We report reversible and spectrally selective fluorescence quenching of quantum dots (QDs) placed in close proximity to graphene. Controlling interband electronic transitions of graphene via electrostatic gating greatly modifies the fluorescence lifetime and intensity of nearby QDs via blocking of the nonradiative energy transfer between QDs and graphene. Using ionic liquid (IL) based electrolyte gating, we are able to control Fermi energy of graphene in the order of 1 eV, which yields electrically controllable fluorescence quenching of QDs in the visible spectrum. Indeed, our technique enables us to perform voltage controllable spectral selectivity among quantum dots at different emission wavelengths. We anticipate that our technique will provide tunable light-matter interaction and energy transfer that could yield hybrid QDs-graphene based optoelectronic devices with novel functionalities, and additionally, may be useful as a spectroscopic ruler, for example, in bioimaging and biomolecular sensing. We propose that graphene can be used as an electrically tunable and wavelength selective fluorescence quencher. Colloidal quantum-dots (QDs) have attracted a tremendous amount of research interest and have a wide landscape of applications ranging from biological imaging to display technologiesF). Unlike pristine graphene, which absorbs around 2.3% over a broad spectrumF for vertical transitions due to Pauli blocking176620et al. demonstrated that modulating charge density on graphene using solid electrolyte, causes modulation of the emission of QDs in infrared frequencieset al. showed that tuning electrostatic doping on graphene using a back-gated transistor, yields relaxation pathways of excited erbium ions placed a few nanometer away from graphene surfaceRecently, controlling intrinsic properties of QDs by electrical means has been introduced by several research groups using graphene, a single atomic layer material with excellent electrical, optical, and mechanical properties89102 layer by drop casting. The average diameter of QDs with the polymer cladding layer is around 6 nm  on the QD-graphene distance, which is varied from 3 nm to 50 nm. The distance-dependence of relative intensity can be modelled by Förster type energy transfer between graphene and quantum dot with a rate, kET, given by:A schematic view of the sample structure used to investigate static fluorescence quenching of QDs on pristine graphene surface is depicted in und 6 nm . Graphenτ0 is the lifetime of the QD in vacuum, d0 is the Förster critical distance, and d is the QD-graphene distancen, depends on the dimensionality of the acceptor. In the case of graphenewhere kr is the rate of energy transfer through radiation. The fit yields the Förster critical distance of d0 = 18 ± 1 nm, which is relatively large in comparison to the critical distance between two dipole emitters (d0 = 6 nm) and comparable with the critical distance between a dipole and 2-D surface (d0 = 25.5 nm)where, a and c are the fitting constants, τ is the lifetime, and β is the stretched exponential parameter. The extracted lifetimes (where ifetimes  vary froF < EQ), the fluorescence of QDs is greatly quenched mainly due to the nonradiative energy transfer between graphene and QDs mediated by interband electronic transitions of graphene. When graphene is heavily doped (2EF > EQ), the suppression of interband transitions results in a significant increase in the intensity of the QDs photoluminescence. During the doping process of graphene, the resistance of graphene varies from 10 to 3 kΩ as the gate voltage is scanned from 0 to +3 V, Now, we would like to study gate-controlled fluorescence quenching of QDs by electrostatic doping of graphene. To elucidate this process, we developed an electrolyte gating scheme (graphene capacitor structure) which gives Fermi energies as large as 1.2 eVn is the index of refraction of the substraten = 1.5), the absorption of graphene is around 1.5% of the incidence light in the visible spectrum. However, as the Fermi energy increases, the transmittance of graphene increases due to the blocking of interband transitions which results in a step-like spectrum with cut-off wavelength of 2EF, F reaches the energy of QDs (2 eV) at a gate voltage of 2.8 V. In fact, this gate voltage agrees very well with the threshold voltage that we observed in the fluorescence measurements. Besides, the variations in optical transmittance, which is due to the blocking of interband transitions are altering the Raman spectra of graphene. Raman spectra of undoped and doped graphene at gate voltages of 0 and 4 V, respectively, are shown in The Fermi energy of graphene provides a useful piece of information about the mechanism of gate-controlled fluorescence quenching. We extracted the Fermi energy of the doped graphene from the optical transmittance spectra. The absorption of single layer undoped graphene on a dielectric substrate is smaller than the theoretical value of 2.3% as 1, E2, and E3 represent energies of the QDs and EF is the Fermi energy of graphene, which is reversibly tunable with the applied gate voltage. In F matches the emission energy of the QDs. Whereas QDs with 655 nm and 625 nm emission wavelengths are switched on at gate voltages of 3 V and 3.7 V, respectively.In order to show the promises of our technique, we would like to generate electrically controllable spectral selectivity among quantum dots with different emission energies. Using step-like behavior of the gate-controlled fluorescence quenching in hybrid graphene-QDs devices, we can successively switch QDs at different gate voltages. We examined this process using several QDs with emission energies of 705 nm (1.75 eV), 655 nm (1.9 eV), and 625 nm (2 eV). In summary, we demonstrated reversible and spectrally selective fluorescence quenching of colloidal quantum dots placed in close proximity to graphene surface in large area. Therefore, here we, for the first time, showed that graphene can be used as an electrically tunable and wavelength selective fluorescence quencher. By tuning the Fermi energy of graphene through efficient electrolyte gating, we were able to modify the florescence intensity of QDs in the visible spectrum. Electrical control of the interband electronic transitions of graphene greatly modifies the fluorescence lifetime and intensity of nearby QDs. Ionic liquid (IL) assisted electrolyte gating enable us to control Fermi energy of graphene in the order of 1 eV yielding electrically controllable fluorescence quenching of QDs. We anticipate that gate-tunable florescence quenching together with its spectral selectivity could provide new electrical means for hybrid QDs-graphene based optoelectronic devises operating in the visible spectrum, and may be useful as a spectroscopic ruler, for example, in bioimaging and biomolecular sensing. Additionally, we anticipate that new generation of electrolytes® 625, 655, and 705 Catalog Number A10197) with emission maxima of ~625 nm, 655 nm, and 705 nm, respectively, purchased from Life Technologies Corporation. The substrates were uniformly coated with QDs by drop casting the diluted ~4 nM QD solution, which was dried with a stream of nitrogen gas.We used CdSe/ZnS core-shell quantum dots (QdotCH4 = 1.5 Torr and PH2 = 3.5 Torr) and flow rate of the gases (JCH4 = 10 s.c.c.m and JH2 = 85 s.c.c.m.) were used to have full coverage single layer graphene on the foils at 1035 °C. After 10 minutes of growth, the samples were left for fast cooling to room temperature.Graphene was synthesized on ultra-smooth copper foils  by chemical vapor deposition. The partial pressure (P2 flow) to pattern the graphene layers.After transfer printing process, we used standard UV photolithography process for the AZ5214 photoresist and oxygen plasma etching , and NH3, respectively. The final film thickness was measured as 65 nm by an atomic force microscope and a surface reflectometer. Same process parameters with 25 °C growth temperature was used to deposit the spacer dielectric layer in order to prevent quantum dots getting damaged from high temperature process.Plasma enhanced chemical vapor deposition (PECVD) was used to grow stress free silicon nitrate film on single crystal silicon (100) substrates as a gate dielectric and spacer. Growth of silicon nitrate film was carried out in PlasmaLab 8510C reactor at 200 °C and the process was carried out under the pressure of 1 Torr and RF power of 9 W. The flow rates of the gases were 25 sccm, 30 sccm, 200 sccm, and 4 sccm for NTime-correlated single photon counting experiments were performed using Horiba, NanoLog series of spectra-fluorometers equipped with Fluoro3PS-Photomultiplier Power Source. NanoLED laser diode with excitation wavelength of 390 nm at repetition rates of 1 MHz was used to excite QDs. The lifetime trajectories of QDs were recorded for 400 ns time durations.Electrical measurements were done by using Keithley Model 2400 and 2401 Source Meter Instrument. The measurements were performed with custom data acquisition software.Fluorescence measurements were performed under a Nikon inverted microscope equipped with Hamamatsu EMCCD camera. To acquire simultaneous electrical and fluorescence measurements, the samples were wire-bonded on a ceramic DIP packages.4 in H2, and 20 sccm NH3. The growth rate of the dielectric was 16 nm/min.Room temperature PECVD deposition was performed in order to prevent QDs to degrade from high temperature. The spacer layer deposition was carried out again in PlasmaLab 8510C reactor at 27 °C (at room temperature) and the process was done under the pressure of 1 Torr and RF power of 9 W. The flow rates of the gases were 200 sccm SiHHow to cite this article: Salihoglu, O. et al. Graphene as a Reversible and Spectrally Selective Fluorescence Quencher. Sci. Rep.6, 33911; doi: 10.1038/srep33911 (2016).


