Publications by authors named "Sebastian Heeg"

16 Publications

  • Page 1 of 1

Anti-Stokes Raman Scattering of Single Carbyne Chains.

ACS Nano 2021 Jul 13. Epub 2021 Jul 13.

Photonics Laboratory, ETH Zürich, 8093 Zürich, Switzerland.

We investigate the anti-Stokes Raman scattering of single carbyne chains confined inside double-walled carbon nanotubes. Individual chains are identified using tip-enhanced Raman scattering (TERS) and heated by resonant excitation with varying laser powers. We study the temperature dependence of carbyne's Raman spectrum and quantify the laser-induced heating based on the anti-Stokes/Stokes ratio. Due to its molecular size and its large Raman cross section, carbyne holds great promise for local temperature monitoring, with potential applications ranging from nanoelectronics to biology.
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http://dx.doi.org/10.1021/acsnano.1c03893DOI Listing
July 2021

Tunable Graphene Phononic Crystal.

Nano Lett 2021 Mar 23;21(5):2174-2182. Epub 2021 Feb 23.

Department of Physics, Freie Universität Berlin, Arnimallee 14, 14195 Berlin, Germany.

In the field of phononics, periodic patterning controls vibrations and thereby the flow of heat and sound in matter. Bandgaps arising in such phononic crystals (PnCs) realize low-dissipation vibrational modes and enable applications toward mechanical qubits, efficient waveguides, and state-of-the-art sensing. Here, we combine phononics and two-dimensional materials and explore tuning of PnCs via applied mechanical pressure. To this end, we fabricate the thinnest possible PnC from monolayer graphene and simulate its vibrational properties. We find a bandgap in the megahertz regime within which we localize a defect mode with a small effective mass of 0.72 ag = 0.002 m. We exploit graphene's flexibility and simulate mechanical tuning of a finite size PnC. Under electrostatic pressure up to 30 kPa, we observe an upshift in frequency of the entire phononic system by ∼350%. At the same time, the defect mode stays within the bandgap and remains localized, suggesting a high-quality, dynamically tunable mechanical system.
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http://dx.doi.org/10.1021/acs.nanolett.0c04986DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7953378PMC
March 2021

Raman Scattering Cross Section of Confined Carbyne.

Nano Lett 2020 09 17;20(9):6750-6755. Epub 2020 Aug 17.

Department of Physics, Freie Universität Berlin, 14195 Berlin, Germany.

We experimentally quantify the Raman scattering from individual carbyne chains confined in double-walled carbon nanotubes. We find that the resonant differential Raman cross section of confined carbyne is on the order of 10 cm sr per atom, making it the strongest Raman scatterer ever reported.
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http://dx.doi.org/10.1021/acs.nanolett.0c02632DOI Listing
September 2020

Resonant, Plasmonic Raman Enhancement of α-6T Molecules Encapsulated in Carbon Nanotubes.

J Phys Chem C Nanomater Interfaces 2019 Apr 2;123(16):10578-10585. Epub 2019 Apr 2.

Institut für Experimentalphysik, Freie Universität Berlin, Berlin 14195, Germany.

Surface-enhanced Raman scattering (SERS) and resonant Raman scattering are widely used techniques to enhance the Raman intensity of molecules and nanomaterials by several orders of magnitude. In SERS, typically, molecules are investigated and their intrinsic resonance is often ignored while discussing the plasmonic enhancement. Here, we study α-sexithiophenes encapsulated in carbon nanotubes placed in the center of a nanodimer. By dielectrophoretic deposition, we place the nanotubes precisely in the center of a plasmonic gold nanodimer and observe SERS enhancement from individual tube bundles. The encapsulated molecules are not subjected to chemical enhancement because of the protective character of the nanotube. Polarization-dependent Raman measurements confirm the alignment of the molecules within the carbon nanotubes (CNTs) and reveal the influence of the plasmonic near field on the molecule's Raman intensity. We investigate the encapsulated molecules in small CNT bundles with and without plasmonic enhancement and determine the molecular and plasmonic resonance by tuning the excitation wavelength. We observe a strong red shift of the maximum Raman intensity under plasmonic enhancement toward the plasmon resonance.
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http://dx.doi.org/10.1021/acs.jpcc.9b01600DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7011763PMC
April 2019

Beam Steering with a Nonlinear Optical Phased Array Antenna.

