CNG Research News

Periodic graphene nanodots

To fabricate transistors and computer chips, today we use photolithography. This approach is well developed, reliable and allows for patterning on the scale of tens of nanometers.
However, going beyond this limit is extremely challenging, and perhaps simply impossible with the current technology.

Self-assembly can be an alternative. In this process, single elements, like atoms or molecules, aggregate spontaneously to create larger, more complex structures.

A group of researchers at Technical University of Denmark (Denmark), Århus University (Denmark), IBM (USA) and Brookhaven National Laboratory (USA), have now shown self-assembly within a two-dimensional sheet.

When two gases are simultaneously dosed on a heated crystal of iridium in a vacuum chamber, the gas molecules dissociate and reorganize on the surface of the metal to make an one-atom-thick membrane. Within this membrane, carbon atoms self-assemble into dots of uniform size of approximately 2 nm.

These carbon nanodots repel each other, thus self-organizing in a periodic array. The research team also has shown that by changing the ratio between the two gases, the periodicity of the nanodot array can be tuned as well.

To study the new material, the team used an advanced microscope, called scanning tunneling microscope, which is available at Århus University. This microscope has a metallic tip that scans over the surface of a material, acquiring information about the structural and electronic properties of the material under study, with atomic precision.

The researchers have also developed a theoretical model able to shed light on the synthesis process. Under the conditions used in the experiments, an ensemble of islands of a given, small size is energetically more stable than a large island formed by the aggregation of the small ones.

The present work paves the way for an extreme form of materials design, providing a radically new material that is expected to have novel optoelectronic properties and a variety of potential device applications.

For further information contact:
DTU: Luca Camilli/Peter Bøggild (lcam@nanotech.dtu.dk/peter.boggild@nanotech.dtu.dk)
Aarhus University: Liv Hornekær (liv@phys.au.dk/61663133).

June 2016

Hot pickup - temperature control makes batch fabrication of 2D layers possible

Researchers at DTU Nanotech have developed a highly robust method to “laminate” atomically thin films with super-clean interfaces.

First there were graphene, an atomically thin film of carbon atoms with record-crushing properties. Then thousands of other atomically thin materials entered the scene which can be layered to create new hybrid supermaterials - socalled van der Waals heterostructures. Until now, only a few leading groups have been able assemble these with sufficiently high quality. With the "hot pickup" stacking method, researchers at DTU Nanotech aim to make atomic scale nanoassembly faster and easier than ever before.

Since graphene was isolated for the first time in 2004, more than 200000 articles and 40000 patents have been published, a testament to the enormous interest from both scientific and commercial communities. Thanks to the long list of extraordinary properties of these atomically thin films of carbon atoms, applications in diverse areas such as digital electronics, solar cells, high speed communications, water filtering, corrosion protection, flexible electronics, quantum computing, polymer reinforcements, catalysis, energy storage, gas sensing and energy harvesting are being developed - to mention just a few. In the past few years, it has become clear that graphene is not at all the only two-dimensional supermaterial. More than 2500 other layered, atomically thin, materials have signed up, ready to be studied and exploited. While these materials cover an amazing range of electrical, chemical, optical and mechanical properties, perhaps the most astounding discovery is that these materials can be combined freely to create altogether new materials, so called van der Waals heterostructures. Since all atoms and molecules attract each other by the ubiquitous van der Waals forces, there are virtually no limitations to how all these new superthin materials can be assembled into stacks – just like LEGO blocks. The difficulty lies in doing this without damaging the thin films, and without incorporating dirt and wrinkles as one may imagine. In collaboration with James Hones group in Columbia University, researchers at the Center for Nanostructured Graphene at DTU Nanotech have developed what they call the “hot pickup” technique for creating stacks of atomically thin films. The trick is to control the temperature of the stack as well as the manipulator - the "nanohand". The nanohand is a a thin plastic coating on a glass slide can be used to both pick up and drop down the thin films, and the temperature is used to tune the adhesion for either picking up or releasing the films.

