Graphene is a promising next-generation conducting material with the potential to replace traditional electrode materials in supercapacitors. Since energy storage in supercapacitors relies on the electrolyte-electrode interface, here we elucidate the interfacial subnanometer structure of a single component liquid composed solely of cations and anions – an ionic liquid- on electrified graphene. We study the effect of applied potential on the interaction between graphene and a silicon tip in an ionic liquid and describe it within the framework of the Derjaguin-Landau-Verwey-Overbeck (DLVO) theory. The energy is stored in an electrical double layer composed of an extended Stern layer, which consists of multiple ion layers over ~2 nanometers, beyond which a diffuse layer forms to compensate the applied potential on graphene. The electrical double layer significantly responds to the applied potential, and it shows the transition from overscreening to crowding of counterions at the interface at the highest applied potentials. It is proposed that surface charging occurs through the adsorption of the imidazolium cation to unbiased graphene (likely due to π-π interactions) and that the surface potential is better compensated when counterion crowding happens. This study scrutinizes the electrified graphene-ionic liquid interface, with implications not only in the field of energy storage, but also in lubrication. Made up of a honeycomb carbon lattice, it has been found to exhibit many unique properties including ballistic electron transport3, large in-plane elastic modulus4, and high thermal conductivity5. Graphene owes its superior electronic properties to its honeycomb lattice carbon network, in which 2s, 2px, and 2py orbitals hybridize such that each carbon atom is bonded to its three neighbors by strong “sigma” bonds. The remaining “π” orbital determines the low-energy electronic structure of graphene. These desirable properties make graphene of considerable interest for potential use in many applications, several of which rely on liquid-graphene interfacial properties, including its use as supercapacitor electrode8. Interfacial water consisting of monolayers and multilayers have been often observed between graphene films and hydrophilic substrates such as mica and sapphire10, and between graphene sheets11. A fundamental understanding of the interfacial liquid properties imposed by electrified graphene is, however, still lacking.Since its discovery in 200412; due to its ionic composition, they are subjected to significant electrostatic interactions. Because of their high charge density and wide electrochemical window, and the possibility to tune intermolecular forces and physicochemical properties, ILs are considered to be ideal electrolytes for energy storage13. ILs display more pronounced ordering than conventional electrolytes due to strong ion-ion interactions -especially of Coulombic, van der Waals and solvophobic origin-, and many form well-defined nanostructures in the bulk phase14, at solid surfaces15 and in confinement16. Experiments18 show that ILs near the solid surface commonly exhibit oscillatory density profiles, which reflect the layered arrangement of the IL ions at the buried liquid-solid interface. Besides out-of-plane order, in-plane order has been also demonstrated by atomic force microscopy and scanning tunneling microscopy.19 Several parameters have been found to influence the interfacial structure, such as the chemical composition of the surface20, the strength of ion-ion interactions, i.e. the IL molecular structure21, and the surface charge and potential23. Increasing surface potential leads to an increase in the strength of the ion-surface Coulombic interaction.24 Further, theoretical studies have shown that, when the electrode polarization increases, the interfacial structure of ILs undergoes a transition from overscreening the surface charge by a single monolayer of counterions to crowding of counterions across more than a single monolayer25.A single component electrolyte composed of ions –an ionic liquid– with a moderate polarizability has been chosen for this study. Ionic liquids (ILs) are organic molten salts with low melting point26. A long-range electrical double layer force with a decay length as large as 13 nm has been measured for several ILs on various substrates31. This finding is important since the interfacial capacitance is inversely proportional to the effective thickness of the electrical double layer where the charge is stored, and hence, it affects the stored energy in the supercapacitor. The proposed “aggregation” model26 assumes that ILs are dielectric solvents composed of a strongly correlated ionic network, where each charge is counterbalanced by the sum of the neighbours’ charge and only a small percentage of the ionic network is effectively dissociated. To reconcile the aggregation model with NMR diffusivity measurements, which fail to find evidence for long lived ion pairs32, the coordinated cation-anion network is proposed to be highly dynamic and transient. Although the temperature-dependence of the electrical double layer force further strengthens the argument in favor of a diffuse layer, the shape of the reported capacitance and ion conductivity in separate works and in different systems contradicts the assumption of low ion dissociation, and it is still a subject of debate26. Nevertheless, our results are consistent with the reported aggregation model.Over the past three years some evidence has accumulated demonstrating that long range surface forces on electrically charged surfaces immersed in ILs are of electrostatic origin and they respond to the characteristics of an electrical double layer force of a dilute electrolyte solution38. The results are contradictory with regard to the composition of interfacial IL layers, since some of them show a significant enrichment of imidazolium cations on uncharged graphene36, while others show a densification of the interfacial layer composed of ion pairs, although, admittedly, π-π interactions are missing in these simulations38. A thorough experimental study of the Stern and diffuse layers of ILs on electrified graphene has not been performed yet. In this work, we have scrutinized the electrical double layer of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (abbreviated as [EMIM][TFSI]) at the graphene plane, for single- and few-layer graphene by probing normal surface forces by atomic force microscopy under applied positive and negative potentials. Statistical analysis of the steps resolved in the force-separation curves and modeling of the surface forces provide molecular insight into the electrical double layer of [EMIM][TFSI].To date, a few simulations, scattering and spectroscopy works have investigated the IL-graphene interfaceA 90 nm thick silicon dioxide layer –that we refer as silica substrate– and a working electrode composed of a 9 nm thick gold layer thermally evaporated on the silica substrate were chosen as substrates for mechanically exfoliated graphene. The number of mechanically-exfoliated graphene sheets was determined by combining Raman microspectroscopy and atomic force microscopy (AFM) imaging of the topography. All graphene samples were first imaged in air and then in [EMIM][TFSI] to exclude changes of topography upon immersion in the IL. Figure After equilibration, normal force-separation curves were measured on edge-free areas of single-layer (SLG), bilayer (2LG), few-layer , and multi-layer (MLG) ( > 7) graphene supported on gold and silica at a constant approach speed of 10 nm/s. Figure 40, i.e. the layered arrangement of the molecules at the buried liquid-graphene interface. The FTT happens when the force required to overcome a maximum in the oscillatory force is reached. In terms of mechanics, if the second derivative of the effective surface interaction potential exceeds the spring constant of the cantilever, the tip will push through an IL layer, thereby leading to a step in the force-separation curve. On unbiased graphene, ~4–7 FTTs are resolved in each force-separation curve, i.e. more than on silica and gold supports (~3–4). While the atomically smooth graphene plane is expected to favor efficient ion packing at the interface compared to silica, the roughness of gold is also sub-nanometer (RMS ~0.3 nm as determined by AFM), and hence, only surface roughness cannot explain the enhanced IL-layering on graphene.Discontinuities in all force-separation curves appear at separations smaller than D ~ 2–3 nm superposed to an attractive force; the “red” regions within the clouds of data points with higher density represent these discontinuities in a bivariate histogram. The discontinuities, which are referred to as film-thickness-transitions (FTT), reflect the oscillatory density profile of liquids at interfacesSince the number of graphene sheets did not significantly influence the interaction between tip and supported graphene in the IL Figure , the inv21. This previous work also detected an attractive force but did not rationalize its origin. We propose that the electrical double layer (EDL) force is attractive for the dissimilar graphene-tip system and that the variation of the force with surface potential reflects the change of the EDL with applied potential. Further, the decrease in the attractive EDL force at the highest applied potentials(~ ± 0.7 V) is related to the onset of crowding, as discussed later.On graphene Fig. , the for28), or the Hamaker constant on gold is much smaller than on graphene as a result of the unique electronic properties of graphene; the discussion of the Hamaker constant in the next section does not support the latter, though. Nevertheless, the consistently lesser sensitivity of the measured force on gold to surface potentials compared to graphene  is expected to be larger than that of graphene, the higher pull-off force on graphene in the absence of an applied potential (OCP) might appear surprising. However, due to the presence of oscillatory entropic solvation forces, multiple adhesive minima are present in confined liquids that arrange in layers. Such adhesive minima depend on the number of confined layers, as already demonstrated for ILs. 42. Figure 43. Asymmetric electrowetting has been also measured for deep eutectic solvents composed of choline chloride in organic solvents like urea, ethylene glycol and glycerol on HOPG, and hence, it is not a unique feature of ILs44.The pull-off force remarkably decreases when the potential goes beyond a threshold , D is the surface separation (in m), ψG is the potential at the Outer Helmholtz Plane (OHP) on graphene (in V), −1), ε is the relative permittivity of the IL (unitless), and εo is the vacuum permittivity (in F/m). The expression for the EDL force is based on Hogg-Healy-Fuerstenau approximation45 and it assumes constant surface potential as boundary condition; neither