arXiv subjects

Stephanie Reich

Publications and source records attributed to Stephanie Reich.

At least 37 records · Page 2Linked to original sources

arXiv2021

Synthesis of Multifunctional Charge Transfer Agents: Towards Single Walled Carbon Nanotubes with Defined Covalent Functionality and Preserved π System

The attachment of well-defined charge transfer agents to the surface of nanomaterials is an efficient strategy to control their charge density and also to tune their optical, electrical, and physicochemical properties. Particularly interesting are charge transfer agents that either donate or withdraw electrons depending on the arrangements of their building units and that promise a non-destructive attachment to delicate nanomaterials like sp$^2$ compounds. In this work, we rationally synthesize molecular moieties with versatile functionalities. A reactive anchor group allows to attach them to carbon nanotubes as defined charge transfer agents while preserving the tube $π$-conjugation. The charge transfer agents were synthesized through the stepwise nucleophilic substitution of either one (monosubstituted series) or two chlorine (disubstituted series) atoms of cyanuric chloride by aniline derivatives containing one, two or three methoxy groups in the para and meta positions. Variation in the number and position of methoxy as an electron transferring group help us to manipulate the electronic and optical properties of the molecular probes and their charge transfer to the SWNTs systematically. The correlation between the optical properties of these molecular probes and their functionality was investigated by experiments and quantum chemical calculations. While the optoelectronic properties of the conjugated charge transfer agents were dominated by the aniline segments, the triazine warrants the ability to nondestructively attach to the surface of SWNTs. This study is one step ahead towards the production of SWNTs with desired optical and electrical properties by covalent $π$-preserving functionalization.

cond-mat.mes-hall↗

arXiv2021

Plasmon-Polaritons in Nanoparticle Supercrystals: Microscopic Quantum Theory Beyond the Dipole Approximation

Crystals of plasmonic metal nanoparticles have intriguing optical properties. They reach the regimes of ultrastrong and deep strong light-matter coupling, where the photonic states need to be included in the simulation of material properties. We propose a quantum description of the plasmon polaritons in supercrystals that starts from the dipole and quadrupole excitations of the nanoparticle building blocks and their coupling to photons. Our model excellently reproduces results of finite difference time domain simulations. It provides detailed insight into the emergence of the polariton states. Using the example of a face centered cubic crystals we show that the dipole and quadrupole states mix in many high symmetry directions of the Brilouin zone. A proper description of the plasmon and plasmon-polariton band structure is only possible when including the quadrupole-derived states. Our model leads to an expression of the reduced coupling strength in nanoparticle supercrystals that we show to enter the deep strong coupling regime for metal fill fractions above $0.8$. In addition to the plasmon-polariton energies we analyse the relative contributions of the dipole, quadrupole, and photonic states to their eigenfunctions and are able to demonstrate the decoupling of light in the deep strong coupling regime. Our results pave the way for a better understanding of the quantum properties of metallic nanoparticle supercrystals in the ultrastrong and deep-strong coupling regime.

physics.optics↗

arXiv2021

Raman Sideband Thermometry of Single Carbyne Chains

We demonstrate Raman sideband thermometry of single carbyne chains confined in double-walled carbon nanotubes. Our results show that carbyne's record-high Raman scattering cross section enables anti-Stokes Raman measurements at the single chain level. Using laser irradiation as a heating source, we exploit the temperature dependence of the anti-Stokes/Stokes ratio for local temperature sensing. Due to its molecular size and its large Raman cross section carbyne is an efficient probe for local temperature monitoring, with applications ranging from nanoelectronics to biology.

cond-mat.mes-hall↗

arXiv2021

Strong light-matter coupling in MoS$_2$

Polariton-based devices require materials where light-matter coupling under ambient conditions exceeds losses, but our current selection of such materials is limited. Here we measured the dispersion of polaritons formed by the $A$ and $B$ excitons in thin MoS$_2$ slabs by imaging their optical near fields. We combined fully tunable laser excitation in the visible with a scattering near-field optical microscope to excite polaritons and image their optical near fields. We obtained the properties of bulk MoS$_2$ from fits to the slab dispersion. The in-plane excitons are in the strong regime of light-matter coupling with a coupling strength ($40-100\,$meV) that exceeds their losses by at least a factor of two. The coupling becomes comparable to the exciton binding energy, which is known as very strong coupling. MoS$_2$ and other transition metal dichalcogenides are excellent materials for future polariton devices.

