes by deep etching through the wafer from the back side. The different thermal expansion coefficients of Si and STO tend to create very high tensile stress in the STO films, preventing the release without breaking the membranes. By optimizing the sputtering parameters, it is possible to decrease the stress to about 300 MPa enabling the release and fabrication of relatively large area free-standing membranes. Fabrication of corrugated membranes using the low stress process is also possible. The resistivity of the Nb doped films with Nb content of about 10 % is in the range of a few mOhmcm. The grain size of the polycrystalline low stress membranes is very small, leading to low thermal conductivity and the thermal conductivity measured by 3w method is only 2.5 W/mK. These values are very promising for thermoelectric applications. In this talk we will report on the fabrication and properties of the thin free-standing STO:Nb membranes deposited by magnetron sputtering and discuss the potential applications.
[1] Ohta et al., Nature Materials 6 (2007) 129; C. Yu et al., Appl. Phys. Lett. 92 (2008) 092118.

Mesoporous zeolitic thin films (MZTFs), containing vertical mesochannels and high thermal stability, are fabricated via decane-induced self-assemblies of beta-zeolite seeds onto flat substrates, including silicon wafer and conducting ceramics. With different ratios in surfactants, the dimensions of the mesochannels are ranged from 2 to 10 nm and their shapes can be tuned from polygons (n=3-8) to hexagons (n=6) only. With intermittent rates of injecting beta-zeolite seeds, bilayered MZTFs are obtained. To realize periodicity of MZTFs on substrates, we use grazing-incidence small-angle scattering (GISAXS) analysis that explains hexagonal packing and superlattice structures of the mesochannels. With complementary X-ray reflectivity (XRR) analysis, in-plain signals suggest the bilayer structures, directly evidenced in cross-sectional TEM and SEM images. MZTFs maintain highly ordered morphology even after high temperature calcination (700 ^oC) and hydrothermal (100 ^oC) treatments, suggesting their enhanced thermal and hydrothermal stabilities. The MZTFs are employed to confine growths (5-20 nm) of metallic nanoparticles (e.g. Ag, Au, Cu) for plasmonic applications, as well as to inhibit biological infection (e.g. E.coli) at room temperature. Our works reveal a facile method of fabricating robust on-substrate thin film materials that demonstrate functionalities for multiple interfacial applications.

Polysulphone (PSF) is one of the most popular thermoplastic materials used in the manufacturing of various types of microfiltration and ultrafiltration membranes. Despite the high structural and chemical stability of PS membranes, the flux produced from such membranes is relatively low. Therefore, an increase in the porosity and flux of PSF membranes is highly desirable to increase the productivity and efficiency of these membranes. In this work, the effect of addition of Arabic gum as a new pore-forming into the casting solution on the properties of the fabricated PS membranes has been studied. PS membranes were cast via phase inversion at different concentrations of Arabic gum (0, 1, 3, 5 and 7%) in PSF and dimethyacetamide (DMAc) solvent. Philos casting system was used for membrane preparation. The effect of Arabic gum on the pore size and water flux of the prepared membranes has been studied. The porosity, pore size and morphology of the developed membranes have been characterized with SEM. Moreover, the oil removal capability of polysulfonic membranes (with Arabic gum) has also been performed when oil-water solution (100 mg/l) have been tested with the new membrane. The use of Arabic gum as a surfactant has been found to successfully remove oil from water (up to 97%) and increase the porosity and flux of the prepared membranes.

It is well-known that only the high-quality graphene possesses its unique physical, chemical and mechanical properties, when it is used in practical applications. However, it is still a challenge to obtain the high-quality graphene in large-scale and low-cost, which hinders its broad applications. In this paper, we introduce an electrochemical exfoliation for producing graphene in large scale, and then follow a spark plasma sintering (SPS) treatment for obtaining the mass high-quality graphene.
Recently, electrochemical exfoliation has drawn great attention as a promising method for producing graphene on an industrial scale with high efficiency, low cost, and non-pollution. In comparison to cationic intercalation, anionic intercalation (primarily in aqueous electrolytes) is more effective and less time demanding, which has predominated the literatures concerning the electrochemical exfoliation of graphite. Sulfuric acid is the most commonly used electrolyte because the interlayer spacing of graphite (0.335 nm) is comparable to the size of sulfate ion (0.46 nm), which facilitates the intercalation of sulfate ions. In addition, the sulfate ions can also act as a surfactant to prevent the re-stacking of graphene product in aqueous solution. However, the induced oxidation and chemical functionalization is unavoidable for the anodic electrochemical exfoliation of graphite in acidic electrolytes.
Herein we proposed a simple and effective route to eliminate the oxygen functional groups by using a spark plasma sintering (SPS) system. SPS is a newly developed sintering method applying high temperature spark plasma generated momentarily. The spark plasma, produced by the large pulse current, has an effect of cleansing impurities of samples and enhancing the heat transfer effect that produces better bonding. During the sintering, the pristine graphene could be effectively converted to high-quality graphene by the elimination of oxygen-containing groups and restoration of its intrinsic structures and properties. Moreover, the output could be readily scaled up due to the rapid sintering process. According to the experimental results, the produced graphene exhibited very low defects density, extremely high carbon to oxygen (C/O) ratios and good processability in various solvents. Free-standing graphene paper (G-paper) was further fabricated from the high-quality graphene, and a high conductivity of 38460 S/m could be attained. The G-paper without any binders, which was used as supercapacitor electrodes, delivered a specific capacitance of 129.0 F/g at 1 A/g, retaining 97% capacitance even after 1000 cycles.

There is great interest in intracellular delivery, recording, and sensing, and significant progress has been made with mammalian cells using various micro/nano-needle platforms^1-3. Additionally, cell transfection has been achieved by applying an electroporation voltage across a nanomembrane pore⁴. These techniques, however, are still in their infancy especially with respect to their application to the plant system, where their rigid cell wall poses additional challenges and complexity. In particular, there is great need to deliver molecules (especially DNA) to plant organisms such as microalgae. Microalgae are single-celled plants that are attractive for applications in biofuel and biochemical production, but the lack of genetic engineering tools has been a major impediment to the research progress⁵. In order to non-destructively pierce the cells, which are 5-10 μm in diameter, a needle must necessarily have sub-micron-scale dimensions and a hollow needle must therefore have precise nano-scale wall thickness.

In this work, we discuss the fabrication of a unique nanomembrane consisting of vertically-protruding hollow needles and its application in interfacing with microalgae cells. The needles are conical, several microns tall, and have tips that are ~100-200 nm in diameter. The wall thickness of the hollow needles is defined by the nanomembrane thickness and is a few tens of nanometers. This platform is capable of penetrating the cell wall of microalgae via a forced piercing event, allowing direct fluidic access to the cytosol. Using fluorescence microscopy and electron microscopy techniques, we demonstrate the ability to deliver reporter molecules to the cell interior, and investigate how the delivery efficiency is affected by such variables as the protruding needle dimensions and the force of the piercing event.

