Dongsuk Lim

Tunable Electrical and Optical Characteristics in Monolayer Graphene and Few-Layer MoS2 Heterostructure Devices

Lateral and vertical two-dimensional heterostructure devices, in particular graphene-MoS2, have attracted profound interest as they offer additional functionalities over normal two-dimensional devices. Here, we have carried out electrical and optical characterization of graphene-MoS2 heterostructure. The few-layer MoS2 devices with metal electrode at one end and monolayer graphene electrode at the other end show nonlinearity in drain current with drain voltage sweep due to asymmetrical Schottky barrier height at the contacts and can be modulated with an external gate field. The doping effect of MoS2 on graphene was observed as double Dirac points in the transfer characteristics of the graphene field-effect transistor (FET) with a few-layer MoS2 overlapping the middle part of the channel, whereas the underlapping of graphene have negligible effect on MoS2 FET characteristics, which showed typical n-type behavior. The heterostructure also exhibits a strongest optical response for 520 nm wavelength, which decreases with higher wavelengths. Another distinct feature observed in the heterostructure is the peak in the photocurrent around zero gate voltage. This peak is distinguished from conventional MoS2 FETs, which show a continuous increase in photocurrent with back-gate voltage. These results offer significant insight and further enhance the understanding of the graphene-MoS2 heterostructure.

Electrical characterization of multilayer HfSe2 field-effect transistors on SiO2 substrate

We fabricated and characterized two-dimensional field-effect transistors (FETs) based on hafnium diselenide (HfSe2) crystalline nanoflakes. The HfSe2 FET exhibits an n-type semiconductor behavior with a high on/off current ratio exceeding 7.5 × 106. In the temperature range of 120 K–280 K, the thermally activated transport is observed at high carrier concentrations, while at low concentrations and low temperatures hopping conduction dominates the transport mechanism. We also observed the metal insulator transition at carrier density of ∼1.8 × 1012 cm−2. This initial report on the physical and electrical characterization of two dimensional HfSe2 material demonstrates the feasibility of this semiconducting material for electronic devices.

Non-degenerate n-type doping by hydrazine treatment in metal work function engineered WSe₂ field-effect transistor.

We report a facile and highly effective n-doping method using hydrazine solution to realize enhanced electron conduction in a WSe2 field-effect transistor (FET) with three different metal contacts of varying work functions-namely, Ti, Co, and Pt. Before hydrazine treatment, the Ti- and Co-contacted WSe2 FETs show weak ambipolar behaviour with electron dominant transport, whereas in the Pt-contacted WSe2 FETs, the p-type unipolar behaviour was observed with the transport dominated by holes. In the hydrazine treatment, a p-type WSe2 FET (Pt contacted) was converted to n-type with enhanced electron conduction, whereas highly n-doped properties were achieved for both Ti- and Co-contacted WSe2 FETs with on-current increasing by three orders of magnitude for Ti. All n-doped WSe2 FETs exhibited enhanced hysteresis in their transfer characteristics, which opens up the possibility of developing memories using transition metal dichalcogenides.

Gate-Tunable Hole and Electron Carrier Transport in Atomically Thin Dual-Channel WSe2/MoS2 Heterostructure for Ambipolar Field-Effect Transistors

An ambipolar dual-channel field-effect transistor (FET) with a WSe2 /MoS2 heterostructure formed by separately controlled individual channel layers is demonstrated. The FET shows a switchable ambipolar behavior with independent carrier transport of electrons and holes in the individual layers of MoS2 and WSe2 , respectively. Moreover, the photoresponse is studied at the heterointerface of the WSe2 /MoS2 dual-channel FET.

Raman shift and electrical properties of MoS2 bilayer on boron nitride substrate.

We have fabricated a bilayer molybdenum disulphide (MoS2) transistor on boron nitride (BN) substrate and performed Raman spectroscopy and electrical measurements with this device. The characteristic Raman peaks show an upshift about 2.5 cm(-1) with the layer lying on BN, and a narrower line width in comparison with those on a SiO2 substrate. The device has a maximum drain current larger than 1 μA and a high current on/off ratio of greater than 10(8). In the temperature range of 100 K-293 K, the two terminal gate effect mobility and the carrier density do not change significantly with temperature. Results of the Raman and electrical measurements reveal that BN is a suitable substrate for atomic layer electrical devices.

