The direct observation of single crystal graphene growth and its shape evolution is of fundamental importance to the understanding of graphene growth physicochemical mechanisms and the achievement of wafer-scale single crystalline graphene. Here we demonstrate the controlled formation of single crystal graphene with varying shapes and directly observe the shape evolution of single crystal graphene by developing a localized-heating and rapid-quenching chemical vapor deposition (CVD) system based on electromagnetic induction heating. Importantly, rational control of circular, hexagonal and dendritic single crystalline graphene domains can be readily obtained for the first time by changing the growth condition. Systematic studies suggest that the graphene nucleation only occurs during the initial stage, while the domain density is independent of the growth temperatures due to the surface-limiting effect. In addition, the direct observation of graphene domain shape evolution is employed for the identification of competing growth mechanisms including diffusion-limited, attachment-limited and detachment-limited processes. Our study not only provides a novel method for morphology-controlled graphene synthesis, but also offers fundamental insights into the kinetics of single crystal graphene growth. In order to achieve large size and high-quality single crystal graphene, a fundamental understanding of precise mechanisms that governs the formation of graphene domains is necessary. Much effort has been placed on the investigation of graphene growth kinetics by using carbon isotope labeling technique13, or the empirical parameters-controlled methods based on the nucleation and growth theory14. It has been widely accepted that the graphene growing process mainly involves surface catalytic reaction for catalyst with low-carbon solubility13. In this case, graphene nucleation on catalyst surface is one of the critical steps in the growth process. Various factors affect the initiation of graphene nucleation process, including the type or surface morphology of catalyst16, carbon source17, carbon segregation from metal-carbon melts18 and parameters in CVD growth20. In general, nucleation densities on polycrystal Cu substrates are nonuniform, representing a key problem in high quality graphene film synthesis. Recently it has been found that uniform nucleation distribution, low nucleation density and highly ordered single crystal graphene films can be obtained by using liquid Cu film as catalytic, which is possibly due to the elimination of Cu grain boundaries22. However, the direct insight of single crystal graphene growth and its corresponding domain shape evolution during the growth process are still lacking. The challenge is to suspend the graphene nucleation and growth process accurately so that one can observe the pristine graphene domain formation. But it is almost impossible to achieve this by using the conventional hot wall tube furnace due to the slow temperature ramping during the heating and cooling processes.Chemical vapor deposition (CVD) has enabled the growth of single layer graphene over large area through the optimization of synthesis conditions24, zinc oxide nanowires and titanium dioxide nanoswords26, the great capacities of rapid heating and quenching characteristics belonging to electromagnetic induction heating have not been fully utilized. Here, for the first time we control the formation of varying shapes of single crystal graphene and directly observe the shape evolution by developing a localized-heating, rapid heating and quenching CVD system based on electromagnetic induction heating. The CVD system based on electromagnetic induction heating is capable of selectively heating the copper foil only and can also cool the copper foil from more than 1000°C to 700°C in ~5 s. By using the electromagnetic induction CVD system, we have identified the competing atomic phenomena, such as diffusion-limited growth, attachment-limited growth and detachment-limited growth, whose balancing effect defines the characteristic single crystal graphene domain density, shapes and crystal quality. Our study provides a novel method for morphology-controlled single crystal graphene synthesis and offers fundamental insights into the kinetics of growth processes.Electromagnetic induction heating is the process of heating an electrically conducting object by electromagnetic induction, where eddy currents are generated within the metal and resistance leads to Joule heating of the metal. Electromagnetic induction heating technique has been widely used in industry, including surface hardening, melting, brazing, sealing and heating to fit, due to its features such as fast-heating, fast-cooling, clean, cheap price and energy-saving. Although the technique has been demonstrated for the synthesis of carbon nanotubes4, which, however, requires more than 1 min for the carbon precursor gases to evacuate the chamber entirely due to the limited pump rate , as shown in By using this ultra rapid cooling method, we have observed some remarkable key phenomena as follows: (1) the graphene domain density and lateral size are independent of growth temperatures ; (2) nuc27. According to the Robinson and Robins model28, the nucleation is the result of competition between carbon species adsorption, surface diffusion and re-evaporation processes29. One of the possible reasons for the temperature-independent nucleation density is that the catalyst used in our study is liquid phase Cu film (at 1100°C and 1150°C) and recrystallized Cu film (at 1050°C). 2H2 can be easily formed from CH4 on the Cu (111) surface30. Therefore, the Cu (111) surface represents a more favorable reaction pathway for nucleation. Moreover, the diffusion of carbon species groups is much faster on (111) surface than that on (200) surface of commonly used polycrystalline Cu foil30. We believe that the nucleation kinetics is depended mostly on the surface state of Cu catalyst rather than on the growth temperature. It is proposed that the temperature-independent nucleation density on recrystallized Cu film is the result of formation of Cu (111) surface. Even though other peaks except (111) also appear for recrystallized Cu film, the (111) peak is sharp and stronger than other peaks. Thus for recrystallized Cu film, the Cu (111) surface is dominant. Furthermore, the carbon species can easily form and nucleate on the dominant Cu (111) surface, which may retrain the graphene nucleation on other surface. Although there is lack of theoretical study explaining the critical role of liquid Cu surface playing in graphene nucleation, the similar nucleation behavior on solid Cu (111) surface and liquid Cu surface demonstrates that the atomic phenomena of carbon species on liquid Cu surface is the same to that on the Cu (111) surface.The density of graphene domains is independent of the growth temperature under our experimental conditions within the growth temperatures range from 1050°C to 1150°C, which is in contrast to the reported literatures27. It is believed that the graphene nuclei will keep on growing or even connect with each other during the conventional slow cooling process . Particularly, when the growth temperature increases, the domain aggregation will become notable, which leads to the seeming result that domain density is decreased with increase of temperature. With our rapid-quenching CVD system, we are able to rule out the interference of domain aggregation on the graphene domain density observation. We investigated the growth of graphene on the recrystallized Cu film from 800°C to 1050°C for 10 s. As shown in the SEM images of 31, the carbon-Cu interaction is sufficiently weak, the decomposed carbon atoms are only able to diffuse on the Cu surface and both the nucleation and growth of graphene are dominated by the surface diffusion of the decomposed carbon atoms. As a consequence, the feedstock cannot be accessed for the catalyst surface covered with carbon species, namely self-surface-limited effect. Therefore, the surface carbon species supersaturation concentration is limited mostly by the Cu surface rather than by the growth temperature, which leads to the temperature-independent nucleation density.As reported, the graphene nuclei density decreases with the increase of growth temperature22, the circular shape graphene domains are rarely reported. It is worth noting that circular shape graphene domains can only be synthesized at high temperatures (above 1150°C). However, due to the relatively low heating and cooling rate of traditional tube furnace, the complete evaporation of Cu substrate would occur during the heating, growing and cooling processes. That is the reason why it is hard to observe circular shape graphene domains by the conventional CVD synthesis methods. The graphene domains are dendritic shape after 60 s growth at 1050°C. For the graphene domains grown at 1100°C and 1150°C for 60 s, the size keeps the same order of magnitudes, but the shape is transformed from dendritic to hexagon and circular, respectively. The time-evolution of domain growth at different growth temperatures could be visualized by rapid cooling the graphene growth process. The graphene domains grown at 1050°C evolve from diamond to tetragonal-star and then hexangular star shape. The domains grown at 1100°C remain hexagon, while the domains grown at 1150°C evolve from quasi-hexagonal to circular shape as they grew. According to the second law of thermodynamics, in the chemical reaction process, including CVD, a chemical reaction will proceed if the change in Gibbs free energy is negative. In other word, the thermodynamic driving force for a chemical reaction is the negative change of Gibbs free energy. In our graphene CVD synthesis process, the carbon atom in a state of gaseous species changed into that in the solid graphene state through the chemical reaction and the graphene domains were growing larger under the suitable condition. Thus the total Gibbs free energy is reduced, which is the thermodynamic driving force to grow graphene. So we try to explain the formation of different graphene domain shapes from the perspective of thermodynamics.Unlike the domain density, the shape of graphene domains, for instance, circular, hexagonal and dendritic, is under the control of growth temperatures. With the increase of growth temperature, the single crystal graphene domains are transformed from dendritic shape (at 1050°C) to hexagonal shape (at 1100°C) and to circular shape (at 1150°C). Although the graphene domains with dendritic and hexagonal shape have been previously synthesized on Cu foil or liquid Cu film32. The change of Gibbs free energy induced by the formation of graphene from gaseous CH4 can be expressed as:According to the Gibbs thermodynamic principle, the shape of an equilibrium crystal is obtained by minimizing the total surface free energy associated to the crystal-medium interfaceA is the area of graphene domain, Ao is the equivalent area of single carbon atom in graphene, Δg is the change of Gibbs free energy from single gaseous CH4 molecule to solid graphene lattice, k is the shape factor, which reflects the anisotropy of graphene, C(A) is the perimeter of graphene domain, which is sensitively dependent on the shape of graphene domain, γedge is the edge free energy of graphene domain, ACuf is the area of Cu-CH4:H2 flow surface, γGf, γGCu, γCuf is the interfacial energy of grapnene-CH4:H2 flow surface, graphene-Cu surface and Cu-CH4:H2 flow surface, respectively. The first term reflects the final chemical energy variation tendency during the growth process, which is the thermodynamic driving force of graphene growth. The second and third terms reflect the thermodynamic impediment of graphene growth by the introduction of additional edge energy and interfacial energy of the total CVD system. The