Ruizhe Wu

Polymer-Embedded Fabrication of Co2P Nanoparticles Encapsulated in N,P-Doped Graphene for Hydrogen Generation

We developed a method to engineer well-distributed dicobalt phosphide (Co2P) nanoparticles encapsulated in N,P-doped graphene (Co2P@NPG) as electrocatalysts for hydrogen evolution reaction (HER). We fabricated such nanostructure by the absorption of initiator and functional monomers, including acrylamide and phytic acid on graphene oxides, followed by UV-initiated polymerization, then by adsorption of cobalt ions and finally calcination to form N,P-doped graphene structures. Our experimental results show significantly enhanced performance for such engineered nanostructures due to the synergistic effect from nanoparticles encapsulation and nitrogen and phosphorus doping on graphene structures. The obtained Co2P@NPG modified cathode exhibits small overpotentials of only -45 mV at 1 mA cm(-2), respectively, with a low Tafel slope of 58 mV dec(-1) and high exchange current density of 0.21 mA cm(-2) in 0.5 M H2SO4. In addition, encapsulation by N,P-doped graphene effectively prevent nanoparticle from corrosion, exhibiting nearly unfading catalytic performance after 30 h testing. This versatile method also opens a door for unprecedented design and fabrication of novel low-cost metal phosphide electrocatalysts encapsulated by graphene.

Oxidized nitinol substrate for interference enhanced Raman scattering of monolayer graphene

We demonstrate the preparation of a controllable and reproducible active substrate for surface enhanced Raman scattering (SERS) using a facile oxidation method that allows us to obtain a titanium oxide (TiO2) capping layer with the desired thickness on nickel–titanium alloy (NiTi). The carefully tuned oxide layer, which is obtained by controlling the annealing time, exhibits the enhancement of the 2D band intensity of graphene up to ∼50 times in comparison to bare nitinol. The dependence of Raman enhancement upon the oxide thickness can be explained by the interference enhanced Raman scattering (IERS) process, and fitted to a multi reflection model (MRM) of the Raman scattering of graphene on a layered structure. Thus our results provide a facile method to enhance Raman signals of graphene by tuning the thickness of the oxide layer at all three lasers (514 nm, 633 nm and 785 nm). The present method can be adapted to exploit the recent advances in molecular vibration study and biomolecular detection due to the versatility of the proposed substrate.

Controlled removal of monolayers for bilayer graphene preparation and visualization

Bilayer graphene, unlike monolayer graphene, provides a tunable electronic energy gap when an electrical field is applied across the layers, and therefore holds great potential in the semiconductor industry. Here, we demonstrate a facile technique to obtain bilayer graphene structures on the growth substrate by controlling oxidation to remove monolayers, while retaining the bilayer electronic properties. We found that this precise oxidation process selectively destructs monolayers while preserves the qualities of bilayers, evidenced by the expected quantum Hall effect and exceptional room temperature carrier mobilities of ∼3500 cm2 V−1 s−1 obtained from an electrical transport measurement. In addition, visualization of the bilayers, which serve as nuclei for graphene growth, opens the door to understanding the actual mechanism of the graphene growth process which eventually can lead to the optimized synthesis.

Recoil Effect and Photoemission Splitting of Trions in Monolayer MoS2

The 2D geometry nature and low dielectric constant in transition-metal dichalcogenides lead to easily formed strongly bound excitons and trions. Here, we studied the photoluminescence of van der Waals heterostructures of monolayer MoS2 and graphene at room temperature and observed two photoluminescence peaks that are associated with trion emission. Further study of different heterostructure configurations confirms that these two peaks are intrinsic to MoS2 and originate from a bound state and Fermi level, respectively, of which both accept recoiled electrons from trion recombination. We demonstrate that the recoil effect allows us to electrically control the photon energy of trion emission by adjusting the gate voltage. In addition, significant thermal smearing at room temperature results in capture of recoil electrons by bound states, creating photoemission peak at low doping level whose photon energy is less sensitive to gate voltage tuning. This discovery reveals an unexpected role of bound states for photoemission, where binding of recoil electrons becomes important.

Characterization of Thermal Resistances Across CVD-Grown Graphene/Al2O3 and Graphene/Metal Interfaces Using Differential 3-Omega Technique

Chemical vapor deposited (CVD) graphene together with a superior gate dielectric such as Al2O3, is a promising combination for next-generation high-speed field effect transistors (FET). These high-speed devices are operated under high frequencies and will generate significant heat, requiring effective thermal management to ensure device stability and longevity. It is thus of importance to characterize the interfacial thermal resistance (ITR) between graphene/Al2O3 gate dielectric and graphene/metal contacts.In this work, ITRs across the single-layer graphene/Al2O3 and the graphene/metal (Al, Ti, Au) interfaces were characterized from 100 K to 330 K using the differential 3ω method. Unlike previous works which mostly used exfoliated single or few-layer graphene, we used CVD large-scale graphene, which is most promising for FET fabrication due to cost and quality control, in the experiments. To ascertain the measured results and reduce uncertainty, different sandwich configurations including metal/graphene/metal, Al2O3/graphene/Al2O3 and metal/graphene/Al2O3 were used for the measurements. The effects of post annealing on different interfaces were also investigated.Measurements of numerous samples showed an average ITR at 300K of 9×10−8 m2K/W for graphene/Al2O3, 6×10−8 m2K/W for graphene/Al, 5×10−8 m2K/W for graphene/Ti, and 7×10−8 m2K/W for graphene/Au interfaces. For the metal interfaces with graphene, the results are within the same order of magnitude as previous measurement results with graphite. However, ITR for graphene/Al2O3 is one order of magnitude higher than those reported for graphene/SiO2 interfaces. The measured ITRs for both metal and dielectric interfaces with graphene are almost temperature-independent from 100 K to 330 K, indicating that phonons are the major heat carrier. Annealing was found to have different effects on different interfaces. For graphene/Ti interfaces, ITR results measured before and after annealing consistently show a reduction of around 20%. However, such improvements on interfacial conductance were not observed for graphene/Al, graphene/Au and graphene/Al2O3 interfaces. The reduction of ITR of graphene/Ti interface is perceived to stem from the formation of Ti-C covalent bonds. However, neither the commonly used maximum transmission model nor the diffuse mismatch model explicitly considers bonding effects at the interface, which is why they poorly predict and explain all the aspects of the measurements. An improvement to the classic anisotropic DMM model was proposed by taking into account different bonding types and bonding area between graphene and Al2O3/metal layer, resulting in a better fitting with the experimental data.Copyright

Stacking Modes-Induced Chemical Reactivity Differences on Chemical Vapor Deposition-Grown Trilayer Graphene

Trilayer graphene (TLG) synthesized by chemical vapor deposition (CVD), in particular the twisted TLG, exhibits sophisticated electronic structures that depend on their stacking modes. Here, we computationally and experimentally demonstrate the chemical reactivity differences of CVD-TLG induced by the stacking modes and corroborated by a photoexcited phenyl-grafting reaction. The experimental results show that the ABA stacking TLGs have the most inert chemical property, yet 30°-30° stacking twisted TLGs are the most active. Further, density functional theory calculations have shown that the chemical reactivity difference can be quantitatively explained by the differences in the number of hot electrons generated in their valence band during irradiation. The activity difference is further verified by the calculated adsorption energy of phenyl on the TLGs. Our work provides insight into the chemistry of TLG and addresses the challenges associated with selective functionalization of TLG with phenyl groups. The understandings developed in this project can also guide the future development of TLG-based functional devices.