Kazunori Fujisawa

Ultrasensitive gas detection of large-area boron-doped graphene

Significance The gas-sensing performance of graphene could be remarkably enhanced by incorporating dopants into its lattice based on theoretical calculations. However, to date, experimental progress on boron-doped graphene (BG) is still very scarce. Here, we achieved the controlled growth of large-area, high-crystallinity BG sheets and shed light on their electronic features associated with boron dopants at the atomic scale. As a proof-of-concept, it is demonstrated that boron doping in graphene could lead to a much enhanced sensitivity when detecting toxic gases (e.g. NO2). Our results will open up new avenues for developing high-performance sensors able to detect trace amount of molecules. In addition, other new fascinating properties can be exploited based on as-synthesized large-area BG sheets. Heteroatom doping is an efficient way to modify the chemical and electronic properties of graphene. In particular, boron doping is expected to induce a p-type (boron)-conducting behavior to pristine (nondoped) graphene, which could lead to diverse applications. However, the experimental progress on atomic scale visualization and sensing properties of large-area boron-doped graphene (BG) sheets is still very scarce. This work describes the controlled growth of centimeter size, high-crystallinity BG sheets. Scanning tunneling microscopy and spectroscopy are used to visualize the atomic structure and the local density of states around boron dopants. It is confirmed that BG behaves as a p-type conductor and a unique croissant-like feature is frequently observed within the BG lattice, which is caused by the presence of boron-carbon trimers embedded within the hexagonal lattice. More interestingly, it is demonstrated for the first time that BG exhibits unique sensing capabilities when detecting toxic gases, such as NO2 and NH3, being able to detect extremely low concentrations (e.g., parts per trillion, parts per billion). This work envisions that other attractive applications could now be explored based on as-synthesized BG.

Ultrasensitive molecular sensor using N-doped graphene through enhanced Raman scattering

N-doped graphene can be used as a substrate for different molecules to effectively enhance their Raman scattering signal. As a novel and efficient surface analysis technique, graphene-enhanced Raman scattering (GERS) has attracted increasing research attention in recent years. In particular, chemically doped graphene exhibits improved GERS effects when compared with pristine graphene for certain dyes, and it can be used to efficiently detect trace amounts of molecules. However, the GERS mechanism remains an open question. We present a comprehensive study on the GERS effect of pristine graphene and nitrogen-doped graphene. By controlling nitrogen doping, the Fermi level (EF) of graphene shifts, and if this shift aligns with the lowest unoccupied molecular orbital (LUMO) of a molecule, charge transfer is enhanced, thus significantly amplifying the molecule’s vibrational Raman modes. We confirmed these findings using different organic fluorescent molecules: rhodamine B, crystal violet, and methylene blue. The Raman signals from these dye molecules can be detected even for concentrations as low as 10−11 M, thus providing outstanding molecular sensing capabilities. To explain our results, these nitrogen-doped graphene-molecule systems were modeled using dispersion-corrected density functional theory. Furthermore, we demonstrated that it is possible to determine the gaps between the highest occupied and the lowest unoccupied molecular orbitals (HOMO-LUMO) of different molecules when different laser excitations are used. Our simulated Raman spectra of the molecules also suggest that the measured Raman shifts come from the dyes that have an extra electron. This work demonstrates that nitrogen-doped graphene has enormous potential as a substrate when detecting low concentrations of molecules and could also allow for an effective identification of their HOMO-LUMO gaps.

Low‐Temperature Solution Synthesis of Few‐Layer 1T ′‐MoTe2 Nanostructures Exhibiting Lattice Compression

Molybdenum ditelluride, MoTe2 , is emerging as an important transition-metal dichalcogenide (TMD) material because of its favorable properties relative to other TMDs. The 1Tu2009 polymorph of MoTe2 is particularly interesting because it is semimetallic with bands that overlap near the Fermi level, but semiconducting 2H-MoTe2 is more stable and therefore more accessible synthetically. Metastable 1Tu2009-MoTe2 forms directly in solution at 300u2009°C as uniform colloidal nanostructures that consist of few-layer nanosheets, which appear to exhibit an approx. 1u2009% lateral lattice compression relative to the bulk analogue. Density functional theory calculations suggest that small grain sizes and polycrystallinity stabilize the 1Tu2009 phase in the MoTe2 nanostructures and suppress its transformation back to the more stable 2H polymorph through grain boundary pinning. Raman spectra of the 1Tu2009-MoTe2 nanostructures exhibit a laser energy dependence, which could be caused by electronic transitions.

Noble-Metal-Free Hybrid Membranes for Highly Efficient Hydrogen Evolution.

Free-standing and flexible WS2 /WO2.9 /C hybrid membranes are synthesized and used as catalytic electrodes for electrochemical hydrogen evolution, exhibiting a high and stable catalytic activity. By virtue of the synergetic effect, a low onset overpotential of 20 mV and a Tafel slope of 36 mV dec-1 are achieved.