final Gibbs free energy of the CVD system is tending to reach the minimal value as the result of the competition among the three parts in the Where 4 on Cu surface during the graphene growth are: (1) active CH4 is broke down into carbon species such as CHx (x = 0–3) through dissociative chemisorption on the Cu surface and diffuse on the Cu surface; (2) once the increasing dynamic concentration of carbon species reaches critical supersaturation level, the nucleation of graphene takes place; (3) part of the supersaturated carbon species with enough energy reach the graphene domain edge and attach to the graphene domain; (4) the CHx species at the unstable graphene edge detached itself from graphene and form dissociative carbon species; (5) Dissociative carbon species combine with hydrogen and are desorbed from Cu surface30.The overall processes of CH32. It is expected that the growth limiting factor for hexagon graphene domain is the attachment of carbon species at the anisotropic growing front of natural graphene nucleus. The dissociative carbon species on the liquid Cu surface (at 1100°C) have enough concentration and high diffusing ability. Because the natural graphene domain exhibits fast growth in the [2īī0] direction and slow growth in the [10ī0] direction, the typical hexagons domain is formed as it growth. As the temperature decreases to 1050°C, the surface diffusing ability of carbon species on solid Cu surface is limited, thus the dendritic graphene is under the control of diffusion-limited growth mechanism33. It should be noted that the dendritic shape has the longest perimeter in comparison with other shapes, which is beneficial for the collection of dissociative carbon species. Moreover, the dendritic shape becomes manifest as the domain grows. The achievement of carbon species attachment to domain edge and incorporation into graphene lattice before their desorption can lower the total Gibbs energy of the CVD system, which is the thermodynamic driving force for the dendritic graphene domain formation. The formation of circular graphene indicates that, with the increase of temperature, attachment and surface diffusion are no longer the limiting factors. As mentioned above, the circular graphene domain grown at 1150°C has the lowest Gibbs energy due to the shortest edge. Although the zigzag edge along [10ī0] direction has the lower edge free energy than that of armchair edge, its longer perimeter will increase the total Gibbs energy of hexagons domain especially when the domain grows larger. Thus the anisotropic edge energy is no longer the limiting factor and the shape is instead governed by the minimization of perimeter. It is supposed that the high temperature makes the active CHx at the hexagonal graphene edge detach from graphene (detachment-limited growth) and relax to the kink with lower energy, which lead to the circular shape on the macro level. The phenomenon of shape evolution of 2-dimensional crystal has also been proved theoretically34.In conclusion, we developed a rapid-quenching CVD system based on electromagnetic induction heating, which is capable of selectively heating the Cu substrate only and rapid cooling the Cu substrate from 1150°C to 700°C in several seconds. The rapid-quenching CVD system makes it possible to terminate the graphene growth rapidly and to study the graphene growth kinetics. Based on the rapid-quenching CVD system, our observation shows that the graphene nucleation only occurs during the initial stage and the domain density is independent of the growth temperatures due to the surface-limiting effect. Taking full advantage of the rapid-heating and rapid-cooling method, controllable circular, hexagonal and dendritic graphene domains were obtained. The evolution of varying shapes with growth time were observed, which are beneficial for the deep identification of competing atomic phenomena, such as diffusion-limited, attachment-limited and detachment-limited mechanism, in single crystal graphene growth.4 and H2, in which CH4 was used as the carbon-containing precursor. A 2.5-cm-diam quartz tube, using as the CVD chamber, is wrapped by the Cu induction coil. The Cu foil  using as graphene growth catalyst was placed on a tungsten (W) plate (1.2 cm × 4 cm), which was put into a W coil (with inner diameter of 1.2 cm and 3-cm-length). The package was then loaded into the quartz tube at the middle of Cu induction coil. When the high frequency power  was applied on the Cu induction coil, the Cu foil was heated to a high temperature rapidly. The temperature was measured by using a thermocouple . Before the growth of graphene, the Cu foils on W plate were firstly annealed at 1150°C for 10 s. During the growing process, 28 sccm H2 and 19 sccm CH4 were introduced to the CVD chamber at pressure of 215 Pa and the high power was applied to the Cu foil to rise up to target temperature. After reaction, the Cu foil was cooled down rapidly with the flow of 28 sccm H2.Graphene was synthesized by electromagnetic induction heating CVD using a gas mixture of CHScanning electron microscopy (SEM) images of graphene on Cu foil were obtained with Hitachi S-3000N and Nova NanoSEM 230 at 5 kV. High resolution transmission electron microscopy (HRTEM) selected area electron diffraction (SAED) patterns were taken at 80 kV with TECNAI G2F20. Atomic force microscopy (AFM) images of graphene on silicon substrate were obtained with Bruker Multimode 8 at tapping mode. X-Ray Diffraction (XRD) patterns of the Cu foil and the recrystallized Cu film were obtained with MiniFlex II.


Even at kHz light modulation frequencies, specific detectivities exceed 108 Jones making them suitable for fast video imaging. The simple device architecture and QD film patterning in combination with a broad spectral sensitivity manifest an important step toward low‐cost, multi‐color infrared cameras.Infrared light detection enables diverse technologies ranging from night vision to gas analysis. Emerging technologies such as low‐cost cameras for self‐driving cars require highly sensitive, low‐cost photodetector cameras with spectral sensitivities up to wavelengths of 10 µm. For this purpose, colloidal quantum dot (QD) graphene phototransistors offer a viable alternative to traditional technologies owing to inexpensive synthesis and processing of QDs. However, the spectral range of QD/graphene phototransistors is thus far limited to 1.6 µm. Here, HgTe QD/graphene phototransistors with spectral sensitivity up to 3 µm are presented, with specific detectivities of 6 × 10 8 Jones, and kHz operation demonstrating potential for fast video imaging.Colloidal quantum dot/graphene phototransistors offer a viable alternative to traditional infrared detector technologies owing to inexpensive synthesis and processing of QDs. Here, the spectral sensitivity is extended up to 3 µm with HgTe QD/graphene phototransistors presenting specific detectivities of 6 × 10 Furthermore, a thermal expansion coefficient mismatch between some materials and the read‐out electronics induces strain in the bump bonds resulting in a low reliability upon repetitive cryogenic cooling, the cooling is generally being required for photodetectors with spectral sensitivities beyond a wavelength of 3 µm. Therefore, bump bonding is a complex technical process, which increases the overall camera fabrication costs impeding the use in emerging applications such as in self‐driving vehicles. Other emerging technologies such as micro‐spectrometer and multi‐color  IR cameras can be used for environmental monitoring, detection of hazardous gases, and for increased object identification making use of material specific spectral absorption, however, patterning of the IR active material is required. In multi‐color cameras, each pixel can distinguish between multiple IR colors and first demonstrations of this include two‐color photodetectors based on stacked HgTe epitaxial thin films, HgTe QD thin films, or hybrid HgTe QDs on silicon. However, the integration of additional colors into these devices drastically increases the fabrication complexity. Alternatively, multi‐color IR cameras can be fabricated by dividing each pixel into color‐sensitive sub‐pixels requiring microfabrication and patterning techniques.Night vision and passive imaging of thermal sources require infrared (IR) light detection with a spectral sensitivity up to 10 µm, while IR spectroscopy of gases and molecules require a spectral sensitivity ideally covering 2–15 µm. For these applications, highly sensitive, but costly, detectors are presently used; in particular, epitaxially grown narrow bandgap semiconductors such as HgCdTe and InGaAs alloys. For camera applications, the detector material is commonly grown on a separate substrate and later integrated into silicon‐based read‐out electronics by bump bonding, which is challenging for pixel pitches below 10 µm as it requires high assembling accuracy.1, 2 simultaneously provides high spectral sensitivities and potentially reduced overall detector costs by combining low‐cost fabrication with patterning techniques such as stamp transfer, and nanoprinting. These phototransistors do not require bump bonding and can readily be integrated into silicon‐based read‐out electronics. High sensitivities have been reported for room‐temperature operation of PbS QD/graphene FETs with a spectral sensitivity determined by the QD size resulting in a spectral cutoff between 1000 and 1600 nm. While expanding the spectral range toward shorter wavelength has been demonstrated by employing QD materials such as ZnO for UV, and perovskite QDs or CdSe QDs for visible light detection, it has remained challenging to extend the spectral sensitivity to longer wavelengths, namely the short‐ and mid‐wavelength‐IR spectral range. For this task, HgTe QDs, with their tunable spectral sensitivity from short‐wavelength into terahertz range, are among the most suitable candidate materials. Various HgTe QD based photodetectors have been demonstrated with spectral sensitivities in the short‐wavelength and mid‐wavelength infrared region. Some photoconductors and photodiodes have implemented the use of graphene electrodes (which is significantly different as compared to graphene field‐effect transistor devices) demonstrating responsivities in the low mA W−1 range. Higher responsivities of 105 A W−1 were reached with phototransistors based on HgTe QDs and MoS2 FETs up to a wavelength of 2.1 µm, but the phototransistor required an intermediate TiO2 buffer layer in between the HgTe QDs and the MoS2 to protect the MoS2 transistor and to generate a pn‐junction at the HgTe QD/TiO2 interface. In fact, the interface between graphene (or MoS2) and HgTe QDs is critical for achieving functional phototransistors, and the development and fabrication of HgTe QD/graphene phototransistors operating beyond 2.1 µm remain unprecedented.A device concept, based on colloidal QDs sensitized graphene field‐effect transistors (FETs),9 simu8 Jones at a wavelength of 2.5 µm, extending the spectral sensitivity beyond 3 µm. Furthermore, the phototransistor reveals operation at kHz light modulation frequencies with potential for fast video imaging.Here, we demonstrate the fabrication and evaluation of HgTe QD/graphene phototransistors with high responsivities, and a specific detectivity of 6 × 10Figure  The HgTe QDs were synthesized following the protocol of Shen et al. to obtain non‐aggregated QDs, which is paramount for avoiding clogging of the printing nozzle. Transmission electron microscopy of a typical synthesis showed a spherical QD shape with QD diameters of about 5 nm . A powder X‐ray diffraction pattern exhibits a zinc‐blende crystal structure  in agreement with literature. Nanoprinting, as a deposition method, enables a precise deposition of QDs on top of the graphene channel . To prevent potential parallel conductance from one