At the same time, the assembly - where the films are “laminated” – is done at high temperatures since this turns out to lead to super-clean interfaces - contamination is simply squeezed out during the nano-lamination process. Perhaps the most wide ranging feature is the ability to assemble already nanopatterned films. Stacking of multiple structured layers allows interconnects and much more complex circuitry, much in the same way that modern computer chips use multiple active layers to achieve high er speed and more functionality – just much, much smaller. The researchers used the method to batch fabricate high quality devices, which is crucial for accelerating research and development of the many emerging layered materials. The work is published in Nature Communications, June 16, 2016, and funded by CNG-Center for Nanostructured Graphene and the Graphene Flagship.

Read the paper here (Open Access)

A video showing "drop-down" without blister formation can be seen here

June 2015

Thinking out of the box in nanoplasmonics

In classical optics, the interaction of metal particles with light can be expressed using the refractive index of the metal. An important question in nanoplasmonics is when this classical theory breaks down and what new effects, perhaps quantum effects, start to play a role.

For tiny metal nanoparticles, the classical theory predicts that the optical resonances will not depend on the particle size, but only on its shape. But this is not seen when shining light on few-nanometer sized metal particles. Instead, for noble metals such as silver and gold, the resonances of smaller particles move to higher frequencies (“blueshift”), while for so-called simple metals such as sodium the resonances shift instead to lower frequencies (“redshift”).

Until now these phenomena could only be reproduced with quantum mechanical ab-initio calculations, which are numerically very costly. Giuseppe Toscano and co-authors from Germany, China, and Denmark have now developed a simpler hydrodynamic theory for light that also predicts the observed frequency shifts. This increases our understanding of the origin of these frequency shifts. And since the new hydrodynamic calculations are less heavy, they can be used to make accurate predictions for more and larger structures than before. The research is published in Nature Communications.

Figure: Electric field and charge densities in a nanosphere, based on the self-consistent hydrodynamic theory by Toscano et al.

So what is the origin of the different frequency shifts? Classical Drude theory assumes that the electrons in a metal are free to move, at least within the “box” defined by the metal geometry. Standard hydrodynamic theory also makes this assumption, and then incorrectly predicts that resonances of smaller particles will always shift to higher frequencies, for whatever kind of metal. Giuseppe Toscano et al. have now “removed the box” from hydrodynamic theory, and describe how electrons will spill out a tiny little bit also outside the box defined by the classical boundary of the metal. (Less than a nanometer, but still.) In noble metals, this spill-out is negligible so that standard hydrodynamic theory is pretty accurate, but in simple metals the spill-out is larger and gives rise to the observed redshifts of optical resonances for smaller particles.

Original publication
Giuseppe Toscano, Jakob Straubel, Alexander Kwiatkowski, Carsten Rockstuhl, Ferdinand Evers, Hongxing Xu, N. Asger Mortensen, and Martijn Wubs, Resonance shifts and spill-out effects in self-consistent hydrodynamic nanoplasmonics, Nature Communications 6, 7132 (2015) (Open Access).

January 2015

The role of edge states for quantum plasmons in graphene nanostructures

Edge states are ubiquitous for many condensed matter systems with multicomponent wave functions and they are known to play a crucial role for quantum-electron transport in zigzag graphene nanoribbons. In a PRB Rapid Communication, Thomas Christensen and CNG coworkers now show in detail how quantum transitions between bulk states and edge states also have a critical influence on the optical response of graphene nanostructures.

Imagine two graphene nanodisks of the same size. Within classical electrodynamics they are seemingly identical even though they might differ in their atomic-scale details such as in their edge termination. Surprisingly, our quantum mechanical calculations reveal significant differences, with the zigzag version exhibiting a redshift of the dipole resonance and an additional broadening not seen for the armchair version.

In addition to having key importance for graphene plasmonics, our findings also connect to a wider class of systems supporting edge or surface states, e.g., topological insulators such as bismuth bilayers or silicene, MoS2 nanotriangles, nanostructures with Ag(111) facets, or indeed any finite bipartite systems which support zero-energy localized states.