cond-mat.mtrl-sci↗

arXiv2020

In situ functionalization of graphene

While the basal plane of graphene is inert, defects in it are centers of chemical activity. An attractive application of such defects is towards controlled functionalization of graphene with foreign molecules. However, the interaction of the defects with reactive environment, such as ambient, decreases the efficiency of functionalization and makes it poorly controlled. Here, we report a novel approach to generate, monitor with time resolution, and functionalize the defects $\textit{in situ}$ without ever exposing them to the ambient. The defects are generated by an energetic Argon plasma and their properties are monitored using $\textit{in situ}$ Raman spectroscopy. We find that these defects are functional, very reactive, and strongly change their density from $\approx 1\cdot10^{13} cm^{-2}$ to $\approx 5\cdot10^{11} cm^{-2}$ upon exposure to air. We perform the proof of principle $\textit{in situ}$ functionalization by generating defects using the Argon plasma and functionalizing them $\textit{in situ}$ using Ammonia functional. The functionalization induces the n-doping with a carrier density up to $5\cdot10^{12} cm^{-2}$ in graphene and remains stable in ambient conditions.

cond-mat.mes-hall↗

arXiv2020

Raman Scattering Cross Section of Confined Carbyne

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 $10^{-22}$ cm$^{2}\,$sr$^{-1}$ per atom, making it the strongest Raman scatterer ever reported.

cond-mat.mes-hall↗

arXiv2020

Impact of Substrate on Tip-enhanced Raman Spectroscopy --- A Comparison of Frequency Domain Simulations and Graphene Measurements

Tip-enhanced Raman spectroscopy (TERS) has reached nanometer spatial resolution for measurements performed at ambient conditions and sub-nanometer resolution at ultra high vacuum. Super-resolution (beyond the tip apex diameter) TERS has been obtained, mostly in the gap mode configuration, where a conductive substrate localizes the electric fields. Here we present experimental and theoretical TERS to explore the field distribution responsible for spectral enhancement. We use gold tips of $40\pm 10 \ \text{nm}$ apex diameter to measure TERS on graphene, a spatially delocalized two-dimensional sample, sitting on different substrates: (i) glass, (ii) a thin layer of gold and (iii) a surface covered with $12\ \text{nm}$ diameter gold spheres, for which $6\ \text{nm}$ resolution is achieved at ambient conditions. The super-resolution is due to the field configuration resulting from the coupled tip-sample-substrate system, exhibiting a non-trivial spatial surface distribution. The field distribution and the symmetry selection rules are different for non-gap vs. gap mode configurations. This influences the overall enhancement which depends on the Raman mode symmetry and substrate structure.

physics.optics↗

arXiv2019

Plasmonic Properties of Close-packed Metallic Nanoparticle Mono- and Bilayers

The self-assembly of metallic nanoparticles is a promising route to metasurfaces with unique properties for many optical applications, such as surface-enhanced spectroscopy, light manipulation, and sensing. We present an in-depth theoretical study of the optical properties of mono- and bilayers assembled from gold and silver nanoparticles. With finite-difference time-domain simulations, we predict the occurrence of two plasmon modes, a bright and a dark mode, which exhibit symmetric and antisymmetric dipole configurations between the layers, respectively. The dark mode resonance energy depends sensitively on the size of the particles and the interparticle gaps. Hotspots with a nearfield intensity enhancement of up to 3000 are expected, which, together with the fact that the dark mode is roughly four times narrower than the bright mode, reveals how promising these materials are for spectroscopy purposes.

cond-mat.mes-hall↗

arXiv2019

Photoswitchable Single-Walled Carbon Nanotubes for Super-Resolution Microscopy in the Near-Infrared

The design of single-molecule photoswitchable emitters was the first milestone toward the advent of single-molecule localization microscopy that sets a new paradigm in the field of optical imaging. Several photoswitchable emitters have been developed but they all fluoresce in the visible or far-red ranges, missing the desirable near-infrared window where biological tissues are most transparent. Moreover, photocontrol of individual emitters in the near-infrared would be highly desirable for elementary optical molecular switches or information storage elements since most communication data transfer protocols are established in this spectral range. Here we introduce a novel type of hybrid nanomaterials consisting of single-wall carbon nanotubes covalently functionalized with photo-switching molecules that are used to control the intrinsic luminescence of the single nanotubes in the near-infrared (beyond 1 $μ$m). We provide proof-of-concept of localization microscopy based on these bright photoswitchable near-infrared emitters.

physics.bio-ph↗

arXiv2019

Atomic-Resolution Visualization and Doping Effects of Complex Structures in Intercalated Bilayer Graphene