gold electrode through the narrow bandgap semiconductor film to the second gold electrode, the QD film was deposited on a smaller area to avoid overlap with the gold electrodes. As a result, the photoactive area is of a smaller dimension (5 × 40 µm) compared with the graphene transistor. For calculations, the full area of graphene (5 × 50 µm) was taken into account, as noise originates over the complete graphene area. After the nanoprinting, the pristine QDs ligands were exchanged with shorter 1,2‐ethanedithiol ligands by a single‐step solid‐state ligand exchange  resulting in a QDs layer thickness reduction from 410 to 340 nm as determined by atomic force microscopy . The ligand exchange may not replace all pristine ligands with 1,2‐ethanedithiol, but it is sufficient enough to result in a functional phototransistor device. Further optimization of this device may be achieved by optimizing the QD layer thickness as it has been previously reported for PbS QD/graphene phototransistors. The phototransistor was electro‐optically characterized by a responsivity setup with modulated light . Figure −1, which is red‐shifted by about 75 meV as compared to a Fourier‐transform infrared spectrometer (FTIR) absorption measurement . The responsivity measurement differentiates from the FTIR measurement in terms of employed ligands for the QD film and the measurement temperature, which both can result in a bathochromic shift. Further extension of the spectral sensitivity into the long wavelength IR can be achieved by employing larger HgTe QDs.Figure −1 (at an applied drain bias of 0.5 V) at a temperature of 80 K which may even increase at lower temperatures and higher applied drain bias. In contrast to commercial IR detectors, the HgTe QD/graphene phototransistor can be fabricated directly on silicon‐based substrates. Furthermore, the HgTe QD/graphene phototransistor can be repetitively thermally cycled for at least 50 cycles between 300 and 80 K without device degradation nor failure , R is the responsivity (A W−1), in is the noise current (A Hz‐1/2), and Ids the drain current. The specific detectivity D* is shown in Figure 8 Jones at a gate voltage of 6 V, light modulation frequency of 67 Hz, and a wavelength of 2.5 µm.8 Jones at a gate voltage of 11 V and a frequency of 45 Hz, and decreases at higher frequencies. While the frequency bandwidth is defined as a 50% drop in the responsivity value, more attention should be given to the bandwidth of the specific detectivity. Phototransistors exhibit high 1/f noise behavior which compensates the responsivity drop to some extent, while photodiodes have a flat noise behavior and a decrease in responsivity leads to the same decrease in the specific detectivity. Here, 50% of the maximum specific detectivity is still achieved at 700 Hz and proves that these hybrid HgTe QD/graphene phototransistors can be operated at moderate frequencies at the cost of lower responsivity. When larger responsivities are required at low light intensities for small area devices, the drain bias can be increased which will simultaneously increase the responsivity and noise current, but leaves the specific detectivity unaffected within an ohmic operational range. Figure ‐12 W) is derived as marked in Figure 8 Jones with an equivalent noise bandwidth of 0.094 Hz (defined by the lock‐in settings), which is highly similar to the value calculated from the responsivity and noise measurements obtained under the same measurement conditions (2.9 × 108 Jones).Both the responsivity and noise are strongly dependent on the light modulation frequency as shown in Figure  but there is an ample space to further optimize the HgTe QD/graphene phototransistors. For example, an improved specific detectivity is expected by QD deposition over the complete graphene channel, and at an optimized QD layer thickness. Furthermore, it is likely that the temperature‐dependent responsivity can be improved by employing different ligands, and optimized QD/ligand combinations may raise the operational temperature improving the specific detectivity. For example, a solution‐based ligand exchange with a combination of organic and inorganic ligands , followed by QD deposition leads to HgTe QD films with higher charge carrier mobilities. The latter leads to longer charge carrier diffusion‐length which is beneficial for the extraction of photoinduced charge carriers out of thicker QD films and likely results in an improved device operational speed. Furthermore, the conduction‐ and valence band positions can be modified by varying the amounts of HgCl2 in the ligand exchange procedure, and can lead to an optimized band alignment with graphene.The demonstrated HgTe QD/graphene phototransistor device concept is accompanied by three main advantages (i) the spectral sensitivity can be tuned by the QD material, (ii) the phototransistor can be patterned by precise, local deposition of the QDs, as required for multi‐color photodetectors, and (iii) the phototransistor can directly be fabricated on silicon‐based substrates with robust phototransistor performance upon thermal cycling. Such a HgTe QD/graphene phototransistor shows an inferior specific detectivity compared to commercial photon detectors,47 but8 Jones at a wavelength of 2.5 µm and a frequency of 67 Hz is obtained. The specific detectivity declines by 50% at moderate light modulation frequencies of 700 Hz, well suitable for fast video recording. Higher specific detectivities may be achieved by increasing the QD layer coverage on graphene, optimization of the QD layer thickness, or utilizing different ligands and, by that, raising the operational temperature that optimizes the responsivity. Extending the spectral sensitivity of QD/graphene phototransistors into the midwavelength IR is a major advance towards fast multi‐color phototransistor arrays, which can benefit from the simple device architecture by fabrication of the phototransistors on a graphene FET platform, local QD deposition, and spectral sensitivity determined by the QD material.In summary, by employing HgTe QDs, the spectral sensitivity of QD/graphene phototransistors has been extended to midwavelength‐IR spectral range up to 3 µm. The phototransistor shows no photosignal at room temperature, but at 80 K a specific detectivity of at least 6 × 10Acetonitrile ; Ammonium persulfate (≥ 98.0% Sigma Aldrich); bis(trimethylsilyl)telluride ; 1,2‐ethanedithiol ; hexane ; anhydrous hexane ; hydrochloric acid ; mercury(II) chloride ; anhydrous methanol (99.8% PanReacAppliChem); oleylamine ; anhydrous tetrachloroethylene ; Tetradecane . All chemicals were used as received, except otherwise stated. Tetradecane and oleylamine were dried and degassed under reduced pressure at a temperature of 120 °C for 1 h before their use.Caution: Highly toxic materials and heavy metals are used and appropriate precautions have to be implemented.) The HgTe QDs were synthesized following a protocol by Shen et al. Briefly, 27 mg (0.1 mmol) HgCl2 were dissolved in 4 mL dried oleylamine at 100 °C for 1 h under inert atmosphere. A mixture of 15 µL (0.05 mmol) of bis(trimethylsilyl)telluride in 0.5 mL anhydrous hexane was injected into the Hg2+ containing solution. After 1 min of QDs growth at 100 °C, the dispersion was quenched in 4 mL tetrachloroethylene. The QDs were precipitated from the dispersion by adding 3.6 mL methanol, centrifuged at 12 100 rpm for 1 min and the precipitate was dispersed in 0.9 mL tetrachloroethylene. The HgTe QDs were precipitated a second time with 0.5 mL methanol and centrifuged 12 100 rpm for 1 min. A small amount of the precipitate was dispersed in tetrachloroethylene, filtered through a 0.2 µm polytetrafluoroethylene syringe filter. The dispersion in tetrachloroethylene has been used to fabricate HgTe QDs thin films and measure their absorbance with Fourier‐transform infrared spectroscopy. The main precipitate was dispersed in 0.5 mL dried tetradecane and filtered through a 0.2 µm polytetrafluoroethylene syringe filter. The QDs dispersion was stored under dry and inert atmosphere.. While the graphene was etched beside the Cu, it remained intact underneath the Cu. In a next step the Cu was etched with a water‐based ammoniumpersulfate solution (1.6 mg mL−1) for 5 min, and the electron beam lithography was repeated followed by an electron beam assisted thermal deposition of 2 nm Cr and 40 nm Au to form the electrodes.Commercially available monolayer graphene  grown by chemical vapor deposition was transferred on to a highly (aq), 200 µL 1,2‐ethanedithiol) was placed on the sample for about 7 s, removed by spinning the sample and rinsed with acetonitrile, followed by hexane. After the ligand exchange had been performed, the sample was stored in an air‐ and water‐free atmosphere.After nanoprinting of the HgTe QDs, the native oleylamine ligands were exchanged with 1,2‐ethanedithiol in a single‐step solid‐state ligand exchange at ambient conditions. A drop of a ligand exchange solution , onto a monochromator  equipped with two gratings . Higher frequency orders were filtered out with longpass filters . The light was collimated with a CaF2 lens, mounted on a stage to adjust for the wavelengths‐dependent focal lengths. The collimated light was split into two beams with a 50/50 polkadot beamsplitter. One beam illuminated a reference detector  mounted behind a CaF2 window and connected to a lock‐in . The other beam illuminated the sample, in equal distance to the beam splitter as compared with the reference detector, mounted in a cryostat  with CaF2 windows and an operating temperature range between 80–300 K. The sample was biased with a source‐measurement unit , and the photosignal was measured as voltage fluctuations with a lock‐in  over a 100 Ω load resistor connected in series with the sample. A gate voltage was applied with a source‐measurement unit  and the measured gate leakage did not exceed 10 nA. Measurements were performed at reduced pressure (10−5 hPa).Light from a globar  was focused with a CaF−5 hPa).A battery‐powered SR570 current amplifier was used to bias the sample and to amplify the current. The current was recorded with a NI USB‐6281 data acquisition board at a sampling rate of 625 kHz, and a 40 kHz lowpass filter. The signal was band‐limited with a tab 100 impulse response digital filter to 78 kHz and downsampled by a factor of four. Ten time‐traces each one second long were recorded and the resulting power spectral densities were averaged. The 50 Hz frequency was manually removed from the frequency spectra. Measurements were performed at reduced pressure  between the nozzle and the sample. The nanoprinting was performed under ambient conditions at 25 °C and 35–40% relative humidity.We followed a previously published protocol.11 Bri‐10 A Hz‐0.5 in the 1/f noise spectrum at a modulation frequency of 1 kHz (taking a drain current of 3.8 × 10−5 A at a drain bias of 0.5 V). At the same measurement conditions, the calculated thermal noise was 1.1 × 10−12 and 0.6 × 10−12 A Hz−0.5 at 300 and 80 K, respectively; and the calculated Shot noise was about 3.5 × 10−12 A Hz−0.5. Therefore, the thermal and shot noise contributed together about 1% or less to the total noise level and their influence could be neglected.The lowest noise level was around 3.8 × 10The authors declare no conflict of interest.M.J.G., I.S., M.V.K. conceived and designed the experiments. M.J.G. fabricated the graphene FETs and carried out the optoelectronic measurements and AFM measurements. M.J.G. synthesized the HgTe QDs. The QDs were nanoprinted by C.U.H. with contributions to the experimental design and discussions with D.P. The noise measurements were conducted by D.B. with assistance of M.C. The paper was written by M.J.G. with contributions by all authors and all authors have given approval to the final version.Supporting InformationClick here for additional data file.