P-doping and efficient carrier injection induced by graphene oxide for high performing WSe2 rectification devices

In this work, we fabricated multi-layer WSe2 rectifying diodes using graphene oxide (GO) as p-doping material on one side of the contacting electrodes. This GO layer can reduce the contact resistance by forming a tunneling barrier for efficient hole injection, while it increases the contact resistance for the injection of electrons. Results of Raman shift spectra and the opto-electric response of the device confirmed the p-doping effect caused by the GO layer and the formation of a barrier, respectively. We observed a gate tunable rectification effect with a forward/reverse current ratio of 104 and low reverse bias current of 10−10 A. Applying a GO layer in the fabrication of two-dimensional transition metal dichalcogenides based devices is a very useful method in the applications in future nanotechnologies.

Molybdenum disulfide nanoparticles decorated reduced graphene oxide: highly sensitive and selective hydrogen sensor

In this work, we report on the hydrogen (H2) sensing behavior of reduced graphene oxide (RGO)/molybdenum disulfide (MoS2) nano particles (NPs) based composite film. The RGO/MoS2 composite exhibited a highly enhanced H2 response (∼15.6%) for 200 ppm at an operating temperature of 60 °C. Furthermore, the RGO/MoS2 composite showed excellent selectivity to H2 with respect to ammonia (NH3) and nitric oxide (NO) which are highly reactive gas species. The composites response to H2 is 2.9 times higher than that of NH3 whereas for NO it is 3.5. This highly improved H2 sensing response and selectivity of RGO/MoS2 at low operating temperatures were attributed to the structural integration of MoS2 nanoparticles in the nanochannels and pores in the RGO layer.

Reduction of persistent photoconductivity in a few-layer MoS2 field-effect transistor by graphene oxide functionalization

We functionalized two-dimensional few-layer MoS2 based FET with graphene oxide (GO) in order to improve its persistent photoconductivity and photoresponse time. Both pristine and GO functionalized devices show n-type semiconductor behavior with high on/off ratio exceeding ∼105. The photoresponse of the GO–MoS2 hybrid device shows almost complete recovery from persistent photoconductivity and a substantial decrease in response time from ∼15 s in the pristine MoS2 device to ∼1 s in the GO–MoS2 device. The reasons behind this improvement have been explored and discussed on the basis of electrostatic and photo interaction between GO and MoS2. As GO is a strong candidate for various sensing applications, therefore this intelligent hybrid system, where GO interacts electrostatically with the underlying MoS2 channel, has tremendous potential to add more functionalities to a pristine MoS2 device for realizing various smart nanoscale FET-based biochemical and gas sensors for myriad applications.

High performance MoS2-based field-effect transistor enabled by hydrazine doping.

We investigated the n-type doping effect of hydrazine on the electrical characteristics of a molybdenum disulphide (MoS2)-based field-effect transistor (FET). The threshold voltage of the MoS2 FET shifted towards more negative values (from -20 to -70 V) on treating with 100% hydrazine solution with the channel current increasing from 0.5 to 25 μA at zero gate bias. The inverse subthreshold slope decreased sharply on doping, while the ON/OFF ratio increased by a factor of 100. Gate-channel coupling improved with doping, which facilitates the reduction of channel length between the source and drain electrodes without compromising on the transistor performance, making the MoS2-based FET easily scalable.

Observation of negative differential resistance in mesoscopic graphene oxide devices

The fractions of various functional groups in graphene oxide (GO) are directly related to its electrical and chemical properties and can be controlled by various reduction methods like thermal, chemical and optical. However, a method with sufficient controllability to regulate the reduction process has been missing. In this work, a hybrid method of thermal and joule heating processes is demonstrated where a progressive control of the ratio of various functional groups can be achieved in a localized area. With this precise control of carbon-oxygen ratio, negative differential resistance (NDR) is observed in the current-voltage characteristics of a two-terminal device in the ambient environment due to charge-activated electrochemical reactions at the GO surface. This experimental observation correlates with the optical and chemical characterizations. This NDR behavior offers new opportunities for the fabrication and application of such novel electronic devices in a wide range of devices applications including switches and oscillators.