Kai Yan

Modulation-doped growth of mosaic graphene with single-crystalline p–n junctions for efficient photocurrent generation

Device applications of graphene such as ultrafast transistors and photodetectors benefit from the combination of both high-quality p- and n-doped components prepared in a large-scale manner with spatial control and seamless connection. Here we develop a well-controlled chemical vapour deposition process for direct growth of mosaic graphene. Mosaic graphene is produced in large-area monolayers with spatially modulated, stable and uniform doping, and shows considerably high room temperature carrier mobility of ~5,000u2009cm2u2009V−1u2009s−1 in intrinsic portion and ~2,500u2009cm2u2009V−1u2009s−1 in nitrogen-doped portion. The unchanged crystalline registry during modulation doping indicates the single-crystalline nature of p–n junctions. Efficient hot carrier-assisted photocurrent was generated by laser excitation at the junction under ambient conditions. This study provides a facile avenue for large-scale synthesis of single-crystalline graphene p–n junctions, allowing for batch fabrication and integration of high-efficiency optoelectronic and electronic devices within the atomically thin film.

Fast and controllable fabrication of suspended graphene nanopore devices

A poly(methyl methacrylate) assisted dry transfer method was developed to transfer graphene microflake onto a suspended SiN chip in an effective and efficient way for further graphene nanopore drilling for DNA analysis. Graphene microflakes can be patterned by e-beam lithography to a designed shape and size on a large scale of a few thousands simultaneously. Subsequently, individual graphene microflakes can be picked up and transferred to a target hole on a suspended SiN membrane with 1xa0µm precision via a site-specific transfer-printing method. Nanopores with different diameters from 3 to 20xa0nm were drilled on the as-transferred graphene membrane in a transmission electron microscope. This method offers a fast and controllable way to fabricate graphene nanopores for DNA analyses.

Site‐Specific Transfer‐Printing of Individual Graphene Microscale Patterns to Arbitrary Surfaces

massless Dirac fermions, [ 3 ] extremely high mobility, [ 4 ] special quantum Hall effect, [ 3 ] and gate voltage tunable optical transitions. [ 5 ] Those remarkable electrical and optical properties make it an attractive candidate for potential applications in integrated bipolar fi eld-effect transistors (FETs), [ 6 ] transparent electrodes for solar cells, [ 7,8 ] as well as other microscale functional devices. [ 9 ]

Free Radical Reactions in Two Dimensions: A Case Study on Photochlorination of Graphene

Graphene, a two-dimensional giant-molecule of sp(2) -bonded carbon atoms, provides a perfect platform for studying free radical reaction chemistry in two-dimensions, which holds promise to control the chemical functionality of graphene. Free-radical photochlorination of graphene is used as an example to investigate the thickness, stacking order, and single- and double-side dependent reactivity in graphene. Anomalously low reactivity is observed in the photochlorination of AB-stacked bilayer graphene in comparison with that of few-layer graphene. Double-sided chlorination of graphene shows higher reactivity and chlorine coverage than single-sided reaction. It is also experimentally and theoretically demonstrated that chlorine free radicals at low coverage prefer to form a stable charge-transfer complex with graphene, which highly enhances graphenes conductivity and simultaneously generates a pseudo-bandgap through noninvasive doping. Moreover, the initial accumulation of chlorine radicals is considered as the rate-determining step of photochlorination of graphene.

Unipolar p-type single-walled carbon nanotube field-effect transistors using TTF–TCNQ as the contact material

We demonstrate herein that organic metal tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ) can serve as an ideal material for source and drain electrodes to build unipolar p-type single-walled carbon nanotube (SWNTs) field-effect transistors (FETs). SWNTs were synthesized by the chemical vapor deposition (CVD) method on silicon wafer and then TTF-TCNQ was deposited by thermal evaporation through a shadow mask to form the source and drain contacts. An SiO2 layer served as the gate dielectric and Si was used as the backgate. Transfer characteristics show that these TTF-TCNQ contacted devices are Schottky barrier transistors just like conventional metal contacted SWNT-FETs. The most interesting characteristic of these SWNT transistors is that all devices demonstrate the unipolar p-type transport behavior. This behavior originates from the unique crystal structure and physical properties of TTF-TCNQ and this device may have potential applications in carbon nanotube electronics.

Epitaxial Growth of Asymmetrically‐Doped Bilayer Graphene for Photocurrent Generation

An asymmetrically doped bilayer graphene is grown by modulation-doped chemical vapor deposition, which consists of one intrinsic layer and one nitrogen-doped layer according to AB stacking. The asymmetrically doped bilayer crystalline profile is found to extend the identical registry as adjacent pristine bilayer region, thus forming single-crystalline bilayer graphene p-n junctions. Efficient photocurrent with responsivity as high as 0.2 mA/W is generated at the bilayer p-n junctions via a hot carrier-assisted mechanism.

Properties of photochlorinated graphene

We report the synthesis and electronic properties of chlorinated graphene, a totally new graphene derivative. The gas-phase photochlorination of graphene, followed by the structural transformation of the C-C bonds from sp2 to sp3 configuration, could remove the conducting π-bands and open up a band gap in graphene. After chlorination, the resistance of graphene increases over 4 orders of magnitude. Moreover, the resistance of chlorinated graphene shows a significant temperature dependence. Electrical transport in chlorinated graphene is well described by variable range hopping in two dimensions due to the presence of localized states in the band gap. The facile, highly efficient and patternable photochlorination method offers a feasible pathway to engineer the band structure of graphene.