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Wearable electronics, as an important branch of modern electronics, demonstrates huge potentials to change our lifestyles in the near future. However, it remains challenging to develop matchable energy harvesting and storage devices to meet the flexible and wearable requirements of wearable devices. Here we have designed a new family of high-performance fiber-shaped energy harvesting and storage devices, and integrated multifarious functions, including high output voltage, magnetic response, self-healing and energy conversion/storage to meet a broader range of applications. Three main systems are described below.

The invention of fiber-shaped supercapacitors with high output voltages. The output voltages of fiber-shaped supercapacitor based on aqueous electrolytes are too low for practical applications, which requires efficient strategies for in-series connection. We have invented fiber-shaped supercapaitors with high and controllable output voltages in mimicking electric eel, the strongest bioelectricity producer in nature. High output voltages up to 1000 V have been achieved in a single fiber, which breaks the voltage limit and maintains the high integration of the resulting devices.

The invention of self-healable fiber-shaped supercapacitors. One concern for fiber-shaped devices lies in the breaking of delicate fibers under deformation, which may cause the failure of the entire module. We have invented a novel family of fiber-shaped supercapacitors with self-healing capacity to extend their lifetime. Even completely cut off, they can quickly self-heal within 10 s at room temperature, and 92% of the capacitance can be recovered.

The invention of integrated fiber-shaped energy harvesting and storage devices. The integration of energy harvesting and storage into one single device is of great importance for practical applications. We have developed a series of fiber-shaped integrated devices with high energy storage capacity and integration, which represent promising candidates in a variety of application fields.

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Global healthcare in its current reactionary format is irrepressibly overwhelmed by a surplus of patients and tremendous medical expenses; associated treatment strategies are progressively insufficient. A paradigm shift centered on nanotechnology is anticipated to enable the gradual transition from reactionary to predictive medicine needed to enhance global wellness. In particular, it is hypothesized that the integration of synthetic immune cell mimics together with modern medical intervention will substantiate the concept of a smart vaccine capable of coordinating aggressive phagocyte and B-cell activities. The objective of the current study is to fabricate synthetic immune cells by exploiting tunable polymersome (Ps) membrane properties. Ps are artificial, biocompatible vesicles that self-assemble via the hydrophobicity interactions of admixed aqueous and organic substances. For the current application, chemical viability studies have shown that a Poly(D,L-lactide)-b-poly(ethylene glycol)-carboxylic acid (PDLLA-PEG-COOH) diblock copolymer ideally self-assembles to form Ps. Strategies were devised to supply a polymeric vesicle affixed by IgG antibodies along the peripheral hydrophilic PEG regime and isolated subcellular material within the aqueous core. The utility of EDC and Sulfo-NHS crosslinking chemistry facilitated the adherence of fluorescein isothiocyanate (FITC)-labeled antibodies to Ps surfaces. Transmission electron microscopy (TEM) and fluorescence microscopy images provide a validation of bilayer formation in addition to an optical quantification of the degree of IgG surface conjugation. As part of a complementary experiment, mitochondria were isolated from human dermal fibroblasts via a rigorous centrifugation and homogenization routine. Structural integrity was ensured through TEM characterization prior to Ps conjugation, and chemical viability was quantified by an assessment of the enzymatic activity of isolated organelles. Associated bacterial and cytotoxicity assays measured the viability of Ps that were functionalized to include antibodies and/or subcellular material. Clinical injection of the composite Ps solution is hypothesized to activate an aggressive B-cell response in which antibodies disperse and mark antigens for destruction. Associated mitochondrial interactions are expected to drive controlled antibody release mechanisms. Accelerated morphological considerations assume the generation of a fully functional immune cell appended by several classes of antibodies. Postliminary studies will follow closely the development of an in-situ sensing device that would enable clinicians to externally steer nanocarriers within the vasculature. Automated design is anticipated to provide a nanodevice with capabilities that far exceed the healing properties shown by innate immune cells. The shift from reactionary to predictive intervention will prompt a new era in which early disease detection and treatment are clinically significant.

A sub-nanoporous carbon thin film has been considered as a promising candidate for a next-generation robust reverse osmosis (RO) membrane. We focused on fabrication of carbon thin films using plasma-enhanced chemical vapor deposition (PECVD) method. The method has many industrial advantages such as high-throughput and large-area production, defect elimination, precise control of film thickness, and flexible design of a multi-layer structure. Although some researchers have reported carbon-based RO membranes in the 1980's, the rational design on making sub-nanopores in a PECVD-made carbon thin film has not been established to date. Here we present how we can introduce sub-nanometer pores in a carbon thin film and control its size and porosity by tuning plasma parameters.

Using PECVD method, a sub-nanometer porous carbon thin film was directly deposited on a porous polymer support at room temperature. Although conventional hydrocarbon monomers did not give water-permeable thin films due to hydrophobicity, a hydrophilic carbon thin film that can quickly permeate water molecules was obtainable using a nitrogen-containing amine monomer. Eighty percent of nitrogen atoms in the monomer was successfully transferred in the carbon film, while atomic ratio of hydrogen decreased largely due to generation of radicals by detaching a hydrogen atom in a monomer, which results in the film deposition by formation of carbon-carbon bonds. A transmission electron microscope revealed amorphous structure of the carbon thin film. Positron annihilation lifetime spectroscopy clearly detected large number of sub-nanometer pores in the film. Since the hydrophilic film can absorb water molecules (typically 5 to 20%), Young's modulus of 8.0 GPa in dry state decreased to 2.0 GPa when the film is wet with water.

Among various plasma parameters, we investigate in detail the effect of input power, monomer pressure, and monomer flow rate. A diagram of deposition rate clearly exhibits different regimes of low-temperature plasma state. By examining wide range of plasma parameter combinations, we found the complicated effects of these plasma parameters could be rationalized using a simple parameter, electron temperature (i.e. electron energy in plasma state). As a result, we can optimize film density by tuning electron temperature. In our experiments, a refractive index value measured with ellispometry was used as an index for a relative evaluation of a film density because the atomic composition does not changed significantly. The refractive index has direct correlation with water permeability. While high-refractive-index carbon film does not permeate water even under high pressure of 4.0 MPa, a carbon film of medium refractive index shows both water permeability and high salt rejection. By optimizing an electron temperature, twenty-nanometer-thick sub-nanoporous carbon thin film realized high water flux and salt rejection (more than 90% for NaCl).

