Qinghong Yuan

Thin Film Field‐Effect Phototransistors from Bandgap‐Tunable, Solution‐Processed, Few‐Layer Reduced Graphene Oxide Films

Thin film field-effect phototransistors (FETs) can be developed from bandgap-tunable, solution-processed, few-layer reduced graphene oxide (FRGO) films. Large-area FRGO films with tunable bandgaps ranging from 2.2 eV to 0.5 eV can be achieved readily by solution-processing technique such as spin-coating. The electronic and optoelectronic properties of FRGO FETs are found to be closely related to their bandgap energy. The resulting phototransistor has great application potential in the field of photodetection.

Fast growth of inch-sized single-crystalline graphene from a controlled single nucleus on Cu–Ni alloys

Wafer-scale single-crystalline graphene monolayers are highly sought after as an ideal platform for electronic and other applications. At present, state-of-the-art growth methods based on chemical vapour deposition allow the synthesis of one-centimetre-sized single-crystalline graphene domains in ∼12 h, by suppressing nucleation events on the growth substrate. Here we demonstrate an efficient strategy for achieving large-area single-crystalline graphene by letting a single nucleus evolve into a monolayer at a fast rate. By locally feeding carbon precursors to a desired position of a substrate composed of an optimized Cu-Ni alloy, we synthesized an ∼1.5-inch-large graphene monolayer in 2.5 h. Localized feeding induces the formation of a single nucleus on the entire substrate, and the optimized alloy activates an isothermal segregation mechanism that greatly expedites the growth rate. This approach may also prove effective for the synthesis of wafer-scale single-crystalline monolayers of other two-dimensional materials.

Synthesis of large single-crystal hexagonal boron nitride grains on Cu–Ni alloy

Hexagonal boron nitride (h-BN) has attracted significant attention because of its superior properties as well as its potential as an ideal dielectric layer for graphene-based devices. The h-BN films obtained via chemical vapour deposition in earlier reports are always polycrystalline with small grains because of high nucleation density on substrates. Here we report the successful synthesis of large single-crystal h-BN grains on rational designed Cu-Ni alloy foils. It is found that the nucleation density can be greatly reduced to 60 per mm(2) by optimizing Ni ratio in substrates. The strategy enables the growth of single-crystal h-BN grains up to 7,500 μm(2), approximately two orders larger than that in previous reports. This work not only provides valuable information for understanding h-BN nucleation and growth mechanisms, but also gives an effective alternative to exfoliated h-BN as a high-quality dielectric layer for large-scale nanoelectronic applications.

Seamless Stitching of Graphene Domains on Polished Copper (111) Foil

Seamless stitching of graphene domains on polished copper (111) is proved clearly not only at atomic scale by scanning tunnelling microscopy (STM) and transmission electron micoscopy (TEM), but also at the macroscale by optical microscopy after UV-treatment. Using this concept of seamless stitching, synthesis of 6 cm × 3 cm monocrystalline graphene without grain boundaries on polished copper (111) foil is possible, which is only limited by the chamber size.

Regulating Infrared Photoresponses in Reduced Graphene Oxide Phototransistors by Defect and Atomic Structure Control

Defects play significant roles in properties of graphene and related device performances. Most studies of defects in graphene focus on their influences on electronic or luminescent optical properties, while controlling infrared optoelectronic performance of graphene by defect engineering remains a challenge. In the meantime, pristine graphene has very low infrared photoresponses of ~0.01 A/W due to fast photocarrier dynamics. Here we report regulating infrared photoresponses in reduced graphene oxide phototransistors by defect and atomic structure control for the first time. The infrared optoelectronic transport and photocurrent generation are significantly influenced and well controlled by oxygenous defects and structures in reduced graphene oxide. Moreover, remarkable infrared photoresponses are observed in photoconductor devices based on reduced graphene oxide with an external responsivity of ~0.7 A/W, at least over one order of magnitude higher than that from pristine graphene. External quantum efficiencies of infrared devices reach ultrahigh values of ~97%, which to our knowledge is one of the best efficiencies for infrared photoresponses from nonhybrid, pure graphene or graphene-based derivatives. The flexible infrared photoconductor devices demonstrate no photoresponse degradation even after 1000 bending tests. The results open up new routes to control optoelectronic behaviors of graphene for high-performance devices.