We analyze the destructive interference in monolayer graphene under Landau quantization in a time-dependent way by using the Bloch-Maxwell formalism. Based on this analysis, we investigate the dynamics control of an infrared probe and a terahertz (THz) switch pulses in graphene. In presence of the THz switch pulse, the destructive interference take places and can be optimized so that the monolayer graphene is completely transparent to the infrared probe pulse. In absence of the THz switch pulse, however, the infrared probe pulse is absorbed due to such a interference does not take place. Furthermore, we provide a clear physics insight of this destructive interference by using the classical dressed-state theory. Conversely, the present model may be rendered either absorbing or transparent to the THz switch pulse. By choosing appropriate wave form of the probe and switch pulses, we show that both infrared probe and THz switch pulses exhibit the steplike transitions between absorption and transparency. Such steplike transitions can be used to devise a versatile quantum interference-based solid-state optical switching with distinct wave-lengths for optical communication devices. The graphene has led to many significant research activities on its fascinating electronic and optical properties due to linear and massless band structure near the Dirac point and chiral character of electron states. The magneto-optical properties and thin graphite layers have triggered multiple absorption peaks and particular selection rules between Landau levels (LLs)3. The influence of the Landau quantization on the carrier dynamics in graphene has been investigated8. Progress in making high-quality epitaxial graphene and graphite with high room-temperature mobility and strong magneto-optical response attracted a lot of interest and showed the promise of new applications in the infrared optics and photonics12. In particular, optical transition between LLs may construct the quantum interference pathways which can be exploited, e.g., to devise a novel-type all-optical switching mechanism.The discovery of graphene opened a new area in material science. Graphene is the first truly two-dimensional (2D) crystal consisting of just a single layer of carbon atoms arranged in a hexagonal lattice15. Although a lots of quantum optical effects have been observed in the past decades and within different physics systems, there is renewed interest in such time-dependent dynamic effects which may include solitons20, creation of short pulses25, and multiwave mixing processes in the ultraslow propagation regime28, etc.31. In particular, time-dependent coherent control of the optical response of certain media that exhibit electromagnetic induced transparency (EIT)15 has also triggered a flurry of interest mainly due to its potential applications in integrated optics and quantum information science. The LLs transitions are particular attractive for these investigations, which is much more practical than that in atomic system because of its flexible design and the controllable LLs energies.On the other hand, quantum optical phenomena where the dynamics of one probe pulse can be modulated in time by another pulse have been the focus of current investigations in quantum optics12, but they mainly focused on the absorption spectra, enhanced nonlinear parametric generation and relaxation dynamics while no time-dependent dynamics of coherent pulses was considered. In ref. In this paper, we investigate the time-dependent coherent control of an infrared probe pulse and a terahertz (THz) pulse in a quantized three-level graphene system that exhibit destructive interference between three LLs transitions. This is a rather relevant issue which has remained so far unexplored, as far as we know, both theoretically and experimentally. A few authors have discussed the topic of coherent control of the LLs transitions in graphene system under a strong magnetic fieldSpecifically, we analyze the destructive interference in monolayer graphene under Landau quantization in a time-dependent way via the Bloch-Maxwell formalism. Based on this analysis, we investigate the dynamics control of an infrared probe and a THz switch pulses in graphene. By solving the Bloch-Maxwell equation, we find that the graphene system is completely transparent to the infrared probe pulse when such an interference induced by the switch pulse take places. However, when the THz switch pulse is switched off, the infrared probe pulse is absorbed. Interestingly enough, the present graphene system may also be rendered either absorbing or transparent to the THz switch pulse. Furthermore, we show that both infrared probe and THz switch pulses exhibit the steplike transitions between absorption and transparency. Such steplike transitions can be used to devise a versatile optical switching based on destructive interference for optical communication devices.ωp and THz switch pulse with frequency ωTH are perpendicularly incident on the single-layer graphene  crystal structure in the x − y plane) placed in a magnetic field B, in which both two optical fields and magnetic field are along the z-axis. Under the action of the external magnetic field, the original linear dispersion relation of graphene results in unequally spaced Landau levels (LLs), their transition energies are proportional to 12.We consider a 2D graphene crystal structure in the presence of a strong magnetic field. The geometry of the proposed scheme is shown in Fig. x − y plane), the effective-mass Hamiltonian12 without external optical field can be written asγ0 ~ 2.8 eV and C-C spacing a = 1.42 Å, e is the electron charge and Bx) for a uniform magnetic field. In general, one can obtain the eigenenergies of discrete LLs for the magnetized graphene by solving the effective mass Schrödinger equations, i.e., n  is the Hermite polynomial. The eigenenergy can be calculated as D electron/hole system with a parabolic dispersion, LLs in graphene are unequally spaced and their transition energies are proportional to m〉, |n〉 are LLs with energy index m and n. The term ϕn are orthogonal, the above expression is nonzero only when |m| − 1 = |n| or |m| = |n| − 1. As a result, the selection rule for the allowed transitions turns out to be Δ|n| = ±1, where n is the energy quantum number.When the magnetic field is perpendicularly applied in a single-layer graphene  polarized photons could be homogeneously absorbed. Conversely, the left-hand circularly (LHC) polarized photons are simultaneously absorbed in case of Δ|n| = +13. It should be noted that the carrier frequencies of optical transition between adjacent LLs turn out to be in the infrared or terahertz (THz) region for a magnetic field in the range of 0.01 − 10 T. The electric field vector of the system can be expressed as ωp(ωTH).Due to the special selection rules in present graphene, the selected transitions are dipole allowed between the appointed energy levels, i.e., Δ|ω31 (ω32) between levels |1〉 and |3〉 (|2〉 and |3〉) as a function of the external magnetic field B. For the external magnetic field B up to 3 T, the frequency ωc is on the order of ω32/2π is about 6.59 THz  and By inserting the complete set of states {|3〉, |2〉, |1〉} in Eq. , we can Then we can obtain the total Hamiltonian of graphene system i.e., γij . A comprehensive treatment of the decay rates would involve other broadening mechanism into the system. However, we have adopted the phenomenological approach of treating the decay just as done in the literatures32. A more fully treatment taking into account of these broadening mechanism has been investigated quite thoroughly by some authors  ) is the slowly oscillating term of the induced polarization in LLs transition between Pp = Nμ31ρ31  = Nμ32ρ32). Note that the expressions of the polarization Pp ) contains a 2D electron density N. The electron density can be expressed as lc. To convert them into the bulk polarization for comparison with other materials, we can divide it by the thickness of one monolayer as ND3 = N/Δz . Of course there is no need to consider propagation of the pulses through a monolayer graphene. However, we have used the Maxwell wave equation to describe the propagation dynamics, which can keep our results general to make it applicable to a multiplayer graphene layer. In the present work, we will focus on the time-dependent control of the infrared and THz pulses for one monolayer thickness, i.e., z ~ Δz.In order to correctly describe the time-dependent dynamics of the infrared and THz pulse in the medium, equations of motion –16) mus mus16) mz = 0. In the limit of a weak probe signal, almost electrons will remain in the state |1〉 and hence we may assume that z = Δz as,i, j = 1, 2, 3), we solve the time-dependent Eqs  can be observed in the absorption curve. When the THz switch beam is on  decrease from γ to 0.5γ. And also, Fig. In order to provide a clear picture for the destructive interference induced by the THz switch pulse, for a fixed external magnetic field 34. When the transition a〉 and |b〉 with λa and λb the corresponding energy eigenvalues, respectively. With the rotating-wave approximation and the electric dipole approximation, the Hamiltonian of the transition TH = 0), the energy eigenvalues of Eqs  and g with durations τp and τTH, respectively. In the local (retarded) frame where ξ = z and τ = t − z/c, according to ρij and the wave equations for the normalized infrared probe and THz switch pulses f and g across the graphene system can be rewritten asξ = 0, i.e., As illustrated in Figs f|2 at output terminal (ξ ~ Δz) for different frequency detuning of the infrared probe pulse, i.e., Δp = 0, 0.5γ and γ, respectively. This is done for two different cases, i.e., without (p = 0). If the frequency detuning Δp is tuned to be off-resonance from the corresponding transition, the absorption of the Gaussian-type probe pulse will be suppressed. In presence of the secondary THz switch pulse with p = 0 due to the existence of the destructive interference, which agrees with the results shown in Fig. 15. However, different from the EIT effect, we extend the destructive interference to the pulsed regime, where the destructive interference between the optical transitions , we, we\docug|2 for different widths of two pulses  with varying the probe pulse intensity 34. As done in Fig. Up to now, we have investigated the temporal evolution of infrared probe pulse which depends prominently on the THz switch pulse. Interestingly, the reverse case can also be realized precisely where a THz switch pulse controlled by a secondary probe pulse is rendered either opaque or transparent to the present graphene system. As an example, we consider in Fig. In conclusion, we have investigated in detail the dynamics control of two coherent pulses, an infrared probe and a terahertz (THz) switch pulses, in monolayer graphene where the destructive interference may take place and be easily tuned. We analyze the physics mechanism of this destructive interference indcued by one of the two pulses in graphene under Landau quantization in a time-dependent way via the Bloch-Maxwell formalism. Based on this analysis, we find that the graphene system can be completely transparent to the infrared probe pulse when such an interference induced by the switch pulse take places. In the absence of the switch pulse, the infrared probe pulse can pass through the monolayer graphene with partial transparency. In the presence of the switch pulse, the absorption can be modulated from optimizing the destructive interference by varying the intensity of switch pulse and frequency detuning of the infrared probe pulse. In addition, we provide a clear physics insight of optimizing destructive interference by using the classical dressed-state theory. Conversely, the present graphene system may be rendered either absorbing or transparent to the THz switch pulse. By choosing the appropriate wave form of the pulses, we have demonstrated that both infrared probe and THz switch pulses can exhibit the steplike transitions between absorption and transparency. The present results illustrated the potential to utilize destructive interference to realize the time-dependent control for either signal or control with distinct wave-lengths which may turn out to be useful in designing quantum interference-based solid-state devices for optical communications.