The emergence of multi-resistant pathogens has encouraged the investigation of new strategies to cope
with this ever-increasing threat to public health. In this context, silver nanoparticles (AgNPs)
were combined with doxycycline (DO) to evaluate the potentiality of this
hybrid as a bactericidal agent against E. coli. Polyvinylpyrrolidone (PVP) was used as a stabilizer to
prevent the excessive growth and agglomeration of AgNPs. With a 2² full-factorial design and with the help of UV-vis and TEM, we found out that the highest concentrations of silver ions and PVP delivered the smallest nanopartilces with narrow size distribution. Interestingly, DO bound directly to PVP and had its concentration increased around the particle as a consequence of this interaction, as evidenced by time-resolved fluoresnce and FTIR measurements. As a result, the AgNPs/DO conjugates presented enhanced bactericidal properties compared to the individual components. Stabilizing agents are generally undesirable on the surfaces of nanoparticles because they block adsorption surface sites. However, we have shown that PVP played a paramount role in concentrating DO around the particle, which culminated in an increased bactericidal activity towards E. coli.

The broad applications of self-rolling nanomembranes in nano-elecromechanical/micro-electromechanical systems (NEMS/MEMS), biomedical devices and optoelectronics have driven extensive research efforts, aiming to control the size and shape of the rollup structure. To achieve this objective, accurate knowledge of the mechanics underlying the rollup process is necessary. In the present study, we formulated a comprehensive analytical model for bidirectional rollup of nanomembranes, in which the contributions of both longitudinal and transverse mismatch strains to the rollup curvature are accounted for. Our model predictions show the transverse mismatch strain can have significant influence on the final rollup curvature, validated through finite element (FE) simulations. Our results explain the considerable discrepancy between experimentally measured and the previous theoretically predicted curvatures. Furthermore, the new model we formulated is shown to be capable to predict potential bistable curvature configurations, providing new guidance in manipulating the rollup direction and curvature. Guided by our model, a novel design strategy based on corner geometry engineering has been proposed to realise the unidirectional rollup, validated by experimental results and FE simulations

Graphene is a two-dimensional carbon material with excellent properties in mechanical strength, atomic thickness and flexibility. Because of these remarkable properties, Graphene has been a prominent candidate as structural material in various fields since its first isolation in 2004. Graphene Oxide (GO) is a derivative of Graphene with oxygen-containing functional groups. Recently, climate change has been emerged as a serious problem, and a lot of effort has been tried to solve this problem in the respects of CO₂ capture, water purification and energy harvesting. Most of all, membrane-based technology has been regarded as a promising method due to simple and economical fabrication process, safety and environmental friendliness. Among them, GO membrane of layered carbon structure is a novel candidate in this field with its exceptional characteristics compared with traditional materials such as polymers and zeolites. It has been reported that GO membrane has fast and selective permeability for the specific matters like water and gases depending on the morphology, thickness, forms of containing groups and so on.
Here, we focus on the effect of the morphology of GO membrane in terms of three different fabrication methods. We used GO suspension with same concentration to produce GO laminates on the Anodisc Aluminum Oxide (AAO) as supporting membrane through vacuum-filtration, spray-coating and spin-coating methods. After every deposition process, the membranes were dried at 60°C for 24 hours. Raman, XRD, SEM and AFM were conducted to analyze the condition and morphology of the structure. Finally we confirmed that the surface of the GO layered structure deposited by vacuum filtration was the most uniform. We attribute this to vacuum pressure which stacks the GO flakes perpendicularly to the membrane. However, further research is needed for understanding and explaining the main reasons in detail.

Electrospun nanofibrous mats have been shown to be useful as membranes for separation and filtration applications, including in the biomedical field for white blood cell filtration, because of their high surface area and interconnected non-woven structure. This study is focused on structure-property relationships of electrospun fibers of poly(butylene terephthalate) (PBT, viscosity averaged molecular weight, M_v = 45,000 g/mol.) with potential use as a filtration membranes. Nanofibers were obtained from a 15% w/v solution of PBT in trifluoroacetic acid and dichloromethane (TFA/DCM = 1/1 by volume) as solvents. Electrospinning was done using a flow rate of 0.04 ml/min and a distance of 12 cm between the needle tip and the counter-electrode with an applied voltage of 20 kV between them. Scanning electron microscopy was used to study the morphology of the fibers. Nanofibers free of defects such as beads were obtained which had an average diameter of 1.9 microns. The crystal structure of the nanofibers was studied using X-ray diffraction, and thermal properties were evaluated using thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and temperature modulated DSC (TMDSC). X-ray diffraction showed the as-spun fibers were slightly crystalline. TGA analysis showed ~0.5% of bound solvent was removed at around 60 °C, and major degradation of the fibers occurred at around 390 °C. Highly stretched PBT fibers tend to shrink during heating through the glass transition temperature (T_g) region. However, a well-resolved glass transition (T_g) step was observed in the TMDSC reversing heat flow, showing T_g occurs at ~52 °C. Future work will be reported on pore size measurements and contact angle, quasi-isothermal TMDSC studies and dielectric relaxation analysis of PBT nanofibers.

The field of flame retardancy for furniture (i.e., foams and textiles) is currently facing a few changes and challenges because the halogenated or phosphorus-based flame retardants have proven to be bioaccumulative and toxic in mammals. Therefore, the study for highly efficient eco-friendly flame retardant products, which are developed by using simple techniques (i.e., layer-by-layer), is lead the researchers towards the development of creditable alternatives. In the past two decades, layer-by-layer (LbL) assembly has been widely used as a convenient and versatile method to fabricate functional thin films. Thin films of cationic starch (CS) and sodium montmorillonite (MMT) clay, prepared via LbL assembly, are eco-friendly and bio-based flame retardant. In order to obtain a better understanding of flame retardant behavior of three different coatings, 5, 10 and 20 bilayers samples were fabricated and evaluated. UV-vis absorbance was used to measure the linear growth of this film as a function of bilayers deposited. Thermal and flame retardant properties were measured by thermogravimetric analysis (TGA) and vertical flame test (VFT), indicating that the char residue at temperatures from 400 to 800 °C and burning time were significantly enhanced as compared with the control sample. SEM analysis of cotton samples before and after VTF revealed the increasing char forming ability by LbL coating layers, showing that the structure of cotton fabric in all coated fabrics were preserved by CS and ceramic surface layer char. This work provided a simple but effective method of enhanced flame retardancy of cotton fabric and can be applied to other polymeric materials.