constant surface charge nor mixed boundary conditions were found to agree with the experimental data. The second term in the EDL force always leads to an attractive force, whereas the first term can cause an attractive or a repulsive force depending on the sign of the diffuse potentials. Calculations were not carried out for DLVO forces between silica or gold supports and the tip due to the small value of the measured surface forces at D > 2 nm.To get more insight into the EDL of [EMIM][TFSI] on graphene, the measured surface forces were modeled according to the DLVO theory. The following expression gives the DLVO force for dissimilar surfaces as the sum of van der Waals and electric double layer forces47. The Lipkin theory48 was used to estimate the Hamaker constant of graphene and tip  in air with 15 s−1, and 15 s−1 46, which yields AG ~ 1.52·10−19 J and is close to reported experimental and theoretical values for HOPG (1.35·10−19 J in ref. Au ~ 1.91·10−19 J for gold (15 s−1). There is no agreement about the influence of the number of graphene sheets on the Hamaker constant yet50, but these results suggest the influence to be small. According to mixing rules, the IL is expected to reduce the Hamaker constant with respect to the values obtained in air39. Gebbie et al.28 argued that surface-adsorbed IL ions can further affect the Hamaker constant and used a value of ~0.9–2·10−20 J for mica-IL-gold systems. An accurate analytical estimation of the Hamaker constant for the AFM probe-IL-graphene system is not possible yet. Satisfactory fits to the force-separation curves with Eq. , as previously determined for [EMIM][TFSI] in the potential range + /− 0.5 V)27. The good fit of the DLVO theory to the measured surface forces under applied potential demonstrates that an EDL force can explain the change in the measured attractive interaction , the stronger attractive force leads to a more negative effective diffuse potential on graphene Fig. . A betteFigure extended Stern layer because it is composed of multiple layers– can be determined through statistically scrutiny of the film-thickness transitions detected in the force-separation curves. The size of a film-thickness transition is defined as 52. 2D histograms with the layer size (∆) and the layering force (F) were constructed to compare the composition of the four IL-layers closer to the hard wall on graphene, and silica and gold supports ; the second and third IL-layers are much larger than on the silica support , while the thickness of the following IL-layers decreases toward ∆ ~ 5 Å. The cartoon in Fig. extended Stern layer of [EMIM][TFSI] on graphene, consisting of a film with 4–7 IL-layers. The characteristic size of the IL-layer at the graphene plane (~3.3 Å) is consistent with a cation-enriched layer, in which the imidazolium ring is parallel to the graphene plane. A cation-enriched adlayer would yield a plane of net positive charge on unbiased graphene with a maximum overscreening of ~1.6 positive charges per nm2 , qualitatively consistent with the positive diffuse potential at OCP .The thickness of the IL-layers on silica-supported graphene significantly deviates from the values measured on the silica support Fig. : the IL-OCP Fig. . Figure 1 ~ 3.1 Å, ∆2 ~ 8 Å and ∆3 ~ 5 Å) were reported for [EMIM][TFSI] on HOPG . Since water molecules preferentially interact with the anion, they might be enriched in the second layer, where the highest expansion was observed on silica-supported graphene.We note that the extended Stern layers built up on gold- and silica-supported graphene are qualitatively similar Fig.  vs. 5B,Cst layer remains essentially unchanged at negative potentials, which is consistent with a cation-rich composition. The size of the second and third IL-layers responds to the applied negative potential through a smaller expansion, likely to result from the increase in near-surface cation population compared to OCP  that differs from the characteristic layer size distribution on silica and graphene surfaces. At positive potentials, the thickness of the 1st layer does not change, which would be consistent with an anion-enrichment at OCP and ∆U > 0. The decrease in size of the 1st layer to ∆1 ~ 3–3.5 Å at negative potentials clearly supports cation-enrichment and parallel orientation of the imidazolium ring relative to the surface (6), as observed for graphene. We note that previous work has shown the specific adsorption of 1-butyl-1-methylpyrrolidinium cations to gold at OCP and even at positive potentials56. However, the interaction between gold and [EMIM]+ is known to be weaker than between gold and 1-butyl-1-methylpyrrolidinium, and hence, this is unlikely to happen here58. The observed increase in the layering force required to push through the IL layer closest to the hard wall under applied potential compared to OCP supports the last resolved layer to be enriched in counterions. The force differs under negative (∆F ~ 0.75–1 nN) and positive (∆F ~ 0.44 nN) potentials, which indicates the different binding strength of the (positively and negatively charged) counterions, justified by the ion pair asymmetry59 and in agreement with other works28. Although no layers were resolved at loads above 2 nN, it cannot be totally excluded that IL remains confined between the AFM probe and the gold substrate at the maximum applied load. Such an effect could explain the smaller pull-off force on gold compared to graphene. Future studies, e.g. with our Surface Forces Apparatus, are needed to test this hypothesis on gold surfaces.The out-of-plane structure of [EMIM][TFSI] on gold is clearly different at all potentials. Figure 26, ion pairs are expected to populate the solid-liquid interface in the absence of surface charge and specific interactions, thereby eliminating overscreening and EDL forces, as it occurs on silica surfaces  are not able to fully screen the surface charge, which leads to a non-zero potential at the Outer Helmholtz Plane; to compensate this potential, a diffuse layer builds-up. Figure 31.The described force measurements provide insight into the EDL of [EMIM][TFSI] on graphene. While EDL forces have been previously measured between charged or biased surfaces in ILsces Fig. . HoweverAn increase in applied potential is reflected in an increasing onset of the EDL force while the decay length remains constant. An important observation is that the thickness of the IL layers decreases. This is likely because a lesser population of co-ions is present within the extended Stern layer. The trend of the EDL force is reversed at the highest potentials, at which the EDL force becomes less attractive, which suggests that the surface potential is better compensated by the counterions. We propose that this reversal coincides with the transition from overscreening to crowding. The phenomenon of crowding implies that the electric field is high enough to dissociate more effectively the ionic network, so that several layers of counterions populate the interface Fig. . Thus, c46 and in water47. The different interfacial structure of [EMIM][TFSI] on graphene compared to gold at the same potential must rely on specific surface IL-interactions. Image forces, i.e. the electrostatic force on the IL in the neighborhood of a conductor, are expected for both graphene and gold, and hence, they are not expected to be the main source for the observed differences, although image forces are expected to play an important role. First-principle density functional theory (DFT) calculations have shown strong donor-acceptor interactions between ions and graphene61 and molecular dynamics simulations suggest π-π interactions to be one of the main mechanisms for adsorption of proteins on graphene63. Such π-π interactions are also possible between the π-orbitals in graphene and in the imidazolium ring, and hence, they may be responsible for the enrichment of cations with planar orientation at the graphene interface. Considering that the extended Stern layer at negative potentials appears to be thicker than at positive potentials, it is likely that π- π interactions between graphene and the imidazolium ring of the IL provide more efficient packing of the cation than of the [TFSI]- ion. Similarly, specific interactions between gold and [EMIM][TFSI] could justify the better compensation of the surface potential on gold compared to graphene and consequently, the less pronounced EDL force at all potentials on gold, but this will be investigated in a separate work.The shielding effect of graphene in the IL is demonstrated through the remarkably different interfacial structure that is resolved on the selected supports, gold and silica. This is reasonable considering the reported shielding effect of graphene in air64, it has been demonstrated that the capacitance is dominated by the EDL in the IL if the graphene electrode is composed of more than four graphene sheets. In this case, the potential-dependent differential capacitance13 could be roughly estimated with the effective Debye length. Our results show that, for this particular IL, the effective thickness of the EDL on graphene is >1 nm and hence, larger than ideally expected for supercapacitors (<1 nm). Importantly, the measured change of the layer size distribution with potential shown in our force measurements supports that the compacity parameter (or packing efficiency) of [EMIM][TFSI] is not constant but it depends on the applied potential, which is not considered in the simplified EDL theory13. The high configurational and conformational entropy of the IL ions, and the ion dissociation as a function of the potential are important parameters to consider for the selection of IL electrolytes for energy storage on graphene electrodes. The observed instability in the force-separation curves  will help to clarify the type of interactions that are responsible for the observed interfacial structure and will contribute to guide the design of IL-based supercapacitors with graphene electrodes.66. No exfoliation of graphene has been observed in [EMIM][TFSI]67, which has been attributed to its lower surface tension (33 mN/m), which is also consistent with the stability of our system for at least 12 hours, which was the maximum duration of the experiment.