This paper explores enterprise development and commercialization in the field of graphene. Firm characteristics and relationships, value chain positioning, and factors associated with product entry are examined for a set of 65 graphene-oriented small and medium-sized enterprises located in 16 different countries. As well as secondary sources and bibliometric methods to profile developments in graphene, we use computerized data mining and analytical techniques, including cluster and regression modeling, to identify patterns from publicly available online information on enterprise web sites. We identify groups of graphene small and medium-sized enterprises differentiated by how they are involved with graphene, the materials they target, whether they make equipment, and their orientation toward science and intellectual property. In general, access to finance and the firms’ location are significant factors that are associated with graphene product introductions. We also find that patents and scientific publications are not statistically significant predictors of product development in our sample of graphene enterprises. We further identify a cohort of graphene-oriented firms that are signaling plans to develop intermediate graphene products that should have higher value in the marketplace. Our findings suggest that policy needs to ensure attention to the introduction and scale-up of downstream intermediate and final graphene products and associated financial, intermediary, and market identification support. The paper demonstrates novel data methods that can be combined with existing information for real-time intelligence to understand and map enterprise development and commercialization in a rapidly emerging and growing new technology. Graphene is an ultra-thin layer of carbon with exceptional properties and the potential for path-breaking applications across a range of areas including strong lightweight materials, next generation electronic devices, specialized coatings, new biomaterials and sensors, and innovative medical applications . In 2015, more than 4700 graphene patent applications were filed worldwide, based on our analysis of Derwent Innovations data Fig. . This reThe rise of Asian industrial interest in graphene, not just in the Japan and South Korea but also in China, has attracted attention. Baglieri et al.  investigWork published by the UK Intellectual Property Office indicates an accumulation of over 13,000 patent family applications relating to graphene by 2014 IPO . AcademiThe absence of cost-effective and scalable graphene manufacturing techniques is a major current challenge. Ideal large-sized mono-crystalline single-layer sheets of graphene are hard to isolate and costly to produce, while much of the graphene now producible has inferior performance levels IDTechEx . AvailabThere is an inverse relationship between scalability of these techniques and their costs, quality, and range of accessible applications IDTechEx . At presAt present, the number of companies involved not just in research but also in the manufacturing and marketing of graphene and graphene-enabled products is relatively small, although growing. Among early studies, IDTechEx  reportedFrom a commercial perspective, graphene generates value when it is input or embodied into products, processes, devices, and systems that can be marketed to users. Wei and Kivioja  identifyFirms that currently produce and supply graphene directly to the open market are generally small to medium-sized. There are now many available formats of graphene including graphene films, layered materials, composites, and powders with variations in scale, purity, and performance characteristics. Graphene can presently be purchased in low volumes and at relatively high prices. Several companies, such as US-based Graphene Laboratories Inc., sell graphene products online . It is anticipated that graphene prices will decrease as industrial-scale-up occurs, although as yet graphene priced on the market appears not to have entered a significant downward price curve. Over the longer run, it continues to be expected that the development of new technologies will facilitate large-scale production and lower costs—and there have been numerous recent media announcements of novel methods that promise cost-effective graphene mass production , allowing the building up of a data series of developments over time  cases, older versions of web pages can be found through use of internet archives such as the Wayback Machine —whereas analysis is most readily done when these data are transformed into a structured format. In this study, the process of converting unstructured data into a structured format that can be analyzed involves web content retrieval, text mining, and statistical analysis. We used and developed a methodology based on Gök et al.  and triaWe included graphene-based SMEs in our database and excluded firms whose primary activity was not related to graphene. We also did not focus on large multinational companies. We used a mixed-methods process to identify companies that could potentially be included in our data set of graphene SMEs. We started by identifying the assignees of graphene-related patents. This search revealed a number of larger corporations that were active in graphene patenting. Some smaller firms with graphene patents were also identified. This was helpful, but this approach clearly did not cover all enterprise activities in graphene because many SMEs do not patent (or have yet to patent). Therefore, we also used other sources to identify these firms including social media, specialist websites, academic publications, business databases, industry reports, and also gray literature. Our initial worldwide data set consisted of 87 graphene-based SMEs.The process of web content mining started with crawling the websites of these graphene-based SMEs. Web crawling  is a computerized method to harvest and extract information from web sites and web pages. We used IBM Watson Explorer for this process. The web crawling process for the data used in this study was initiated in September 2014 and updated in January 2015. Our review during this crawling process led us to focus on 65 firms . We excluded firms that on closer investigation were really focused on consulting services rather than graphene-related manufacturing. Other firms that dropped out were ones that appeared to no longer be in business at the time of crawling or for whom little useful information could be obtained from their web sites. The final data set does comprise graphene SMEs who are in the business of advancing and developing graphene technologies. As we show later, while there are differences in detailed characteristics, most of these graphene SMEs are upstream in the value chain, making graphene materials, some intermediate products, and equipment for graphene manufacturing. The data set by definition does not include SMEs who do not publicly present online what they are doing in graphene. The crawling process targeted web addresses or, more specifically, Uniform Resource Locators (URLs). In total, some 74,038 URLs were initially crawled. After deleting duplicates, the number of unique URLs associated with the data set was 67,672. There are variations among the firms: some enterprise web sites comprised a few web pages, while others had very large websites in excess of one thousand web pages.After IBM Watson Explorer finished crawling, we indexed the corresponding corpus by using the same tool. This indexing process discarded web pages that did not contain any valuable information. We indexed 11,285 pages. Most of these pages (about 70 %) were in standard HyperText Markup Language (HTML) format. The data set also included text extracted from files in Portable Document Format (PDF), plain text, Extensible Markup Language (XML), Microsoft PowerPoint, and other file types. The resulting corpus comprised more than 9.45 million words. The indexing process included the following four components: (1) language detection, which removes non-English text not filtered out in the crawling process; (2) URL rule-based annotation, where sections of the corpus are tagged by firm names based on extensive rules; (3) linguistic analysis—the corpus is divided into paragraphs and sentences, phrases and words. Each word is stripped of its stemming . Words are tagged by their part of speech ; and (4) named entity recognition annotations: locations, person names, and organizations are tagged in the text by using an advanced UIMA (unstructured information management architecture)-based algorithm. We utilized IBM Watson Explorer Studio in creating some of the UIMA based rules.We exported the indexed corpus into XML and CSV  files so that it could be imported into VantagePoint software. We also created a conceptual map of the variables we sought to extract from the data by using a variety of sources including technical academic and non-academic publications, consultations with graphene experts, and other studies of graphene. A list of the variables and associated extraction rules we used is presented in Appendix Table The final stage of our method involves statistical analysis. We imported the structured table created by VantagePoint into Stata and SPPS. We further processed the data by creating new variables and modifying the existing ones for statistical and cluster analysis. In the statistical analysis, we introduce a normalization process. The websites varied greatly in terms of web pages  and words . We thus normalized all variables by the total number of words they contained then multiplied by a factor of 1000 for ease of comparison.Our validated data set comprised 65 small and medium-sized graphene-based