As the global populations grow, water demand and pollution of water resources will increase. As a consequence, water borne disease outbreaks are on the rise and current disinfection methods have been shown to be ineffective in inactivating all pathogens during water treatment. Aluminum oxide nanoparticles (Al₂O₃ NPs) have been shown to poses antimicrobial properties due to oxidative stress and particle interaction with cell walls that lead to rupture and cell death. Also, Al₂O₃ has high thermal and chemical stability, which makes these NPs an excellent candidate for water treatment applications. Thus, the objective of this work is to asses the bactericidal properties of Al₂O₃ NPs synthesized using a polyol-based process in presence of polyvinylpyrrolidone (PVP) as a growth inhibitor reagent. For practical applications nanoparticles must be immobilized in a medium to ensure that particles are not dispersed into the treated water. For this reason, synthesized nanoparticles were dispersed in electrospun polyacrylonitrile (PAN) membranes to also evaluate the bactericide capacity. X-Ray Diffraction (XRD) and Fourier Transform Infrared Spectroscopy (FT-IR) analysis suggests that synthesized nanoparticles are γ-Al₂O₃ after annealing at 800°C for 6 hours. Scanning Electron Microscopy (SEM) characterization was used to determine the morphology and size of synthesized nanoparticles. Composite electrospun membranes where also characterized by XRD, FT-IR, and SEM. The bactericide activity of the synthesized γ-Al₂O₃ NPs, commercially available Al₂O₃ particles and Al₂O₃-PAN composite electrospun membranes against E. coli bacteria was evaluated by the disc diffusion method. Both synthesized and industrially produced particles exhibited antibacterial activity against E. coli, but polyol-based synthesized nanoparticles demonstrated better bactericide properties. Al₂O₃ Nanoparticles embedded in electrospun PAN also exhibited antibacterial properties. Results suggest that the polyol-synthesized γ-Al₂O₃ embedded in electrospun membranes have the potential to be used in alternative water disinfection processes.

Particulate matter (PM) pollution and bacteria transmission have raised serious concerns for public health. We know outdoor individual protection could be achieved by facial masks. In this research, various materials nanofiber membranes can be used to purify the air pollution and bacteriostatic. We use the surface effect and active energy of the nanofibers surface to remove micro-particle and eliminate bacteria. This work creates another field for nanomaterial application.

A novel nanofiltration membrane was produced by reacting polyethersulfone (PES) functionalized with acyl chloride (PES-COCl) and m-phenylenediamine (MPDA). Membranes composed of PES and PES-COCl were fabricated using a non-solvent-induced phase separation process, and then MPDA was reacted with acyl chloride in the PES-COCl on the membrane surface. The PES-COCl was synthesized by reacting the aminated PES with trimesoyl chloride (TMC), and the formation of PES-COCl and reactions between PES-COCl and MPDA was confirmed using FT-IR, XPS, and FE-SEM analyses. Contents of acyl chloride groups in the membranes was controlled by changing mixing ratio of PES and PES-COCl. Membranes composed of PES and PES-COCl exhibited salt rejection lower than 10% against MgSO₄, while those fabricated by reacting MPDA with acyl chloride in the PES-COCl exhibited salt rejection higher than 50% against MgSO₄. Salt rejection increased by increasing acyl chloride content in the membrane.

Ultrafiltration membranes composed of polyethersulfone (PES) and multi-walled carbon nanotube grafted with carboxylated polyethersulfone (MWCNT-PES-COOH) were fabricated to provide hydrophilic and antibiotic properties to PES membranes. Carboxylated PES (CPES), which was prepared by reacting PES, N-butyllithium, and carbondioxide, was reacted with trimesoyl chloride (TMC) to functionalize with acyl chloride. Aminated MWCNT (MWCNT-NH₂), was prepared by reacting carboxylated MWCNTs with (3-aminopropyl)triethoxysilane (APTES). The formation of MWCNT-NH₂ and MWCNT-PES-COOH was confirmed by FT-IR, NMR, XPS, and FE-SEM analyses. The hydrophilicity and water flux of the PES/ MWCNT-PES-COOH membranes increased with increasing MWCNT-PES-COOH content. No antibacterial activity was observed for PES membranes, while PES/MWCNT-PES-COOH membranes containing at least 5 wt% MWCNT-PES-COOH displayed antibacterial activity (=6.1). Membranes having antibacterial activity, high water flux, and improved fouling resistance without a loss in solute rejection could be fabricated by blending PES with MWCNT-PES-COOH.

Transient electronics has attracted great attention due to its unique capability to disappear instantly and/or after programmed time by external triggering. This key advantage generates new opportunities on the field of information security. The data stored in nonvolatile memory can be electrically erased for security purpose, however, they can be easily restored. The information in missed or stolen memory devices cannot be protected. In these cases, chemical/physical destruction of the data storage module can be a potential solution for the permanent data erasure and reliable information security. Here, we demonstrate new information security technology by developing a novel class of nanomembrane-based transient resistive random access memory (RRAM) integrated with photo-acid generators (PAGs) and upconverting nanoparticles (UCNPs). The ultrathin transient RRAM is composed of 5-nm-thick zinc oxide nanomembrane and has advantage of low-power consumption and fast switching. The device is coated with an acid-generating matrix which is composed of ultraviolet (UV)-responsive PAGs and the multi-dye sensitized UCNP in polyethylene oxide. UCNPs are synthesized and integrated to use wide wavelength range photons for triggering the physical disappearance of the device. Therefore, illumination of NIR and/or visible, UV light on UCNPs triggers the emission of UV, resulting in the generation of photo-acid. Due to the ultrathin nature of RRAM, fast and complete chemical destruction of stored data in the memory device could be achieved. This integrated system provides a novel concept of information security in nonvolatile data storage devices as well as new opportunities in mobile electronics and defense applications.

As a strategy to save the cost of expensive substrates in semiconductor processing, the technique called “layer-transfer” has been developed. In order to achieve real cost-reduction via the “layer-transfer”, the followings need to be insured: (1) Reusability of the expensive substrate, (2) Minimal substrate refurbishment step after the layer release, (3) Fast release rate, and (4) Precise control of a released interface. Although a number of layer transfer methods have been developed including chemical lift-off, optical lift-off, and mechanical lift-off, none of those three methods fully satisfy conditions listed above. In this talk, we will discuss our recent development in a “graphene-based layer-transfer” process that could completely fullfill the above requirements, where epitaxial graphene can serve as a universal seed layer to grow single-crystalline III-N, III-V, II-VI and IV semiconductor films and a release layer that allows precise and repeatable release at the graphene surface.

Graphene, a monolayer of carbon atoms in honeycomb configuration, has become a subject of active study since its discovery in 2004. Many potential applications that exploits its mechanical properties and gas impermeability [1] have been proposed such as pressure sensing [2]. Moreover, suspending graphene on circular cavities or trenches prevents substrate effects and enables electro- or opto-mechanical actuation [3].

Single-layer graphene drums have been extensively studied, and several groups have reported the statistical variations of their properties by measuring few drums with laser interferometry, Raman spectroscopy, and atomic force microscopy. However, a parallel, non-invasive, and affordable characterization technique is necessary for any attempt to commercialize graphene mechanical sensors. Furthermore, CVD SLG usually contains gas permeable lattice defects and nanoscale pores due to its growth on imperfect substrates, which blocks its application in gas pressure sensing devices that require impermeable membranes. A possible route to overcome this difficulty is to stack several CVD layers to reduce the probability of having nanopores from different layers aligned on the same spot [4].