[EMIM][TFSI]  was purchased with > 99% purity. Recent work has shown that for an IL to flow in between graphene sheets, the surface tension of the IL must be in the range 37–45 mN/m, i.e. closely match the surface energy of graphene2. The wafers were then thermally oxidized at 1100 °C for 22 minutes, which led to an oxide layer of ~90 nm that we refer to as silica support or surface. Prior to graphene transfer, the silica surfaces were rinsed with ethanol, dried, and treated in a UV-ozone cleaner for 40 minutes. The gold (Au) working electrode was prepared on the silica support. A chromium (Cr) layer with a thickness of 1 nm and a gold layer with a thickness of 9 nm were thermally evaporated on the silica support with an E-beam evaporator. The Au/Cr film was kept at ~10 nm to yield a semi-transparent film allowing for the fast identification of single- and few-layer graphene with an optical microscope. Prior to exfoliation, the gold electrodes were cleaned for 10 min in a UV-ozone cleaner, followed by thorough rinsing with milliQ water for 1 minute and ethanol for an additional minute. The gold electrodes were then dried with dry N2, cleaned by UV-ozone for additional 5 minutes, and rapidly immersed into an ethanol bath for one hour to reduce the gold surface. After reduction, the electrodes were removed from the ethanol bath, and dried with dry N2 prior to graphene exfoliation.Graphene was prepared by mechanical exfoliation using Kish graphite, Grade 200 . Silicon wafers were cleaned in a base piranha solution for 20 minutes at 85 °C, rinsed thoroughly with milliQ water, followed by ethanol rinsing, and dried with dry NRaman spectroscopy and atomic force microscope imaging of the topography were used to detect the number of graphene sheets on silica and gold supports. Raman microspectroscopy measurements were collected with a Nanophoton Raman 11  microscope using a 532 nm laser. Laser power was set to at least 1 mW, with exposures lasting at least 15 seconds per spot. The graphene was exposed to ambient laboratory air for ~3–4 hours during identification of suitable graphene samples for experiments, and during Raman spectroscopy. The samples were then stored in vacuum (−0.5 bar) until force measurements were performed, which happened within 24 h to minimize aging.68.The atomic force microscopy (AFM) studies were performed on a JPK Nanowizard Ultra  under ambient laboratory conditions . Prior to experiments, all AFM tips were cleaned via UV-Ozone for at least 40 minutes. Graphene images were acquired first in tapping mode in air with a backside gold coated tip  with a resonant frequency of ~300 kHz. The images were collected at scan rates between 1–2 Hz. Following imaging in air, the AFM tip was changed to a contact mode tip, and graphene samples and tip were immersed in vacuum-dry [EMIM][TFSI] and the IL was allowed to equilibrate for 40 minutes prior to force measurements. Images of the graphene samples immersed in the IL were typically collected in contact mode with sharp Si-tips  with a spring constant of ~0.5 N/m and a nominal tip radius < 12 nm, at an applied load of 5 nN and scan rates between 1–1.5 Hz. The spring constant was determined according to the thermal noise method3N4-tips. For the normalization of the force, the tip radius was measured by scanning electron microscopy after the experiment.Force measurements were performed on edge-free areas on single-layer (SLG), few-layer , and multi-layer (MLG) ( > 7) graphene supported on gold and silica, and the control measurements were carried out on silica and gold supports, all in [EMIM][TFSI], in an open cell. Force maps were collected at a constant approach speed of 10 nm/s and an applied normal load that did not exceed 10 nN within scan areas that were at least 500 nm by 500 nm, with at least 64 force isotherms collected per scan area. Although most of the experiments were performed with a Si-tip, some force measurements were conducted with Si2. The platinum counter (dia. = 0.5 mm) and reference electrodes (dia. 2 mm) were sonicated in dilute (1 M) HCl for 5 mins, followed by rinsing with milliQ water and ethanol, and dried with dry N2.The electrochemical AFM cell consists of a gold working electrode and platinum counter and reference electrodes. The volume of the electrochemical cell (~1.3 mL) was filled with the IL to ensure immersion of the counter and reference electrodes. The diameter of the gold working electrode is ~15.6 mm, resulting in a working electrode area of ~1.9 cmWater uptake cannot be excluded during the duration of the experiment, but after 12 h it did not exceed 0.45 wt% and 0.1 wt% in the experiments with silica- and gold-supported graphene, respectively, as obtained gravimetrically. The reason for the different water content is the different volume of the open AFM cell used in both experiments (1.3 mL for gold-supported graphene and 20 µL for silica-supported graphene) and the uptake rate.The datasets generated and/or analyzed during the current study are available from the corresponding author on reasonable request.Supplementary Material


This paper describes the methodological developments of the sexual health items included in the Health Behaviour in School-aged Children (HBSC) study since their mandatory inclusion in the study in 2002. The current methodological, ethical and pedagogical challenges in measuring young people’s sexual health behaviours are discussed along with the issues associated with the sexual health items introduced to the HBSC study in 2002. The development and piloting of new cross-national items for use in the 2013/14 HBSC data collection are presented and discussed.An international pilot study was undertaken to determine the impact of these proposed changes. Questionnaires and classroom discussion groups were conducted in five pilot countries in 2012/2013  with a total of 612 school-aged children .2 = 22.831, df = 9, p = .007, V = .117). Qualitative feedback suggests that for the vast majority of students the phrasing and age-targeting of the questions were considered appropriate. With the exception of a small number of respondents who commented on the clarity and/or personal nature of the content, no specific issues with the questions were identified.The majority of participants in each country provided positive feedback about the appropriateness of the questions. Some small cross-national differences were found in the self-reported quantitative data relating to the appropriateness of the questions (χThese findings provide guidance on the answerability (including the extent of missing and inconsistent data), understandability, acceptability (including in different cultures) and relevance of questions to potential participants. The findings from the pilot study suggest that in general, the questions are understandable, acceptable, and of a high priority to the target population, and that the simplification has significantly reduced the proportion of missing data. The new developments thus enhance the capacity of the questions to measure cross-nationally, sensitive aspects of young people’s sexual behaviour. These questions were included in the 2013/2014 round of the HBSC survey and will continue to be used to monitor trends in adolescent sexual health and behaviours, and to inform and influence health services and health education policy and practice at local, national and international levels. The Health Behaviour in School-aged Children (HBSC) study is a cross-national research project carried out in collaboration with the World Health Organization (WHO). Founded in 1982, the study initially aimed to understand smoking behaviours in England, Finland and Norway. The HBSC study is now undertaken every four years in over 40 countries and regions across Europe and North America and has expanded to cover a wide range of indicators including health behaviours, risk behaviours and wellbeing [The study uses an anonymous, pen-and-paper, self-complete, internationally standardised questionnaire; administered in classrooms to gain the perspectives of a representative proportion of 11, 13 and 15 year old school-going children. In some countries, the protocol is also extended to other age groups. The study seeks to advance the scientific field of adolescent health internationally whilst acting as a monitoring tool to inform and influence health services and health education policy and practice at local, national and international levels .Sexually active young people aged 15–24 years are at a greater risk of experiencing adverse health and social outcomes including unplanned pregnancy, early maternity, abortion or sexually transmitted infections (STIs) than older sexually active populations –5. EarlyFew cross-national studies have been conducted to explore the sexual behaviour and contraceptive use of young people , 14, andin extremis . In an effort to reduce such circumstances, the sexual health questions were recommended for administration only to 15-year-old students [The sexual health questions were designed to be mandatory . However, each country or region was given the opportunity to opt-out of including these questions students –24. At tstudents .The 2005/2006 HBSC study included the same sexual health questions as those used in 2001/2002. Of the 41 participating countries, data were collected from 30 countries on experience of sexual intercourse, 31 on contraceptive pill use, and 30 on condom use , 25. In The inclusion of standardised, mandatory sexual health questions in the HBSC surveys resulted in the publication of internationally comparable data on sexual initiation, , 22, 26,Despite the inclusion of standardised, mandatory sexual health questions, considerable challenges remained in the interpretation of the international sexual health data. Some countries have introduced the use of skip patterns to avoid students completing questions which are not applicable. Although there are benefits associated, this has led to inconsistencies in the quality of the data collected.Missing data currently poses the biggest issue for the collection of information about young people’s sexual health. Potentially sensitive, the questions are consequently only asked to participants aged 15 years and older. At the beginning of the questionnaire, participants are reminded that their responses are both anonymous and confidential, and they are encouraged to answer questions individually. Nevertheless, young people may refuse to answer the questions for reasons related to cultural norms, religious barriers, embarrassment or peer pressure, thus generating missing data. Many students who reported having engaged in sexual intercourse did not respond to subsequent questions relating to contraceptive methods also generating missing data. Further, the layout of the questions . Similarly, some participants’ responses contradicted answers within the contraceptive use questions . Inconsistencies were also identified on the two separate questions addressing condom use .Inconsistent, inaccurate, and missing data have posed difficulties for the comparison of data across HBSC survey rounds and across countries. Efforts to maintain continuity across countries and time periods have been made by the generation of an internationally comparable data file by the HBSC Data Management Centre. Countries who have asked questions that significantly deviate from the HBSC protocol are removed from the data set for those particular questions. There remained however some subjectivity about the further development and use of data at a country