firms. Identifying SMEs is inherently a moving target. The number of graphene companies has increased over the past few years, but there is also underlying churn as new firms are established, some are taken over by other companies and merged, while others may go out of business. Our validation process excludes wholesalers and larger firms. In some countries, particularly China, we have not detected all SMEs, and this underrepresentation should be kept in mind when comparing across the regions. In the other regional locations , our coverage is better. Overall, we do have representation of SMEs from the world’s major regional locations for graphene research and innovation and, based on our available knowledge, the data set represents the largest set of graphene SMEs subject to web mining to date.The 65 small and medium-sized graphene-based firms in our data set are based in 16 different countries. We constructed four regional groupings of firms for further analysis. The North America group constitutes around half of the data set and comprises 32 USA and two Canadian firms. A second group (16 % of the data set) is comprised of ten UK firms. The third group (19 % of the data set) comprises 12 firms located elsewhere in Western Europe, of which six are based in Spain, two in Norway, and one each in Germany, Italy, the Netherlands, and Sweden. The fourth group (17 % of the data set) is comprised of 11 companies located in East Asia and emerging economies, of which three are based in China, two in South Korea, and one each in India, Japan, Malaysia, Russia, Thailand, and Turkey. There is some logic in clustering these countries together in that the varieties of economic management in the countries of the East Asia and emerging economies group emphasize government industrial targeting and control that is more intense than typically found in Western European and North American economies.About three-quarters of the 65 graphene SMEs in our data set were established after 2000 , the extent to which R&D activities are mentioned, the properties of graphene highlighted, graphene production methods, and mentions of manufacturing and service activities.As would be expected from the sample selection, the firms in our data set make many references to graphene. Relatively , graphene intensity is lower for US-based firms, while it is higher for firms based in Western Europe and in East Asia and Emerging countries  have either patented or published. This highlights the point that if we selected firms based only on conventional publication and patent data, we would miss around two-thirds of the graphene firms included in our web content mining sample. Nonetheless, graphene-based SMEs depend on science and technology, and they frequently mention other research and development activities in addition to papers and publications. Firms from North America tend to mention research activities relatively more, while UK firms tend to mention R&D relatively less frequently Fig. . Among tWe have extracted the applications of graphene mention on the websites of the SMEs included in our sample Table . CurrentA summary comparative analysis highlights the key similarities and differences of the characteristics of the firms by the four major regions Fig. , top. InRelationships with other firms, universities, and government are strategically important to the development of high-technology SMEs. Similarly, access to private sector finance is also strategically important for development and growth of SMEs. Our analysis of graphene SME enterprise web sites allow us to discern what these companies present on these vital relationships. We do find that there is frequent mention of linkages with other businesses, universities, and government. In general, mention of linkages with businesses and universities is more common. There are also regional variations in linkages. Firms from East Asia and emerging economies more frequently report business links than firms elsewhere, suggesting that they are part of broader business clusters and alliances. UK and Western European graphene SMEs are relatively less likely to mention other external business relationships. Western European SMEs cite government linkages relatively less frequently. The UK graphene SMEs tend to mention university linkages less frequently than firms elsewhere. North American and UK graphene firms are more likely to mention government linkages. Interestingly, UK firms are more likely to mention linkages with external sources of private finance, including venture capital and equity capital. Other Western European firms are relatively less likely to mention external private sector finance sources Fig. , bottom.We have also examined online ways in which companies market and communicate. Firms in the UK and, to some extent, in East Asia and emerging economies are more frequently likely to report an online sales capability on their web site. Marketing through direct online sales may suggests that firms are more likely to be focused on low-volume commodity graphene materials for research laboratories. It could also suggest capability and trust in online sales processes. We find that many graphene-based enterprises use social media to provide information on their activities and products. An exception is that firms based in East Asia and emerging economies are less likely to use social media. Compared with the UK firms, graphene SMEs in East Asia and emerging economies are more likely to highlight relationships with other businesses, but less likely to emphasize external sources of private finance. These companies may be able to raise funding internally, informally, or from other business partnerships—and they are relatively less willing than UK companies to report their sources of finance on their web sites. Graphene companies in all four regions report relationships with universities, but the UK graphene companies are slightly less likely to stress such relationships than companies elsewhere. North American and UK companies are more likely to report government relationships on their web sites. UK companies are more likely to report online sales capabilities. As a side note, we found that graphene firms based in the UK and East Asia and emerging economies are more likely to mention the graphene Nobel Prize in Physics than firms based elsewhere Fig. .Value chain analysis is a tool for systematically examining the positioning of firms and identifying sources of competitive advantage Porter . It has Material producers firms producing nanoscale graphene-based structures in unprocessed form, e.g., graphene powder, nanoplatelets, dispersion, graphene oxideIntermediate producers firms producing products with nanoscale features that are further incorporated into other products, e.g., graphene-based composites, coatings, inks, battery additives, transistorsEquipment manufacturers firm producing tools and equipment used to visualize, manipulate, and model matter at the nanoscale, e.g., CVD graphene-producing machineFinal product manufacturers firms producing finished goods incorporating graphene, e.g., solar cell, paints, DNA sequencing devicesWe use a value chain position typology developed by Lux Research  originalWe categorized firms according to their existing or potential products in each value chain positions Table . We valiThe analysis reported so far organizes the graphene firms in our sample in regional groupings and value chain positions. There are of course other ways to analyze the web-mined data that we have assembled. One approach is to treat each company as an individual unit, then compare their characteristics and strategies with those of other firms to see where there are similar groupings of these factors. In this approach, similarities and differences among the characteristics of the firms are used to construct groups, rather than using a predetermined group typology. The statistical method used is two-step cluster analysis—this allows us to distinguish archetypes of graphene firms based on key variables. We use the SPSS statistical package to conduct the cluster analysis. Two-step cluster analysis allows for clustering on the basis of both scale and categorical variables. On the basis of the Akaike’s information criterion (AIC) statistics, we selected a five-cluster model. This solution resulted in a “silhouette measure of cohesion and separation” of 0.25 which is generally considered as within the acceptable range. The largest cluster comprises 23 firms, with five firms in the smallest cluster . Most of these firms were founded before 2000. They are not necessarily only based on graphene, but most of these companies have graphene materials and intermediate products in their current and prospective product portfolio. All of the limited numbers of companies that produce final products also belong to this cluster. Therefore, they are spread over the value chain. These companies have very good access to finance. They are also competent in marketing, especially through social media. All of the graphene-related scientific publications were produced by these firms. This cluster includes half of the UK companies.Cluster 2 Graphene-Focused New Entrants: Materials (20 firms). Almost all of these companies are active in producing graphene material. Their linkages to universities, financial markets, and government are relatively lower.Cluster 3 Multi-Material New Entrants: 2D Materials (5 firms). These firms produce a wide variety of 2D materials including graphene as their mention of 2D materials is greatly higher. They are significantly less R&D active. Their links to other businesses and financial markets are particularly lower, while they mention government in their websites significantly more, most probably due to strict health and safety regulations they need to adhere. They tend to mention production methods for graphene and other 2D materials more than other firms. The websites that these companies maintain are considerably larger than other firms. Most companies in this cluster are based in North America.Cluster 4 Equipment Manufacturers (9 firms). Vast majority of these firms are equipment manufacturers, but some of them are also active in producing graphene material. They have good links with financial markets. They also often mention their relationships with other firms and universities (probably as their significant customers). Most companies in this cluster are based in North America.Cluster 5 Science-IP-Oriented Firms (8 firms): These firms are very young—most of them were established after 2010. They are very research active and they produced the majority of the graphene patents filed by graphene-based SMEs. They have very high linkages with universities as well as other businesses. They tend not to mention government in their websites, while almost all of them underline the Nobel Prize in 2010. They also highlight the properties of graphene more than other firms. Some of these firms have started producing materials and intermediate products, but most of them have not yet introduced products or revealed