Xuebin Tan

Improved carrier mobility in few-layer MoS2 field-effect transistors with ionic-liquid gating

We report the fabrication of ionic liquid (IL)-gated field-effect transistors (FETs) consisting of bilayer and few-layer MoS2. Our transport measurements indicate that the electron mobility μ ≈ 60 cm(2) V(-1) s(-1) at 250 K in IL-gated devices exceeds significantly that of comparable back-gated devices. IL-FETs display a mobility increase from ≈ 100 cm(2) V(-1) s(-1) at 180 K to ≈ 220 cm(2) V(-1) s(-1) at 77 K in good agreement with the true channel mobility determined from four-terminal measurements, ambipolar behavior with a high ON/OFF ratio >10(7) (10(4)) for electrons (holes), and a near ideal subthreshold swing of ≈ 50 mV/dec at 250 K. We attribute the observed performance enhancement, specifically the increased carrier mobility that is limited by phonons, to the reduction of the Schottky barrier at the source and drain electrode by band bending caused by the ultrathin IL dielectric layer.

Mobility enhancement and highly efficient gating of monolayer MoS 2 transistors with polymer electrolyte

We report electrical characterization of monolayer molybdenum disulfide (MoS2) devices using a thin layer of polymer electrolyte (PE) consisting of poly(ethylene oxide) (PEO) and lithium perchlorate (LiClO4) as both a contact-barrier reducer and channel mobility booster. We find that bare MoS2 devices (without PE) fabricated on Si/SiO2 have low channel mobility and large contact resistance, both of which severely limit the field-effect mobility of the devices. A thin layer of PEO/LiClO4 deposited on top of the devices not only substantially reduces the contact resistance but also boost the channel mobility, leading up to three-orders-of-magnitude enhancement of the field-effect mobility of the device. When the PE is used as a gate medium, the MoS2 field-effect transistors exhibit excellent device characteristics such as a near ideal subthreshold swing and an on/off ratio of 106 as a result of the strong gate-channel coupling.

Electrowetting on dielectric experiments using graphene

We report electrowetting on dielectric (EWOD) experiments using graphene; a transparent, flexible and stretchable nanomaterial. Graphene sheets were synthesized by chemical vapor deposition, and transferred to various substrates (including glass slides and PET films). Reversible contact angle changes were observed on the Teflon-coated graphene electrode with both AC and DC voltages. Nyquist plots of the EWOD reveal that the graphene electrode has higher capacitive impedance than gold electrodes under otherwise identical conditions, suggesting a lower density of pin-holes and defects in the Teflon/graphene electrode than in the Teflon/gold electrode. Furthermore, we have observed reduced electrolysis of the electrolyte and smaller leakage current in the dielectric layer (Teflon) on graphene electrodes than on Au electrodes at the same Teflon thickness and applied voltage. We expect that the improved EWOD properties using graphene as an electrode material will open the door to various applications, including flexible displays and droplet manipulation in three-dimensional microfluidics.

A sandwich substrate for ultrasensitive and label-free SERS spectroscopic detection of folic acid / methotrexate

A highly sensitive surface enhanced Raman scattering (SERS) substrate with particle-film sandwich geometry has been developed for the label free detection of folic acid (FA) and methotrexate (MTX). In this sandwich structure, the bottom layer is composed of a copper foil decorated with silver nanoparticles synthesized by the galvanic displacement reaction, and top layer is constituted by silver nanoparticles. The FA and MTX molecules are sandwiched between the silver nanoparticles decorated copper film and the silver nanoparticles. The plasmonic coupling between the two layers of the sandwich structure greatly enhances the SERS spectra of FA and MTX. SERS activity of the substrate was studied and optimized by adjusting the time of galvanic displacement reaction. The SERS spectra of the FA and MTX showed the minimum detection concentration of 100 pM. The identification of methotrexate and folic acid analogs was also carried out by SERS spectra analysis.

Electrowetting on flexible, transparent and conducting single-layer graphene

This paper reports the results of using graphene as a novel electrode material in the experiment of electrowetting on dielectric (EWOD). The device has a Teflon layer coated CVD graphene electrode which is patterned on transparent and flexible substrates such as PET films. By applying a potential difference between a liquid droplet and a graphene electrode, we observed a change in contact angle from 117° to 86° as the voltage is increased from 0V to 40V. We also demonstrated reversible electrowetting. By utilizing graphene as an electrode material we have achieved high breakdown voltage, transparency and flexibility, all of which will be beneficial for future applications such as flexible display technology and cell manipulation.

A high-throughput microfluidic chip for size sorting of cells

We present a microfluidic cell sorter that enables high-throughput, label-free, size separation of cells (ranging from few microns to hundred microns) in heterogeneous biological samples. The separation technique we use here involves microsieves filtration and time-sequential flow rate control (back-and-forth flow) separation. The results show that our design can achieve high separation efficiency (99%) in a high-throughput manner (100∼1000cells/s).

Control and enhancement of graphene sensitivity by engineering edge defects

This paper presents a novel design to control defect sites and to enhance sensitivity in graphene-based pH and biological sensors. We showed that pristine graphene was not sensitive to the pH. We demonstrated that the pH sensitivity of graphene could be controlled by varying the patterns and lengths of edges that were created using simple processes of photolithography and dry etching.

Graphene based digital microfluidics

We demonstrated a novel electrowetting-on-dielectric (EWOD) device for digital microfluidic using a flexible, transparent, and conducting graphene electrode. We showed that a graphene electrode had a lower leakage current and a higher breakdown voltage compared to a gold electrode when they were under the same dielectric layer such as Teflon. The droplet movement was manipulated on graphene-based EWOD using a sequential switching voltage controlled by a programmable logic controller (PLC) relay array.