Also, we show a 100-fold enhanced photodetection due to highly focused (up to a record factor of |E/E0| ≈ 20 for graphene) electromagnetic fields induced by electrically tunable multimodal plasmons, spatially localized in self-similar fashion on the metasurface. Our findings give direct insight into the physical processes governing graphene plasmonic fractal metamaterials. The proposed structure represents a promising route for the realization of a broadband, compact, and active platform for future optoelectronic devices including multiband bio/chemical and light sensors.Metamaterials have recently established a new paradigm for enhanced light absorption in state-of-the-art photodetectors. Here, we demonstrate broadband, highly efficient, polarization-insensitive, and gate-tunable photodetection at room temperature in a novel metadevice based on gold/graphene Sierpinski carpet plasmonic fractals. We observed an unprecedented internal quantum efficiency up to 100% from the near-infrared to the visible range with an upper bound of optical detectivity of 10 Notwithstanding the high level of maturity, the large scale, and diversity of application areas, the need for a compact photodetection platform with higher performance in terms of speed, efficiency, spectral range, as well as flexibility, semitransparency, and compatibility with complementary metal-oxide-semiconductor (CMOS), is becoming more eminent2 . This leads to the concept of metadevices2, a logical extension of the metamaterial paradigm, where interactions are nonlinear and the response is dynamic, using systems with spatially variable elements. Generally, such metadevices can operate at low voltages, which is a competitive advantage over conventional technology exploiting bulk and expensive electro-optical crystals, often non-integrated in silicon photonics. However, the bandwidth of such metamaterials is generally narrow due to their resonant nature and most of them are working in the infrared band. Mastering control in the optical regime is technologically a more challenging task, due to the expensive and demanding fabrication of nanostructures, resulting from the need of high-resolution electron- or ion-beam techniques. Also, devices working in a wide spectral range are important for multiband photodetection3, on-chip sensing of multiple analytes4, as well as multiplexed fluorescence and Raman detection5. Hybridization of low-dimensional carbon allotropes with plasmonic metamaterials16 is known to provide broadband and strong light-matter interactions, improvement of the nonlinear response due to the electromagnetic field enhancement given by the metamaterial, and compatibility with well-established silicon photonic and CMOS technologies18, therefore realizing a platform for high-speed19 light modulation and detection on the same chip.Such features can be greatly fulfilled by combining functional systems with metamaterials20, with a strong focus on various photodetection frameworks1. The versatility of this exceptional material enables its application in a number of areas including ultrafast and electrically tunable detection of light over a wide spectral range from ultraviolet to THz30. However, single-layer graphene absorbs 2.3% of the incident light in a large spectral range31, which is remarkably high for an atomically thin material, but it leads to a low photocarrier absorption efficiency that constitute a limiting factor for highly sensitive state-of-the-art photodetectors. By coupling plasmonic metamaterials with graphene, localized surface plasmons32 (LSP) — collective charge oscillations at the interface between a subwavelength metal object and a dielectric — can be excited in photodetectors to improve graphene absorption by the resulting field enhancement, therefore increasing the device responsivity. However, plasmonic structures with periodic geometries can be used only for filtering light polarization and selective absorption at designed frequencies33, restricting their application field. Recently, the first graphene plasmonic fractals7 have been realized, reporting a polarization-insensitive, gate-tunable, and enhanced photodetection. So far, such metamaterials are still narrow-band and have low photodetection efficiency with poor electrical modulation.In this regard, graphene — a two-dimensional system composed of carbon atoms arranged in a hexagonal lattice — has rapidly established itself as intriguing building block for optoelectronic applications34 fabricated on a graphene/SiO2/Si substrate. The gold/graphene (Au/G) fractal absorbs light 25 times stronger than an Au SC. As a result of the strong electromagnetic field enhancement (up to |E/E0| ≈ 20) within the interstructure gaps, the graphene-light interaction is greatly enhanced (up to an unprecedented factor of |E/E0|4 ≈ 105) at designed frequencies. This provides a multimodal plasmonic response that covers a broad range of the electromagnetic spectrum (VIS–MIR). Owing to the continuous nature of the Au/G fractal metamaterial, both its absorption intensity and bandwidth can be controlled electrically by varying the charge carrier concentration in graphene, through a gate voltage. This approach allows modulation of the device optical reflectance up to 23% in intensity and 60% in photon energy, much higher than that reported by Aygar et al.6. We observed in the optical range up to 100% internal quantum efficiency, remarkably higher than that reported by Fang et al.7, with a gain up to 106 and an upper bound of optical detectivity of 2 × 1011 cm Hz1/2W−1. Moreover, we present up to 100-fold enhancement of the responsivity, with respect to an unpatterned graphene photodetector.Here, we show that plasmonic fractal metamaterials can be designed to significantly improve broadband and polarization-independent light absorption in graphene photodetectors, thus enabling multiband photodetection. A novel graphene-based electrically active metadevice for efficient control and detection of radiation in the visible (VIS)–mid-infrared (MIR) range was realized. The metamaterial consists of an Au Sierpinski carpet (SC) fractal35 are self-similar objects generated by geometrical rules, having a non-integer (Hausdorff-Besicovitch) dimension. Sierpinski carpets can be generated by a recursive geometrical algorithm employing a Lindenmayer system implementation34. We designed Au/G SCs starting from a graphene unit cell of side L0 = 10 μm that is divided into a 3 × 3 array of sub-cells of lateral size L1 = L03−1, with an Au square placed in the central sub-cell. By iteratively applying the same rule to the generated sub-cells of size t of complexity34. In this way, we indirectly patterned graphene, thereby realizing an Au/G fractal metamaterial that can be thought as a discrete Au SC and a complementary continuous graphene SC. At each iteration the side of the sub-cells is reduced by a factor 34.Deterministic fractals2 substrate , we fabricated the first five orders of an Au/G SC on a Si/SiO34, the Au SC can be regarded as a metallo-dielectric photonic quasi-crystal. In Fig. 36 reciprocal lattice of the SC for t = 5 order is shown, by computing the fast Fourier transform of its SEM micrograph. Such a reciprocal lattice is self-similar as its direct counterpart34. It follows that the diffraction patterns and the related optical spectra of such fractals are themselves self-similar36. Also, the SC has a larger number of points in the reciprocal space than a periodic array, since its fractal reciprocal lattice is a superposition of five periodic lattices with different constants at = 3Lt, under the periodic approximation37.As we have previously reported in ref. r of an Au and Au/G SC divided by the reflectance rsub of the SiO2/Si substrate is shown. The reflectance of the substrate is dominated by a minimum around 0.7 eV due to thin-film interference effects between SiO2 and Si. The spectra of Au and Au/G SCs exhibit multiple features in the VIS–MIR range, which for the Au SC appear as maxima ωn owing to the thin-film interference effects between Au and graphene. In ref. 34, we have reported that the optical spectrum of an Au SC at a given fractal order t mainly manifests t diffraction-mediated LSP resonances n = 1,2,…t, for small LSP wavevectors kp. For large kp, ωsp = 4.17 eV the Au surface plasmon32 /RAu between the two samples is up to 25% at ω3, which means a remarkable relative absorption variation (RAu/G − RAu)/(1 − RAu) of 2500%. Therefore, graphene can be exploited as an ultrathin, broadband, anti-reflective coating. From the comparison with the reflectance spectra of Au/G periodic arrays in Fig. at of the Au structures. Also, the SC resonances in Fig. 34. In particular, the ω3 mode overlaps with the SiO2 minimum. From data in Fig. kp = 2π/Lt = 6π/at is the self-similar LSP wavevector of the Au SC. From the plot we observe that the out-of-plane coupling between the Au SC LSP modes and graphene is two times stronger than the in-plane coupling between the LSP modes of the Au SC squares /RCNP, for the five LSPs can be enhanced up to 23% (2d). Also, we noticed that only the position in energy EF of the graphene interband threshold , . This is an important feature in order to realize polarization-independent devices7, since other aperiodic