their plans for future releases.We analyzed the typical (average) characteristics of these five clusters. These cluster groups represent sets of companies with broad within-group similarities in characteristics and strategies, and broad between-group dissimilarities with the other groups. The five clusters are:For our Graphene SME sample, there are relatively more UK Established Movers-In than for other geographical regions. Western Europe and Asia have relatively more graphene-focused materials new entrants. North American graphene SMEs are distributed across all categories Fig. .Achieving the economic and commercial benefits anticipated for graphene is contingent on producing products that are demanded in the marketplace. Four-fifths (53) of the firms in our sample have introduced a product to the market, while the remainder (12 firms) plan to introduce products. Once a graphene product is available, a variety of factors will affect its subsequent success, including the nature of demand, the ability to scale-up production and to develop distribution channels, manufacturing processes, and quality, the advantages of the new product over incumbent technologies, competition, access to finance, and possibly regulatory factors. We cannot assess the future likelihood of success from enterprise web sites, but we can develop insights about the factors that influence initial product introduction. To do this, we developed a model predicting the factors influencing introducing a product in the market.We employed a binary logistic regression approach. This approach is appropriate for predicting the influence of various independent factors on a binary-dependent variable where two outcomes are possible. In this case, the two possible outcomes are product introduced or not introduced. Our first model examines influences on introducing products in general. To investigate the factors influencing product introduction in specific value chain positions, we developed three additional models. We break out material production and equipment manufacturing and combine intermediate and final products , it does influence measures of statistical significance.Our results suggest that, in general, access to finance and the firms’ location are significant factors that are associated with graphene product introductions. Graphene SMEs that report access to financial sources, including venture and equity capital, have a higher probability of reporting products currently available. The attraction of external finance may signal firms that have more promising applications, and firms may benefit from the review, guidance, and credibility that is associated with external finance. Graphene SMEs located in the UK also had a more significant likelihood of product introduction in general, although for the upstream graphene materials location in Asia was also significant. Mentioning other 2D materials turned out to be a significantly negative predictor of introducing a product into the market. In other words, focusing on graphene was more likely to be associated with a product introduction—perhaps because other 2D materials are as yet further away from being ready for the market or because focusing on multiple materials in a resource-constrained SME might diffuse or slow down commercialization capabilities. We also found that patents and scientific publications were not statistically significant predictors of product development in our sample of graphene SMEs. In terms of individual value chain positions, being in the material production value chain position was positively related with being located in East Asia and emerging economies. Furthermore, firm age was significantly related to being active in final and intermediate products production.A number of web-based variables including linkages with other businesses, government, and universities are also not significant predictors of introducing a product into the market. There might be two effects working here separately or in combination. First, it might be the case that linkages are in fact not good predictors. Second, it might be that for some firms linkages do matter, but they do not publicize these prominently on their websites.In terms of individual value chain positions, introducing a product in the material production value chain position is positively related with being located in East Asia and emerging economies. Government links are also relatively important for material producers . Access to finance is a critical factor for product introductions by equipment makers, while mention of any other 2D materials is a significant and negative predictor for this group of firms. Finally, firm age is significantly related to introducing products in the intermediate and final part of the value chain. Again, graphene patents and publications are not shown as statistically significantly linked with product introductions in any of the value chain positions Table .Table 3GOur review of current developments in graphene research and commercialization presents a broad picture of growth and advancement. There has been widespread and worldwide recognition of the original path-breaking UK research which first isolated graphene. In recent years, there has been a massive expansion in scientific research and advancements in understanding graphene and its properties. This has generated further research on related two-dimensional materials. There has also been great interest in acquiring intellectual property protection across a wide range of potential graphene processes and applications, as demonstrated by the expansion of patent applications. Universities, public research institutions, and companies in Europe, Asia, and the USA, as well as in other countries, are active in graphene research and commercialization. A growing number of companies have entered the graphene domain, with some early products already on the market. Policy initiatives and programs to stimulate graphene research and commercialization have been launched in multiple countries.Yet, at the same time, there is also uncertainty about the trajectory and character of graphene commercialization. Market expectations and forecasts vary. Manufacturing and industrial-scale up are current areas of concern—and while there is an expectation that these can be resolved with further research, development, investment, and experience, the performance-price competitiveness of graphene compared with other incumbent (and new) materials remains a longer-term issue. The significance of potential barriers to commercialization, whether graphene-specific, such as intellectual property thickets and regulatory developments or broader issues such as access to finance, is also undefined. While there is an emergent graphene value chain, we have yet to see how this will develop and, in particular, what will be the relative roles of large incumbent firms and new small start-up companies in this process. To date, relatively few products enabled by graphene are available in the market and these products mostly offer incremental improvements over existing technologies. The process of developing not only more graphene-enabled devices and products, but also more transformational outputs, is still at an early stage. Such products will need not only to be producible at scale, but also commercially viable, with a premium of price and/or performance over incumbents, and able to gain market acceptability .Uncertainty and ambiguity are, of course, inherent in the process of technological development, especially for novel and disruptive technologies. Moreover, as the path-breaking nature of the underlying research on graphene and its novel properties is acknowledged, it is perhaps inevitable that there will be hype and over-expectation as to the scale and scope of potential commercial applications. The use of hype to boost an emerging technology is evident in elsewhere in nanotechnology and in other emerging technologies Meyer , and theIn this paper, we have further explored a promising new approach to provide intelligence and insight for the analysis of business developments and innovation in emerging technological fields—through mining information from unstructured online sources such as enterprise web pages . This approach is appropriate for emerging technologies that have developed in conjunction with the widespread deployment of web pages by companies, for example synthetic biology. Available structured sources are not comprehensive in capturing the innovation strategies of technology-oriented companies, especially smaller firms who may not patent extensively and who are often privately owned. Yet, increasing amounts of information about such firms are available through unstructured online sources. These data are more readily extracted for SMEs than for larger companies. SMEs are often vital in pioneering new technological innovations in emerging sectors. The online information that is publicly and openly reported by SMEs can be further combined with established structured databases including data on patenting, industry reports, and case studies to facilitate real-time and ongoing monitoring, mapping, and analysis.However, there are limitations. There are difficulties in applying the web content analysis method to large multi-nationals (and their large and complex web sites). The interpretation of what is reported on enterprise web sites is suggestive but can benefit from further validation using other methods. In the current study, we are also limited by the target enterprise population size. We have focused on an emerging technology domain that is of great interest and we have captured detailed data on more graphene SMEs than prior studies. However, at this stage of development, the number of SMEs that are actively engaged in graphene development is relatively small. In further development of enterprise content analysis, it would be important to target domains where there are thousands, if not tens of thousands, of enterprises. This would require requisite scale-up in data analytic capabilities. In this study, we were able to capture the presence of enterprise business and technology terms; in future work, opportunities should be explored to use more sophisticated content analytic methods that can capture topic context as well as the presence to improve interpretation of meaning and significance. Improved capabilities to analyze non-English enterprise websites are also needed. In short, enterprise web content analysis is still exploratory and has limitations as well as advantages. Nonetheless, this approach adds a useful further and new dimension to the information sources available to track and monitor developments in new technologies. While we have discussed the case of graphene companies, it is an approach that can be applied to other areas of emerging technology as part of a larger process of exploring what can  be additionally learned for innovation management and policy from text mining and data analytic approaches using the expanding array of online sources that now are available.