Read the entire article here

November 2014

Electrically Continuous Graphene from Single Crystal Copper Verified by Terahertz Conductance Spectroscopy and Micro Four-Point Probe

Electrical performance of CVD graphene transferred to insulating surfaces may be compromised by extended defects, including for instance grain boundaries, cracks, wrinkles, and tears. In this study, we experimentally investigate and compare the nano- and microscale electrical continuity of single layer graphene grown on centimeter-sized single crystal copper with that of previously studied graphene films, grown on commercially available copper foil, after transfer to SiO2 surfaces. The electrical continuity of the graphene films is analyzed using ultrabroadband terahertz timedomain spectroscopy and micro four-point probe. Ultrabroadband terahertz time-domain spectroscopy measures the complex conductance response in the frequency range 1−15 terahertz, covering the entire intraband conductance spectrum, and reveals that the conductance response for the graphene grown on single crystalline copper intimately follows the Drude model for a barrier-free conductor. In contrast, the graphene grown on commercial copper foil shows a distinctly non-Drude conductance spectrum that is better described by the Drude−Smith model, which incorporates the effect of preferential carrier backscattering associated with extended, electronic barriers with a typical separation on the order of 100 nm. Micro four-point probe resistance values measured on graphene grown on single crystalline copper in two different voltage−current configurations show close agreement with the expected distributions for a continuous 2D conductor, in contrast with previous observations on graphene grown on commercial copper foil. The presented results demonstrate that the graphene grown on single crystal copper is electrically continuous on the nanoscopic, microscopic, as well as intermediate length scales.

Read the entire article here.

Jonas D. Buron, Filippo Pizzocchero, Bjarke S. Jessen, Timothy J. Booth, Peter F. Nielsen, Ole Hansen, Michael Hilke, Eric Whiteway, Peter U. Jepsen, Peter Bøggild, and Dirch H. Petersen.

October 2014

Science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems

The European Graphene Flagship project is one of the largest research grants given in the European research scene, with more than 120 research groups and a total budget of 1000 Million euro over 10 years. The project is covering many different areas of graphene and 2D material research from fundamental science to applications, from chemistry and physics to biology and environmental research. To give the public and the scientific community a chance to learn more about the ambitions and contents of this monstrous and manyheaded project, Andrea Ferrari and Francesco Bonaccorso from Manchester University has led the tremendous and very complicating task of coordianting the writing of the review paper Science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems. The article seeks to give a comprehensive overview of the research field and more specifically describe what routes the flagship will be sailing - a manifest of the next decade(s) of graphene research on the European scene. Peter Bøggild of CNG has contributed to the chapter 8 on sensors, which is one of the smaller chapters in the more than 200 pages and 2334 references extensive article.

Read the entire article here.

Decoration with DNA molecules of a graphene field effect device can affect the chemoresistive response according to the specific DNA sequence.

September 2014

Revealing origin of quasi-one dimensional current transport in defect rich two dimensional materials

The presence of defects in graphene have for a long time been recognized as a bottleneck for its utilization in electronic and mechanical devices. We recently showed that micro four-point probes may be used to evaluate if a graphene film is truly 2D or if defects in proximity of the probe will lead to a non-uniform current flow characteristic of lower dimensionality. In this work, simulations based on a finite element method together with a Monte Carlo approach are used to establish the transition from 2D to quasi-1D current transport, when applying a micro four-point probe to measure on 2D conductors with an increasing amount of line-shaped defects. Clear 2D and 1D signatures are observed at low and high defect densities, respectively, and current density plots reveal the presence of current channels or branches in defect configurations yielding 1D current transport. A strong correlation is found between the density filling factor and the simulation yield, the fraction of cases with 1D transport and the mean sheet conductance. The upper transition limit is shown to agree with the percolation threshold for sticks. Finally, the conductance of a square sample evaluated with macroscopic edge contacts is compared to the micro four-point probe conductance measurements and we find that the micro four-point probe tends to measure a slightly higher conductance in samples containing defects.

The paper is featured in Applied Physics Letters 105, 053115 (2014)
DOI: 10.1063/1.4892652

Mikkel R. Lotz, Mads Boll, Ole Hansen, Daniel Kjær, Peter Bøggild, and Dirch H. Petersen

Histograms of the simulated resistance ratio.

