Ruiqi Zhao

Universal Segregation Growth Approach to Wafer-Size Graphene from Non-Noble Metals

Graphene has been attracting wide interests owing to its excellent electronic, thermal, and mechanical performances. Despite the availability of several production techniques, it is still a great challenge to achieve wafer-size graphene with acceptable uniformity and low cost, which would determine the future of graphene electronics. Here we report a universal segregation growth technique for batch production of high-quality wafer-scale graphene from non-noble metal films. Without any extraneous carbon sources, 4 in. graphene wafers have been obtained from Ni, Co, Cu-Ni alloy, and so forth via thermal annealing with over 82% being 1-3 layers and excellent reproducibility. We demonstrate the first example of monolayer and bilayer graphene wafers using Cu-Ni alloy by combining the distinct segregation behaviors of Cu and Ni. Together with the easy detachment from growth substrates, we believe this facile segregation technique will offer a great driving force for graphene research.

Inverse relationship between carrier mobility and bandgap in graphene

A frequently stated advantage of gapless graphene is its high carrier mobility. However, when a nonzero bandgap is opened, the mobility drops dramatically. The hardness to achieve high mobility and large on∕off ratio simultaneously limits the development of graphene electronics. To explore the underlying mechanism, we investigated the intrinsic mobility of armchair graphene nanoribbons (AGNRs) under phonon scattering by combining first-principles calculations and a tight-binding analysis. A linear dependence of the effective mass on bandgap was demonstrated to be responsible for the inverse mobility-gap relationship. The deformation-potential constant was found to be determined by the strain dependence of the Fermi energy and the bandgap, resulting in two mobility branches, and is essential for the high mobility of AGNRs. In addition, we showed that the transport polarity of AGNRs can be switched by applying a uniaxial strain.

Two‐Dimensional Layered Heterostructures Synthesized from Core–Shell Nanowires

Controlled stacking of different two-dimensional (2D) atomic layers will greatly expand the family of 2D materials and broaden their applications. A novel approach for synthesizing MoS2 /WS2 heterostructures by chemical vapor deposition has been developed. The successful synthesis of pristine MoS2 /WS2 heterostructures is attributed to using core-shell WO3-x /MoO3-x nanowires as a precursor, which naturally ensures the sequential growth of MoS2 and WS2 . The obtained heterostructures exhibited high crystallinity, strong interlayer interaction, and high mobility, suggesting their promising applications in nanoelectronics. The stacking orientations of the two layers were also explored from both experimental and theoretical aspects. It is elucidated that the rational design of precursors can accurately control the growth of high-quality 2D heterostructures. Moreover, this simple approach opens up a new way for creating various novel 2D heterostructures by using a large variety of heteronanomaterials as precursors.

Widely tunable carrier mobility of boron nitride-embedded graphene.

The carrier transport in boron nitride-embedded graphene (BNG) is studied using density functional theory coupled with the Boltzmann transport equation. Under a phonon scattering mechanism, the intrinsic carrier mobility of BNG at room temperature is tunable from 1.7 × 10(3) to 1.1 × 10(5) cm(2) V(-1) s(-1) when the bandgap is between 0.38 and 1.39 eV. Some specific BNG materials even show ultrahigh mobility up to 6.6 × 10(6) cm(2) V(-1) s(-1) , and the transport polarity (whether it is electron or hole transport) can be tailored by the application of a uniaxial strain. The wide mobility variation of BNG is attributed to the dependence of the effective mass and the deformation potential constant on the carbon concentration and width. The results indicate that BNG can have both a large on-off ratio and high carrier mobility and is thus a promising material for electronic devices.

Bandgap opening in Janus-type mosaic graphene

We demonstrate a novel Janus-type mosaic graphene (J-MOG) for achieving a ubiquitous bandgap opening by asymmetrical modification with covalently bonded H, F, Cl, and Br on opposing sides of graphene sheet. The theoretical capacity of J-MOG is shown to break the pattern restrictions, giving a robust non-zero gap. Our approach provides an effective pathway for the bandgap engineering of graphene for various electronic applications.

The transition metal surface passivated edges of hexagonal boron nitride (h-BN) and the mechanism of h-BN's chemical vapor deposition (CVD) growth.