level, even if recoding syntaxes are centrally provided as guidance. This variability led to concerns about the reliability and comparability of these items.In addition to these methodological issues, broader ethical and pedagogical issues have also been raised relating to the methods of contraception stated as pregnancy prevention. Concerns arose relating to the implied statement that withdrawal is an adequate method of pregnancy prevention. This issue may not be in isolation; in order to ensure the questionnaire is culturally relevant to young people, and to match some national programs, some countries add ‘national items’. Seven countries added ‘national items’ which included contraceptive methods considered inappropriate for this age group .The issues associated with the sexual health questions across the past waves of the HBSC study prompted important changes to the instruments measuring sexual health and behaviour. The changes are presented below along with the findings from a cross-national pilot study of these proposed changes.Three mandatory sexual health items are included in the 2013/2014 HBSC survey. To maintain comparability, these are largely similar to the mandatory questions from the 2001/2002, 2005/2006 and 2009/2010 surveys. The questions still measure participants’ experience of sexual intercourse, age at first intercourse, and the use of contraception at last sexual intercourse  and are still only asked of students aged 15 years old and older. The main changes for the 2013/2014 international mandatory sexual health questions are the addition of a skip pattern and alterations to the layout of the contraceptive methods question.A skip pattern has been introduced after the first question “Have you ever had sexual intercourse ?” Respondents who have not had sexual intercourse are directed to the next applicable question. The response option “I have never had sexual intercourse” was removed from subsequent questions. This skip pattern simplifies coding, reduces inconsistencies across items, increases comparability between digital and offline surveys,The aim of the pilot was to provide guidance to the international network about the following dimensions of the proposed items; answerability (including the extent of missing and inconsistent data), understandability, acceptability (including in different cultures and both genders) and relevance to potential participants. The findings have been used to guide the inclusion of items for the 2013/2014 HBSC study.n = 76), Hungary (n = 188), Ireland (n = 228), Portugal (n = 30) and Romania (n = 90), representing a wide range of cultural backgrounds. Ethical approval was sought from the University Ethics Boards or other authorities associated with the research team in each country. Schools were recruited based on a purposive convenience sampling method to reflect a range of public, private, rural and urban schools, and those with varying levels of family and socio-economic backgrounds. Class selection was left at the discretion of the school, based on the age range of participants required for the pilot study. All young people in the selected classes were invited to participate. Table A pilot of the proposed new mandatory items was conducted between 2012 and 2013 in five countries; France . All participants were asked to provide active informed consent prior to participation. The mixed methods pilot study involved a questionnaire administered to students followed by a qualitative classroom exploration. All questionnaires contained the proposed items relating to experience of sexual intercourse, contraception use at last intercourse and age at first intercourse. Additional questions relating to sexual behaviour were asked during the piloting process. These included experience of romantic relationships, use of contraception at first intercourse, age of partner at first intercourse, use of alcohol/other substances at first intercourse and perception of timing of first intercourse . All questions underwent a translation and back-translation process, in accordance with the HBSC international protocol, to ensure international consistency and comparability. In Ireland and France, additional questions on bullying, alcohol use behaviour and self-harm were also piloted. All participants were asked socio-demographic questions. At the end of the questionnaire participants were asked “when you filled in the questions, did you think…?”, with response options “none,” “some,” “most,” or “all of the questions are inappropriate for young people your age.” An open-text feedback box at the end of the questionnaire was also provided, reading “In the box below you can say anything you like about these questions. Tell us if you understood the questions or if you feel they are okay to ask young people your age. Remember it is totally anonymous and nobody except the researchers will see what you write.” Prior to filling in the questionnaire participants were assured that they did not have to answer any questions that they were uncomfortable with and were encouraged to highlight this issue in their feedback if it was the case.Based on shared guidelines, qualitative data collection followed the individual completion of the questionnaire. Participants were first asked to underline any words/phrases which were difficult to understand and to provide written feedback about each item and/or items they found problematic or inappropriate. Subsequently participants were invited to participate in a collective class discussion. Participants were first given a chance to have an open discussion about the questionnaire. They were then asked to discuss whether they felt that the overall topic was acceptable, relevant and appropriate to young people their age. Each item was then considered individually, and young people were asked open response questions as to whether they felt the questions were appropriate, relevant and what was able to be understood by young people their age. Quantitative data were collected from France, Hungary, Ireland and Romania  with a total of 582 participants. Qualitative data were collected from all countries with a total of 612 participants.n = 112) indicated “yes”. A small proportion  did not provide an answer to this question.When asked if they had ever engaged in sexual intercourse, 19.2% of the participants (Table n = 109), 66.1% reported condom use, 28.6% reported that a condom was not used and 2.7% did not know whether they or their partner used a condom at last intercourse. Three participants who reported sexual initiation (2.7%) did not answer this question.Table n = 107), 17% reported contraceptive pill use at last intercourse, 62.5% reported no use of birth control pills and 16.1% of respondents did not know if birth control pills were used at last intercourse. Five participants who reported sexual initiation (4.5%) did not answer this question.Of the participants who reported experience of sexual intercourse and answered the question on birth control pill use (n = 105), 9.8% reported some other method of contraception at last intercourse, 67.0% reported no other method of contraception was used, and 17.0% did not know if another method of contraception was used at last intercourse. Seven participants who reported sexual initiation (6.3%) did not respond to this question.Of those participants who reported experience of sexual intercourse and answered the question on other contraceptive methods use (n = 108) reported the age at which they first had intercourse. Four participants who reported sexual initiation (3.6%) did not respond to this question.Of those respondents who reported sexual initiation, 96.4%  reported “using a condom at last intercourse,” three (0.7%) reported “don’t know” in relation to condom use and three (0.7%) reported “don’t know” in relation to the use of other contraceptive methods at last intercourse. One participant (0.2%) reported using a condom at last intercourse, despite leaving the question on experience of sexual intercourse blank. One participant (0.2%) reported using the contraceptive pill while reporting never having engaged in intercourse. According to the skip logic, this may appear inconsistent; however it is possible that this participant was indeed on the contraceptive pill without having engaged in sexual intercourse. Finally, three participants (0.7%) reported that they did not know if birth control pills were used at last intercourse, while saying that they did not have sexual intercourse. No participants who reported never having engaged in sexual intercourse reported an age of first intercourse .“All the questions are clear, do not change a thing” “This questionnaire is very easy to get” “Easily understood and straightforward” During the classroom discussion the overwhelming majority of students understood the terms ‘sexual intercourse’ and the colloquial terms used. Students generally reported no problems understanding questions relating to contraceptive methods or the age of first intercourse. When asked to provide “general comments” in the open text box at the end of the questionnaire, 219 respondents (37.6%) provided responses. A total of 174 respondents (79.4%) recorded that the questions were clear and/or easy to understand.“I found the ‘now skip to question…’ a bit confusing. The questions are okay” “Sometimes a bit confusing to follow.” .Only five respondents (2.3%) reported the questions as unclear or confusing.Although the skip logic reduced the proportion of missing and inconsistent data, skip patterns in general can create some challenges in surveys of adolescents. In this pilot, the small number of participants who expressed confusion about the items indicated that the skip pattern instructions were the unclear aspect.2 = 22.831, df = 9, p = .007). Young people across all four regions were similar in the proportion of those reporting that “some” or “all” of the questions are age-inappropriate. Fewer Romanian participants indicated that they found “none” of the questions inappropriate, whereas fewer Hungarian respondents reported that “most” of the items were inappropriate. Cramér’s V was used to provide an estimate of the effect size of the association, V = .117, p = .007. This indicates that despite the significant association between region and ratings of age-inappropriateness, it is a very low effect. No significant association was found between the acceptability of the questions and age  or gender of participants . Cross-cultural or even school-level differences may confound the association, and the results should be treated with caution due to the low sample sizes. Given that additional questions relating to sexual behaviour were asked during the piloting process, and that in Ireland and France, additional questions on other health behaviours were also included, it is important to acknowledge that comments relating to the appropriateness of questions may relate to these supplementary questions on the questionnaire, even when the researchers facilitating the focus group discussion were asked to clearly separate the topics.Of the 559 respondents to the question relating to the inappropriateness of the questions, 64.3% reported that none of the questions were inappropriate while 19.6% reported that some of the questions were inappropriate. A total of 7.4% reported that most of the questions were inappropriate, whereas 4.5% reported that all of the questions were inappropriate. Chi-square tests identified significant cross-national differences in the responses (χn = 3) was that the question was limited to penetrative sexual intercourse.