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An array of embedded graphene ribbons, whose edges connect to a buffer layer, can be grown on 4HSiC(0001). The Raman D peak intensity of the armchair edge of the ribbon shows the same polarization dependence as that of the non-connected armchair edge of graphene. Considering the Raman scattering process of the D peak, this polarization dependence indicates that electrons and holes in the embedded graphene by incident photons are scattered back at the boundary of the embedded graphene ribbon and buffer layer. These results indicate polarized Raman scattering spectroscopy is useful for investigating the edge structure of embedded graphene.

Vertically stacked two-dimensional van der Waals (vdW) heterostructures with specific interlayer angles exhibit peculiar physical properties. Nowadays, most of the stacked layers are fabricated by mechanical exfoliation followed by precise transfer and alignment with micrometer spatial accuracy. This stringent ingredient of sample preparation limits the productivity of device fabrication and the reproducibility of device performance. Here, we demonstrate the one-pot chemical vapor deposition growth of hexagonal boron nitride (hBN)/graphene bilayers with a high-purity moiré phase. The epitaxial intercalation of graphene under a hydrogen-terminated hBN template leads to convergent interlayer angles of less than 0.5°. The near 0° stacking angle shows almost 2 orders of magnitude higher likelihood of occurrence compared with angles larger than 0.5°. The bilayers show a substantial enhancement of carrier mobility compared with monolayer graphene owing to protection from the top hBN layer. Our work proposes a large-scale fabrication method of hBN/graphene bilayers with a high uniformity and controlled interlayer rotation and will promote the production development for high-quality vdW heterostructures.

Hexagonal boron nitride (hBN) is an ideal insulating substrate and template for other two-dimensional (2D) materials. The combination of hBN and 2D materials of group IV atoms, such as graphene, is interesting, because it can offer attractive physical properties and promising applications. Here, we demonstrate the unique behavior of tin (Sn), one of the group IV elements, on multilayer hBN which was grown by chemical vapor deposition (CVD). At high temperatures, triangular nanoplates formed after thermal deposition of Sn on the hBN surface, with their orientations determined by the hBN lattice. The triangular Sn nanoplates moved on the hBN surface, leaving monolayer-deep nanotrenches. Low-energy electron microscopy (LEEM) revealed that the nanotrenches are aligned in the armchair directions of the hBN. Furthermore, an additional Ar annealing without supplying Sn vapor induced the structural change of the linear trenches to triangular pits, indicating the preferential formation of zigzag edges in the absence of Sn. Our work highlights the unique behavior of Sn on hBN and offers a novel route to engineer the hBN surface.

Controlling the stacking order in bilayer graphene (BLG) allows realizing interesting physical properties. In particular, the possibility of tuning the band gap in Bernal-stacked (AB) BLG (AB-BLG) has a great technological importance for electronic and optoelectronic applications. Most of the current methods to produce AB-BLG suffer from inhomogeneous layer thickness and/or coexistence with twisted BLG. Here, we demonstrate a method to synthesize highly pure large-area AB-BLG by chemical vapor deposition using Cu-Ni films. Increasing the reaction time resulted in a gradual increase of the AB stacking, with the BLG eventually free from twist regions for the longer growth times (99.4% of BLG has AB stacking), due to catalyst-assisted continuous BLG reconstruction driven by carbon dissolution-segregation processes. The band gap opening was confirmed by the electrical measurements on field-effect transistors using two different device configurations. The concept of the continuous reconstruction to achieve highly pure AB-BLG offers a way to control the stacking order of catalytically grown two-dimensional materials.

