Monolayer transition metal dichalcogenides are materials with an atomic structure complementary to graphene but diverse properties, including direct energy bandgaps, which makes them intriguing candidates for optoelectronic devices. Various approaches have been demonstrated for the growth of molybdenum disulphide (MoS 2 ) on insulating substrates, but to date, growth of isolated crystalline flakes has been demonstrated at random locations only. Here we use patterned seeds of molybdenum source material to grow flakes of MoS 2 at predetermined locations with micrometre-scale resolution. MoS 2 flakes are predominantly monolayers with high material quality, as confirmed by atomic force microscopy, transmission electron microscopy and Raman and photoluminescence spectroscopy. As the monolayer flakes are isolated at predetermined locations, transistor fabrication requires only a single lithographic step. Device measurements exhibit carrier mobility and on/off ratio that exceed 10 cm 2 V À 1 s À 1 and 10 6 , respectively. The technique provides a path for in-depth physical analysis of monolayer MoS 2 and fabrication of MoS 2 -based integrated circuits.

Growth of transition metal dichalcogenide (TMD) monolayers is of interest due to their unique electrical and optical properties. Films in the 2H and 1T phases have been widely studied but monolayers of some 1T′-TMDs are predicted to be large-gap quantum spin Hall insulators, suitable for innovative transistor structures that can be switched via a topological phase transition rather than conventional carrier depletion [Qian et al. Science 2014, 346, 1344–1347]. Here we detail a reproducible method for chemical vapor deposition of monolayer, single-crystal flakes of 1T′-MoTe2. Atomic force microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, and transmission electron microscopy confirm the composition and structure of MoTe2 flakes. Variable temperature magnetotransport shows weak antilocalization at low temperatures, an effect seen in topological insulators and evidence of strong spin–orbit coupling. Our approach provides a pathway to systematic investigation of monolayer, single-crystal 1T′-MoTe2 and implementation in next-generation nanoelectronic devices.

Scalable production of all-electronic DNA biosensors with high sensitivity and selectivity is a critical enabling step for research and applications associated with detection of DNA hybridization. We have developed a scalable and very reproducible (>90% yield) fabrication process for label-free DNA biosensors based upon graphene field effect transistors (GFETs) functionalized with single-stranded probe DNA. The shift of the GFET sensor Dirac point voltage varied systematically with the concentration of target DNA. The biosensors demonstrated a broad analytical range and limit of detection of 1 fM for 60-mer DNA oligonucleotide. In control experiments with mismatched DNA oligomers, the impact of the mismatch position on the DNA hybridization strength was confirmed. This class of highly sensitive DNA biosensors offers the prospect of detection of DNA hybridization and sequencing in a rapid, inexpensive, and accurate way.

All-electronic DNA biosensors based on graphene field-effect transistors (GFETs) offer the prospect of simple and cost-effective diagnostics. For GFET sensors based on complementary probe DNA, the sensitivity is limited by the binding affinity of the target oligonucleotide, in the nM range for 20 mer targets. We report a ∼20 000× improvement in sensitivity through the use of engineered hairpin probe DNA that allows for target recycling and hybridization chain reaction. This enables detection of 21 mer target DNA at sub-fM concentration and provides superior specificity against single-base mismatched oligomers. The work is based on a scalable fabrication process for biosensor arrays that is suitable for multiplexed detection. This approach overcomes the binding-affinity-dependent sensitivity of nucleic acid biosensors and offers a pathway toward multiplexed and label-free nucleic acid testing with high accuracy and selectivity.

Electronic DNA‐biosensor with a single nucleotide resolution capability is highly desirable for personalized medicine. However, existing DNA‐biosensors, especially single nucleotide polymorphism (SNP) detection systems, have poor sensitivity and specificity and lack real‐time wireless data transmission. DNA‐tweezers with graphene field effect transistor (FET) are used for SNP detection and data are transmitted wirelessly for analysis. Picomolar sensitivity of quantitative SNP detection is achieved by observing changes in Dirac point shift and resistance change. The use of DNA‐tweezers probe with high‐quality graphene FET significantly improves analytical characteristics of SNP detection by enhancing the sensitivity more than 1000‐fold in comparison to previous work. The electrical signal resulting from resistance changes triggered by DNA strand‐displacement and related changes in the DNA geometry is recorded and transmitted remotely to personal electronics. Practical implementation of this enabling technology will provide cheaper, faster, and portable point‐of‐care molecular health status monitoring and diagnostic devices.

We predict and demonstrate that a disorder induced carrier density inhomogeneity causes magnetoresistance (MR) in a two-dimensional electron system. Our experiments on graphene show a quadratic MR persisting far from the charge neutrality point. Effective medium calculations show that for charged impurity disorder, the low-field MR is a universal function of the ratio of carrier density to fluctuations in carrier density, a power-law when this ratio is large, in excellent agreement with experiment. The MR is generic and should occur in other materials with large carrier density inhomogeneity. PACS numbers:Introduction -The classical magnetoresistance of a material arises when the Lorentz force caused by an applied magnetic field has a component acting against the direction of electron motion thereby decreasing the conductivity of an electronic material. This property has long been of interest both as a tool to probe the fundamental properties of an electronic material (such as the topology of the electron bands) [1] and also for technological applications such as its use in magnetic memory read-heads [2]. A well known result is that single electronic bands (in systems with a spatially homogenous carrier density) will have no magnetoresistance, while the presence of two or more electronic bands with different carrier mobilities readily gives rise to a classical magnetoresistance. This classical effect is different from weak localization [3] (a quantum interference effect present at low temperatures) or Abrikosov's quantum magnetoresistance [4] (that occurs in gapless semiconductors in the high field Landau quantized regime).Graphene is an example of a material with more than one electronic band. This single-atom-thick sheet of carbon comprises an electron band and a hole band each with a linear dispersion that touch at a topologically protected Dirac point [5,6]. It was shown by Ref. [7] that if both bands are occupied, one then expects a classical magnetoresistance even if the electron and hole bands have the same electronic mobility (this is by virtue of the Lorentz force being of opposite sign for the electron and hole carriers). While

We developed a high-yield synthesis of highly crystalline bilayer graphene (BLG) with two preferential stacking modes using a Ni-Cu gradient alloy growth substrate. Previously reported approaches for BLG growth include flat growth substrates of Cu or Ni-Cu uniform alloys and "copper pocket" structures. Use of flat substrates has the advantage of being scalable, but the growth mechanism is either "surface limited" (for Cu) or carbon precipitation (for uniform Ni-Cu), which results in multicrystalline BLG grains. For copper pockets, growth proceeds through a carbon back-diffusion mechanism, which leads to the formation of highly crystalline BLG, but scaling of the copper pocket structure is expected to be difficult. Here we demonstrate a Ni-Cu gradient alloy that combines the advantages of these earlier methods: the substrate is flat, so easy to scale, while growth proceeds by a carbon back-diffusion mechanism leading to high-yield growth of BLG with high crystallinity. The BLG layer stacking was almost exclusively Bernal or twisted with an angle of 30°, consistent with first-principles calculations we conducted. Furthermore, we demonstrated scalable production of transistor arrays based crystalline Bernal-stacked BLG with a band gap that was tunable at room temperature.

A method based on dark field transmission electron microscopy is developed to quantitively investigate the layer number and stacking order of multilayer graphene, demonstrated here on multilayer crystalline graphene synthesized by chemical vapor deposition. Our results show that the relative intensities of first- and second-order diffraction spots and contrast in corresponding dark field images are sufficient to identify the layer number and stacking order of graphene with layer number up to seven (7) or more with few-nanometer spatial resolution.