metamaterial16 and periodic metamaterials14 usually depend strongly on light polarization.Additionally, in ref. 36. When a mode of the Au fractal is excited, a considerable field enhancement not only near the squares but also across the graphene layer is observed. In fact, graphene represents a unique model system where nonlinear applications, such as surface enhanced Raman spectroscopy (SERS), can be studied in detail43. In Fig. t = 5 order of the Au/G SC and for an unpatterned graphene sample (t = 0) are plotted. A SERS enhancement factor on the Au/G SC can be evaluated in Fig. at for the graphene G and 2D phonon bands. As for Au SCs, the SERS enhancement factor follows a power law34, but for Au/G SCs is 10-fold higher for the smallest feature, achieving a value of 105. Additionally, as shown in Fig. t = 5 order Au/G SC a thin layer of Brilliant Cresyl Blue (BCB) dye as a probe . Graphene is the active material of the device and the Au fractal acts as a plasmonic sensitizer to improve the overall light sensing. In the inset of Fig. G = 4.5 eV20 and ΦAu = 4.8 eV46, respectively. For the same reason, the Au fractal also provides an hole charge-transfer to graphene 2 exhibits a nonlinear dependence on the incident power of light P, due to electrostatic and thermal effects, trapping, and recombination occurring at high values of power47. The photocurrent response results linear up to 6 mW/cm2 for VG = −50 V at a photon energy of 1.94 eV. Furthermore, by photogating effect, the device produces a DC photovoltage that is dependent on the incident photon energy and power . In a metal-graphene-metal configuration, in which graphene is contacted with metal electrodes as source and drain, a p-n junction occurs close to the contacts because of the difference in the work functions between the metal and graphene49. Consequently, a charge transfer with a shift of the graphene Fermi level in the region below the metal pads occurs. The junction results in an internal electric field able to separate the light-induced electron-hole pairs (photovoltaic effect). Unless the contacts are made of different materials, the photocurrent produced at both the contacts will be of opposite polarity for symmetry reasons, resulting in zero net current at small gate voltages close to the CNP19. Usually, in such devices a light-induced photogating and photothermoelectric effect are also present at gate voltages close to the CNP, while at larger gate voltages the bolometric effect is typically dominant49. We observed the photovoltaic and photogating effects in the unpatterned graphene photodetectors , where the photothermoelectric and photogating effects become smaller, the bolometric effect should be dominant. Therefore, we propose the following explanation for the photogeneration mechanism in Au/G SC photodetectors. When light is shone on the metadevice, it is absorbed by Au fractal LSPs generating electron-hole pairs in the sensitizer (gating layer), which are transferred to graphene (channel layer). This results in a photocurrent and a photovoltage due to the change in electronic temperature between the Au SC and graphene regions, thus a variation of the carrier density Δn = CΔVG  of the device bandwidth is 50 in graphene  growth of large area graphene52 would grant integration over wafer scale at a low cost of graphene-based photodetectors. Finally, the wide-range and high-gain spectral response of the Au/G fractal and its ability to hierarchically localize strong electromagnetic fields on a sub-wavelength scale could pave the way to a new generation of materials for integrated optics and optoelectronics, such as multiband photosensor pixel arrays for imaging cameras and optical communication devices.In summary, we presented a novel graphene-based, plasmonic metamaterial having a multiresonant optical response hierarchically distributed on the fractal structure, which can be employed to realize a highly sensitive and polarization-independent, broadband photodetector. The metadevice is compact and can operate at room temperature under small voltages, with low power consumption. The detectivity could be further improved by depositing the Au fractal on high-mobility graphene-based van der Waals heterostructures, such as G/hBN52. The foil was spin-coated by a PMMA membrane and the graphene was electrochemically delaminated from the growth substrate in 1 M NaOH solution. Using a frame, the graphene/PMMA membrane was removed from the electrolyte, rinsed in deionized water and dried in ambient conditions. The membrane was deposited on the target Si/SiO2 substrates at 120 °C and the PMMA carrier polymer was removed in acetone. Graphene and Au SCs were patterned by EBL (Raith 150-Two) on Si wafers (1–5 mΩ cm) with a tox = 285 nm tick layer of thermal oxide. A layer of approximately 160 nm of 950 K poly-methyl-methacrylate  was deposited by spincoating on the substrates, which were then post-baked for 7 min on a hot plate at 180 °C. The electron beam was operated at 20 kV with a current of 35 pA. After exposure, graphene was etched by O2 (5 sccm) reactive ion etching (Sentech instruments ICP-RIE 500) for 30 s at a RF power of 35 W and temperature of 15 °C in an Ar inert atmosphere (80 sccm). The SC patterns were exposed with a varying dose inversely proportional to the size of the squares of the fractal. Contacts pads and probes were exposed for the electrical measurements. The resist was developed for 30 s in a cold (8 °C) 1:3 mixture of MIBK:IPA. A 35/5 nm thick Au/Ti film was deposited on the substrates at a rate of 0.2 Å/s by electron-beam evaporation (Kenosistec) at an operating pressure of 10−6 mbar. Lift-off was then performed by hot (80 °C) acetone. A wire bonder (F&S Bontec Series 56) was used to wire the contact pads to an external circuit board. For SERS measurements, SCs were dip-coated for 1 h in 1 mM BCB [(C17H20ClN3O)2ZnCl2] aqueous solution, then rinsed in deionized water to wash the excess molecules in order to form a thin layer 12 ± 2 nm thick, and finally dried in nitrogen flow.Single-layer, single-crystal graphene was grown on copper foil by CVD using an Aixtron BM cold-wall reactor at a temperature of 1060 °C and a pressure of 25 mbar. A semi-dry transfer approach was used to transfer the graphene from Cu to the target substrate34. Scanning electron microscopy micrographs of Au/G SC samples deposited on Si/SiO2 were acquired by FEI Helios NanoLab DualBeam 650. Atomic force microscopy height profiles of the samples deposited on Si/SiO2 were measured in tapping mode by Bruker Innova in combination with v-type cantilever and SiN tips. Optical spectroscopy in reflection mode was performed with unpolarized light on SCs deposited on CaF2 substrates, by Thermo Fisher FTIR spectrometer and Thermo Fisher Nicolet Continu μm microscope equipped with NO2-cooled MCT and Si detectors, KBr and quartz beamsplitters, and a 15× (0.58 N.A.) Cassegrain objective. Surface enhanced Raman spectroscopy was carried out by Renishaw inVia micro-Raman microscope equipped with a 100× (0.85 N.A.) and 150× (0.95 N.A.) objectives, and 514 nm and 532 nm lasers at power 1.2 W/cm2 and 0.1 W/cm2 respectively, and integration time 1 s. All spectra were calibrated with respect to the first-order silicon LO phonon peak at 520 cm−1 and recorded in backscattering geometry at room temperature. Raman measurements were performed on Au/G SCs and as a reference on bare graphene deposited on the same Si/SiO2 substrate. Raman maps of graphene G and 2D, and BCB ω* = 1655 cm−1 vibrational bands were scanned at 0.3 μm steps in both the directions in the plane of the sample. Renishaw WiRE 3.0 software was used to analyze the collected spectra, whose baseline was corrected to the third-order polynomial. Two- and four-probe electrical measurements in van der Pauw configuration were performed in vacuum (10−6 mbar) by Keithley 2612 A digital source-measure unit and Agilent 34410 A digital voltmeter. In order to estimate the value of the residual carrier density n0 ≈ 1011 cm−2, two-probe resistance curves were fitted by the formula Rtot = 2Rc + Rsh, with L, W the FET channel length and width, respectively. For four-probe resistance curves Rc = 0 and Rsh = 4.53Rtot, due to the van der Pauw geometrical correction. Carrier mobility was then evaluated by μe = σ/ne. Photocurrent was measured using 0.80 eV, 1.46 eV, 1.94 eV, and 2.38 eV laser sources at different powers. The output power density was calibrated by a power meter. Responsivity was measured by using the following custom optical setup. An ASB-XE-175 source emitted panchromatic light passing through a SP CM110 monochromator controlled by PC. Emerging monochromatic light was coupled to an optical fiber, modulated at 173 Hz by a mechanical chopper, and focused by a lens first on the samples, then on Si and Ge photodiodes. The AC current from the sample was measured through a current pre-amplifier (DL 1211) with a amplification factor C = 10−6 A/V, whose output was read by a Signal Recovery 7265 lock-in amplifier. The photocurrent was given by the relation π/4 is the fundamental sine wave Fourier component of the square wave produced by the chopper, and Vlock-in the amplitude of the signal read by the lock-in.Samples were characterized as in ref. Supplementary Information.