Optical identification of sulfur vacancies: Bound excitons at the edges of monolayer tungsten disulfide

Bound exciton is a signature of sulfur vacancies, and thus, it can be used to investigate defects in atomically thin materials. Defects play a significant role in tailoring the optical properties of two-dimensional materials. Optical signatures of defect-bound excitons are important tools to probe defective regions and thus interrogate the optical quality of as-grown semiconducting monolayer materials. We have performed a systematic study of defect-bound excitons using photoluminescence (PL) spectroscopy combined with atomically resolved scanning electron microscopy and first-principles calculations. Spatially resolved PL spectroscopy at low temperatures revealed bound excitons that were present only on the edges of monolayer tungsten disulfide and not in the interior. Optical pumping of the bound excitons was sublinear, confirming their bound nature. Atomic-resolution images reveal that the areal density of monosulfur vacancies is much larger near the edges (0.92 ± 0.45 nm−2) than in the interior (0.33 ± 0.11 nm−2). Temperature-dependent PL measurements found a thermal activation energy of ~36 meV; surprisingly, this is much smaller than the bound-exciton binding energy of ~300 meV. We show that this apparent inconsistency is related to a thermal dissociation of the bound exciton that liberates the neutral excitons from negatively charged point defects. First-principles calculations confirm that sulfur monovacancies introduce midgap states that host optical transitions with finite matrix elements, with emission energies ranging from 200 to 400 meV below the neutral-exciton emission line. These results demonstrate that bound-exciton emission induced by monosulfur vacancies is concentrated near the edges of as-grown monolayer tungsten disulfide.

Light‐Emitting Transition Metal Dichalcogenide Monolayers under Cellular Digestion

2D materials cover a wide spectrum of electronic properties. Their applications are extended from electronic, optical, and chemical to biological. In terms of biomedical uses of 2D materials, the interactions between living cells and 2D materials are of paramount importance. However, biointerfacial studies are still in their infancy. This work studies how living organisms interact with transition metal dichalcogenide monolayers. For the first time, cellular digestion of tungsten disulfide (WS2 ) monolayers is observed. After digestion, cells intake WS2 and become fluorescent. In addition, these light-emitting cells are not only viable, but also able to pass fluorescent signals to their progeny cells after cell division. By combining synthesis of 2D materials and a cell culturing technique, a procedure for monitoring the interactions between WS2 monolayers and cells is developed. These observations open up new avenues for developing novel cellular labeling and imaging approaches, thus triggering further studies on interactions between 2D materials and living organisms.

Photoluminescence Enhancement of Titanate Nanotubes by Insertion of Rare Earth Ions in Their Interlayer Spaces

The optical properties of titanate nanotubes (TiNts) intercalated with rare earths (RE) ions were intensively investigated in this study. To prepare the nanomaterials, sodium titanate nanotubes (Na-TiNts) were submitted to ion exchange reactions with different rare earth elements (RE: Pr3+, Er3+, Nd3+, and Yb3+). To the best of our knowledge, it is the first time that these RE-TiNts were synthesized. All samples were characterized by Raman spectroscopy, X-ray powder diffraction (XRD), transmission electron microscopy (TEM), and energy dispersive X-ray spectroscopy (EDS). Furthermore, the optical properties were examined using photoluminescence spectroscopy (PL) and UV-Vis-NIR absorption spectroscopy. The PL intensity (visible range) of the RE-TiNt samples showed a strong dependence when the temperature was decreased down to 20źK. This PL enhancement probably was promoted by electronic modifications in titanate layer band gap and/or interface charge transfers due to RE ions intercalation.

Solution synthesis of few-layer WTe2 and MoxW1−xTe2 nanostructures

Transition metal ditellurides are gaining widespread attention due to their rich polymorphism and distinct electronic properties when compared to transition metal disulfides and diselenides. Here, we report the direct synthesis of Td-WTe2 nanostructures via a low-temperature solution-based method. A comprehensive structural characterization reveals the anisotropic pathway by which the few-layer WTe2 nanostructures grow, as well as the co-existence of multiple stacking motifs. The versatility of this synthetic approach is further expanded to access nanostructured MoxW1−xTe2 alloys across the entire solid solution that spans the 1T′-MoTe2 and Td-WTe2 end members, where the elemental composition and crystal structure can be systematically tuned by modulating the metal reagent ratios. The ability to access transition metal ditellurides using solution synthesis methods complements traditional gas-phase fabrication techniques and provides an alternative strategy for manufacturing an expanded library of composition-tunable TMD nanostructures.

Characterization of second-order nonlinear optical properties of two-dimensional transition metal dichalcogenides (Conference Presentation)

Two-dimensional transition metal dichalcogenides (TMD), such as WS2 and MoS2, have been shown to exhibit large second order optical nonlinearity due to their non-centrosymmetric crystalline symmetry in few odd- and mono-layers, and resonance enhancement. Here we study the second-order nonlinear susceptibility of 2D TMDs through second harmonic generation (SHG) and sum frequency generation (SFG). Using a wavelength-tunable femtosecond laser, we can characterize SHG of TMDs to obtain the second-order nonlinear susceptibility at multiple wavelengths. Along with the experimental studies, theoretical investigation of the second-order nonlinear susceptibility is also performed. With this we explore the estimation of the second-order nonlinear susceptibility of 2D TMD layered materials based on their first-order susceptibility through the experimental and theoretical verification of Miller’s Rule for these materials. Additionally, we characterize the second-order nonlinear susceptibility of 2D TMD alloys through the SFG spectroscopy.

Nonlinear characterization of two dimensional materials (Conference Presentation)

Two-dimensional materials have attracted significant interest recently for their unique optical properties compared to their bulk counterparts. Specifically, the family of transition metal dichalcogenides (TMD), such as MoS2 and WS2, have large second order nonlinear susceptibility. Extraordinary second harmonic generation and sum frequency generation have been observed. Here we investigate the second order nonlinearity of 2D materials, including TMD layered materials with dopants and defects. Experimental results and preliminary theoretical analysis will be discussed.