June 2014

Study of extremely confined gap surface-plasmon modes reported in Nature Communications

Ultra narrow and deep grooves in metal surfaces are known to support gap surface-plasmon (GSP) modes, as observed in experiments with optical excitation. Such modes are typically featuring very strong absorption which can render otherwise shiny gold surface completely black.
Søren Raza, Nicolas Stenger and CNG coworkers have now in collaboration with SDU and AAU explored such gap surface-plasmon modes with electron-beam excitation. The team used high-spatial and energy resolution electron energy-loss spectroscopy (EELS) – available at DTU Cen – to probe extremely confined GSP modes excited by swift electrons in nanometre-wide gaps. Using this unique approach they were able to reveal the resonance behavior associated with the excitation of the antisymmetric GSP mode for extremely small gap widths, down to 5 nm.

While the explored antisymmetric GSP mode is far too lossy for any waveguide application, its existence plays a crucial role in experimental realizations of non-resonant light absorption by ultra-sharp convex grooves with fabrication-induced asymmetry being inevitably present. The occurrence of the antisymmetric GSP mode along with the fundamental GSP mode exploited in plasmonic waveguides with extreme light confinement is a very important factor that should be taken into account in the design of nanoplasmonic circuits and devices.

The paper is featured in Nature Communications 5, 425 (2014), DOI: 10.1038/ncomms5125

S. Raza, N. Stenger, A. Pors, T. Holmgaard, S. Kadkhodazadeh, J.B. Wagner, K. Pedersen, M. Wubs, S.I. Bozhevolnyi & N.A. Mortensen, “Extremely confined gap surface-plasmon modes excited by electrons”.

News_june

Beam of swift electrons traversing a metal nanogroove (artwork by Nanna Bild).

May 2014

Nanopatterned Graphene as Ultrasensitive NO₂ Sensing Material

Nitrogen dioxide (NO₂) is an important atmospheric pollutant due to the widespread presence of both natural (e.g. lightning and volcanos) and in particular anthropogenic sources (e.g.: internal combustion engines, thermal power stations, kerosene burners…). NO₂ can be a respiratory irritant limiting lung functionalities, but it also contributes to creation of smog inside cities. Smog is considered a major cause of respiratory diseases, which were related to more than 16% of the total worldwide deaths in 2008.

In their recently published article in Nano Research, Alberto Cagliani, David Mackenzie and coworkers from CNG and DTU-Nanotech show the use of nanopatterned graphene as ultrasensitive NO₂ sensing material. They achieved homogeneous nanopatterning of cm-scale areas by spherical block copolymer lithography which generated locally hexagonal arranged nanoholes with 10 to 50 nm in diameter. The nanopatterned samples showed sensitivities for NO₂ of more than one order of magnitude higher than for non-patterned graphene. NO₂ concentrations as low as 300 (parts per trillion) ppt were detected with an ultimate detection limit of tens of ppt. This is so far the smallest value reported for not UV illuminated graphene chemiresistive NO₂ gas sensors. The drastic improvement in the gas sensitivity is believed to be due to the high adsorption site density, thanks to the combination of edge sites and point defect sites, achieved by tuning of the nanopatterning process parameters. The nanopatterned graphene devices open a new way for scalable fabrication of NO₂ sensors with more than one order of magnitude higher response than CVD graphene in the sub-100ppb range.

The paper is featured on the cover of the May issue of Nano Research (Nano Research, 7(5), 743–754, 2014, DOI: 10.1007/s12274-014-0435-x).

Nanopatterned graphene and gas sensing measurements.

April 2014

Generalized nonlocal plasmonic theory reported in Nature Communications

With the rapidly maturing of nanofabrication capabilities and novel experimental techniques, the plasmonic response of noble-metal nanostructures is now being intensively explored at length scales down to few nanometers or even atomic dimensions. At such extreme length scales, classical electrodynamics is naturally speculated to become challenged which has made a call for quantum descriptions of plasmons and light-matter interactions.

Indeed, recent experimental studies have fueled this development with observations for both nanoparticles and dimers that seem impossible to explain with classical electrodynamics and the celebrated Drude local-response description of plasmons. Is this exposing the fundamental limitations of classical electrodynamics and the onset of quantum dynamics of plasmons?

Much unexpected, N. Asger Mortensen and co-workers (from CNG and in collaboration with SDU and AAU) have shown how the applicability of classical electrodynamics may be extended to these extreme dimensions by semi-classically accounting for nonlocal dynamics of the electron gas associated with both quantum-pressure driven convection and diffusion of the electromagnetically induced charge.