“Most people our age have done everything but sex, just saying” When reviewing the data from the written comments on the questionnaires, the main concern expressed about the question asking whether participants had ever engaged in sexual intercourse .“I feel that some of the questions were quite invasive and too personal.” “I want to say that some items are too private and they should not be used because they refer to your private life. Some students are shyer and they don’t talk about their life.” “The questions are clear but sometimes childish; teenagers in my age can talk about sex seriously.” Of the 219 participants who provided responses to the “general comments” section at the end of the questionnaire, seven said that the questions were private or personal, but did not specify whether this was a problem. A total of 25 students stated that the questions were inappropriate, too personal or private or felt that they were not adapted to people of their age group “These questions were perfect to present to teenagers. They tackle all important issues in a teenagers life, and they were really well laid out and clear” “Your questionnaire is well done, adapted to the teenagers” A total of 178 students reported that they felt the questions were appropriate, suitable for their age or relevant to young people.n = 13) raised concerns about whether students would answer honestly.“Some people are not going to say if they did some of the things mentioned” “The questions are clear, but I don’t think everyone answered honestly” A small number of students .“Should have more questions for virgins” “Should include more about boy-girl friendships” A total of 28 participants provided suggestions for additional, more detailed sexual health/behaviour questions that they felt would also be important to young people .A test-retest study was carried out in three schools in Ireland consisting of a total of 63 participants. Kappa statistics were calculated for items measuring engagement in sexual intercourse (κ = 0.832), age of sexual initiation (κ = 0.788) and condom (κ = 0.730), birth control pill (κ = 0.781) and other contraceptive (κ = 0.451) use at last intercourse. Apart from the question on other contraceptive used at last intercourse, all contained a sufficient level of inter-rater reliability  indicating their suitability for use in the HBSC survey.An international pilot study conducted in five countries suggests that despite some small cross-national differences in the self-reported quantitative data relating to the appropriateness of the questions, the majority of participants in each country reported positive feedback about the appropriateness of the questions. Qualitative feedback suggests that, overall, the phrasing and age-targeting of the questions were considered suitable by the vast majority of students. With the exception of a small number of respondents who commented on the clarity and/or personal nature of the content, no specific issues with the questions were identified.2 = 29.651, df = 3, p < .001). This positively impacts on data quality for future HBSC survey rounds of data collection. Some countries have also introduced digital questionnaire completion. This would further reduce the problem of inconsistent responses, preventing those who do not report having had sexual intercourse from spending time on any related questions. Online testing would also simplify the skip question instructions mentioned by some as a problem. This benefit would go beyond the sexual health questions if introduced as a future HBSC standard.Missing and inconsistent data posed two of the most significant issues with previous HBSC survey rounds. The newly introduced skip pattern and alterations to the layout of the contraceptive methods question have reduced the potential for participants to provide missing and inconsistent data. When comparing the consistent, inconsistent and missing responses from the 2009/10 HBSC study data (limited to the four target countries) and the 2013-pilot data, the proportion of the students who responded in an inconsistent way has reduced, although not significantly across the study sweeps (1.4% in 2010 and 1.0% in the 2013 pilot). However, simplification of the item structure resulted in a significant decrease in the number of missing responses, from 6.5 to 3.1% , understandability, acceptability (including in different cultures) and relevance to potential participants.The pilot study is not however without limitations. First it should be acknowledged that the sample size is quite small and not representative at country level, despite an effort to collect data in contrasting schools. Completion of the questionnaire required self-report data from young people about the potentially sensitive topic of sexual behaviour. Although participants completed questionnaires both anonymously and confidentially, and every effort was made to ensure that young people completed the questionnaire individually , the limitations of collecting self-report data about sensitive topics has been well documented . In addiAnother limitation is the current wording of the question relating to sexual intercourse. It has been shown that with the help of the information given in parenthesis, young people interpret this as penetrative vaginal sex only , 34, howFinally, it must also be acknowledged that the pilot questionnaire contained additional questions relating to romantic relationships and the circumstances surrounding first sexual intercourse , and in Ireland and France, further additional questions on bullying, alcohol use and self-harm. The evaluation of the sexual health items may also partially reflect an evaluation of these questions or be altered by the evaluation of these questions.To summarise, following the issues highlighted in relation to the mandatory sexual health questions included in the HBSC survey since 2002, changes have been made to the HBSC survey questions. The core meaning of the questions has not changed, but findings from the pilot study suggest that the simplification has significantly reduced the proportion of missing data, thus enhancing the capacity of the questions to measure sensitive aspects of young people’s sexual behaviour cross-nationally. As such the adapted questions have been included in the 2013/2014 round of the HBSC survey. The Sexual Health Focus Group within the HBSC Research Network will continue to monitor trends in adolescent sexual health and behaviours and improve their measures, with the goal to inform and influence health services, decision makers in health education policy and all professionals caring for adolescents at local, national and international levels.


PD-L1 genetic abnormalities, including its 3′-UTR disruption.Immune checkpoint blockade therapy using antibodies against programmed cell death 1 (PD-1) and PD-1 ligand 1 (PD-L1) is revolutionizing cancer treatment . These a− tumors, raising questions about the utility of these immunological markers as an ideal selection criterion for PD-1/PD-L1 blockade therapy  in non-small cell lung cancer (NSCLC) patients. Moreover, candidate neoantigens were experimentally validated using a high-throughput multimer screening to identify neoantigen-specific T cells. In one responder, neoantigen-specific T-cell reactivity paralleled tumor regression . In addiBRCA2 mutation, which is related to DNA repair and replication. In patients with melanoma treated with pembrolizumab, BRCA2 mutations were enriched in those who were responsive to PD-1 blockade , from within a largely unresponsive population. Especially, genetic abnormalities in or PD-L2 .PD-L1-involving genetic alterations have also been reported in various other cancer types. For example, PD-L1 amplification or copy number gain is observed in a subset of B-cell non-Hodgkin's lymphomas and stomach adenocarcinoma. Another genetic mechanism inducing PD-L1 activation reported in primary mediastinal B-cell lymphoma is utilization of an ectopic promoter caused by chromosomal translocation (PD-L1 3′-UTR were found in a wide variety of tumor histologies, including diffuse large B-cell lymphomas and stomach adenocarcinomas (PD-L1 transcripts (in vitro and in vivo experiments using CRISPR/Cas9 technology, which showed that the 3′-UTR disruption can accelerate immune evasion of tumor cells (PD-L1 itself, should be exploited as a potential genomic biomarker to select patients who are expected to achieve long-term durable response to PD-1/PD-L1 blockade therapy. The genomics-based approaches will hopefully allow for development of reliable biomarkers to better guide immune checkpoint blockade therapy for cancer patients.location . Initiallocation , structurcinomas . Resultior cells . These g