Several transition metals other than the largely used Cu and Ni can be, in principle, employed to catalyze carbon precursors for the chemical vapor deposition of graphene, because the thermodynamics of their alloying with carbon is well known. For example, the wealth of information in the Co-C phase diagram can be used to predict the properties of graphene grown in this way. It is, in fact, expected that growth occurs at a temperature higher than in Ni, with beneficial consequences to the mechanical and electronic properties of the final product. In this work, the growth of graphene onto Co film is presented together with an extensive Raman characterization of the structural properties of the material so far obtained. Previous results reporting the full coverage with negligible defective areas, in spite of discontinuities in the underlying metal, are confirmed, together with the occurrence of strain in the graphene sheet. Strain is deeply investigated in this work, in view of possible employment in engineering the material properties. The observed strain is ascribed to the high thermal mismatch with the substrate, even if an effect of the crystallographic transition of Co cannot be excluded. The thermodynamic approach has been largely employed in the study of the different techniques for the growth of innovative materials, since the birth of material science.G = H − TS , and its derivative with respect to the variation of the quantity of matter, the so-called chemical potential M = δG/δn, allow discriminating between ordered or disordered growth, or by crystalline or epitaxial processes. This approach, however, had no great success in the case of the growth of graphene onto catalyzing metals by chemical vapor deposition (CVD), probably because of the great complexity of this mechanism.Thermodynamic potentials, like the Gibbs’ free energy 104s1}, which minimizes reciprocal repulsions. The 3d7 and 3d8 orbitals of Co. and Ni are between the most unstable electronic configuration (Fe) and the most stable one (Cu). Based on this, several investigators considered Cu and Ni as the most suitable catalysts for graphitic carbon formation, whereas Co received less attention [The most used catalyzing metals employed in the CVD of graphene are Ni and Cu. Copper has the lowest affinity to carbon, in fact, it does not form any carbide phases , and hasttention ,3,4,5,6,Tracking carbon during the growth process by means of isotope labeling in conjunction with Raman spectroscopic mapping ,8, demonContinuously imaging the C monolayer coverage on Cu using low-energy electron microscopy confirmed no C precipitation or island growth during cooling , suggestIn contrast, Co and Ni can dissolve much more C atoms. The graphene growth comes also from the precipitation during the cool-down of the process and “polygraphene” was detected in most cases , as a covacancy sinks. In the case of Co and Ni, on the other hand, an extra, effective mechanism for vacancy reduction is provided by the C atoms precipitating from the substrate [From another point of view, cooling can be considered as an additional way to reduce the entropy of the system. In fact, after the isothermal stage of exposure to hydrocarbons, the entropy of vacancies in Cu-grown graphene can be reduced only by their diffusion during cooling, since no additional atoms are available to saturate them ,10. It cubstrate . G = H − TS, it is straightforward that G is minimized with a low-entropy contribution when the deposition temperature is high. It is then advisable that most of the graphene growth occurs during the first stages of cooling. Baraton et al. [T, a lateral growth of high quality graphene occurs at surface curvatures of the catalyzer, and assisted by a diffusive flux of C atoms, whereas, when decreasing T, surface precipitation of C occurs, giving rise to nanocrystalline graphene with high entropy.According to the definition of the Gibbs’ free energy, n et al. , supportT in the case of Ni:D0 = 2.48 cm2 s−1, the activation energy DE = 1.74 eV, and Bk is Boltzmann’s constant. Calculation gives a diffusion length for C atoms in Ni of the order of few μm in the first 1 s of the cooling stage. This means that C atoms can freely redistribute in the growing film to reduce the entropy of the system. From a microscopic point of view, the kinetics of precipitation is influenced by the diffusivity of C atoms, which has been reported by Lander et al.  to have Another aspect is the control of precipitation of C atoms onto the surface of the catalyzing metal. If their number is too large, polygraphene is formed. There is intrinsic difficulty in theoretically describing the precipitation of foreign atoms  in phaseT of the C solubility σ into the metallic substrate, depicted in Δ is the heat of precipitation and β0 is an entropic pre-factor related to the density of sites into the metal able to host the solute C atoms. Lander et al. [Δ = 0.42 eV, β0 = 5.33 × 1022 atoms cm−3. It has to be noted, in passing, that the value for the heat of precipitation of Cu is comparable, but pre-factor is 2 orders of magnitude lower. The data for Co from the fits in Δ = 0.791 eV, and β0= 9.06 × 1023 atoms cm−3, meaning that Co can in principle host ˜10 times more C atoms than Ni, with approximately a double heat of precipitation that makes the T-dependence of solubility more abrupt. This means that in Co, C atoms precipitate at temperature higher than in Ni. This is somehow equivalent to having a much more rapid cooling process, and, according to the discussion above, to grow graphene at higher T, which means increased C atom diffusivity with reduction of the entropy of the growth product.The steeper dependence on r et al. , experimIn the following sections of the paper, several experimental findings on graphene grown on Co will be presented and discussed basing on a simple thermodynamic model. It will be shown how departures from this vision can be explained in terms of other mechanisms, like the different thermal expansion of graphene and substrate. Raman maps of graphene areas, directly taken on the Co growth substrate will be analyzed, with relevance to the structural and mechanical properties of graphene grown in this way.The mobility of electrical carriers in CVD graphene is known to be limited by scattering with vacancies, grain boundaries, and distorted bonds, even at the nanoscale : such de−1) of graphene is the well-known G peak, characteristic for sp2-hybridized carbon-carbon bonds. The second prominent feature (≈2690° cm−1) originates from a double-resonance process, which creates an electron-hole pair that recombines after two inelastic scattering events involving phonons with opposite momenta and is labeled as the 2D peak. If defects are present, one of the two scattering events can occur elastically and the D peak, observed in this case, exhibits only half the Raman shift (1350° cm−1). Saturated, but distorted, bonds can be present even in case the D peak is not detected. These sources of entropy have been recently revealed [2D linewidth, since it contains valuable information on bond-length variations in graphene in the nanometer range, well below the laser spot size.The first Raman feature  has been explained by Hasebe et al. [CT of Co, , but hiG of a graphene layer in which the vacancies left by the incomplete, isothermal growth have to be annihilated by the successive precipitation step, can be written by adding a term G−1, the energy for a vacancy annihilation, to the energy balance equation given in [n is the number of vacancies, G0 is the free energy of the perfect crystal, G1 the energy needed to generate one vacancy, and G2 = −T·Sc . The total free energy given in : G = G0 δG/δn = 0) are reported in [vc as a function of the entropy variation ΔS.The calculations following by imposing the equilibrium condition , but deviations from ideality are obvious, and related to additional effects competing with the C atoms precipitation, segregation above all. Additionally, a further dependence of β on the cooling rate cannot be excluded. For sake of simplicity, we will consider β as a phenomenological constant, reasonably lower than the pre-exponential factor of C solubility, and related to the availability of C atoms dissolved into substrate, obtaining [Such vacancy concentration will decrease thanks to C atoms precipitating from the metal substrate according to Equation (2), which gives the btaining :ΔS = −, both terms are positive in sign and the total entropy variation ΔS is negative, as expected from the annihilation of one vacancy. The important point is that a larger value of Δ yields the same entropy variation ΔS, but at higher T, which means a larger reduction of G.Equation (5) provides the reduction of entropy when 4 as a precursor gas either in a chamber of a rapid thermal processing (RTP) apparatus [−2 mbar Ar+ pressure, deposition rate around 3 Å/s) or by thermal evaporation onto Si substrates with 500 nm thick SiO2. Substrates, sonicated in acetone and ethanol and then inserted in the reactor chamber, were exposed to a H2 atmosphere for 5 min at 1000 °C, to remove the residual Co oxide, then 10 standard cubic centimeters per minute (SCCM) of CH4 and 20 SCCM of H2 were delivered for 5 min at the same temperature and a pressure of 6.7 mbar. Cooling was carried out in two ways: by switching the lamps off in the RTA system or by sliding the sample out of the heated zone in the tubular furnace. In both cases a flux of few hundreds SCCM inert gas (Ar/N2) was delivered into the chamber. The cooling