Hexagonal boron nitride (h-BN), an insulating two-dimensional (2D) layered material, has attracted increasing interest due to its electrical screening effect, high-temperature-resistant gas barrier properties, and other unique applications. However, the presence of grain boundaries (GBs) in h-BN is a hindrance to obtain these properties. Here, we demonstrate the epitaxial growth of monolayer h-BN by chemical vapor deposition (CVD) on Ni(111) thin films deposited on c-plane sapphire. The Ni(111) films showed higher thermal stability than Cu(111) and Cu-Ni(111) alloy films, allowing us to perform CVD growth at a high temperature of 1100 °C. This resulted in an increase of the h-BN grain sizes to up to 0.5 millimeter, among the highest reported so far, and in a well-defined triangular grain shape. Low-energy electron microscopy (LEEM) revealed the epitaxial relationship between h-BN and the underlying Ni(111) lattice, leading to a preferential alignment of the h-BN grains. Both the large grain size and the alignment are expected to facilitate the synthesis of h-BN with a low density of GBs. We also found that the addition of N2 gas during the CVD improves the crystalline shape of the h-BN grains, changing from an irregular, truncated to a sharp triangle. The growth behavior of monolayer h-BN is further discussed in terms of the dependences on growth temperature and pressure, as well as on the structural evolution of the Ni metal catalyst. Our findings not only help understand the h-BN growth mechanism but also offer a new route to grow high-quality, monolayer h-BN films.

Electron-hole symmetry breaking of Dirac electrons has been evidenced as a well-resolved cyclotron resonance (CR) peak splitting at magnetic fields around 300-500 T in epitaxial monolayer graphene. The magnetic field was swept up to 560 T for the infrared CR by means of the electromagnetic flux compression technique in a 1000-T class megagauss generator. The authors argue that it is the CR mode repulsion, caused by the electron-electron correlation, which is responsible for the observed CR intensity transfer from the hole to the electron.

Aligned growth of transition metal dichalcogenides and related two-dimensional (2D) materials is essential for the synthesis of high-quality 2D films due to effective stitching of merging grains. H...

Quasi free standing monolayer graphene (QFMLG), grown on SiC by Si evaporation from the Si-rich SiC(0001) face and H intercalation, displays irregularities in STM and AFM images appearing as localized features, associated with vacancies in the H layer coverage. Size, shape, concentration, and distribution of these features depend on hydrogenation conditions. In order to understand this phenomenology and possibly control it, we perform a Density Functional Theory study of QFMLG with defects in the H coverage of different size and shape and arranged in different configurations. We show that H vacancies generate localized states with highly specific electronic structure. Based on the comparison of simulated and measured STM images we are able to associate different vacancies of large size (7-13 missing H) to the observed features, unraveling the structural diversity of defects of H coverage in QFMLG. Our study indicates a tendency of single H vacancies to aggregate and to locate on a regular superlattice, providing insight into the kinetics of the hydrogenation process. The energy of the localized states associated with these vacancies depends on their size and shape, showing dependence of the electronic properties on the environmental conditions during the sample production.

The vibrational properties of different Ge phases formed on Ag(111) thin films by the Ge segregation method were investigated using in situ Raman spectroscopy. As the segregation temperature is increased, the striped phase, the less-ordered phase, and the long-range ordered phase appear in this order. While the less-ordered and long-range ordered phases showed Raman peaks assignable to germanene, featureless Raman spectra of the striped phase indicate that the striped phase is Ag-Ge surface alloy. The germanene Raman peaks shift to higher wavenumbers with the segregation temperature, which could be due to the reduction of the tensile strain in germanene.

We demonstrate that dark-field (DF) optical microscopy is a powerful tool to visualize grain boundaries (GBs) and grain structure of graphene grown by chemical vapor deposition (CVD). Copper oxide nanoparticles sparsely formed along the graphene GBs by postgrowth mild oxidation allow one to determine the position and structure of the GBs by the DF microscope. As DF imaging offers a much higher sensitivity than bright-field (BF) microscopy, some GBs were observed even without the postgrowth oxidation. We found that periodic Cu steps formed below graphene can be also used to visualize the grain structure of the as-grown graphene by DF microscopy. Moreover, DF imaging is applicable to study of the gas barrier property of CVD graphene. Interestingly, the dissolved oxygen inside Cu foil enhanced oxidation of the Cu surface below graphene in spite of the fact that the graphene protects the underlying Cu from the exterior gas. Our work highlights the wide availability of DF optical microscopy in characterizing g...