Nano Lett 2019 Sep 26;19(9):6097-6103. Epub 2019 Aug 26.

Photonics Laboratory , ETH Zürich , 8093 Zürich , Switzerland.

Transition metal dichalcogenides (TMDCs) exhibit high second harmonic (SH) generation in the visible due to their noncentrosymmetric crystal structure in odd-layered form and direct bandgap transition when thinned down to a monolayer. In order to emit the SH radiation into a desired direction, one requires a means to control the phase of the in-plane nonlinear polarization. Here, we couple the SH response of a monolayer MoS to an optical phased array antenna and demonstrate controllable steering of the nonlinear emission. By exploiting the intrinsic SH generation by the phased array antenna we achieve uniform emission efficiency into a broad angular range. Our work has relevance for novel optoelectronic applications, such as programmable optical interconnects and on-chip LIDAR.
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http://dx.doi.org/10.1021/acs.nanolett.9b02029DOI Listing
September 2019

Probing hotspots of plasmon-enhanced Raman scattering by nanomanipulation of carbon nanotubes.

Nanotechnology 2018 11 16;29(46):465710. Epub 2018 Nov 16.

We present a two-step procedure to probe hotspots of plasmon-enhanced Raman scattering with carbon nanotubes (CNTs). Dielectrophoretic deposition places a small CNT bundle on top of a plasmonic Au nanodimer. After 'pre-characterizing' both the nanotubes and dimer structure, we subsequently use the tip of an atomic force microscope to push the bundle into the plasmonic hotspot located in the 25 nm wide dimer gap, characterize its location inside the gap, and observe the onset of plasmon-enhanced Raman scattering. Evidence for the activation of the CNT's double-resonant D-mode by the near-field of the plasmonic hotspot is discussed.
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http://dx.doi.org/10.1088/1361-6528/aaded9DOI Listing
November 2018

Carbon Nanotube Chirality Determines Properties of Encapsulated Linear Carbon Chain.

Nano Lett 2018 09 15;18(9):5426-5431. Epub 2018 Aug 15.

ETH Zürich, Photonics Laboratory , 8093 Zürich , Switzerland.

Long linear carbon chains (LLCCs) encapsulated inside double-walled carbon nanotubes (DWCNTs) are regarded as a promising realization of carbyne, the truly one-dimensional allotrope of carbon. While the electronic and vibronic properties of the encapsulated LLCC are expected to be influenced by its nanotube host, this dependence has not been investigated experimentally so far. Here we bridge this gap by studying individual LLCCs encapsulated in DWCNTs with tip-enhanced Raman scattering (TERS). We reveal that the nanotube host, characterized by its chirality, determines the vibronic and electronic properties of the encapsulated LLCC. By choice of chirality, the fundamental Raman mode (C-mode) of the chain is tunable by ∼95 cm and its band gap by ∼0.6 eV, suggesting this one-dimensional hybrid system to be a promising building block for nanoscale optoelectronics. No length dependence of the chain's C-mode frequency is evident, making LLCCs a close to perfect representation of carbyne.
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http://dx.doi.org/10.1021/acs.nanolett.8b01681DOI Listing
September 2018

Minimizing residues and strain in 2D materials transferred from PDMS.

Nanotechnology 2018 Jun 12;29(26):265203. Epub 2018 Apr 12.

Photonics Laboratory, ETH Zürich, 8093 Zürich, Switzerland.