In this work, we introduce a new non-invasive optical technique to characterize the mechanical properties and the permeability of large arrays of suspended graphene membranes. We observe Newton's rings on a suspended CVD double-layer graphene (DLG) drumhead when applying a pressure step between inside and outside of the cavity, which allows us to study the deformation of this mechanical system. Based on these observations, the permeability of the DLG membrane is determined and found to be similar to that of pristine graphene.

REFERENCES:
[1] Bunch, J. S., & Verbridge, S. S. (2008). Impermeable atomic membranes from graphene sheets. Nano Letters, 8(8), 2458–2462.
[2] Dolleman, R. J., Davidovikj, D., Cartamil-Bueno, S. J., Van Der Zant, H. S. J., & Steeneken, P. G. (2016). Graphene Squeeze-Film Pressure Sensors. Nano Letters, 16(1), 568–571.
[3] Ferrari, A. C., Bonaccorso, F., Falko, V., Novoselov, K. S., Roche, S., Bøggild, P., … Kinaret, J. (2014). Science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems. Nanoscale, 7(11), 4598–4810.
[4] Celebi, K., Buchheim, J., Wyss, R. M., Droudian, A., Gasser, P., Shorubalko, I., … Park, H. G. (2014). Ultimate Permeation Across Atomically Thin Porous Graphene. Science, 344(6181), 289–292.

Determining the chemical state of a catalyst under realistic reaction conditions is of crucial importance in designing catalytic systems with improved activity and selectivity towards sought after products, and a key step in developing or improving existing industrial processes. Ambient pressure X-ray photoelectron spectroscopy (APXPS), based on analyzers that incorporate a differentially pumped lens system, has proved a powerful technique for providing quantitative, surface sensitive information on the chemical composition of surfaces/interfaces under reaction conditions.[1] Whilst this approach proves practical up to the tens of mbar regime, significant gas phase scattering of photoelectrons at higher pressures makes measurement impractical. However numerous reactions of interest occur at atmospheric pressures and above, and thus the behavior observed in existing APXPS systems may not be truly representative of such reactions.
Here we demonstrate atmospheric pressure XPS using single-layer graphene membranes as photoelectron-transparent barriers that sustain pressure differences in excess of 6 orders of magnitude.[2] The graphene-based membranes are produced by transferring graphene grown by chemical vapour deposition (CVD)[3,4] onto metal coated(Au or Al), perforated silicon nitride grids using a polymer-free transfer technique. The suspended graphene then serves as a support for catalyst nanoparticles under atmospheric pressure reaction conditions (up to 1.5 bar), where XPS allows the oxidation state of Cu nanoparticles and gas phase species to be simultaneously probed. We thereby observe that the Cu²⁺ oxidation state is stable in O₂ (1 bar) but is spontaneously reduced under vacuum. We further demonstrate the detection of various gas-phase species (Ar, CO, CO₂, N₂, O₂) in the pressure range 10-1500 mbar including species with low photoionization cross-sections (He, H₂). Pressure-dependent changes in the apparent binding energies of gas-phase species are observed, attributable to changes in work function of the metal-coated grids supporting the graphene. We expect this graphene membrane approach to be a valuable tool for studying nanoparticle catalysis under atmospheric pressure reaction conditions, as well as a promising technique for studying solid-liquid interfaces during electrochemical reactions.[5]

(1) Eren et al. J. Am. Chem. Soc. 2016 (ASAP)
(2) Weatherup et al. J. Phys. Chem. Lett. 2016, 7, 1622–1627
(3) Weatherup et al. J. Am. Chem. Soc. 2015, 137, 14358–14366.
(4) Weatherup et al. J. Am. Chem. Soc. 2014, 136, 13698-13708
(5) Velasco-Velez et al. Angew. Chemie Int. Ed. 2015, 54, 14554–14558.

Novel single-component ultrathin nanocomposites were fabricated via non-conventional layer-by-layer (LbL) assembly of graphene oxide (GO) flakes using hydrophobic solvent binder in order to establish the uniform layered growth and furnish strong complementary interactions. Ultrastrong freestanding graphene oxide (rGO) LbL nanomembranes having low thickness of 3 nm (3 monolayers of GO), which can be transferred over a large surface area across tens of square centimeters by using a facile surface tension-assisted release technique, were concocted by eliminating organic and regular polymeric binders from assembly process or needs for the intermediate surface chemical modification. These ultra-smooth and highly uniform rGO nanomembranes demonstrate outstanding elastic modulus of 120 GPa and mechanical strength of 0.5 GPa, which are several times stronger than other reported regular rGO films, with high electrical conductivity (up to 3000 S/m after additional chemical reduction) and high transparency (up to 93% at 550 nm before reduction). Furthermore, a flexible freestanding protected noble metal monolayers having surface enhanced Raman scattering (SERS) properties can be constructed by sandwiching up to 94 wt% of silver nanoplates between 5 nm GO layers. With the superior mechanical properties, high optical transmittance and high conductivity, these transferrable flexible rGO/Ag/rGO nanomembranes can be valuable for potential applications in the protective molecular coatings, flexible electronic devices, SERS sensing elements, energy harvesting, optical sensors, and ion separators.

Graphene and CNTs have been studied for gas sensing applications due to their outstanding physical, electronic, mechanical and optical properties. The high surface-to-volume ratio (giving a large exposed area) and mono-atomic thickness of graphene sheets makes it suitable for sub-ppm gas sensing as graphene membranes are easily deflected under small pressure, resulting in change in its electronic properties. In this work, graphene sheets are used as piezoresistive materials to detect the change in the electrical resistance due to strain caused by a pressure load. The graphene used was grown by CVD machine using Cu as catalyst. First step was the integration of Graphene in the desired sensor architecture, the membrane structure. This sensor was tested under atmospheric pressure in order to be tested at a later stage in vacuum. A vacuum setup was put in place for the testing, the integrity of this setup was achieved for a base pressure of 2.5 mTorr. The device was fabricated using a silicon wafer with 1µm PECVD SiO₂. Photolithography was used to pattern the oxide and then etched into cavities using reactive ion etching (RIE). The cavities were of the size of 5µm, 10µm, 25µm, 35µm and 50µm and etched 723 nm deep using CHF₃ gas in the reactive ion etching (RIE). The graphene was transferred over the substrate and on top Metal contacts of Titanium/Aluminum were deposited. The optical images showed that the graphene sheet transferred to the substrate was not continuous nor completely flat due to the exfoliation process used to transfer the graphene from Cu to the substrate. Electrical characterization in a kelvin like 4-points probe was carried out on the graphene membrane to ascertain conductive path within the metal contact. The resistance was found to be about 1.3kΩ for a voltage sweep of 0-1 V. This is a result of the good electrical conductivity of graphene. A sweep of the temperature from 21.5C to 47C was applied on the graphene to check the stability in accordance to the temperature. The resistance of the graphene increase slightly with increasing temperature and become unstable for a temperature higher than 45C. The graphene was cooled down and brought back to the initial room temperature of 21.5C and the resistance was found to be 1.3kΩ. Which leads us to the conclusion that the graphene is able to reset to the initial resistance value after increase in temperature. Thus the temperature do not affect the physical properties of the graphene. Raman spectroscopy was performed on Graphene on substrate and Graphene on top of cavity, the intensity ratio of the peak I(2D)/I(G) decrease from 4 to 1.7.We have been able to successfully fabricate a membrane structure to serve as a sensor for gas. The resulting electrical characterization shows the material can be used as a sensing material. Ongoing work involves the integration of the membrane structure and testing it in vacuum using the setup already installed and test with different gases.