In the present work, we developed a novel method for transferring monolayer graphene onto four different commercial hydrophilic micro/ultra-filtration substrates. The developed method used electrostatic charging to maintain the contact between the graphene and the target substrate intact during the etching step through the wet transfer process. Several measurement/analysis techniques were used in order to evaluate the properties of the surfaces and to assess the quality of the transferred graphene. The techniques included water contact angle (CA), atomic force microscopy (AFM), and field emission scanning electron microscopy (FESEM). Potassium chloride (KCl) ions were used for the transport study through the developed graphene-based membranes. The results revealed that 70% rejection of KCI ions was recorded for the graphene/polyvinylidene difluoride (PVDF1) membrane, followed by 67% rejection for the graphene/polyethersulfone (PES) membrane, and 65% rejection for graphene/PVDF3 membrane. It was revealed that the smoothest substrate was the most effective in rejecting the ions. Although defects such as tears and cracks within the graphene layer were still evolving in this new transfer method, however, the use of Nylon 6,6 interfacial polymerization allowed sealing the tears and cracks within the graphene monolayer. This enhanced the KCl ions rejection of up to 85% through the defect-sealed graphene/polymer composite membranes. The unique structure of graphene and its distinct physical and chemical properties offer a great prospect for numerous industrial applications such as touch screens, solar cells, smartphones, nanomaterials, sensors, and batteries ,2,3. In Although there are many graphene preparation methods, there is no single method that is adequate for all applications. That is because each method comes with advantages and challenges ,19. For Graphene transfer could be classified into two main categories: the wet transfer method (WTMs) -which is commonly used- and the dry transfer method (DTM). The WTM uses etching solution in order to remove the Cu substrate. Typical etching solutions used are: iron nitrate , ammoniuIn this work, we introduced a new efficient protocol to transfer graphene monolayer onto polymeric hydrophilic substrates, this being a vital step towards developing a high-performance membrane for water purification applications. The method utilized electrostatic charging to maintain the contact of graphene/polymeric substrates intact during the etching step. The quality of transferred graphene was characterized using field emission scanning electron microscopy (FESEM), and the performance of graphene/polymeric substrate composites was evaluated using KCl ionic transport study. wt/vol) was mixed with de-ionized water. The APS was purchased from Eurostar Scientific LTD, in Germany. The copper etchant was utilized to remove Cu during the transfer process. The transport studies were performed using potassium chloride salt (KCl), which was procured from Merck Chemicals .Four different hydrophilic commercial substrates were utilized in this study, ACS Material, a USA based company, supplied the CVD monolayer graphene. To ensure that the monolayer graphene covered the Cu substrate, Raman spectroscopy analysis was used. wt/vol) diluted APS etchant in order to remove the one-sided graphene. The graphene was floated over the etchant during this process, as shown in wt/vol) concentrated APS etchant was used to remove Cu from the composite, by floating the Cu/graphene/substrate over the etchant. The graphene stayed attached to the substrate during this step, as displayed in The Cu etchant has a quick tendency to wet hydrophilic substrates. Hence, the wet etching transfer methods are not suitable. To address this issue, we developed a new approcah to allow the graphene to be transferred onto hyrdophilic substrates. The new approcah used a 5%  purchased from Permegear Inc., Pennsylvania, USA as shown in Before placing the graphene memberane in the cell, both chambers of the cell were cleaned using de-ionized water, and then allowed to air-dry. The active side of the graphene was placed, so that it was facing the the left side chamber. In order to ensure that no water bubles accumelate at the surface of the memberane, both cell’s chambers were washed with ethanol, after tighening the cell. Furthermore to ensure that no excess ethanol was trapped on the membrane, a 0.5 M KCl solution was used to wash the left side chamber that faces the membrane, and using de-ionized water to wash the membrane side facing the right chamber. This was repeated for three times. Using the osmosis process caused by the concentrtion difference, the KCl ions moved from the left chamber to the right chamber diffusing through the composite membrane layers. In order to reduce the effect of concentration polarization during the diffusion, magnetic stirring was employed on the solutions in both side of the cell. In order to aquire and monitor the diffusion rate of the ions, we measured the change in conductivity with time using a special condctivity probe. The elctrode probe was manufactured by eDAQ Pty Ltd . The electrode was placed in the de-ionized water (right chamber) as shown in Defects such as cracks and tears might arise inside graphene layer throughout the transfer process. These defects need to be repaired- through plugging and sealing- to produce a defect-free graphene membrane. Defects plugging was performed using interfacial polymerization (IP) of Nylon 6.6 in defects spaces according to the procedure described in . In ordeThe IP process was performed only for 5 min. Once completed, the upper part was rinsed with hexane and ethanol multiple times. This is to ensure the removal of any residual species within the composite membrane. Sealed graphene composite membrane was then removed and washed with de-ionized and ethanol water baths before air drying.The surface characteristics of the considered substrate affect the quality of transferred graphene, which in turn impacts the performance of the developed membrane. A smooth surface with a minimal average root mean square (rms) is preferred for the graphene/polymer membrane over a rough surface. The value of the contact angle is another measuring criterion related to the final product quality. The degree of hydrophobicity is crucial to maintain the graphene/substrate contact during the transfer process. Stable and reliable graphene/substrate contact prevents etchant solution from penetrating and delaminating the adhered layers. In wet graphene transfer methods, it is rather difficult for graphene to be attached onto a hydrophilic surface because it can be easily wetted by the etchant solution. This wetting action causes delamination even before the completion of the transfer process. Four commercial hydrophilic substrates were utilized in the new transfer process. FESEM and AFM micrographs analyses were carried out in order to characterize the as-received substrates, in terms of their surface morphology, and surface roughness (root mean square) as shown in °. PVDF 2 has the roughest surface (RMS = 32.1 nm) with average contact angle ~73.5°.As shown in When a substrate surface is hydrophilic, the etchant can penetrate between the Cu/graphene layer and the polymeric substrate, causing graphene to detach from the composite. The process of graphene detachment is shown in 2. A high quality graphene can be easily be seen in the The results of the above process confirm that having sufficient hydrophobicity for the polymeric substrate is necessary to ensure that etchant solution does not penetrate between the graphene and the substrate interface. To avoid the delamination process coined with the conventional transfer process, the method explained in 2). Although copper residues  showed the lowest ions rejection, while the smoothest substrate (PVDF 1) provided the best ions rejection. It is worth noting that the newly developed electrostatic transfer method offers a viable route for using hydropholic substrates as it does not involve any mechanical forces that could potentially damage the monolayer graphene layer by inducing additional defects such as tears and cracks.Nevetheless, the graphene transferred using electrostatic charging method developed in this study was still prone to produce some cracks and tears within graphene monolayer. Upon sealing these induced defects using Nylon 6.6 via the IP process, KCl ions rejections of 85%, 85%, 84% and 83% were obtained for graphene/PES, graphene/PVDF 2, graphene/PVDF 1 and graphene/PVDF 3 composite membranes, respectively. Although the rejection was remarkably improved, the composites were still leaking around 15%. Apart from the 10% leakage typically due to the intrinsic defects generated during graphene synthesis, optimizing the IP process parameters could yield 90% rejection. In conclusion, a better control of the CVD process parameters combined with the optimized IP process may lead towards synthesizing an impervious graphene/polymer composite membrane.