Edge structure and stability are crucial in determining both the morphology and the growth behaviours of hexagonal boron nitride (h-BN) domains in chemical vapour deposition (CVD) growth under near thermal equilibrium conditions. In this study, various edges of h-BN on three typical transition metal surfaces used for h-BNs CVD growth, Cu(111), Ni(111) and Rh(111), are explored with density functional theory calculations. Different from that in vacuum, our study shows that the formation of non-hexagonal rings, such as pentagon, heptagon or their pairs, is energetically not preferred and both zigzag (ZZ) edges are more stable than the armchair (AC) edge on all the explored catalyst surfaces under typical conditions of h-BNs CVD growth, which explains the broad experimental observation of triangular h-BN domains. More importantly, our results indicate that, instead of the pristine ZZ edge terminated with nitrogen atoms (ZZN), the triangular BN domains observed in experiments are likely to be enclosed with ZZ Klein edges having dangling atoms, ZZB + N or ZZN + B. By applying the theory of Wulff construction, we predicted that the equilibrium shape of a BN domain could be a hexagon enclosed with nitrogen-rich AC edges, triangles enclosed with two different types of ZZ Klein edges or a hexagon enclosed with boron-rich AC edges if the growth is in a N-rich, neutral or B-rich environment, respectively. This study presents how the edges and equilibrium shapes of h-BN domains can be controlled during the CVD synthesis and provides guidelines for further exploring the growth behaviours and improving the quality of CVD-prepared h-BN films.

Sequential assembly of metal-free phthalocyanine on few-layer epitaxial graphene mediated by thickness-dependent surface potential

AbstractDue to strong interactions between epitaxial graphene and SiC(0001) substrates, the overlayer charge density induced by the interface charging effect is much more attenuated than that of exfoliated graphene on SiO2. We report herein a quantitive detection of the charge properties of few-layer graphene by surface potential measurements using electrostatic force microscopy (EFM). A minor difference in surface potential is observed to mediate a sequential assembly of metal-free phthalocyanine (H2Pc) on monolayer, bilayer and trilayer graphenes, as demonstrated by scanning tunneling microscopy (STM). In order to understand this, we further executed density functional theory (DFT) calculations which showed higher adsorption energies for Pc on thinner graphenes. In this case, we attribute the unique growth behavior of Pc to its variable adsorption energies on few-layer graphene, and in turn the layer charge variations from the viewpoint of energy minimizations. This work is expected to provide fundamental data useful for related nanodevice fabrications.

Investigation on thermo-mechanical behaviors of artificial muscle films

IntroductionA recently presented model by de Gennes et al. [1]described the underlying principle of electro-thermo-dynamics in ionic polymers based on internal transportphenomena and electrophoresis. Ionic polymer metalcomposites (IPMCs) show a great potential as soft roboticactuators, artificial muscles, and dynamic sensors in themicro-to-macro scale [2]. Its performance is expected togreatly exceed the conventional actuators such as shapememory alloys and piezoelectrics. However, practical gelactuators have not been developed yet primarily becausemost of hydrogels matrix stiffness is quite weak [3] andtheir shape or volume change sensitivity to applied voltageis so low. Although a polyvinyl alcohol gel swollen withdimethyl sulfoxide has a relatively stiff matrix and exhibitsa fast shape changing upon applied voltage, the improve-ment of its matrix stiffness is still necessary and highervoltage bearing-capacity than several hundred volts is alsorequired [4]. Nafion, which has a relatively stiff matrix,was found to exhibit a fast self-bending upon a smallapplied voltage around 1 V or less. Investigations havebeen performed to develop practical actuators made bysuch ionic polymer-metal composite [5, 6]. Extensiveworks have been done on Nafion-based IPMC, includingoptimization of its mechanical properties and workingstability [7, 8], some excellent results on modeling ofIPMC with water have also been reported [9–12]. How-ever, few works have been done on another importantapplication such as an artificial muscle or artificial skin aswell as biosensor etc. In this work, the precise measuredmechanical properties and coefficients of thermal expan-sion (CTEs) for two typical films, matrix-Nafion117 filmand [Pt(NH