Two-dimensional (2-D) semiconductors exhibit excellent device characteristics, as well as novel optical, electrical, and optoelectronic characteristics. In this talk, I will present our recent advancements in defect passivation, contact engineering, surface charge transfer doping, and heterostructure devices of layered chalcogenides. We have developed a defect repair/passivation technique that allows for observation of near-unity quantum yield in monolayer MoS₂. The work presents the first demonstration of an optoelectronically perfect monolayer. Forming Ohmic contacts for both electrons and holes is necessary in order to exploit the performance limits of enabled devices while shedding light on the intrinsic properties of a material system. In this regard, we have developed different strategies, including the use of surface charge transfer doping at the contacts to thin down the Schottky barriers, thereby, enabling efficient injection of electrons or holes. We have been able to show high performance n- and p-FETs with various 2D materials. Additionally, I will discuss the use of layered chalcogenides for various heterostructure device applications, exploiting charge transfer at the van der Waals heterointerfaces. I will also present progress towards achieving tunnel transistors using layered semiconductors.

Deterministic self-assembly, demanded in nanotechnology, needs a highly precise control of driving forces and energy minimization at the nanoscale, which has been applied in the macro-level, for example, capillary origami and 3D devices. We utilize dry-releasing approach via rapid thermal annealing and manipulate the surface tension of nanodroplets to assist the rolling of strained nanomembranes, and thus push the downscale limitation of rolled-up tubes. The capillary torque generated by the nanoparticles on the nanomembrane surface can efficiently reduce the diameter of rolled-up tubes and thus overcome the downscale limitation (below 100 nm), which is consistent with our theoretical predication. Furthermore, such small tubes embedded with Pt nanoparticles can work as nanoengines and exhibit a dramatic acceleration in speed compared to those with smooth Pt surface due to enhanced mass transfer and large surface area.
We also demonstrated that the unique geometry of the whispering-gallery plasmonic nanotubular cavities, which are fabricated by a strain-engineered self-rolling approach, can significantly enhance the surface plasmon resonance as a result of highly concentrated optical fields. Such synchronous and coherent coupling of the plasmonic and whispering-gallery resonance greatly enhances Raman signals, which could potentially be a simple and robust method towards single-molecular detection with good optimization. The tunability of the coupling effect could open up novel ways to develop new device concepts for high performance deep-sub-wavelength optoelectronic devices such as plasmonic waveguides, single cell photonic nanoprobes, and hyperbolic metamaterials. Our methodology offers a great opportunity for mechanical deformation, such as folding, bending, buckling, and zipping, in nanoscale self-assembly, and may enable solid nanomembranes becoming an essential building blocks in flexible electronics, and lab-on-a-chip micro/nano-electromechanical systems (MEMS/NEMS).

Stronger and more robust nacre-like laminated GO (graphene oxide)/SF (silk fibroin) nanocomposite membranes are obtained by enhancing the interfacial interaction of “bricks”-GO sheets and “mortar”-silk interlayers via controlled water vapor annealing. This annealing has been utilized to relax the confined silk backbones that resulted in significant increase in all mechanical properties measured by bulging test: ultimate strength (by up to 41%), Young’s modulus (up to 75%) and toughness (up to 45%). The highest value achieved for the nanocomposite membrane after annealing shows ultimate stress of 460 ± 65 Mpa, Young’s modulus of 105 ± 10 GPa and toughness of 2.1 ± 0.3 MJ/m³, all characteristics among the highest values for GO based nacre-mimic nanocomposites. Reinforcement mechanism is further investigated. We suggest that though water vapor annealing, SF molecules recrystallization by using hydrophobic surface regions of GO as nucleation sites for beta-sheet formation and directly assembled into nanofibrils that strongly cling on GO layer in situ. Well-ordered packing of GO layers and SF nanofibrils with aligned b-sheet domains results in stronger interfacial interactions with enhanced shear strength across SF layer. The work presented here not only gives the better understanding of SF and GO interfacial interactions, but also provides insight on how to enhance the mechanical properties for the nacre-mimic composites by focusing on adjusting the delicate interactions of heterogeneous “bricks” and adaptive “mortar”.

Single layer graphene is a promising system as a separation membrane. We consider mechanisms and methods of analysis for gas transport through graphene. We also present a detailed analysis of experimental gas permeation data through single layer graphene membranes under batch depletion conditions parametric in starting pressure for He, H₂, Ne, and CO₂ between 100 and 670 kPa. We show mathematically that the observed intersections of the membrane deflection curves parametric in starting pressure are indicative of a time dependent membrane permeance (pressure normalized molecular flow). Analyzing these time dependent permeance data for He, Ne, H₂, and CO₂ shows remarkably that the latter three gases exhibit discretized permeance values that are temporally repeated. Such quantized fluctuations (called “gating” for liquid phase nanopore and ion channel systems) are a hallmark of isolated nanopores, since small, but rapid changes in the transport pathway necessarily influence a single detectable flux. We analyze the fluctuations using a Hidden Markov model to fit to discrete states and estimate the activation barrier for switching at 1.0 eV. This barrier is and the relative fluxes are consistent with a chemical bond rearrangement of an 8–10 atom vacancy pore. Furthermore, we use the relations between the states given by the Markov network for few pores to determine that three pores, each exhibiting two state switching, are responsible for the observed fluctuations; and we compare simulated control data sets with and without the Markov network for comparison and to establish confidence in our evaluation of the limited experimental data set.

Membrane separations play an important role in the mitigation of global problems, such as water shortage, or air pollution. Nanoporous graphene membranes have significant potential to advance membrane technologies for gas separation, water desalination, chemical separation and nanofiltration. Understanding the mechanical strength of porous graphene is critical because membrane separations often involve high pressures. We studied the burst strength of CVD graphene membranes placed on porous supports at applied pressures up to 100 bar by monitoring the gas flow rate across the membrane as a function of pressure. Increase of gas flow rate with pressure allowed for measurement of the fraction of graphene that failed under increasing pressure. SEM and AFM images acquired before and after the burst test were in good agreement with the gas flow rate measurements, and revealed that wrinkles in graphene were prone to failure whereas non-wrinkled areas could sustain high pressure. Graphene membranes consisting of graphene on support membranes with smaller pore sizes tend to have better overall quality, since the extent of damage due to wrinkles is limited. As an essential aspect of the graphene membranes for separations, the effect of created defects on mechanical strength is also studied. We find that the porous graphene membranes with created defects are still ultra-strong, even though finite decrease of the strength occurs. Our study shows that polycrystalline CVD graphene has ultra-high burst strength under applied pressure, suggesting the possibility for its use in high-pressure membrane separations.