I have my neck on a tree singing will I die or will I survive? Somehow, I survived but not without an uncountable number of malaria episodes with chloroquine injections and the adverse itching side effects.I have always been curious about arthropods and the diseases they transmit to humans. Growing up in a malaria-endemic area  exposed my family members and me to the revulsions of malaria disease. Children were the worst hit. We had all sorts of symptoms  and sometimes death. Chief among my symptoms was convulsions. The story goes that my malaria episodes and convulsions were so frequent that I literally formed a song that says Aedes (Stegomyia) aegypti and more recently for my PhD, I took on Anopheles gambiae—our resident enemy. Starting with the study community’s commonest illness—“Iba Maria” , I set out on full time research.I became interested in mosquito bionomics, aspiring to study them and get to the bottom of the illness that prevented us from going to school and playing with our friends. Aspiration became a reality when for my master’s degree project I worked on In the course of the project, I had to pick up indoor biting and resting mosquitoes between the hours of 06:00 and 09:00 over a period of 2 years (2015–2016), four times in a year. Prior advocacy visits prepared the occupants for the early morning exercise. The occupants would step out on our arrival; we would move in and cover the furniture and floor with white sheets. Next, we shut the windows and doors and sprayed the room with insecticides. Then, we stepped out for a bit and reentered after about 10 minutes to pick up knocked down mosquitoes on the white sheets.The knocked down insects were readily visible on the white sheet but not all the communities accepted the white sheets, the reason being that it is generally used for burial ceremonies—more specifically to overlay corpses. So, we bought and replaced the white sheets with golden brown/yellowish sheets. With these new sheets, we were warmly welcomed and made good progress in the first one and half years of the project.Way into the second year, we headed out to the study communities for the last field visit and met head on another challenge. The story about the death of a family from the use of adulterated spray had made the rounds and set the stage for resistance from affected communities. The seller in the local market from whom we bought the sprays had confirmed the news and suggested three brands that may still be acceptable. Although the project brand was one of the three brands suggested, the affected communities were unwilling to let us spray their sleeping places. We needed time for this community to trust us again. Therefore, we continued our project with nearby communities that were unaffected and used that to win back the trust of the affected communities. The project participants in the affected communities were as apprehensive as we were. My fears were predicated on two trains of thought: what if there were underlying issues or other confounders besides the spray that could have been responsible for the unproven deaths; and, should anything go wrong, who takes responsibility?Nothing prepared me for what was coming next. As we moved from community to community, I discovered some families had no nets, whereas others had surplus, with some intact in their packs unused. Without planning, I started to ask questions about redistributing the nets; I wanted to recover the unused nets and pass them on to those who didn’t have any. Mosquito bed nets provide both personal and community protection. My unplanned questions received spontaneous answers. Whereas some were diligent with daily use, others abhorred the nets. Families reported that they were having adverse reactions such as skin irritation or red and teary eyes from the insecticide-treated bed nets. I was truly shocked when they told me they thought that these nets kill newborn babies!On getting back, I went through the semistructured questionnaires used to elicit information on malaria-related knowledge, attitudes, and practices during the cross-sectional survey. Among the reasons recovered from the survey questionnaires for nonuse of these nets or preference for nontreated bed nets was the adverse effect of the treated bed nets. One respondent actually wrote that long-lasting insecticide-treated nets (LLINs) almost suffocated her to death. Two respondents with secondary-level education in the urban community vowed never to use the LLINs because it is lethal. Long-lasting insecticide-treated nets are one of the World Health Organization’s core control tools and a major intervention strategy driving the national elimination goal. As a researcher and a mother whose daughters battled malaria over the years, a barrage of questions poured through my mind. Did the bereaved mothers air the nets, following through on the usage instructions? What brand of insecticide was used to treat the nets in question? Were the nets obtained from the approved distribution channels  or the open market? How do we enhance the integrity of LLINs among these people? Could this be a contributing factor to the high degree of human–vector contact established in the study? The deceased babies’ voices are silent forever, but their mothers’ voices continue to speak for the pediatric population in that locality. The mothers echoed the gnawing pain of loss spreading their theory, albeit with no evidence, while sharing a real and an irreparable loss. Personally, my daughters would break out with skin irritations—redness, swelling on the face, and teary eyes. To avoid or reduce the severity of the discomfort, we would air the nets for 48–72 hours and sometimes more before first use.pm. Of course, I had seen several, but I thought well, they could be airing the nets just like I do or just a few cases of unorthodox use. She said outdoor bites were now a huge challenge which had led to outdoor use of mosquito bed nets for evening sit-outs!One more challenge, and this left me with a key lesson learnt: It is important to be accessible, to engage, and to actively listen to the people in the study communities beyond the first advocacy visit. I was so engrossed with my indoor mosquitoes that I was too blind to see mosquito nets hung outdoors. Fixated on my project objectives, I ignored complaints about outdoor bites until two forthright mothers stopped me right in my tracks and lashed out at me. I was told that I was always coming to pick indoor mosquitoes but that outdoor biting mosquitoes were much more troublesome at the time. It was a case of my perceived need versus their felt need. On sharing this experience with a senior member of the Ministry of Health at the local government level, she advised that I observe more closely nets hung on trees/sticks outdoors and revisit the study communities between 6:00 and about 10:00


Both the basal amygdala (BA) and the bed nucleus of the stria terminalis (BNST) can participate in contextual fear, but it is unclear whether contextual fear engrams involve a direct interaction between these two brain regions. To determine whether dorsal BNST (dBNST)-projecting neurons in the BA participate in contextual fear engrams, we combined the TetTag mouse with a retrograde tracer to label dBNST-projecting cells in the BA. We identified a population of neurons located in the anterior subdivision of the BA (aBA) that was activated during fear conditioning and reactivated during retrieval but that did not project to the dBNST. In contrast, dBNST-projecting neurons located in the posterior BA (pBA) were activated during contextual fear conditioning but were not reactivated during retrieval. Similarly, we found neurons in the oval BNST subdivision (ovBNST) that were activated during contextual fear conditioning without being reactivated during retrieval. However, the anterodorsal BNST (adBNST) subdivision was not activated during either contextual fear conditioning or retrieval, underscoring the divergent functionality of these two dBNST subdivisions. Finally, we found that the ovBNST receives a monosynaptic projection from neurons located in the BA. Our results indicate that aBA neurons that do not project to the dBNST participate in contextual fear engrams. In contrast, dBNST-projecting neurons in the BA do not appear to participate in contextual fear engrams, but might instead contain a BA → ovBNST pathway that is active during the initial encoding of contextual fear memories. Both the basal amygdala (BA) and the dorsal bed nucleus of the stria terminalis (dBNST) can participate in contextual fear, but it is unclear whether this reflects a direct interaction between these two brain regions. BA neurons that do not project to the dBNST were found to be active during both the encoding and retrieval of a contextual fear memory, indicating their participation in a contextual fear engram. In contrast, BA neurons that do project to the dBNST were found to be active during the encoding, but not the retrieval of a contextual fear memory. These findings suggest a direct interaction between the BA and dBNST during the initial encoding, but not the subsequent storage of contextual fear memories.A context previously paired with an aversive experience can induce a sustained and anxiety-like fear that is associated with similar brain activation patterns across vertebrate species . IdentifBoth the BA and BNST are composed of subnuclei with distinct functional heterogeneity and connectivity. The BA is divided into the anterior BA (aBA) and the posterior BA (pBA), which have been shown to contribute differently to the support of a memory dependent on emotional valence . The dorIn an effort to characterize the role of BA-BNST circuitry in contextual fear memory, we used a transgenic mouse called the TetTag mouse to fluorescently tag neurons in the BA and the dBNST that were activated during contextual fear conditioning , and combined this with immunolabeling of an immediate early gene (IEG) to detect neuronal activity on re-exposure to the context. This allowed us to look for BA and dBNST neurons that were active during both fear conditioning and retrieval sessions, which would indicate their participation in a contextual fear engram . We combined this TetTag approach with circuit-tracing to identify the possible participation of BA → dBNST projecting neurons in contextual fear engrams.Our findings identify a population of cells in the aBA that participate in the contextual fear engram but that do not project to the dBNST. A separate population of dBNST-projecting cells in the pBA were found to be activated during contextual fear conditioning without being reactivated during retrieval. Interestingly, our functional and anatomic data indicate the possibility that these pBA → dBNST neurons might activate neurons located in the ovBNST subdivision during contextual fear conditioning.ad libitum and were individually housed beginning immediately after surgery, or one week before fear conditioning in non-surgical cases. Doxycycline chow was replaced with standard chow (no doxycycline) for all mice (fear conditioned group and home cage group) 4 d before fear conditioning. High doxycycline chow (1-g doxycycline/kg) was provided for all mice overnight following the conclusion of fear conditioning, then replaced with regular doxycycline chow for the remainder of the experiment.All animal procedures were performed in accordance with the National Institutes of Health Guide for the Care and Use of Laboratory Animals and were approved by Tufts University Animal Care and Use Committee. Adult female and male TetTag mice were used, with each experimental group consisting of approximately equal numbers of females and males. As no sex differences were observed, the data from females and males were combined. Mice were housed with water and regular doxycycline chow (40-mg doxycycline/kg) provided TetTag mice were stereotactically injected with either 45 nl (for ovBNST) or 150 nl (for dBNST) of 0.05% cholera toxin subunit B (CTB) Alexa Fluor 647 conjugate  dissolved in sterile PBS. Bilateral injections were performed to target the dBNST  using a 0.5-μl Neuros Syringe  fitted with a 32-gauge blunt-point needle. The CTB solution was injected at a rate of 100 nl/min. The needle was slowly removed 10 min after the completion of injection. Mice were given a 10-d postsurgery recovery period before starting behavioral experiments.C57BL/6 mice were stereotactically injected with 300 nl of AAV2-CMV-GFP . Bilateral injections were performed to target the posterior BA  using a 5-μl syringe  fitted with a 33-gauge blunt-point needle. The AAV was injected at a rate of 100 nl/min. The needle