A graphene-silicon hybrid waveguide with a dielectric spacer is proposed to enhance the nonlinear response in ultra-wide wavelength range by applying graphene’s broadband highly nonlinear optical properties. The chemical potential of the graphene layer is tuned to satisfy the resonance condition and hence a low propagation loss is obtained. The dielectric spacer is used for avoiding additional free-carrier-absorption loss due to carrier interchange between the silicon core and the graphene layer. Aiming at the special waveguide structure with ultra-thin graphene layer, a full-vectorial theoretical model is developed to analyze its nonlinear properties. The waveguide dimensions are optimized in terms of the nonlinear parameter. The proposed hybrid waveguide exhibits high nonlinearity enhancement in an ultra-broad wavelength region covering near-infrared and mid-infrared bands. The conversion efficiency for a degenerate four-wave mixing process reaches −18.5 dB only with a pump power of 0.5 W and a waveguide length of tens of microns. In the wavelength range of 1.3–2.3 μm, the conversion efficiency can be kept stable by adopting suitable waveguide geometry and length. The corresponding 3-dB bandwidth can reach 40–110 nm for each fixed pump. The graphene-silicon hybrid waveguide has the potential to support chip-scale nonlinear applications in both near- and mid-infrared bands. Despite the two-dimensional and one-atom thick nature of graphene, the complementary metal oxide semiconductor (CMOS)-compatible integration processes in subwavelength dimensions make it a promising candidate for chip-scale optoelectronics. In 2010, the first observation of the coherent nonlinear optical response of graphene at visible and near-infrared frequencies was reported and the exceptionally large third-order susceptibility is ~10−7 esu4. Since graphene is a film material and the interacting distance is short when light propagates through the film, which will limit the nonlinear effects in graphene. In order to solve this problem, hybrid waveguides considering the composition of graphene and other materials may become ideal choices. In recent years, some kinds of graphene-based photonic devices have been demonstrated ranging from hybrid graphene-silicon waveguide for photodetection5 and modulation8, to graphene-clad fiber for polarization9 and wavelength conversion10, and to graphene-silicon hybrid optical cavity for regenerative oscillation and four-wave mixing11. These hybrid structures can greatly increase the interacting length through the coupling of the evanescent field into the graphene layer. In the past ten years, silicon waveguides has been developed extensively for many nonlinear applications12 and exhibit excellent light-confinement ability due to the large refractive index contrast and also high nonlinear response13. It will be very attractive to combine graphene and silicon-on-insulator (SOI) nanowire waveguides to form graphene-silicon hybrid waveguides for broadband nonlinear optical purpose.Graphene, a single layer of carbon atoms arranged in a honeycomb lattice with linear, massless band structure, has attracted great interest by its remarkable electrical and optical properties14. In graphene hybrid waveguide structures, the in-plane absorption will become much heavy since the interacting length is greatly lengthened15. Large optical loss will affect the efficiency of the nonlinear effects in the waveguide. Fortunately, the absorption of graphene has the chance to be tuned by shifting its electronic Fermi level17 via electrical gating or chemical doping. As for the broadband nonlinear enhancement of graphene-silicon hybrid waveguides, the dependence of the graphene nonlinearity on the chemical potential and the working wavelength is eager to be explored. The third-order susceptibility of graphene has been experimentally measured in previous contributions21, but their measured results cannot agree well with each other and strongly depend on the light frequency, the measurement method, and the sample preparation. Theoretically, the optical nonlinearity of graphene has been calculated by perturbative treatment through various quantum theories27. Though their results are quite different, the third-order nonlinearity of graphene is related to these physical condition: chemical potential, realistic scattering rate, and temperature. Based on the single-particle assumption and perturbative calculation, Mikhailov gave a full expression of the third-order conductivity of graphene, which unified the existing theories proposed so far27 and provided a clear physical insight of the complex nonlinear optical effects in graphene. Therefore, the loss and nonlinear performance of graphene can be both controlled by suitably setting its chemical potential.As we know, a monolayer of pristine graphene exhibits a considerable wavelength-independent absorption (≈ 2.3% per layer) in an ultra-broadband wavelength region involving infrared and visible optical band29. However, the emerging graphene-silicon hybrid waveguides  here exhibit strong guidance with a large longitudinal component of modal fields along the propagation direction, and traditional treatment is not suitable to describe the wave propagation in it accurately. The inhomogeneous and full vectorial solutions of the governing Maxwell’s equation have to be pursued. Without the precondition of weak guidance approximation, a vectorial nonlinear Schrödinger equation for nonlinear pulse propagation has been derived in ideal lossless waveguides with complex transverse structure30, which provides a platform for generalizing nonlinear processes like self-phase modulation (SPM), cross-phase modulation (XPM), four-wave-mixing (FWM), Raman and Brillouin scattering. Nevertheless, the impact of optical loss is a non-negligible factor in graphene-silicon hybrid waveguides and will strongly affect its nonlinear efficiency. In particular, we find that, as the wavelength increases, graphene’s imaginary part of the third-order susceptibility may change its sign near the resonance points of photon absorption, thus the loss of graphene cannot be simply treated as either two-photon absorption (TPA) or absorption saturation.It is quite challengeable to determine the nonlinear wave propagation in the graphene-silicon hybrid waveguide due to the graphene’s one-atom thickness, although the graphene layer can be typically treated as an ultra-thin film with an equivalent bulk nonlinear susceptibility. In conventional waveguides and optical fibers, the linear and nonlinear processes are characterized by using the well-known scalar Helmholtz equation, which is founded on the weak guidance approximation and homogeneous cross-section assumptionIn this paper, we propose a geometry-optimized graphene-silicon hybrid waveguide with a dielectric spacer for optical nonlinear enhancement at near- and mid-infrared frequencies. The dielectric spacer is used to block the carrier exchange and hence reduce the additional loss. The full vectorial model is developed for the analysis of the nonlinear continuous wave propagation by presenting an effective nonlinear parameter that combines the nonlinear phase shift term and the nonlinear loss term. Numerical results show the graphene-silicon hybrid waveguide has an ultra-highly nonlinear response. The degenerated FWM (DFWM) effect in the proposed graphene-silicon hybrid waveguide shows broadband performance and the nonlinear enhancement can cover both the near- and mid-infrared bands.ω, relaxation parameter Γ, temperature T, and chemical potential 32. In the linear optical response, the equivalent complex relative permittivity of the Δ-thick graphene can be expressed as 33. At optical frequencies, graphene’s complex refractive index Graphene’s complex surface conductivity consists of intraband and interband parts and depends on the parameters including the radian frequency 27), for a given chemical potential there are certain frequency ranges where the nonlinear response of graphene are exceptionally large due to the resonance enhancement of photon absorption. At high (infrared) frequencies, the imaginary part of third-order conductivity Γ is a small value 24, the position of high-frequency resonance mainly depends on the Fermi level, i.e. 27, in a broad band (0.9 ~ 3 μm), Fig. et al.4 by two orders of magnitude and have weak dependence on the relaxation parameter in the range of  meV. In practice, we adopt the nonlinear refractive index ε denotes the material’s permittivity). As both graphene’s linear and nonlinear conductivities have considerable real parts and imaginary parts, the derivation of nonlinear refractive index 34. For a given chemical potential, the complex 21. For a monolayer of pristine graphene with −13 ∼ 10−12 and essentially dispersionless over the broad wavelength range. By tuning graphene’s Fermi level, the peak response of 10.As for the nonlinear case, we only consider the third-order conductivity 2O3 spacer6. This insulating layer blocks the carrier interchange between the silicon core and the graphene layer as the photoexcited electrons may transfer from graphene to silicon and cause additional free carrier absorption35. Then a gate voltage can be applied to this hybrid waveguide for actively tuning the Fermi level of the cladding graphene film. Further considering the effective interaction between the light field and the graphene in the particular band centered at Figure 36)s is in the waveguide cross-section plane and ω, we introduce two sets of electromagnetic fields: the vectors  only 21 elements are nonzero, which depend on only four independent quantities  increases/decreases. So the bandwidth of nonlinear response is limited by the wavelength-selected performance of the waveguide structure itself. Therefore, the best nonlinear parameter in the wavelength band interested can be achieved through: firstly, tuning the Fermi level of graphene to match the resonant condition at the pump wavelength −1m−1, an order of magnitude larger than that in graphene-silicon hybrid cavity11. The waveguide’s linear loss From the derivation above, all the effective parameters of the coupled equations can be considered as the optical properties of the corresponding material averaged over an inhomogeneous cross-section weighted by the field distribution. In order to achieve the best nonlinear performance of a DFWM process in the graphene-silicon hybrid waveguide, the cross section dimensions should be designed to maximize the nonlinear parameter at the pump wavelength η versus the pump power (L), where the signal wavelength 7 V/m24 which corresponds to about 1.2 W input light power in our case. For the validity of perturbation assumption of the whole theoretical model and avoid higher order nonlinearity in much tenser light intensity regime, we limit the pump power below 0.5 W in our numerical calculation.Conversion efficiency of the DFWM process, which is defined as the ratio of the output idler power with respect to the incident signal power, is an important parameter which needs to be analyzed under various constrains. According to the coupled equations –13), th, th13), W and height H are shown in Fig. L0 and the maximum conversion efficiency that supported for the Fermi-level-tuned graphene (solid marks) and pristine graphene (hollow marks) cases when the pump and signal powers are 0.5 W and 1 mW, respectively. When the pump wavelength L0 increases linearly and the corresponding maximum conversion efficiency Above the nonlinearity enhancement is demonstrated in near-infrared (telecommunication) band centering at 1.55 μm. Actually, according to the tunable low-loss bands of graphene and silicon, the designed graphene-silicon hybrid waveguide structure is expected to enhance the nonlinearity in a wide wavelength range, covering both near- and mid-infrared bands. For the applications in different bands, the pump wavelengths should be different, and the chemical potential of the graphene layer needs to be tuned to match the resonant condition  method. The coupled equations (36 and solved by a fourth order Runge-Kutta method with a length step Δz = 0.3 μm. The simulation parameters have been given in our paper.By considering the graphene layer as a thin film with thickness of 0.5 nmquations –13) is  is 33, iAll data generated or analyzed during this study are included in this article.