We analyzed the surfaces of hydrogen-intercalated graphene on SiC(0001) by photoelectron spectroscopy and Raman spectroscopy after applying electric currents to the graphene for carrier mobility measurements while the sample was exposed to air. We found that Si at the interface between graphene and SiC oxidizes very gradually, whereas the graphene does not. The Si of this Si oxide is mainly divalent, and the oxide is attributed to SiO molecules. These molecules are volatile, and they have not been observed either on the oxidized surface of SiC(0001) or on oxygen-intercalated graphene/SiC(0001) formed by annealing in air or oxygen. This anomalous oxidation, which is triggered by the application of current to the graphene overlayer, could be observed because of the presence of the graphene overlayer and the oxidation at room temperature.

Decreasing the nucleation density of graphene grown on copper (Cu) foil by chemical vapor deposition (CVD) is essential for the synthesis of large-area single-crystalline graphene. Here, the behavior of the copper oxide layer and its impact on the graphene growth have been investigated. We found that a small amount of oxygen dissolves into the Cu when the oxide layer decomposes during the heating up in a non-reducing Ar environment. The remaining oxygen in the Cu foil can play an important role in decreasing the graphene nucleation density. The dissolved oxygen can withstand at high temperatures even in reducing H 2 environments without completely losing its effectiveness for maintaining a low graphene nucleation density. However, heating up in a H 2 environment significantly reduces the copper oxide layer during the very first moments of the process at low temperatures, preventing the oxygen to dissolve into the Cu and significantly increasing the nucleation density. These findings will help to improve the graphene growth on Cu catalyst by increasing the grain size while decreasing the grain density.

Dynamical energy and momentum mapping for intervalley electron transfer in WSe2 was investigated with high-harmonic-based ultrafast ARPES. The measured ultrashort scattering time of 30 fs will provide a guide to lightwave control of valley pseudospin.

Controlling the stacking order in bilayer graphene (BLG) allows realising unique physical properties. In particular, the possibility of tuning the band gap in AB-stacked BLG (AB-BLG) has a great technological importance for electronic and optoelectronics applications. Most of current methods to produce AB-BLG suffer from inhomogeneous layer thickness and/or coexistence with twisted BLG. Here, we demonstrate a method to synthesise highly pure large-area AB-BLG by chemical vapour deposition (CVD) using Cu-Ni films. Increasing the reaction time resulted in a gradual increase of the AB stacking, with the BLG eventually free from twist regions for the longer times (99.4 % of BLG has AB stacking), due to catalyst-assisted continuous BLG reconstruction driven by carbon dissolution-segregation processes. The band gap opening was confirmed by the electrical measurements. The concept of the continuous reconstruction to achieve highly pure AB-BLG offers a new strategy to control the stacking order of catalytically grown two-dimensional materials.

In spite of recent progress of graphene growth using chemical vapor deposition, it is still a challenge to precisely control the nucleation site of graphene for the development of wafer-scale single-crystal graphene. In addition, the postgrowth patterning used for device fabrication deteriorates the quality of graphene. Herein we demonstrate the site-selective nucleation of single-crystal graphene on Cu foil based on spatial control of the local CH4 concentration by a perforated Ni foil. The catalytically active Ni foil acts as a CH4 modulator, resulting in millimeter-scale single-crystal grains at desired positions. The perforated Ni foil also allows to synthesize patterned graphene without any postgrowth processing. Furthermore, the uniformity of monolayer graphene is significantly improved when a plain Ni foil is placed below the Cu. Our findings offer a facile and effective way to control the nucleation of high-quality graphene, meeting the requirements of industrial processing.