Molecules intercalating two-dimensional (2D) materials form complex structures that have been mostly characterized by spatially averaged techniques. Here we use aberration-corrected scanning transmission electron microscopy and density-functional-theory (DFT) calculations to study the atomic structure of bilayer graphene (BLG) and few-layer graphene (FLG) intercalated with FeCl$_3$. In BLG we discover two distinct intercalated structures that we identify as monolayer-FeCl$_3$ and monolayer-FeCl$_2$. The two structures are separated by atomically sharp boundaries and induce large but different free-carrier densities in the graphene layers, $7.1\times10^{13}$ cm$^{-2}$ and $7.1\times10^{13}$ cm$^{-2}$ respectively. In FLG, we observe multiple FeCl$_3$ layers stacked in a variety of possible configurations with respect to one another. Finally, we find that the microscope's electron beam can convert the FeCl$_3$ monolayer into FeOCl monolayers in a rectangular lattice. These results reveal the need for a combination of atomically-resolved microscopy, spectroscopy, and DFT calculations to identify intercalated structures and study their properties.

cond-mat.mtrl-sci↗

arXiv2017

Electronic band gaps of confined linear carbon chains ranging from polyyne to carbyne

Ultra long linear carbon chains of more than 6000 carbon atoms have recently been synthesized within double-walled carbon nanotubes, and they show a promising new route to one--atom--wide semiconductors with a direct band gap. Theoretical studies predicted that this band gap can be tuned by the length of the chains, the end groups, and their interactions with the environment. However, different density functionals lead to very different values of the band gap of infinitely long carbyne. In this work, we applied resonant Raman excitation spectroscopy with more than 50 laser wavelengths to determine for the first time the band gap of long carbon chains encapsulated inside DWCNTs. The experimentally determined band gaps ranging from 2.253 to 1.848 eV follow a linear relation with Raman frequency. This lower bound is the smallest band gap of linear carbon chains observed so far. The comparison with experimental data obtained for short chains in gas phase or in solution demonstrates the effect of the DWCNT encapsulation, leading to an essential downshift of the band gap. This is explained by the interaction between the carbon chain and the host tube, which greatly modifies the chain's bond length alternation.

cond-mat.mtrl-sci↗

arXiv2017

Evaluating arbitrary strain configurations and doping in graphene with Raman spectroscopy

Raman spectroscopy is a powerful tool for characterizing the local properties of graphene. Here, we introduce a method for evaluating unknown strain configurations and simultaneous doping. It relies on separating the effects of hydrostatic strain (peak shift) and shear strain (peak splitting) on the Raman spectrum of graphene. The peak shifts from hydrostatic strain and doping are separated with a correlation analysis of the 2D and G frequencies. This enables us to obtain the local hydrostatic strain, shear strain and doping without any assumption on the strain configuration prior to the analysis. We demonstrate our approach for two model cases: Graphene under uniaxial stress on a PMMA substrate and graphene suspended on nanostructures that induce an unknown strain configuration. We measured $ω_\mathrm{2D}/ω_\mathrm{G} = 2.21 \pm 0.05$ for pure hydrostatic strain. Raman scattering with circular corotating polarization is ideal for analyzing strain and doping, especially for weak strain when the peak splitting by shear strain cannot be resolved.

cond-mat.mtrl-sci↗

arXiv2017

The First Precise Determination of Graphene Functionalisation by in situ Raman Spectroscopy

We report, for the first time, a comprehensive study involving in situ Raman spectroscopy supported by quantum mechanical calculations to exactly monitor the covalent binding to graphene with unprecedented precision. As a model reaction we have chosen the hydrogenation of reduced graphite ($KC_8$) with $H_2O$ and compared it with the corresponding exposure to $H_2$ and $O_2$. The early stages of graphene hydrogenation are accompanied by the evolution of a series of so far undiscovered D-bands ($D_1$-$D_5$). Using quantum mechanical calculations, we were able to unambiguously assign these bands to distinct lattice vibrations in the neighborhood of the covalently bound addend. Interestingly, the exposure of $KC_8$ to $H_2$ and $O_2$ didn't cause covalent binding, but intercalation of molecular $H_2$ or partial oxidation, respectively. A combination of $H_2O$ and $O_2$ treatment led to the formation of additional hydroxyl (-OH) functionalities. The latter reaction represents a very suitable model for the decomposition of graphenides under ambient conditions (hydrogenation and hydroxylation). We have applied this Raman analysis to simulate and satisfactorily characterize a series of additional covalently functionalised graphene derivatives prepared as bulk materials with different composition (e.g. degree of functionalisation and the nature of covalent addend) demonstrating the generality of the concept and the fundamental value for graphene chemistry.

cond-mat.mtrl-sci↗

arXiv2015

Transport, Magnetic and Vibrational Properties of Chemically Exfoliated Few Layer Graphene

