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Ultramicroelectrodes (UMEs) are electrochemical working electrodes with at least one dimension less than 25 μm. Here, we demonstrate a method to fabricate pristine (contamination and doping-free) monolayer graphene UMEs and characterise its electrochemical properties. Graphene is particularly sensitive to both contamination from device fabrication and doping from the substrate, and these effects are detrimental to accurate measurements of fundamental electrochemical properties as well as electrochemical electrode applications. We demonstrate a UME design and fabrication steps which ensure that the electro-active surface of the graphene never comes into contact with contaminants such as process polymers and solvents, has a well-defined area due to self-aligned fabrication, and we shield the converse side of the graphene monolayer with the highly inert dielectric hexagonal boron nitride (h-BN). Cyclic voltammetry demonstrates the expected steady-state behaviour for a UME in the hemispherical diffusion regime. The reduction of IrCl62− in weak KCl electrolytes was employed to investigate the electron transfer characteristics of the pristine graphene UMEs and the reproducibility of the measurements. Values of the standard rate constant, k0 and the transfer coefficient, α were measured to be 3.04 ± 0.78 × 10−3 cm s−1 and 0.272 ± 0.024 respectively. These values are consistent for monolayer graphene with reduced charge doping from contaminants and the substrate. This is further confirmed by Raman spectroscopy measurements of the graphene UME before and after the electrochemical measurements.

The electromagnetic enhancement theory describes surface-enhanced Raman scattering (SERS) as a Raman effect that takes place in the near-field of a plasmonic nanostructure. The theory has been very successful in explaining the fundamental properties of SERS, modelling the performance of different metals as enhancing materials and optimizing SERS hotspots for strongest possible enhancement. Over the last decade, a number of carefully designed experimental studies have examined predictions of the electromagnetic theory like the size and shape of SERS hotspots, the absolute magnitude of the enhancement and the width of the SERS resonance. Although the overall picture was quite satisfactory, the theory failed to predict key aspects of SERS, for example, the absolute magnitude of the plasmonic enhancement. We scrutinize these experiments and review them focusing on the results that require going beyond the electromagnetic enhancement theory. We argue that the results of these experiments create the need to develop the theory of SERS further, especially the exact role of plasmonic enhancement in inelastic light scattering.

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 cm2 sr−1 per atom, making it the strongest Raman scatterer ever reported., Nano Letters, 20 (9), ISSN:1530-6984, ISSN:1530-6992

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., Comment: 12 pages, 4 figures, including Supplementary Information with 6 pages, 6 figures and 1 table

Carbon, 139, ISSN:0008-6223

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 realize low-dissipation vibrational modes and enable applications towards mechanical qubits, efficient waveguides, and state-of-the-art sensing. Here, we combine phononics and two-dimensional materials and explore the possibility of manipulating phononic crystals via applied mechanical pressure. To this end, we fabricate the thinnest possible phononic crystal from monolayer graphene and simulate its vibrational properties. We find a bandgap in the MHz regime, within which we localize a defect mode with a small effective mass of 0.72 ag = 0.002 $m_{physical}$. Finally, we take advantage of graphene's flexibility and mechanically tune a finite size phononic crystal. Under electrostatic pressure up to 30 kPa, we observe an upshift in frequency of the entire phononic system by more than 350%. At the same time, the defect mode stays within the bandgap and remains localized, suggesting a high-quality, dynamically tunable mechanical system., Comment: 19 pages, 4 figures; Supplementary Information of additional 19 pages and 9 figures

Nano Letters, 19 (9), ISSN:1530-6984, ISSN:1530-6992

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., The Journal of Physical Chemistry C, 123 (16), ISSN:1932-7455, ISSN:1932-7447

Long linear carbon chains encapsulated inside carbon nanotubes are a very close realization of carbyne, the truly one-dimensional allotrope of carbon. Here we study individual pairs of double-walled carbon nanotubes and encapsulated linear carbon chains by tip-enhanced Raman scattering. We observe that the radial breathing mode of the inner nanotube correlates with the frequency of the carbon chain's Raman mode, revealing that the nanotube chirality determines the vibronic and electronic properties of the encapsulated carbon chain. We provide the missing link that connects the properties of the encapsulated long linear carbon chain with the structure of the host nanotube., Comment: keywords: linear carbon chains; carbyne; carbon nanotubes; tip-enhanced Raman scattering; TERS; Significant changes compared to first version of the manuscript. Current version includes Supporting Information