Integrating layered two-dimensional (2D) materials into 3D heterostructures offers opportunities for novel material functionalities and applications in electronics and photonics. In order to build the highest quality heterostructures, it is crucial to preserve the cleanliness and morphology of 2D material surfaces that come in contact with polymers such as PDMS during transfer. Here we report that substantial residues and up to ∼0.22% compressive strain can be present in monolayer MoS transferred using PDMS. We show that a UV-ozone pre-cleaning of the PDMS surface before exfoliation significantly reduces organic residues on transferred MoS flakes. An additional 200 C vacuum anneal after transfer efficiently removes interfacial bubbles and wrinkles as well as accumulated strain, thereby restoring the surface morphology of transferred flakes to their native state. Our recipe is important for building clean heterostructures of 2D materials and increasing the reproducibility and reliability of devices based on them.
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http://dx.doi.org/10.1088/1361-6528/aabd90DOI Listing
June 2018

Plasmonic enhancement of SERS measured on molecules in carbon nanotubes.

Faraday Discuss 2017 12;205:85-103

Department of Physics, Freie Universität Berlin, 14195 Berlin, Germany.

We isolated the plasmonic contribution to surface-enhanced Raman scattering (SERS) and found it to be much stronger than expected. Organic dyes encapsulated in single-walled carbon nanotubes are ideal probes for quantifying plasmonic enhancement in a Raman experiment. The molecules are chemically protected through the nanotube wall and spatially isolated from the metal, which prevents enhancement by chemical means and through surface roughness. The tubes carry molecules into SERS hotspots, thereby defining molecular position and making it accessible for structural characterization with atomic-force and electron microscopy. We measured a SERS enhancement factor of 10 on α-sexithiophene (6T) molecules in the gap of a plasmonic nanodimer. This is two orders of magnitude stronger than predicted by the electromagnetic enhancement theory (10). We discuss various phenomena that may explain the discrepancy (including hybridization, static and dynamic charge transfer, surface roughness, uncertainties in molecular position and orientation), but found all of them lacking in enhancement for our probe system. We suggest that plasmonic enhancement in SERS is, in fact, much stronger than currently anticipated. We discuss novel approaches for treating SERS quantum mechanically that appear promising for predicting correct enhancement factors. Our findings have important consequences on the understanding of SERS as well as for designing and optimizing plasmonic substrates.
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http://dx.doi.org/10.1039/c7fd00127dDOI Listing
December 2017

Dual-Scattering Near-Field Microscope for Correlative Nanoimaging of SERS and Electromagnetic Hotspots.

Nano Lett 2017 04 30;17(4):2667-2673. Epub 2017 Mar 30.

IKERBASQUE, Basque Foundation for Science , 48013 Bilbao, Spain.

Surface-enhanced Raman spectroscopy (SERS) enables sensitive chemical studies and materials identification, relying on electromagnetic (EM) and chemical-enhancement mechanisms. Here we introduce a tool for the correlative nanoimaging of EM and SERS hotspots, areas of strongly enhanced EM fields and Raman scattering, respectively. To that end, we implemented a grating spectrometer into a scattering-type scanning near-field optical microscope (s-SNOM) for mapping of both the elastically and inelastically (Raman) scattered light from the near-field probe, that is, a sharp silicon tip. With plasmon-resonant gold dimers (canonical SERS substrates) we demonstrate with nanoscale spatial resolution that the enhanced Raman scattering from the tip is strongly correlated with its enhanced elastic scattering, the latter providing access to the EM-field enhancement at the illumination frequency. Our technique has wide application potential in the correlative nanoimaging of local-field enhancement and SERS efficiency as well as in the investigation and quality control of novel SERS substrates.
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http://dx.doi.org/10.1021/acs.nanolett.7b00503DOI Listing
April 2017

Graphene Oxide promotes embryonic stem cell differentiation to haematopoietic lineage.

Sci Rep 2016 05 20;6:25917. Epub 2016 May 20.

Cancer Research UK Stem Cell Biology Group, Cancer Research UK Manchester Institute, University of Manchester, Manchester, United Kingdom.