We study the vibrational, magnetic and transport properties of Few Layer Graphene (FLG) using Raman and electron spin resonance spectroscopy and microwave conductivity measurements. FLG samples were produced using wet chemical exfoliation with different post-processing, namely ultrasound treatment, shear mixing, and magnetic stirring. Raman spectroscopy shows a low intensity D mode which attests a high sample quality. The G mode is present at $1580$ cm$^{-1}$ as expected for graphene. The 2D mode consists of 2 components with varying intensities among the different samples. This is assigned to the presence of single and few layer graphene in the samples. ESR spectroscopy shows a main line in all types of materials with a width of about $1$ mT and and a $g$-factor in the range of $2.005-2.010$. Paramagnetic defect centers with a uniaxial $g$-factor anisotropy are identified, which shows that these are related to the local sp$^2$ bonds of the material. All kinds of investigated FLGs have a temperature dependent resistance which is compatible with a small gap semiconductor. The difference in resistance is related to the different grain size of the samples.

cond-mat.mtrl-sci↗

arXiv2015

Quantum Nature of Plasmon-Enhanced Raman Scattering

We report plasmon-enhanced Raman scattering in graphene coupled to a single plasmonic hotspot measured as a function of laser energy. The enhancement profiles of the G peak show strong enhancement (up to $10^5$) and narrow resonances (30 meV) that are induced by the localized surface plasmon of a gold nanodimer. We observe the evolution of defect-mode scattering in a defect-free graphene lattice in resonance with the plasmon. We propose a quantum theory of plasmon-enhanced Raman scattering, where the plasmon forms an integral part of the excitation process. Quantum interferences between scattering channels explain the experimentally observed resonance profiles, in particular, the marked difference in enhancement factors for incoming and outgoing resonance and the appearance of the defect-type modes.

cond-mat.mes-hall↗

arXiv2013

Nanotube caps on Ni, Fe, and NiFe nano particles: A path to chirality selective growth

Carbon nanotubes have properties depending on the arrangement of carbon atoms on the tube walls, called chirality. Also it has been tried to grow nanotubes of only one chirality for more than a decade it is still not possible today. A narrowing of the distribution of chiralities, however, which is a first step towards chirality control, has been observed for the growth of nanotubes on catalysts composed of nickel and iron. In this paper, we have calculated carbon-metal bond energies, adhesion energies and charge distributions of carbon nanotube caps on Ni, Fe and NiFe alloy clusters using density functional theory. A growth model using the calculated energies was able to reproduce the experimental data of the nanotube growth on the alloy catalysts. The electronic charge was found to be redistributed from the catalyst particles to the edges of the nanotube caps in dependence of the chiral angles of the caps increasing the reactivity of the edge atoms. Our study develops an explanation for the chirality enrichment in the carbon nanotube growth on alloy catalyst particles.

cond-mat.mtrl-sci↗

arXiv2012

Chirality-Dependent Growth Rate of Carbon Nanotubes - A Theoretical Study

We consider geometric constraints for the addition of carbon atoms to the rim of a growing nanotube. The growth of a tube proceeds through the conversion of dangling bonds from armchair to zigzag and vice versa. We find that the growth rate depends on the rim structure (chirality), the energy barriers for dangling bond conversion, and the growth temperature. A calculated chirality distribution derived from this minimalistic theory shows surprisingly good agreement with experiment. Our ideas imply that the chirality distribution of carbon nanotubes can be influenced by external parameters.

cond-mat.mtrl-sci↗

arXiv2012

Nanotube bundles and tube-tube orientation: A van der Waals Density Functional Study

We study the binding energy, intertube distance and electronic structure of bundles consisting of single walled carbon nanotubes of the same chirality. We model various nanotube structures (chiralities) and orientations with van der Waals density functional theory. The orientation of the tubes in the bundle strongly influences the properties of the bundles if the chirality of the tubes shares symmetry with the trigonal bundle structure, meaning chiralities which have a $60^\circ$-rotational symmetry (C$_6$-axis), e.g. ($12,0$) bundles. The bundle structure breaks the symmetry depending on the arrangement of the neighboring tubes. Pseudogaps open in the electronic density of states and intertube distances ($\pm5$-10%) vary in dependence of the relative orientation of the tubes in the bundle. Bundles of C$_6$-axis armchair tubes have metallic configurations. A $15^\circ$ rotation off the high symmetry configuration (AA-stacked) of a ($6,6$) bundle shows metallic behavior and has higher binding energy than the high symmetry configuration. We find binding energies between $19 meV/atom$ and $35 meV/atom$, depending on the chirality of the tubes. The intertube distances are between $3.2 Å$ and $3.4 Å$ but independent of orientation for non C$_6$-axis tubes.

cond-mat.mtrl-sci↗