Owing to its unique properties, graphene has attracted tremendous attention in many research fields. There is a great space to develop graphene synthesis techniques by an efficient and environmentally friendly approach. In this paper, we report a facile method to synthesize well-dispersed multilayer graphene (MLG) without using any chemical reagents or organic solvents. This was achieved by the ozone-assisted sonication of the natural graphite in a water medium. The frequency or number of ozone treatments plays an important role for the dispersion in the process. The possible mechanism of graphene exfoliation and the introduction of functional groups have been postulated. The experimental setup is unique for ozone treatment and enables the elimination of ozone off-gas. The heat generated by the dissipation of ultrasonic waves was used as it is, and no additional heat was supplied. The graphene dispersion was stable, and no evidence of aggregation was observed---even after several months. The characterization results show that well-dispersed MLG was successfully synthesized without any significant damage to the overall structure. The graphene obtained by this method has potential applications in composite materials, conductive coatings, energy storage, and electronic devices. Graphene, miracle material, is a two-dimensional carbon allotrope with a thickness of only one atom ,2,3,4. TSeveral graphene synthesis methods have been developed since Novoselov and Geim discovered isolated graphene in 2004. The most common methods are liquid phase exfoliation, the reduction of graphene oxide, micromechanical exfoliation, chemical vapor deposition, and the exfoliation of graphite intercalation compounds in organic solvents ,10,11,12Surfactant-free graphene synthesis in water is extremely difficult, and very little research has been done ,31. In tNatural graphite powder  with an average particle size of 6 μm and purity of over 99% was used as a starting material. Commercial grade oxygen was used as a gas source to produce ozone. Ultrapure water  was used as a solvent in all steps. An ultrasonic bath sonicator with frequency range 23 kHz and power of 80 watts was used to exfoliate natural graphite powders. The graphite powder (0.06 g) was added to 60 mL of ultrapure (millipore direct Q) water. During the sonication of graphite dispersion in water, the ultrasonic waves propagate through the graphite suspension and result in alternating high-pressure and low-pressure cycles. While the low-pressure cycle occurs, acoustic or ultrasonic waves create small vacuum bubbles in the suspension. Subsequently, these bubbles absorb the energy and collapse intensely during a high-pressure cycle. The effect of this cavitation phenomenon induces high-velocity micro liquid jets and shock waves which will generate normal and shear forces on the graphite . Under t2 (100 nm)/Si substrates were used for all of the above-mentioned characterizations. The substrates were cleaned with acetone in an ultrasonic bath and subsequently rinsed with isopropanol and blown dry using a nitrogen gun. X-ray photoelectron spectroscopy  analysis was performed to observe the functional groups and oxygen content of the exfoliated samples. Dynamic light scattering and zeta potential measurements  were used to measure the size distribution and the dispersibility of the exfoliated samples.Scanning Electron Microscopy  was used to characterize the morphology of the exfoliated samples. The number of graphene layers was determined by atomic force microscopy  in tapping mode. Raman spectroscopy analysis was also conducted to investigate the bond quality and confirm the number of graphene layers. The SiOThe morphologies and sizes of pristine and exfoliated graphite observed by SEM are shown in −1. The deconvolution of the 2D band in graphite shows two sub-peaks at 2655 and 2700 cm−1, as shown in −1. Likewise, two similar sub-peaks appear at 2655 and 2700 cm−1 in the deconvoluted spectra of MLG, as seen in I2D/IG intensity ratio is very common and helpful to determine the number of graphene layers. The intensity ratio, I2D/IG, for single-layer graphene is around 2, bi-layer graphene is around 1, and multilayer graphene is less than 1. Pristine natural graphite exhibits I2D/IG ratio of 0.31 with very low D band peak. Exfoliated graphene reveals higher I2D/IG ratio (0.42) with a little higher D band peak compared to graphite. Based on the described intensity ratio, it can be determined that the exfoliated graphene is multilayer graphene with well-maintained graphitic structure. However, another confirmation like AFM measurement is necessary for clear understanding of the number of graphene layers.The Raman spectra of pristine graphite and exfoliated MLG are shown in The XPS measurement was performed to understand the quantitative elemental, chemical state, and functional groups. The detailed C 1s analysis of pristine graphite and MLG are shown in The size distributions of the obtained MLG were measured with a Zetasizer Nano Series particle size analyzer by the dynamic light scattering method. The disposable cuvette container (DTS 0012) filled with 3 mL of dispersed MLG was used for the measurement. As shown in The stability of the dispersed MLG was measured by a zeta potential analyzer (the same device as for the DLS measurement). Disposable folded capillary cells (DTS 1070) filled with 2 mL of dispersed MLG were used for measurement. The zeta potential (ZP) is the key indicator to describe the stability of dispersed MLG and is caused by the net electrical charge in the medium. The magnitude of ZP describes the degree of electrostatic repulsion between the charged particles in a dispersed solution. Generally, a ZP value less than 30 mV (negative or positive) shows instability behavior of a dispersed solution and the occurrence of aggregation owing to the higher attractive force than repulsive force. In contrast, a ZP value higher than 30 mV indicates good stability, and will resist aggregation or flocculation. This study revealed a facile method to synthesize well-dispersed MLG without using any chemical reagents or organic solvents. The obtained MLG was found to be 13 nm thick and a few hundred nanometers wide with negligible structural damage. The dispersed MLG was stable, and no evidence of aggregation was observed, even after several months. These results were achieved using sonication-assisted ozone treatment. The number of ozone treatment times (frequency) is the key to achieving good dispersion. It can also be concluded that a minimum of 15 h sonication with intermittent ozone treatment of 6 × 10 min is required to synthesize well-dispersed MLG. The experimental setup was unique for this kind of ozone treatment—not only of graphene, but also many varieties of materials. This process does not produce ozone off-gas or any waste products to the environment. The MLG herein obtained can be used in composite materials, conductive films, nanofluids, and electronic applications.


However, based on density functional theory, we find that Mn, one of transition metals, embedded in single or double vacancy (Mn@SV and Mn@DV) in a graphene monolayer induces a dipole moment perpendicular to the sheet, which can be switched from up to down by Mn penetration through the graphene. Such switching could be realized by an external stimuli introduced through the tip of a scanning probe microscope, as already utilized in the studies of molecular switches. We estimate the energy barriers for dipole switching, which are found to be 2.60 eV and 0.28 eV for Mn@SV and Mn@DV, respectively. However, by applying biaxial tensile strain, we propose a mechanism for tuning the barrier. We find that 10% biaxial tensile strain, which is already experimentally achievable in graphene-like two-dimensional materials, can significantly reduce the barrier to 0.16 eV in Mn@SV. Moreover, in agreement with previous studies, we find a high magnetic moment of 3  Despite its unique electronic properties due to Dirac points in the corners of the Brillouin zone, it is intrinsically non-polar due to its centrosymmetric crystal structure, which hinders its application in nanoelectromechanical systems. However, recent experimental and theoretical works have confirmed that polarity—consequently piezoelectricity—can be engineered by breaking the inversion symmetry in various ways; such as creating holes of the right symmetry that results in a strain gradient under a uniform stress4, adsorption of foreign atoms on the surface5, or chemical modification like oxidization6, or hydrogen and fluorine codecoration on the surface9. Note that creating holes with right symmetry is experimentally challenging, whereas chemical modifications drastically change the electronic properties of graphene9. On the other hand, due to the weak interaction between adatoms and graphene5, adsorbed foreign atoms have tendency to migrate on the surface with a low energy barrier of order of 0.1–0.3 eV. At room temperature or high actuation frequency, such low migration barrier may cause the adatoms mobile on the surface or even may lead to desorption, causing potential failures in practical applications. Moreover, polarization switching—a signature of ferroelectricity and an essential property for memory devices—might not be feasible in such structures, because penetration barrier for adatoms through the graphene is quite high, and rotating C-H/F bonds in codecorated graphene could require high energy8.Graphene—one atomic layer of C atoms arranged in a hexagonal lattice with many unique and extraordinary properties—has been an object of intense research activities that might revolutionize next generation nano scale devices11. However, due to depolarization field, arising from the uncompensated charges at the interfaces, polarization perpendicular to the film surface in ultra thin ferroelectric films is suppressed, which stems a critical thickness for ferroelectricity11. Interestingly, a new class of two-dimensional (2D) honeycomb binary monolayers has been reported very recently12. Those free standing buckled monolayers are theoretically predicted to be stable with a high polarization. However, those monolayers are yet to be realized experimentally. Moreover, a robust ferroelectricity perpendicular to its plane has also been predicted in a distorted 1T-MoS2 monolayer under a spontaneous symmetry breaking, although it has not been experimentally conformed yet13. Promisingly, experimental observation  of piezoelectricity in MoS2 membranes with the odd number of layers has already been reported for nano-electro-mechanical systems (NEMS)15. As a consequence of device miniaturization, here it should be mentioned that molecular switches16 –mainly metallorganic phthalocyanine molecules could become an alternative paradigm to the conventional ferroelectric materials for data storage and logic circuits. However, for practical realization of such switch, it is essential to assemble these molecules on a surface without forming any strong bonds16. Under those circumstances, as periodicity does not hold perpendicular to the graphene sheet, hence engineering a switchable dipole moment in graphene in a way that is experimentally feasible might pave the way to open up a new possibility of a class of nanoscale “dipoletronics”13 devices.High-performance ferroelectric materials at nanoscale hold a great potential for miniaturization of devices, high-density nonvolatile memory storage, and discovery of exotic physical phenomena20. Realization and control over the high-spin state of an isolated single atom magnet are ultimate goals for spintronics, which may lead the magnetic storing devices to super storing capacity19. As a result, a considerable