rates of 3.5 °C/s and 10 °C/s have been estimated in the RTP system and in the conventional tube, respectively. In a previous publication, we have shown that our Co-grown graphene can be transferred by means of the well-known wet method consisting of immersing the sample into a metal etchant and picking up the floating graphene flake by means of the destination substrate [4 V cm2 s−1 have been measured in a large sample portion displaying typical monolayer characteristics [The experiment consisted in synthesizing graphene via CVD, using CHpparatus , or in aubstrate . This prubstrate , the comeristics . P ≈ 2 mW. The spectra were collected at power density Ψ ≈ 6.4 × 104 W/cm2 and acquisition time tacq = 60 s. Micro-Raman analysis has been performed in a homemade system ,22. All While copper emits strong photoluminescence when irradiated with green light, so hindering reliable Raman measurement on the as-grown layer, cobalt is optically inert in the typical wavelength range of dispersive Raman spectroscopes. This allows the ability to extract information about the properties on the as-grown layer and on the possible effect of the transfer step. In , a carefThe final result is that defects provoked by our transfer method of Co-grown graphene are less than those induced when transferring the Cu-grown, even when using the supporting PMMA layer.Light scattered from the sample has been analyzed by means of a circular polarizer  placed at the entrance of the optical fiber that delivers the emission to the monochromator. It selects a linearly polarized component of the light emitted by the sample, and then through a quarter-wave plate, polarizes it circularly, avoiding any accidental intensity attenuation due to the interaction with the fiber, or the monochromator optics. The  polarization of the pump laser beam has been kept fixed to avoid the same kind of problem with the microscope optics. Polarized Raman analysis has then been performed by rotating the analyzer, and, if necessary, the sample under the microscope .After deposition, scanning electron microscopy has been performed as a first morphological characterization step, evidencing the presence of circular/elliptical holes in the metal substrate, due to dewetting . The cov4. In fact, in this step, the holes in the metal are still in their incubation stage [The preliminary conclusion about the mechanism of graphene growth is that it starts during the isothermal exposure to CHon stage ,25 in thon stage , which iAnother important aspect to be taken into account is the crystallographic transition, occurring in Co at temperatures around 420 °C. As displayed in G occurs by means of the increase of S consequent to the break of some atomic bonds. At these strain levels this phenomenon has not been observed, even in the PMMA-free transfer, and strain has been reported also on the destination substrate [Recently, Pudikov et al.  describeubstrate . This obubstrate , and theA typical Raman map on graphene still lying onto its Co substrate is reported in 2D/G intensity ratio. The reason lies on the fact that the difference in wavelength of the two peaks is 40 nm, and the interference effect can greatly change their relative intensities [D and G peaks are only 8 nm far, then, that ratio can still work for measuring the defect content of graphene. The enhancement of the signal in correspondence of the substrate holes does not allow to directly measure of the number of layers by means of the ensities . On the G and 2D as a consequence for the weakening of bonds and the decrease of the vibration frequency. In the case of uniaxial strain, however, the deformation of bonds along perpendicular directions will be opposite: this leads to a splitting of the Raman modes, in particular, the double-resonant one, since it involves transitions between two Dirac cones [2D phonon broadening and softening is then detected in strained graphene, but it can be somehow confused with the presence of multilayered graphene. To shed more light onto this phenomenon, the Raman analysis has been applied to several holes with different shape and dimension. In fact, the simple model proposed in [On the other hand, the optical enhancement allows for very sensitive measurements on the suspended areas. In a previous paper , we repoac cones ,30. 2D pposed in  suggests−5 K−1 at room T [γ = −Co. xo/dλ0, where Coλ = 1.6 10−5 K−1 is the thermal expansion coefficient of the Co film, ox is a length somehow related to the average distance between holes, and d0 is the hole’s dimension. We expect that smaller holes expand more than graphene when cooling, producing tensile stress, whereas for larger hole diameters, the opposite happens and compressive stress is applied. The dimension of the hole at the crosspoint heavily depends on the parameter ox, which is quite arbitrary, and an experiment to confirm this effect is necessary. The negative thermal expansion coefficient in graphene has been reported to be of the order of 10t room T ,32. In at room T  as γ = −2D in this case) allows the recognition of holes in the film underneath, as described. When mapping the position of the 2D mode, however, a large shift towards higher energies is observed on the large hole, indicating the presence of a large compressive strain therein. Our apparatus measures the 2D mode of unstrained graphene grown on Cu and transferred onto SiO2 to be at 2684 cm−1. Hence, a shift of several tens of cm−1 is observed in this case. The map of the intensity ratio between the D and the G modes does not report on the presence of defects in the graphene portion onto the large hole, rather, a large fragmentation is observed through an exceptional intensity of the D mode onto the other, smaller hole at coordinates . The fact that no strain is detected here suggests that some fracture occurred, caused by the intense (maybe tensile) stress provoked by the expansion of the smaller hole. On the larger one, instead, compressive stress takes place, in agreement with the predictions of our model based on the different thermal expansion of graphene and its substrate.G and 2D, undergo splitting, as a consequence for the application of stress. Bonds oriented in different directions can become longer or shorter, then vary their frequency accordingly. The so-called G+ and G, or 2D+ and 2D− features will then appear in the Raman spectra, with consequent broadening and softening of the peaks. Experimentally, a larger splitting is observed in the case of the 2D mode, and this is the reason why we deeply investigated this feature. In fact, a large splitting is, in principle, observable only in the case of uniaxial stress: when subjected to isotropic stress, the difference of the bond lengths is only due to differences of the Young moduli along the principal directions, armchair and zigzag. As mentioned before, both the two main Raman features of graphene, G feature does not always allow for a reliable deconvolution of its components, which is possible, instead, in the case of the 2D mode. It cannot be excluded, however, that some uniaxial strain takes place along some preferential directions. The polarized Raman analysis has been carried out reveal the presence of such uniaxial components. Our samples show typical isotropic strain, then, the fair splitting of the The fit of the two Lorentzian components of the Raman modes has been carried out by means of the Levenberg-Marquard method, combined with a Monte-Carlo routine for randomly selecting the starting values of the fitting parameters. 2D peak corresponds to a relatively small splitting, an observation consistent with the presence of isotropic strain. This analysis has been applied to spectra taken on the same spot  at different rotation angles of the polarization analyzer, from 0° to 360°.A typical result is displayed in In another approach, polarized Raman maps have been collected at fixed analyzer positions.The full characterization of strain in materials requires the knowledge of almost three among the direction of polarization of the incident and emitted light, the direction of uniaxial strain, and one well-defined crystallographic direction . In theG or the 2D peaks. It can be observed that, in the first case, the two components have a phase difference of roughly 90°, which is consistent with the evidence that the linear polarization of radiation diffused by the two phonon modes are orthogonal, independently if G− (G+) is oriented along the armchair (zigzag) direction or the opposite.The angular dependence of polarized light has been measured at intervals of 10°, by manually rotating the analyzer. In the case shown in The polar plot in 2D+ rotates of about 90° coherently with the optical reference system, clearly indicating no variations of the polarizability tensor, which, on the contrary, varies for the 2D− component, since it rotates 50°, and the intensity of the mode becomes comparable with the 2D+. It could be concluded that shortened bonds, which are responsible for the higher energy 2D+ sideband, do not have any preferential direction of vibration, while the vibration of stretched bonds indicates a preferential alignment of tensile strain along the hole minor axis, in agreement with the model which relates the build-up of the tensile stress to the expansion of small holes.The two components show a rather different behavior: 2D feature have been carried out at different directions of polarization of the scattered light. A sample area of 20 μm × 20 μm with several holes has been selected. In the intensity map of the