Strain engineering has long been recognized as an effective approach for tuning electronic and transport properties of electronic devices as well as for fabrication of nanostructures. In this talk, I will discuss a novel concept of strain engineering to tune spin transport via mechanical bending of a 2D topological insulator (TI). It works by the physical principle that the spin orientations of helical edge states of a 2D TI nanoribbon change continuously when the nanoribbon is bent into a curved shape, as shown by the solution of Kane-Mele model in a curved graphene. Using TI nanofilm of Bi/Cl/Si (111) as a material example, we further demonstrate by first-principles calculations that the relative spin orientations between the two edge states of a Bi/Cl/Si(111) nanoribbon change gradually from antiparallel to parallel when it self-bends from a planar structure to a half cylinder driven by inherent strain. This novel approach of strain engineering of spins via “topological nanomechanical architecture” affords a promising route towards realization of robust spin injectors with 100% spin polarization and large spin current density.

In this study, we report the abnormal and significant thickness-dependent optical property modulation of Methlyammonium Lead Bromide (MAPbBr₃) up to 100 nm grown from Van der Waals (VDW) epitaxy on mica substrates. Single crystalline perovskite square films with thickness ranging from four to several hundreds of nanometers have been obtained which display a blue shift of the photoluminescence peak - 150 meV. The XRD study and TEM characterization reveal clearly the VDW epitaxial relation between MAPbBr₃ and mica. Multiple explanations including the epitaxial strain, band-filling, size-dependent Stokes shift and photon recycling effects have been explored after which the strain factor turns out to play a significant role. Our study is the first attempt to reveal how significant VDW strain could be in such materials system. It sheds light on how we should design halides materials growth via vacuum technology.

Graphene-based materials have attracted considerable interest due to their potential use in a broad range of applications including membrane separations, electrical devices and biomaterials. Their performance in many of these applications relies on the ability to precisely control the layer number and film density of the materials produced. Chemical vapor deposition (CVD) is one of the most promising methods to produce single layer graphene. However, the high cost, limited choice of substrates and error-prone transfer processes limit its application in large-scale roll-to-roll fabrication. On the other hand, laboratory-scale film processing approaches like spin-coating, filtration and drop-casting are incapable of producing films with nanometer-scale control of film thickness and uniformity and/or over the large length scales required for many applications. Recently, the Langmuir-Blodgett (LB) deposition technique has been applied to graphene and graphene oxide with some success. In this method, the material is dispersed at the air-water interface, the floating material, held up by surface tension, and is densified by the compression of two floating barriers. The resulting film is transferred onto substrates by dip coating. While fine control over layer number and film density has been achieved, the technique is currently limited to small area films and suffers from the challenge of losing up to 99% of the material used for transfer. In our work, we developed a directed assembly process for producing graphene oxide monolayers at the air-water interface which assemble into densely tiled films without the need for adjustable barriers. By simply changing the spreading solvent, we obtain high-yield transfer and are able to measure Langmuir surface areas as high as 800 m²/g. Furthermore, we use both in situ Brewster angle microscopy and a custom-made Langmuir-Adam balance to study the mechanism of film formation. The process has also been successfully extended to other 2D nanomaterials such as MoS₂. We believe that the directional film growth mechanism and its applicability to a variety of 2D materials make it a promising process for continuous roll-to-roll deposition for a wide range of applications.

Despite their widely recognized potential in gas storage, catalysis, separation methods, etc., metal-organic frameworks (MOFs) have not yet found widespread industrial applications, mostly on account on the inability to upscale these unique materials. Typically, MOFs come as small disjoint crystallites with the largest reported dimensions in millimeters¹. Recently, there has been significant effort to develop methods that would deposit MOFs over large surfaces but they have not, so far, yielded freestanding or formable/moldable structures. Here, we report synthesis of freestanding, porphyrin-based MOF films at 6-inch-wafer scales, with preferred crystalline orientation, and with the ability to form such structures over arbitrary surface topographies, including micropatterns. Other unique features of this approach are that (1) the formation of the pre-complex promoting MOF growth and the growth itself take place simultaneously, in one pot, and without any need for prior surface activation/functionalization; (2) the orientation of the crystallites in the film can be controlled by the substrate and (3) the method is easily extended to various porphyrin-based systems. Demonstrations in controlling surface wettability (including both water-pinning Wenzel² and water-repellant^3,4 Cassie states), in the recovery of oil spilled over seawater and Volatile Organic Compounds (VOCs), and in large–area chemo or chemo-resistive sensors illustrate the practical potential of truly macroscopic MOF materials.

Over the last years substrate-supported composite films of ligand-stabilized or cross-linked gold nanoparticles (GNPs) attracted significant interest due to their intrinsic, tunneling-based charge transport mechanism, which is highly sensitive to external stimuli. For example, such films were used for the fabrication of highly sensitive strain gauges or resistive sensors for chemical vapors and gases.
In a recent publication we demonstrated the lift-off of alkanedithiol cross-linked GNP thin films and their transfer to 3D electrode microstructures, producing freestanding, conductive nanomembranes with thicknesses in the 20 to 100 nm range and lateral dimensions of several hundreds of micrometers.[1] The tunable elasticity as well as their unique charge transport properties make these membranes highly interesting for the application as functional materials in nano-/microelectromechanical systems (NEMS/MEMS), such as sensors or actuators.

Here, we present the focus of our current research, which is the exploration of novel NEMS/MEMS devices, exploiting the unique properties of cross-linked GNP nanomembranes.
In a recent paper[2] we reported the fabrication of a novel resistive pressure sensor, employing a 1,6-hexanedithiol (6DT) cross-linked GNP nanomembrane as both, diaphragm and strain sensitive transducer. The nanomembrane was deposited over a microfabricated cavity featuring proximal electrodes, suitable for monitoring its resistance. An applied pressure difference resulted in deflection and strain of the nanomembrane and a corresponding resistance change. The elasticity as well as good strain sensitivity of the membrane material enabled a pressure sensitivity which outranges values reported for resistive graphene and silicon based sensors.
Furthermore, we were able to show that freely suspended GNP nanomembranes, placed closely above a counter electrode, can be deflected by electrostatic forces.[3] Electrostatic actuation is a versatile principle, which is prevalently used in NEMS/MEMS. Applying AC voltages, we utilized this actuation mechanism to excite oscillations of the GNP nanomembranes, which were characterized using interferometry. By variation of the excitation frequency, different vibrational modes of the nanomembranes could be observed and imaged by mapping experiments. For circular nanomembrane resonators (diameters of 50 and 100 µm) resonance frequencies in the high kHz to low MHz range and quality factors of up to ~2000 were observed.[4] We present current investigations, exploring the applicability of these devices as microgravimetric sensors.