Light interacting with metallic nanoparticles creates a strongly localized near-field around the particle that enhances inelastic light scattering by several orders of magnitude. Surface-enhanced Raman scattering describes the enhancement of the Raman intensity by plasmonic nanoparticles. We present an extensive Raman characterization of a plasmonic gold nanodimer covered with graphene. Its two-dimensional nature and energy-independent optical properties make graphene an excellent material for investigating local electromagnetic near-fields. We show the localization of the near-field of the plasmonic dimer by spatial Raman measurements. Energy- and polarization-dependent measurements reveal the local near-field resonance of the plasmonic system. To investigate the far-field resonance we perform dark-field spectroscopy and find that near-field and far-field resonance energies differ by 170 meV, much more than expected from the model of a damped oscillator (40 meV).

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$_{2}$ flakes transferred using PDMS. We show that a UV-ozone pre-cleaning of the PDMS surface before exfoliation significantly reduces organic residues on transferred MoS$_{2}$ flakes. An additional 200$^{\circ}$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., Comment: To appear in Nanotechnology (2018)

Discussions.

We present the Raman spectra of metallic carbon nanotubes in solution and deposited onto a Si/SiO2 substrate. In the different nanotube samples, we observe qualitatively similar behavior: an upshifted and narrowed metallic LO peak in solution compared to the spectrum on the substrate. This behavior is due to the Fermi energy shifting away from the intrinsic value in the solutions, which we confirm by in situ electrochemical Raman measurements. Our results have implications for all spectroscopic methods that probe nanotubes in aqueous solutions. Raman spectra of metallic nanotubes in solution (orange) and on a Si/SiO2 substrate (black). The inset depicts the position of the Fermi energy in both situations.

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 106 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 (104). 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.

We study the photoluminescence and photoluminescence excitation intensities of single-walled carbon nanotubes. To derive the chirality fractions of semiconducting tubes from these optical methods requires a thorough analysis of the experimental parameters and the lifetimes of the excitonic states. We obtain the intensity distribution from the product of the widths of the first and second excitonic transition and the intensity of the absorption of the second excited state. Compared with previous analysis protocols our intensity distribution points towards a chirality abundance which is (i) narrower in diameter, (ii) centred at smaller diameters and (iii) more strongly dominated by armchair-like tubes. We present a recipe for the fit of luminescence data.

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.

We investigated the spectral data of three carbon nanotube (CNT) species obtained by Raman spectroscopy and photoluminescence (PL) measurements. The corresponding relative signal intensities without further corrections yielded significantly different relative distributions of the CNT species. Theoretical calculations of optical transition probabilities and electron-phonon coupling were included, providing simple models in order to estimate the relative distribution of the three species within the sample. We proposed the product of PL and PL excitation intensities to be a candidate for quantitative analysis of CNT species. Applying the models, we confirmed that both spectroscopic methods agree on one nanotube species dominating the distribution.

We propose to understand surface-enhanced Raman scattering (SERS) as a higher- order Raman process that contains the plasmonic excitation. The SERS amplitudes are calculated with third- and fourth-order perturbation theory. Treating the plasmonic excitation as a quasiparticle, we derive analytic expressions for all coupling matrix elements. This leads to a general theory of plasmonic enhancement in SERS that can be applied to arbitrary plasmonic nanostructures. We obtain the plasmon eigenvectors of a gold nanosphere and a nanosphere dimer. They are used to calculate the enhancement of the Raman cross section of a molecule coupled to the dipole plasmon mode. The enhancement of the cross section is up to three orders of magnitude stronger than predicted by the theory of electromagnetic enhancement. The difference is most pronounced in vacuum and decreases with increasing dielectric constant of the embedding medium. The predictions from understanding SERS as a higher- order Raman process agree well with recent experiments; they highlight the dominance of plasmonic enhancement in SERS.

The longitudinal optical phonon of metallic nanotubes shifts by 23 cm${}^{\ensuremath{-}1}$ to lower energies when the nanotubes are deposited from a solution onto a substrate. The linewidth increases by 13 cm${}^{\ensuremath{-}1}$. The changes are explained in terms of shifts in the Fermi energy that influence the Kohn anomaly in the longitudinal optical phonon branch in metallic nanotubes. Using in situ electrochemical Raman measurements we show that the Fermi energy is 0.16 eV below its intrinsic value in metallic nanotubes in solution. Our results impact the application of Raman spectroscopy to distinguish between metallic and semiconducting tubes by examining the high-energy mode line shape.

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.