region, 6 holes have been localized. In G peak. The z-axis is normalized to the maximum intensity value, so, the different colors of the holes truly represent the different peak amplitudes. This is not related to some spatial variations of the structural properties of the graphene sheet, but rather to different depths of the holes beneath, which tune the interference enhancement of the Raman signal. The Raman mapping of the two components of the 2D peaks have been collected and analyzed according to the method described above. The results displayed in 2D− component, normalized to the integral of the whole peak. Then, the spatial distribution of 2D+ can be inferred as complementary. The two differently polarized maps have been extracted by rotating the analyzer parallel and perpendicular to the polarization of the incident laser beam without changing the orientation of the sample with respect to the optical system. Spectra of the 2D mode dominates. It is interesting to observe that the prevalence of the 2D− mode occurs in the intermediate region in between the holes (see panel b), as a composition of forces originating in the holes. However, when an orthogonal polarization state for the emitted radiation is chosen (panel c), the intensity of the 2D− mode is quenched in the area between the holes. This is a clear indication that strain is uniaxial there. It is shown that all the graphene regions onto holes of this area undergo tensile strain, since the low energy sideband of the k1 and k2 have the same surface S1= S2 at h > TlT, when their temperature is decreased to lT, their surface areas will be S′1≈ S1(1 − 2k1ΔT), S′2≈ S2(1 − 2k2ΔT), and their difference in surface will be |ΔS| ≈ 2|S1k1 − S2k2|ΔT, proportional to the temperature step. This is what happens to the Co-graphene system, which is formed at T higher than in Ni.A question that may arise is why graphene grown on Ni does not show strain at this level, since the thermal expansion coefficient of Ni differs of only a few % only from that of Co. The answer can be given by applying the law of thermal expansion to two two-dimensional solids. Assuming the two solids with different expansion coefficients In the previous section, we mentioned about another possible origin for strain in graphene. A possible role of the crystallographic transition occurring in Co at temperature around 450 °C from fcc to hcp has already been suggested . This isA drawback of the volume growth of graphene with respect to the case of Cu is the difficulty of controlling precipitation along the cooling stage. From this point of view, Co offers the possibility of growing graphene at higher temperatures, reducing precipitation at moderate temperatures. This is due to the larger heat of precipitation of C atoms in Co. The thermodynamic treatment suggests the reduction of entropy in the graphene sheets grown in this way with respect the case of Ni, while recent results ,26 confi


Two-dimensional (2D) materials possess outstanding lubrication property with their thicknesses down to a few atomic layers, but they are easily susceptible to sliding induced degradation or ubiquitous chemical modification. Maintaining the superior lubricating performance of 2D materials in a harsh working environment is highly desirable yet grandly challenging. Here we show that by proper alignment of graphene on a Ge(111) substrate, friction of graphene could be well preserved at an ultra-low level even after fluorination or oxidation. This behaviour is experimentally found to be closely related to the suppression of molecular-level deformation of graphene within the moiré superlattice structure. Atomistic simulations reveal that the formation of an interconnected meshwork with enhanced interfacial charge density imposes a strong anchoring effect on graphene even under chemical modification. Modulating molecular-level deformation by interfacial confinements may offer a unique strategy for tuning the mechanical or even chemical properties of 2D materials. Two-dimensional materials show remarkable lubrication properties, yet chemical modifications may hinder such capabilities. Here, the authors show that when graphene is aligned on a Ge(111) substrate, ultra-low friction can be preserved even after graphene fluorination or oxidation. Friction exists almost in every mechanical system with moving parts, and it becomes more prominent for small-scale devices due to their high surface to volume ratio. For many applications, it is often desirable to control and, in many cases, to minimize friction134678994In this work, we demonstrate that the moiré superlattice formed between graphene and germanium can impose a strong anchoring to cement the graphene to Ge substrate at the atomic scale. The interfacial confinement effectively suppresses the molecular-level deformation of graphene, and preserves the ultra-low-friction state even when the surface is actively fluorinated or oxidized. Therefore, our study suggests that the mechanical or even chemical properties of 2D materials can be tuned through interfacial confinements by modulating molecular-level deformation of 2D materials.6 plasma and its top surface was fluorinated as confirmed by Raman and X-ray photoelectron spectroscopies  heterostructure, we have taken detailed friction measurements across the base (high friction) and the island (low friction) regions, respectively. As shown in the lateral force image in The impact of superlattice structure on friction was further investigated by scanning tunnelling microscopy (STM). The STM image from the island region see  indicate15165181920nF structures formed during fluorination process could open the energy bandgap of graphenenF structures with narrower energy bandgap can be induced by relatively low-bias voltages of STM, while high-bias voltages are required for the tunnelling of CnF structures with wider energy bandgap. For as-grown graphene on Ge(111), a moiré pattern is clearly visible by STM with sweeping bias voltages (100–500 mV) (Vb (100–800 mV) shows obvious tunnelling protrusions (increased tunnelling current) induced by fluorine attachment and the protrusion density increases markedly with increasing sample bias as show in 22Recent studies have reported that presence of moiré pattern may noticeably affect the self-assembly process of adatom clusters–500 mV) . Howeversp2–sp3 rehybridization. In the nearly aligned case, the local out-of-plane deformation is noticeably suppressed due to the anchoring effect; while the anchoring effect is attenuated in the misaligned case, resulting in a larger deformation  calculations of graphene on Ge(111) substrate were performed. The relaxed morphology -I shows 26Different anchoring effect in the regions with and without moiré pattern might result in different dynamic responses when the graphene surface was slid by a tip. This in turn might cause different velocity dependence of friction on different regions. However, as shown in The anchoring effect, which originates from the interconnected domain wall meshwork of the moiré pattern in the graphene/Ge(111) heterostructure, may be generally applied to preserve the ultra-low-friction state of graphene when utilized in other chemically aggressive environments. This was indeed confirmed by the friction behaviour of oxidized graphene/Ge(111) structure: the regions with moiré superlattice did preserve ultra-low-friction state of graphene as well see . Therefo273132332: CH4: Ar=0.73: 23: 230 s.c.c.m. at the growth temperature of 916 °C for 300 min. Fluorination of graphene was performed by exposing the sample into the SF6 plasma for 3 min in the OXFORD Plasma Lab system-100. To suppress the effect of ion bombardment to graphene, the sample was placed ‘face down' (graphene faced towards the bottom electrode) using a quartz holder. The SF6 flow rate was set as 40 s.c.c.m. and the plasma power was 50 W (bias voltage ∼23 V). During fluorination, the chamber pressure and temperature of the sample stage were 96 mTorr and 60 °C, respectively.P-type Ge(111)  substrates with resistivity of 0.01–0.05 Ohm cm were used in the experiments. The graphene film was synthesized by chemical vapour deposition in a horizontal tube furnace with H−1 and the spring constant at the room temperature is 0.35 N m−1. Lateral force microscopy images were obtained simultaneously with the topographic images. The friction force was calculated by subtracting the forward and backward friction signals on each scan line and divided by two. The normal and lateral force were calibrated using the ramp mode2 environment with a Si tip . STM images were acquired with a platinum–iridium (Pt–Ir) alloy tip in normal ambient under the constant-current mode.Bruker Multimode 8 system was applied to perform the friction, height, pull-off force measurements and STM at ambient conditions . During lateral force microscopy measurements, silicon probes with nominal tip radius of curvature ∼2 nm  were chosen. Deflection sensitivity of the probe is 33.66 nm Vab initio Simulation Package−1. All the calculations were done using experimental Ge lattice constant (a=5.6575 Å).The DFT calculations with a non-local optB86b-vdW exchange-correlation functional37The data that support the findings of this study are available from the corresponding authors on request.How to cite this article: Zheng, X. et al. Robust ultra-low-friction state of graphene via moiré superlattice confinement. Nat. Commun.7, 13204 doi: 10.1038/ncomms13204 (2016).Supplementary Figures 1-11, Supplementary Notes 1-4 and Supplementary References