body of theoretical as well as experimental works has been devoted on an isolated transition metal (TM) atom in a graphene sheet, which is considered as an ideal situation because of non-magnetic graphene21. In this regard, recent experimental demonstration by electron spectroscopy on the realization of the single atom spin state at atomic defects in graphene is a great achievement17. Such structures have been studied theoretically as well20. However, electric polarity of such structures originated from the fact that TM atoms at defect sites locally break the flatness of graphene, has not been investigated yet.On the other hand, the effect of localized magnetic impurities in graphene on its electronic and induced magnetic properties have been extensively studied both experimentally and theoretically in the context of spintronics and unconventional Kondo effect25. Moreover, we also estimate the energy barrier for dipole switching from up to down by Mn penetration throughout the graphene. The barrier for Mn@SV penetration is found to be high, however reasonably low for Mn@DV. Note that the impenetrability throughout pristine graphene has been already shown for helium and hydrogen atoms experimentally26 as well as theoretically27. However, to tune the penetration barrier for Mn@SV, we propose a mechanism that employs a biaxial strain. We show that 10% biaxial tensile strain, which could be experimentally achievable for graphene like 2D materials29, can significantly reduce the barrier indicating a way to switch the dipole moment as such structures are already experimentally available17. We also study the effect of substrate on our systems, for the purpose we consider a layer of graphene underneath the Mn-embedded system. We find a significant change in the energy barrier while no large change in the magnetic moment. Hence we expect that our findings may stimulate more studies to find an experimental way to realize such switching of dipole moment in TM embedded graphene along side with its magnetism, and may find applications in nanoscale memory devices.In this study we investigate the polarity of a Mn atom embedded in a single or double vacancy of a graphene sheet. We find a considerably large dipole moment, which is comparable with other 2D polar materials20, we consider the flat configuration where a Mn atom is located in the same plane of carbon in the graphene. Optimized configurations are shown in Fig. Eb), we find that polar configurations are always more stable than the flat ones because of the larger atomic radius of Mn atom compared to C. In agreement with the previous studies20, the height (h) of the Mn atom over the graphene plane significantly reduces from 1.44 Å for Mn@SV to 0.74 Å for Mn@DV due to the fact that the DV opens a larger space under the Mn atom. However, even the flat configurations have significantly large binding energies  with a Mn atom outwardly protruded from the graphene surface17. Figure We also investigate the migration of a Mn atom across the graphene with single or double vacancy. In experiments, initially a Mn atom can be adsorbed anywhere on the graphene with SV or DV as metal atoms are deposited on the pretreated surface by using thermal evaporation. Here it should be mentioned that usually high energy electron beam irradiation is used in experiments to create such defects in graphene30. A significant charge transformation of 0.90 |e| from Mn to graphene is found for both polar configurations, indicating a local redistribution of electrons and consequently a local dipole moment directing from graphene to Mn. Note that such charge transformation and dipole moment are also found for metal adatoms on perfect graphene. For instance, Mn atoms adsorbed at hollow sites of perfect graphene31 transfer 0.84 |e| to graphene.As the polar configurations are energetically more favorable than the flat configurations, here we investigate the polarity of such polar configurations. First, we compute the charge transformation between Mn and graphene, employing Bader charge analysisP  perpendicular to the graphene sheet is computed asρ(z) is the planar-averaged electron density along z-direction, Zi is the net atomic number of i-th ion with zi being the z coordinate in the unit cell of area A, Lz stands for the length of a unit cell along the z-direction, and e is the absolute value of an electronic charge. A similar definition was used in our previous studies23. The calculated values are shown in Table As adsorbed Mn atoms in ‘polar’ configurations break the flatness of graphene as well as redistribute electrons by charge transformation, an electric dipole moment has induced. The electric polarization 5. Now we show that a reasonably larger polarization directed from the graphene to Mn is found in Mn@SV in comparison with Mn@DV; mainly because the height of the Mn on the graphene is significantly reduced in Mn@DV. Obviously being completely flat,’flat’ configurations exhibit no out-of-plane dipole moment. Note that despite the low dimensionality of our configurations, such polarizations down to the nanoscale are not negligible, in fact comparable with others. We first compare the polarizations with other 2D polar materials. Our estimated values are quite comparable with the values for group IV binary monolayers , however slightly lower than the estimated values for III-V binary monolayers (AlSb and GaAs)12. Note that the polarizations in hydrogen and fluorine co-decorated graphene8, silicene23, and h-BN22 are an order of magnitude larger. Now, considering the height of our configurations as the thickness  predicted for ferroelectric polarization in distorted MoS2 monolayer13. However, the polarizations are about four times lower than the polarization in ferroelectric polymers 32 the polarization is about 8 μC/cm2). We also find that our estimated polarizations are also comparable to those of wurtzite semiconductors such as GaN (3.4 μC/cm2) or InN (4.2 μC/cm2)33, and an order of magnitude larger than the measured values in 2D freely suspended ferroelectric smectic-C films24 and nematic liquid crystal monolayer25. For further comparison, our estimated values are slightly larger than the polarizations of improper ferroelectrics34 Al2O6 (1.80 μC/cm2) and Al2O6 (1.78 μC/cm2); moreover, are in the same order of magnitude with the polarizations of hexagonal RMnO3 (where R stands for rare earth elements) with a typical value of a few μC/cm2 35. Also, the polarization of about 5 μC/cm2 in multiferroic double-perovskite Bi2NiMnO6 structure36 is twice larger than that in our configurations.Note that the lack of band gap in our configurations does not prohibit an out-of-plane electric polarization, as conductivity is only in the plane. Perpendicular to the graphene sheet, the configurations can be considered as insulating. Such a unique property of 2D materials was also used in other previous studies12. Here we emphasize that such dipole switching could be considered as analogous to the molecular switching16 where a molecular state can be switched from one to another under an external stimuli usually through a scanning probe microscope with a threshold voltage of around 2–4 eV. Hence, employing the similar technique, even switching in Mn@SV could be a possibility. However, tuning the barrier may introduce an additional control over the switching between configurations. We find that a biaxial tensile strain can be an effective way to tune the barrier, as the space beneath the Mn increases due to the strain. Figure 29. To this end, it should be noted that energetically non-equivalent states might arise in the presence of a substrate. Hence for real device applications, we suggest equivalent electrodes for instance top and bottom graphene electrodes.Now we turn our attention to the possibility of the polarization switching, where polarization changes sign by the penetration of Mn throughout the graphene sheet. As the up and down polarizations states–where the Mn atom is right above or under the vacancy-site–are symmetric with respect to the flat state, therefore we only consider the barrier path from up to flat configurations. In order to estimate the energy barrier for a dipole moment switching between initial polar to final flat configurations, we perform the NEB calculations with four images between the initial (polar) and final (flat) configurations , the presence of graphene decreases the electric dipole moment of the down state (0.18 |e|Å), although the dipole moment of the up state (0.29 |e|Å) has changed a little. On graphene the dipole moment of the down state of Mn@DV (0.31 |e|Å) is larger than that of free standing Mn@DV (0.14 |e|Å). However, the dipole moment of the up state (0.14 |e|Å) remains the same as the value of free standing. On the other hand, magnetic moments remain almost the same during the electric polarization switching 37. Also note that under a gate voltage substrates can affect the electric and magnetic moments as electric field can be a good way to control the charge transfer between Mn@Gr and the substrate. Additionally, it will be interesting to know the effect of substrates for the proposed gate controlled Kondo effect18 and GSDM37, which is beyond the scope of this paper and left for the further studies.Additionally, a significant change in electric dipole moment during the switching has also been observed in the presence of graphene. Now up and down polarization states have not only different energies but also different electric dipole moments. Compared to free standing Mn@SV  containing 49 (for Mn@SV) or 48 (for Mn@DV) C atoms with a Mn atom are used to simulate the isolated single Mn atom on graphene. As periodic boundary conditions are imposed in all three directions, the distance between two neighboring periodic images in the normal direction is kept larger than 20 Å in order to avoid interactions between periodic atoms. A large enough cutoff energy of 500 eV for the plane-wave expansion is used in all calculations to avoid the basis set incompleteness. All structures are fully relaxed until the Hellmann-Feynman forces on all the atoms are less than 10−3 eV/Å. The geometry optimization of configurations is carried out employing the conjugated gradient technique. The criterion for the total energy is set as 10−7 eV. The Brillouin zone is sampled with a Γ-centered k-point mesh of 7 × 7 × 1 for geometry optimizations, while a denser grid of 15 × 15 × 1 is used for charge density calculations. Dipole correction41 is used to cancel out the artificial electric field that arises from the periodic boundary conditions in polar surface calculations. For Mn embedded graphene on graphene, the Grimme’s DFT-D2 method42 is used to capture the van der Waals interactions.Our first-principles calculations are performed by the spin-polarized density functional theory using projector augmented wave (PAW) potentials to describe the core electrons and the generalized gradient approximation (GGA) of Perdew, Burke, and Ernzernhof (PBE) for exchange and correlation as implemented in the Vienna Ab initio Simulation Package (VASP)43 with four images along the penetration path. The binding energy (Eb), an essential quantity closely related to the binding strength between Mn and graphene, is calculated as Eb = Er−SV/DVG + EMn − EMn@SV/DV; where EMn@SV/DV is the total energy of a Mn atom in SV or DV of graphene, EGr−SV/DV is the total energy of the graphene sheet with a SV/DV, and EMn is the energy of an isolated Mn atom. A positive value of Eb with larger amplitude means that the configuration (Mn@SV/DV) is thermodynamically favorable.Calculations to estimate the penetration barrier of Mn throughout the graphene are performed using the nudged elastic band method (NEB)