The generalized nonlocal optical response (GNOR) theory places established observations of size-dependent damping into the context of nonlocal response and offers an accurate classical explanation of spectral broadening in metallic nanoparticle dimers without invoking quantum-mechanical tunneling.

More details of the theory and its consequences were published on 2 May in Nature Communications:

N.A. Mortensen, S. Raza, M. Wubs, T. Søndergaard, and S.I. Bozhevolnyi, “A generalized nonlocal optical response theory for plasmonic nanostructures”, Nature Communications 5, 3809 (2014).

Nonlocal broadening in plasmonic dimer with a sub-nanometer gap (artwork by Nanna Bild).

[For a popularizing account in Danish, see blog by Jakob Rosenkrantz de Lasson]

January 2014

Conductance mapping of graphene using a dual-probe STM (Phys. Rev. Lett. 112, 096801 (2014))

Mikkel Settnes and co-workers from CNG have theoretically investigated the possibilities offered by multiprobe STM setups. Recent experimental progress has demonstrated the use of multi-probe STM configurations with probe separations down to 50-100 nm. In their very recent paper in Physical Review Letters – this issue features one of their figures as the cover image - (full paper available athttp://arxiv.org/abs/1401.8156), Settnes et al. consider a dual-probe STM setup on graphene and calculate the conductance between the two probes. Electrons are injected at one probe and propagate through the sample to be collected at the other probe. The transmission between the different points on the sample yields much more information than what can be extracted by a standard single probe STM experiment which reflects the topographic fea-tures and local density of states.

Fixing one probe and using the other to scan the sample allows for a real space mapping of the conductance. This exhibits an anisotropic behavior depending on the crystalline directions. The features are analyzed using analytic expressions for the graphene Green function allowing for transparent explanations of the observed behavior. If the probe separation is less than the inelastic mean free path of graphene quantum interference effects will not be washed out by dephasing and a characterization of defects and edges by their scattering patterns in both real and Fourier space becomes possible, in analogy to optical diffraction principles.

Using a gate to vary the Fermi energy, the energy dependent conductance can be calculated. The energy sweeps show different fingerprints depending on the crystalline direction (armchair or zig-zag). Furthermore, dual-probe spectroscopy also appears to be a promising tool for characterizing individual structuring of the graphene sample such as holes or grain boundaries.

The present work paves the way to treating larger interprobe separations, more easily obtainable experimentally, and predicts the influence of nanostructuring of graphene on the conductance of multiprobe setups.

Cover page of Physical Review Letters,
7 March 2014

December 2013

Strong plasmon-phonon coupling in graphene disk and antidot arrays

Xiaolong Zhu and co-workers from CNG have explored strong plasmon-phonon coupling in the mid-infrared regime. Unprecedented large-area graphene nanodot and antidot optical arrays are fabricated by nanosphere lithography, with structural control down to the sub-100 nanometer regime. The interaction between graphene plasmon modes and the substrate phonons is experimentally demonstrated and structural control is used to map out the hybridization of plasmons and phonons, showing coupling energies of the order 20 meV.

The study reveals the importance of polar substrates and their intrinsic optical phonons for both understanding and exploring the plasmon dispersion. Coupling between the substrate phonons and graphene plasmons is significant and may extend the optical tunability of graphene plasmons beyond that offered by electrostatic gating.

For more details, see arXiv:1312.2400

Strong hybridization of graphene plasmons and SiO2 substrate phonons.

September 2013

Enhanced light-matter interaction in hybrid graphene-gold plasmonic nanostructures

Xiaolong Zhu and co-workers from CNG have explored hybrid graphene-gold plasmonic nanostructures for enhanced light-matter interaction. Gold surfaces with nanoscale variations in the surface topography may support plasmons with strongly enhanced electrical fields. Such structures are widely promoted for studies of enhanced light-matter interactions in general and for surface-enhanced spectroscopy in particular. In their work just accepted for publication in Nano Letters (http://dx.doi.org/10.1021/nl402120t), Zhu et al. placed a single layer of graphene in the near-field of the plasmonic host-structure, thus opening a doorway to enhance interactions between the graphene and the plasmons supported by the nanostructured gold substrate. In their experiments, the strong proximity between the noble-metal plasmons and the graphene is witnessed by both a very distinct frequency shift of plasmon resonances themselves (allowing for accurately quantifying even the number of graphene sheets in case of a multi-layer coverage) as well as pronounced plasmon-enhanced Raman response from vibrations in the graphene layer itself (up to a 700-fold enhancement of the characteristic Raman spectrum of graphene).