We show millimeter-scale graphene single crystals synthesized on commercial Cu foils by the atmospheric pressure chemical vapor deposition (CVD) method, which does not involve the routine use of a specially designed CVD reactor or long-term processes. Upon the designed annealing step in the Ar environment, the natural oxide layer covering on Cu catalysts is to a large extent maintained and is further used to protect the surface passivation and restrict the graphene nucleation. Moreover, for Cu foils placing on certain solid supports, we found that the graphene deposition is highly related to the environments proximate to each surface (referred to as open or confined space, respectively). For instance, the domain size of as-grown graphene is larger (smaller), while the nucleation density is higher (lower) on the back (top) surface. The possible mechanism to interpret the discrepancy on either side is discussed in the frame of the graphene nucleation and growth kinetics. At the nucleation stage, the thermal...

Graphene and related two-dimensional (2D) materials are attracting huge attention due to their wide-range potential applications. Because large-scale, high-quality 2D crystals are prerequisites for many of the applications, crystal growth of 2D materials has been intensively studied. We have also been conducting research to understand the growth mechanism of 2D materials and have been developing growth techniques of high-quality materials based on the understandings, in which detailed structural characterizations using low-energy electron microscopy (LEEM) have played essential roles. In this paper, we explain the principles of obtaining various structural features using LEEM, and then we review the status of our current understanding on the growth of graphene and hexagonal boron nitride.

Band gap opening in bilayer graphene (BLG) under a vertical electric field is important for the realization of high performance graphene-based semiconductor devices, and thus, the synthesis of uniform and large-area BLG is required. Here we demonstrate the synthesis of a highly uniform BLG film by chemical vapor deposition (CVD) over epitaxial Cu−Ni (111) binary alloy catalysts. The relative concentration of Ni and Cu as well as the growth temperature and cooling profile was found to strongly influence the uniformity of the BLG. In particular, a slow cooling process after switching off the carbon feedstock is important for obtaining a uniform second layer, covering more than 90% of the total area. Moreover, low-energy electron microscopy (LEEM) study revealed the second layer grows underneath the first layer. We also investigated the stacking order by Raman spectroscopy and LEEM and found that 70–80% of bilayer graphene has Bernal stacking. The metastable 30°-rotated orientations were also observed both i...

We report on a new protein sensor built on pristine graphene. Graphene grown by chemical vapor deposition and supported on a SiO2 surface was modified with a sequence consisting of pyrene, DNA aptamer, and dye. Here, an aptamer is a nucleic acid having a specific base sequence that recognizes and forms a complex with a target molecule such as a protein. In our protein sensing system, the recognition of a target protein triggers the fluorescence of a dye tethered on the graphene surface. This is based on the effective fluorescence quenching property of graphene via fluorescence resonance energy transfer (FRET). We first demonstrate that the graphene FRET aptasensor yields fluorescence when there is a target protein in the sample using prostate specific antigen (PSA, a cancer marker) as the target. We confirm that the fluorescence intensities vary depending on the PSA concentrations. We also discuss the selectivity and limit of detection using a microchannel configuration built on a graphene FRET aptasensor...

In-situ X-ray diffraction was used to investigate initial stages of GaN growth on graphene-covered SiO2 substrates with or without AlN buffer layers by plasma-assisted molecular beam epitaxy. The in-plane lattice parameter of GaN remained almost constant during growth without AlN buffer layers, confirming weak interactions between GaN and graphene. On the substrate with AlN buffer layers, the GaN lattice parameter continuously changed from AlN value to GaN value, indicating that the AlN islands act as seeds for GaN growth. Besides, GaN has preferential orientations epitaxial to graphene. Graphene can be used as a template of GaN growth on amorphous substrates.