Pluripotent stem cells represent a promising source of differentiated tissue-specific stem and multipotent progenitor cells for regenerative medicine and drug testing. The realisation of this potential relies on the establishment of robust and reproducible protocols of differentiation. Several reports have highlighted the importance of biomaterials in assisting directed differentiation. Graphene oxide (GO) is a novel material that has attracted increasing interest in the field of biomedicine. In this study, we demonstrate that GO coated substrates significantly enhance the differentiation of mouse embryonic stem (ES) cells to both primitive and definitive haematopoietic cells. GO does not affect cell proliferation or survival of differentiated cells but rather enhances the transition of haemangioblasts to haemogenic endothelial cells, a key step during haematopoietic specification. Importantly, GO also improves, in addition to murine, human ES cell differentiation to blood cells. Taken together, our study reveals a positive role for GO in haematopoietic differentiation and suggests that further functionalization of GO could represent a valid strategy for the generation of large numbers of functional blood cells. Producing these cells would accelerate haematopoietic drug toxicity testing and treatment of patients with blood disorders or malignancies.
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http://dx.doi.org/10.1038/srep25917DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4873758PMC
May 2016

Plasmon-enhanced Raman scattering by carbon nanotubes optically coupled with near-field cavities.

Nano Lett 2014 12;14(4):1762-8. Epub 2014 Mar 12.

Department of Physics, Freie Universität Berlin , 14195 Berlin, Germany.

We realize the coupling of carbon nanotubes as a one-dimensional model system to near-field cavities for plasmon-enhanced Raman scattering. Directed dielectrophoretic assembly places single-walled carbon nanotubes precisely into the gap of gold nanodimers. The plasmonic cavities enhance the Raman signal of a small nanotube bundle by a factor of 10(3). The enhanced signal arises exclusively from tube segments within the cavity as we confirm by spatially resolved Raman measurements. Through the energy and polarization of the excitation we address the extrinsic plasmonic and the intrinsic nanotube optical response independently. For all incident light polarizations, the nanotube Raman features arise from fully symmetric vibrations only. We find strong evidence that the signal enhancement depends on the orientation of the carbon nanotube relative to the cavity axis.
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http://dx.doi.org/10.1021/nl404229wDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4055469PMC
March 2015

Polarized plasmonic enhancement by Au nanostructures probed through Raman scattering of suspended graphene.

Nano Lett 2013 Jan 11;13(1):301-8. Epub 2012 Dec 11.

Department of Physics, Freie Universität Berlin, 14195 Berlin, Germany.

We characterize plasmonic enhancement in a hotspot between two Au nanodisks using Raman scattering of graphene. Single layer graphene is suspended across the dimer cavity and provides an ideal two-dimensional test material for the local near-field distribution. We detect a Raman enhancement of the order of 10(3) originating from the cavity. Spatially resolved Raman measurements reveal a near-field localization one order of magnitude smaller than the wavelength of the excitation, which can be turned off by rotating the polarization of the excitation. The suspended graphene is under tensile strain. The resulting phonon mode softening allows for a clear identification of the enhanced signal compared to unperturbed graphene.
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http://dx.doi.org/10.1021/nl3041542DOI Listing
January 2013

Selective bundling of zigzag single-walled carbon nanotubes.

ACS Nano 2011 Apr 23;5(4):2847-54. Epub 2011 Mar 23.

Institut für Nanotechnologie, Karlsruhe Institute of Technology, D-76021 Karlsruhe, Germany.

A simple, high throughput fractionation procedure for aqueous/SDS (sodium dodecyl sulfate) suspensions of single-walled carbon nanotubes (SWNTs) is presented, which yields thin bundles of semiconducting-SWNTs with small chiral angles. To demonstrate this we show the photoluminescence signatures of nanotube suspensions that contain almost exclusively zigzag and near-zigzag tubes. Starting suspensions and resulting fractions were characterized using optical absorption, resonance Raman and photoluminescence spectroscopies as well as scanning force microscopy. Taken together with literature observations, our findings suggest that near zigzag edge tubes of similar diameters in a bundle are harder to separate from each other than for other chiral index combinations. We discuss the implications of these observations for SWNT growth and dispersion.
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http://dx.doi.org/10.1021/nn1033746DOI Listing
April 2011
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