[1] H. Schlicke, J. H. Schröder, M. Trebbin, A. Petrov, M. Ijeh, H. Weller, T. Vossmeyer, Nanotechnology 2011, 22, 305303.
[2] H. Schlicke, M. Rebber, S. Kunze, T. Vossmeyer, Nanoscale 2016, 8, 183-186.
[3] H. Schlicke, D. Battista, S. Kunze, C. J. Schröter, M. Eich, T. Vossmeyer, ACS Appl. Mater. Interfaces 2015, 7, 15123–15128.
[4] H. Schlicke, C. J. Schröter, T. Vossmeyer, submitted.

As always in physics the response of a material is strongly altered once the dimensionality is reduced. This is especially true when electronic and thermal transport processes are considered. Consequently, the dimensionality can be gauged by the mean free path of e.g. an electron or a phonon as compared to the size of the system. This translates into the condition of two-dimensionality for a nanomembrane when the membrane is made to be thinner than the phonon mean free path L_ph at room temperature, i.e. 300 nm for silicon. Hence, such nanomechanical membranes offer novel applications, such as mass sensing of large bio-molecules with extraordinary sensitivity. The general idea is that perfect mechanical mass sensors should be of extremely small size to achieve zepto- or yocto-gram sensitivity in weighing single molecules similar to a classical scale. However, the small effective size and long response time for weighing biomolecules with a cantilever restricts their usefulness as a high-throughput method. Commercial mass spectrometry (MS) on the other hand, such as electro-spray ionization (ESI)-MS and matrix-assisted laser desorption/ionization (MALDI)-time of flight (TOF)-MS and their charge amplifying detectors are the gold standards to which nanomechanical resonators have to live up to. These two methods rely on the ionization and acceleration of biomolecules and the following ion detection after a mass selection step, such as time-of-flight (TOF). The principle we are describing here for ion detection is based on conversion of kinetic energy of the biomolecules into thermal excitation of CVD diamond nanomembranes via phonons, followed by phonon-mediated detection via field emission of thermally emitted electrons. We fabricated ultrathin diamond membranes with large lateral dimensions for MALDI-TOF MS of high mass proteins. These diamond membranes are realized by straightforward etching methods based on semiconductor processing. With a minimal thickness of 100 nm and cross sections of up to 400 x 400 µm² the membranes offer extreme aspect ratios. Ion detection is demonstrated in MALDI-TOF analysis over a broad range from Insulin to BSA. The resulting data in detection shows much enhanced resolution as compared to existing detectors, which can offer better sensitivity and overall performance in resolving protein masses.

Biological lipid bilayer membrane is an ideal example for precise and efficient molecular separation. One of its characteristics is a free-standing property with molecular thickness, and molecular scale phenomena become dominant in the direction of the membrane thickness. Thus, artificial membrane with a free standing properties and nanometer thickness would be a unique property different from conventional thicker membrane. Based on this idea, we have developed functional free-standing nanomembranes with a centimeter-scale of lateral size. These membrane are manipulable macroscopically, event its thickness is a few tens nanometers.
We have succeeded to prepare a free-standing and ultrathin membrane with precise molecular filtration ability by designing nanochannels structures across a membrane. Our next target is to separate further small molecules, including CO₂ and gaseous molecules, because membrane separation of CO₂ is one of promising CO₂ capture technologies. In this scope, we have developed membranes composed of polymer and inorganic materials.
In polymeric nanomembranes, we have investigated cross linkable materials, such as an epoxy resin, urea and melamine derivatives, for the preparation of nanomembrane. In all case, we have succeeded to prepare free-standing membrane with a few tens nanometer thick, and the gas permeance of each membrane was investigated.
In inorganic membrane, we employed the composite materials composed of titanium alkoxide carboxylic derivatives, such as phthalic acid, to control the gas selectivity of the membrane. Based on a spin-coating process, titania composite membrane with the thickness of 100 nm or less was prepared on a PDMS support. Some composite membrane, show preferential CO₂ permeation over nitrogen.
In membrane separation, the thickness plays an important role for the efficient separation. Further thinning to reach the thickness of a biological lipid membrane is our challenge to create ideal membrane separation based on molecular dynamics.

Addressing water pollution arising from oil spillage and chemical leakage is still challenging and much progress has been made currently toward separating oil-water mixture with high flux by super-wetting membranes [1]. As permeation theory predicts that filtration flux is inversely proportional to the membrane thickness, membrane with thickness at nanometer scale has been a trend to obtain high performance in both selectivity and permeability [2, 3]. However, durability of ultrathin films is a major tough challenge, especially for the ability to undergo external mechanical forces or damages. Moreover, biofouling resulted from growth of unwanted marine organisms cannot be effectively avoided and causes detrimental effects on membrane separation systems [4]. Therefore, a new approach for fabricating a durable micro- or nano-membrane that can withstand a series of harsh conditions, while ensuring high-flux molecular transport, has been presented.

Herein, an extremely robust carbon nanofiber-polydimethylsiloxane (CNFs-PDMS) network inlay-gated stainless steel mesh (SSM) that shows superhydrophobic property is subtly designed and prepared by improved vacuum-based suction. Carbon nanofibers with 100 nm diameter were deposited into SSM pores to form network membrane. The resulting SSM/CNFs-PDMS membrane exhibits excellent resistance to harsh environments such as acid, salt, organic, biofouling, and mechanical abrasion treatments. Particularly, mechanical damage to the CNFs-PDMS network can be avoided using the protective support of SSM to ensure super-wetting performance. Compared to previous superhydrophobic membranes, the thickness significantly decreases, leading to enhanced oil-in-water emulsion separation flux. The membrane shows a gravity-driven water-in-oil emulsion separation with flux up to 2970 L/m²h¹. Even if the SSM/CNFs-PDMS membranes are eroded and abraded, high separation performance was still maintained accordingly. Moreover, this technique has been applied for preparing highly durable ultra-thin carbon nanotubes (CNTs) nano-membrane with high flux in present research. Thus, this work can provide a brand new route for creating durable and high-flux separation systems by combining nano-membranes with protective mesh with inlay-gated structure.
References
[1] B. Wang et al. Chem. Soc. Rev. 336-361(2015) 44
[2] S. Karan et al. Science 1347-1351 (2015) 348
[3] Z. Shi et al. Adv. Mater. 2422-2427 (2013) 25
[4] J. A. Callow et a