Upright Standing Graphene Formation on Substrates

We propose integrating graphene nanoribbons (GNRs) onto a substrate in an upright position whereby they are chemically bound to the substrate at the basal edge. Extensive ab initio calculations show that both nickel (Ni)- and diamond-supported upright GNRs are feasible for synthesis and are mechanically robust. Moreover, the substrate-supported GNRs display electronic and magnetic properties nearly the same as those of free-standing GNRs. Due to the extremely small footprint of an upright GNR on a substrate, standing GNRs are ideal building blocks for synthesis of subnanometer electronic or spintronic devices. Theoretically, standing GNR-based microchips with field-effect transistor (FET) densities up to 10(13) per cm(2) are achievable.

Exploiting Differential Electrochemical Stripping Behaviors of Fe3O4 Nanocrystals toward Heavy Metal Ions by Crystal Cutting

This study attempts to understand the intrinsic impact of different morphologies of nanocrystals on their electrochemical stripping behaviors toward heavy metal ions. Two differently shaped Fe3O4 nanocrystals, i.e., (100)-bound cubic and (111)-bound octahedral, have been synthesized for the experiments. Electrochemical results indicate that Fe3O4 nanocrystals with different shapes show different stripping behaviors toward heavy metal ions. Octahedral Fe3O4 nanocrystals show better electrochemical sensing performances toward the investigated heavy metal ions such as Zn(II), Cd(II), Pb(II), Cu(II), and Hg(II), in comparison with cubic ones. Specifically, Pb(II) is found to have the best stripping performance on both the (100) and (111) facets. To clarify these phenomena, adsorption abilities of as-prepared Fe3O4 nanocrystals have been investigated toward heavy metal ions. Most importantly, combined with theoretical calculations, their different electrochemical stripping behaviors in view of facet effects have been further studied and enclosed at the level of molecular/atom. Finally, as a trial to find a disposable platform completely free from noble metals, the potential application of the Fe3O4 nanocrystals for electrochemical detection of As(III) in drinking water is demonstrated.

How Graphene Islands Are Unidirectionally Aligned on the Ge(110) Surface

The unidirectional alignment of graphene islands is essential to the synthesis of wafer-scale single-crystal graphene on Ge(110) surface, but the underlying mechanism is not well-understood. Here we report that the necessary coalignment of the nucleating graphene islands on Ge(110) surface is caused by the presence of step-pattern; we show that on the preannealed Ge(110) textureless surface the graphene islands appear nonpreferentially orientated, while on the Ge(110) surfaces with natural step pattern, all graphene islands emerge coaligned. First-principles calculations and theoretical analysis reveal this different alignment behaviors originate from the strong chemical binding formed between the graphene island edges and the atomic steps on the Ge(110) surface, and the lattice matching at edge-step interface dictates the alignment of graphene islands with the armchair direction of graphene along the [-110] direction of the Ge(110) substrate.

How a Zigzag Carbon Nanotube Grows

Owing to the unique structure of zigzag (ZZ) carbon nanotubes (CNTs), their ring-by-ring growth behavior is different from that of chiral or armchair (AC) CNTs, on the rims of which kinks serve as active sites for carbon attachment. Through first-principle calculations, we found that, because of the high energy barrier of initiating a new carbon ring at the rim of a ZZ CNT, the growth rate of a ZZ CNT is proportional to its diameter and significantly (10-1000 times) slower than that of other CNTs. This study successfully explained the broad experimental observation of the lacking of ZZ CNTs in CNT samples and completed the theory of CNT growth.

Chemical Trends of Electronic Properties of Two-Dimensional Halide Perovskites and Their Potential Applications for Electronics and Optoelectronics

Current thermoelectric (TE) materials often have low performance or contain less abundant and/or toxic elements, thus limiting their large-scale applications. Therefore, new TE materials with high efficiency and low cost are strongly desirable. Here we demonstrate that, SiS and SiSe monolayers made from non-toxic and earth-abundant elements intrinsically have low thermal conductivities arising from their low-frequency optical phonon branches with large overlaps with acoustic phonon modes, which is similar to the state-of-the-art experimentally demonstrated material SnSe with a layered structure. Together with high thermal power factors due to their two-dimensional nature, they show promising TE performances with large figure of merit (ZT) values exceeding 1 or 2 over a wide range of temperatures. We establish some basic understanding of identifying layered materials with low thermal conductivities, which can guide and stimulate the search and study of other layered materials for TE applications.