Aligned growth of transition metal dichalcogenides and related two-dimensional (2D) materials is essential for the synthesis of high-quality 2D films due to effective stitching of merging grains. Here, we demonstrate the controlled growth of highly aligned molybdenum disulfide (MoS

We propose a molecular design for a biomolecular probe to realize an on-chip graphene oxide (GO) aptasensor with enhanced sensitivity. Here, GO works as an excellent acceptor for fluorescence resonance energy transfer. We inserted a rigid double-stranded DNA as a spacer between the GO surface and the aptamer sequence to extend the distance between a fluorescence dye and the GO surface during molecular recognition. We examined the dependence of the sensitivity on the length of the spacer quantitatively by using a 2×2 linear-array aptasensor. We used the modified aptamer with 10 and 30 base pair (bp) double-stranded DNA spacers. The signal with a 30bp-spacer was about twice as strong that with a 10bp-spacer as regards both thrombin and prostate specific antigen detections. The improvement in the sensitivity was supported by a model calculation that estimated the effect of spacer length on fluorescence recovery efficiency.

The growth of single-layer graphene on Cu metal by chemical vapor deposition (CVD) is a versatile method for synthesizing high-quality, large-area graphene. It is known that high CVD temperatures, close to the Cu melting temperature (1083 °C), are effective for the growth of large graphene domains, but the growth dynamics of graphene over the high-temperature Cu surface is not clearly understood. We investigated the surface dynamics of the single-layer graphene growth by using heteroepitaxial Cu(111) and Cu(100) films. At relatively lower temperatures, 900–1030 °C, the as-grown graphene showed the identical orientation with the underlying Cu(111) lattice. However, when the graphene was grown above 1040 °C, a new stable configuration of graphene with 3.4° rotation became dominant. This slight rotation is interpreted by the enhanced graphene–Cu interaction due to the formation of long-range ordered structure. Further increase of the CVD temperature resulted in graphene which is rotated with wide angle distr...

The versatility of an on-chip graphene oxide (GO) aptasensor was successfully confirmed by the detection of three different proteins, namely, thrombin (TB), prostate specific antigen (PSA), and hemagglutinin (HA), simply by changing the aptamers but with the sensor composition remaining the same. The results indicate that both DNA and RNA aptamers immobilized on the GO surface are sufficiently active to realize an on-chip aptasensor. Molecular selectivity and concentration dependence were investigated in relation to TB and PSA detection by using a dual, triple, and quintuple microchannel configuration. The multiple target detection of TB and PSA on a single chip was also demonstrated by using a 2 × 3 linear-array GO aptasensor. This work enables us to apply this sensor to the development of a multicomponent analysis system for a wide variety of targets by choosing appropriate aptamers.

We studied the surface structures of chemical vapor deposited (CVD) graphene on Cu(103). The graphene covered Cu surface had (103) facets parallel to the Cu[010] direction, on which triangular patterns were formed. In contrast, the bare Cu surface showed no facets. Post-growth thermal annealing in an ultra-high vacuum induced surface changes on the Cu(103) facets. The reorganization of the Cu surface by the post-growth thermal annealing led to a change in the lattice strain and hole doping level of the CVD-grown graphene.

Monolayer graphene epitaxially grown on SiC(0001) was etched by H-plasma and studied by scanning tunneling microscopy and spectroscopy. The etching created partly hexagonal nanopits of monatomic depth as well as elevated regions with a height of about 0.12 nm which are stable at $T$ = 78 K. The symmetric tunnel spectrum about the Femi energy and the absence of a $6\times6$ corrugation on the elevated regions suggest that in these regions the carbon buffer layer is decoupled from the SiC substrate and quasi-free-standing bilayer graphene appears at originally monolayer graphene on the buffer layer. This is a result of passivation of the SiC substrate by intercalated hydrogen as in previous reports for graphene on SiC(0001) heat treated in atomic hydrogen., Comment: 6 pages, 5 figures, ACSIN-14 and ICSPM-26 proceedings

We demonstrate time-resolved ARPES based on the 27th harmonic pulse with sub-10-fs duration. The measurement for the initial stage of carrier photoexcitation near the Dirac point of graphite confirms duration of 8 fs.