was slowly removed 10 min after the completion of injection. Mice were killed two weeks after injection to allow sufficient time for virus expression.On day 1 of the behavioral experiment, mice were subjected to contextual fear conditioning, which consisted of three training sessions each separated by 3 h. At the start of each fear conditioning session, the mouse was transferred to a plexi-glass box with a grid floor  contained within an isolation chamber. Foot shocks  were delivered at 198, 278, 358, and 438 s, with a total session time of 500 s. Mice were returned to their home cage in between each session. On day 4, mice were subjected to a 500-s retrieval test. Mice were placed in the context used for fear conditioning but did not receive foot shocks during the testing session. The sessions were recorded with an above digital camera and freezing behavior was quantified using Actimetrics FreezeFrame software. The bout length of freezing was set to 1 s, and the threshold for freezing was determined by an experimenter blinded to group. Mice in the home cage group remained in their cage throughout the duration of the experiment.Mice were anesthetized and transcardially perfused with ice cold 4% paraformaldehyde 80 min after the start of the retrieval session. Home cage mice, which did not undergo a retrieval session, were perfused on the same day, staggered between perfusions of fear conditioned mice throughout the day. Brains were dissected and postfixed overnight in 4% paraformaldehyde, then sunk in 30% sucrose for 3 d. Brains were sliced into 20-μm coronal sections on a cryostat. Free-floating tissue sections were rinsed three times for 15 min in PBS with 0.25% Triton X-100 (PBS-T), then transferred to a blocking solution of PBS-T with 10% normal goat serum for 1 h at room temperature. Sections were incubated in a primary antibody solution of rabbit anti-Zif268 , or rabbit anti-SynapsinI  combined with mouse anti-PSD95 . Primary antibodies were diluted in the blocking solution, incubated at 4°C for 72 h, and rinsed three times for 15 min in PBS-T. Secondary antibodies  were diluted in the blocking solution and applied to the sections for 2 h at room temperature. Sections were mounted on slides and cover-slipped using DAPI mounting media to label cell nuclei and stored at 4°C.Images for the BNST TetTag experiment were acquired on an epifluorescent TissueFAXS Whole Slide Scanning System using a 20× air objective.Z sections per image field. The maximum intensity projection image was used for subsequent analysis.All other images were acquired on a Nikon A1R confocal laser scanning microscope using a 20× air, 40× oil, or 60× oil objective. Image stacks were acquired at 2-μm step sizes for a total of 8–10 For the BNST TetTag experiment, image analysis was performed manually by identifying and counting GFP+, Zif+, or GFP+Zif+ cells, which were normalized to the number of DAPI cells [estimated by region of interest (ROI) area] for each subdivision.For the CTB TetTag experiment, image analysis was performed using ImageJ software. Quantification of GFP+ and Zif+ cells were done by thresholding the image and filtering for particle size. Masks of GFP+, Zif+, and colocalizing GFP+Zif+ cells were superimposed on the corresponding CTB image and GFP+Zif+CTB– and GFP+Zif+CTB+ cells were manually identified and counted for each section. Left and right BA were analyzed for each section, with three to six sections per mouse. Final percentages of cell types are averages per mouse. Chance level of colocalization between GFP and Zif was calculated as [/] × [/] × 100%. Base cells were either DAPI, CTB+, or CTB– cells. Comparison with chance level was used to identify engram neurons that have significant reactivation that cannot be explained by random overlap of GFP and Zif .Synaptic colocalization was analyzed using Imaris 3D imaging software. Colocalizing synaptic markers were identified by creating “spots” based on SynapsinI puncta and filtering for spots that contained above-threshold intensity pixels for PSD-95 puncta. A 3D mask of GFP+ axons was used to select for overlapping spots.t tests were used for normally distributed variables to evaluate significance with p < 0.05 as the level of statistical significance. Data that were not normally distributed were analyzed with non-parametric tests: the Mann–Whitney test for unpaired mean comparisons and the Wilcoxon matched signed-rank test for paired mean comparisons. Data in graphs are presented as mean ± SEM. Normality, type of test, group means with SEM, and p value for data comparisons are presented in Data were analyzed in GraphPad Prism 6. Datasets were tested for normality with the Shapiro–Wilk test. Paired or unpaired two-tailed c-fos-based reporter mouse (egr-1/zif268 (Zif), was used to identify fear neurons that were reactivated during retrieval of the fear memory . Combined, the GFP and Zif markers provide two temporally defined activation profiles for the same set of cells within the brain ROI, allowing us to distinguish cells that were activated during fear conditioning alone, retrieval alone, or during both. One group of mice was submitted to a contextual fear conditioning paradigm , while the other group remained in their home cage as controls . The FC group was assessed for contextual fear memory 3 d later in a retrieval session and perfused shortly after to allow for sufficient Zif expression  or GFP+Zif+ cells above chance level  within the CTB+ population of the pBA  had a significant increase in GFP+ neurons when compared with the HC group (n = 7), indicating activation of ovBNST neurons during contextual fear conditioning , or an increase in GFP+Zif+ cells above chance level  during training, but not if the foot shock was more tightly paired with placement in the context (1 min after context placement). This suggests that the level of participation of the BNST in retrieval of contextual fear is dependent on the imminence of the perceived threat . In our Our study has some limitations that must be considered when interpreting the results. Because of the absence of a context-only (no-shock) group for comparison, we cannot exclude certain factors that may have contributed to the activation of tagged cells other than fear conditioning. For example, the possibility remains that the GFP+ cells in the pBA were not activated during the fear conditioning sessions themselves, but on the mouse’s return to the home cage after the fear conditioning sessions. If this were the case, it is more likely that these pBA cells project to the anxiolytic adBNST rather than the anxiogenic ovBNST and may belong to a predefined population of cells that play a role in reward behavior . AlternaIn summary, our study confirms functionally distinct and projection-specific cellular populations in the BA. Specifically, we observed that the role of BA projection neurons in contextual fear memory differentiates across BA subdivisions (aBA vs pBA) and across downstream projection targets (dBNST vs non-dBNST). Fear engram cells were preferentially localized in the aBA and did not project to the dBNST. Furthermore, our data point to a pBA → ovBNST pathway that is activated during contextual fear conditioning, but that is not incorporated into the fear engram. Future optogenetic and pharmacogenetic studies of the BA → ovBNST pathway can directly test its necessity and sufficiency for the initial storage of contextual fear memories.


Single-crystal graphene has attracted much attention due to its excellent electrical properties in recent years, and many growth methods have been proposed, including the copper pockets method. In the copper pockets method, a piece of copper foil is folded into a pocket and put into a chemical vapor deposition (CVD) system for the growth of graphene. The dynamic balance of evaporation and deposition of copper on the inner surfaces of the copper pockets avoids high surface roughness caused by the evaporation of copper in open space, such as the outer surfaces of copper pockets. Much lower partial pressure of methane in the copper pockets and lower surface roughness reduce the nucleation density of graphene and increase the size of single-crystal graphene. It is found that the growth pressure is closely related to the size of single-crystal graphene prepared by the copper pockets method; the higher the growth pressure, the larger the size of single-crystal graphene. It is also found that the growth pressure has an effect on the inner surface roughness of the copper pockets, but the effect is not significant. The main factor affecting the size of the single-crystal graphene is the change in the volume of the copper pockets caused by the change in the growth pressure, and the volume of the copper pockets determines the content of methane in the copper pockets. According to the above law, the size of single-crystal graphene prepared by the copper pockets method can be enlarged by increasing the growth pressure. The size of single-crystal graphene can be enlarged in a wide range as the growth pressure can be increased in a wide range. In our experiments, when the growth pressure reached 450 Pa, single-crystal graphene with a diameter of 450  Graphene, the thinnest carbon material with atoms arranged in a two-dimensional hexagonal lattice, was first isolated by mechanical exfoliation from highly oriented pyrolytic graphite (HOPG) and has exceptional chemical stability, superior mechanical stability, very high thermal conductivities and electronic conductivities ,2,3. BecGrain boundaries produced during material synthesis affect both the intrinsic properties of materials and their potential for high-end applications. This effect is commonly observed in graphene film grown using chemical vapor deposition. Large-area graphene films are generally merged from small single-crystal domains and, therefore, have lattice defects, which result in challenges in the consistency of graphene materials. Based on this, the performance of graphene devices is uneven, and accordingly, it is necessary to study the growth and preparation of single-crystal graphene. Therefore, the growth of single-crystal graphene has aroused great interest in recent years. Graphene grain boundaries (GGBs), which form in regions where graphene domains with different orientations merge, are observed as line defects in graphene film and result in degradation of the thermal, electrical, and chemical properties of graphene ,17,18,19There are two ideas to improve the size of single-crystal graphene, which are the single-seed method and the multi-seed method. The key of the single-seed method is to control the nucleation density of graphene. It is found that higher surface roughness and impurity pollution will increase the nucleation density because these factors will increase the number of active sites on the surface of the substrate and then increase the nucleation probability of graphene ,22. Thery merged ,33,34. By merged , unlike  quality . After t quality ,38. It wIn this paper, we use the copper pockets method to prepare single-crystal graphene. It is found that the growth pressure has a positive correlation with the size of the single-crystal graphene, and the mechanism is discussed. In our experiments, single-crystal graphene with diameter of 450 ectrodes . Due to ectrodes .Graphene was synthesized by low-pressure CVD in a tube furnace. As shown in Figure 3b, a Cu pocket was made by first bending a copper foil 2S2O8) aqueous solution . After etching, the PMMA/graphene was rinsed in DI water several times. The PMMA/graphene was dried in a desiccator and then placed on the SiO2/Si substrate. The PMMA layer was dissolved using hot acetone.A thin film of polymethyl methacrylate (PMMA)  was spin-coated onto the graphene/Cu (600 r/min for 6 s then 3000 r/min for 40 s). The etching of Cu usually takes 3 h. Then, the PMMA/graphene/Cu thin film was put into 30 mg/mL ammonium persulfate nCH4In each experiment, we used the same growth temperature, so the temperature The volume of the copper pockets graphene . The decIn conclusion, growth pressure will affect the establishment of a dynamic balance of evaporation and deposition of copper in copper pockets when single-crystal graphene is prepared using the copper pockets method, and it will then affect the inner surface roughness of copper pockets. This effect has been shown, as the higher the growth pressure, the smaller the inner surface roughness of the copper pockets, but this effect is small. The growth pressure also affects the volume of the copper pockets. The larger the growth pressure, the smaller the volume of the copper pockets stretched by the copper vapor and the less