The tremendous enhancement factors on rough metallic surfaces have made the technique of surface-enhanced Raman spectroscopy (SERS) potentially very interesting for bio-chemical detection with an accuracy approaching even the single-molecule level. However, the enhancement comes at a price. The rough and spatially irregular surface topography is the key to SERS while at the same time opposing the technological desire to work with well-defined, flat, and cleanable surfaces. The present work opens a very interesting new direction: adding a layer of graphene to the metallic structures leaves the impression of an atomically flat and chemically well-defined surface without jeopardizing the giant field-enhancement supported by the plasmon in the underlying nanostructured metallic surface. By adding SERS active molecules (rhodamine molecules) on top of the hybrid graphene-gold structure, this hypothesis is indeed confirmed by experiments.

News of the month september

Artist’s impression of SERS response from molecules on a hybrid graphene-gold substrate.

August 2013

Nonlocal theory of plasmonic waveguiding featured on the cover ofNanophotonics

Giuseppe Toscano, Søren Raza, Wei Yan, and co-workers from CNG used the hydrodynamic model to formulate a novel wave equation that incorporates nonlocal effects in a straightforward and exact manner through a Laplacian correction in the commonly used local-response wave equation. With this new development, nonlocal effects can be readily accounted for in existing computational frameworks.

The power of the new formulation was exemplified by studies of waveguiding with extreme light confinement, revealing the upper limitations on the light confinement in groove and wedge metal waveguides. The limitation on light-matter interactions translates into an upper limit for the corresponding Purcell factors, and thus has important implications for quantum plasmonics.

The paper is featured on the cover of the July issue of the high-impact journal Nanophotonics (De Gruyter) [Nanophotonics 2(3), 161-166 (2013)] and the activities on nonlocal theory have stimulated considerable interest with calls to present the work as invited talks at both the MRS Spring Meeting 2013, ICMAT 2013, and PIERS 2013.

June 2013

Graphene plasmonics: enabling coupling to graphene-plasmon polaritons by subwavelength silicon gratings

Xiaolong Zhu, Wei Yan, and co-workers from CNG have experimentally demonstrated graphene-plasmon polariton (GPP) excitation in a continuous graphene monolayer. Intrinsically, the GPPs are non-radiating because of the momentum mismatch to free-space radiation and to optically excite GPPs one has to break the continuous translational symmetry in one or the other way. While previous work has overcome this by a cumbersome nanostructuring of the graphene itself, our present work elegantly paves a new way: just place the graphene on top of an artificial two-dimensional subwavelength silicon grating!

The paper was recently published in Applied Physics Letters and it has already lead to a significant interest with a prominent position on the ‘Top 20 Most Read Articles in April 2013’ and an invitation to Sanshui Xiao to present the work at PIERS 2013 in Stockholm.

Artist’s impression of continuous graphene layer resting on top of an artificial silicon grating.

May 2013

EELS studies of plasmon blueshift effects in silver nanoparticles

Søren Raza, Nicolas Stenger and co-workers from CNG used state-of-the-art electron-energy loss spectroscopy (EELS) to study the confinement of plasmons in nanoscale silver particles where quantum properties and nonlocal response of the plasma are anticipated to become important. Larger particles (>25 nm) exhibit classical dynamics, while the smallest particles (~3.5 nm) show significant resonant blue shifts of 0.5 eV. This observation cannot be explained by classical local-response theory, thus providing intriguing evidence for the importance of quantum effects and nonlocal dynamics in light-matter interactions at the nanoscale.

The paper has been accepted for publication in the recently launched high-impact journal Nanophotonics. Our results were recently presented at Nanometa 2013 and together with the preprint arXiv:1210.2535 the work has stimulated considerable interest with calls to present the work as invited talks at both META’13 and SPP6.

Updated by Christina Rochat on 17 January 2020

Responsible editor: Christina Rochat