We investigated the ultrafast terahertz (THz) nonlinearity in a monolayer graphene under the strong magnetic field using THz pump-THz probe spectroscopy. An ultrafast suppression of the Faraday rotation associated with inter-Landau level (LL) transitions is observed, reflecting the Dirac electron character of nonequidistant LLs with large transition dipole moments. A drastic modulation of electron distribution in LLs is induced by far off-resonant THz pulse excitation in the transparent region. Numerical simulation based on the density matrix formalism without rotating-wave approximation reproduces the experimental results. Our results indicate that the strong light-matter coupling regime is realized in graphene, with the Rabi frequency exceeding the carrier wave frequency and even the relevant energy scale of the inter-LL transition.

A thorough understanding of the stability of graphene under ambient environmental conditions is essential for future graphene-based applications. In this paper, we study the effects of ambient temperature on the properties of monolayer graphene using terahertz time-domain spectroscopy as well as time-resolved terahertz spectroscopy enabled by an optical-pump/terahertz-probe technique. The observations show that graphene is extremely sensitive to the ambient environmental conditions and behaves differently depending on the sample preparation technique and the initial Fermi level. The analysis of the spectroscopic data is supported by van der Pauw and Hall effect measurements of the carrier mobility and carrier density at temperatures comparable to those tested in our THz spectroscopic experiments.

We demonstrate a new bottom-up approach to synthesize graphene nanoribbons (GNRs) on a Cu(100) film by chemical vapor deposition (CVD) without the use of any lithography and etching processes. Ambient pressure CVD with a low concentration CH4 feedstock produced a number of GNRs with widths of 40–50 nm on a heteroepitaxial Cu(100)/MgO(100) substrate. These nanoribbons are confined inside the nanoscale trenches formed on the Cu surface, and their orientations are highly controlled by the crystallographic orientation of the Cu(100) lattice. Raman spectra taken after the transfer indicated the growth of high-quality, single-layer GNRs. Moreover, low-energy electron microscopy revealed that all these aligned GNRs have the hexagonal orientations whose edges are terminated with zigzag edges. The GNR growth was not observed on Cu foil, and we discuss the growth mechanism of the oriented GNRs over epitaxial Cu(100) film. Our bottom-up approach offers a new method to grow single-layer GNRs which are oriented in a s...

The energetics for the Si desorption and the C adsorption at a [11-20] step on SiC(0001) surface are studied using the first-principles calculation. It is found that the [11-20] step is stable and nonreactive. The stability of the step is thought to govern the surface morphology during the graphene formation. It is shown that the Si pressure and the temperature are the control parameters for the surface morphology and the graphene quality.

Graphene is widely recognized as an outstanding and multi-functional material in various application fields such as electronics, photonics, mechanics, and life sciences. We propose a neurotransmitter sensor with ultra-small volume for detecting the photonic light-matter response. Such detection can be achieved using surface-activated monolayer graphene sheets and CMOS-compatible silicon-photonic circuits. Patterned pieces of CVD-grown graphene are integrated on the top of a silicon micro-ring resonator, which induce the adsorption of catecholamine molecules originated from the π-stacking effect. We used dopamine to demonstrate such detection and examine the sensitivity of graphene-dopamine coupling. To avoid high optical insertion loss and degradation of resonance characteristics caused by a graphene’s extremely high optical absorption coefficient in the near infrared region, a ring resonator with adjusted coupling design is used to compensate for the drawbacks. Owing to the advanced nano-sensing platform and measurement system, an activated graphene-sensing surface of only ∼30 µm2/ch enables π coupling to dopamine with enough sensitivity to detect less than 10-µM solution concentration. The detection mechanism through the surface reaction is also verified by optical simulation and atomic force microscopy measurement, revealing that the flowing dopamine molecules can only occupy the outermost surface of graphene. We expect this sensor to contribute to the development of an innovative label-free and disposable bio-sensing platform with accurate, sensitive, and fast response.