In-plane gate graphene transistor with epitaxially grown molybdenum disulfide passivation layers.

We demonstrate in-plane gate transistors based on the molybdenum disulfide (MoS2)/graphene hetero-structure. The graphene works as channels while MoS2 functions as passivation layers. The weak hysteresis of the device suggests that the MoS2 layer can effectively passivate the graphene channel. The characteristics of devices with and without removal of MoS2 between electrodes and graphene are also compared. The device with direct electrode/graphene contact shows a reduced contact resistance, increased drain current, and enhanced field-effect mobility. The higher field-effect mobility than that obtained through Hall measurement indicates that more carriers are present in the channel, rendering it more conductive.

Persistent charge storage and memory operation of top-gate transistors solely based on two-dimensional molybdenum disulfide.

We fabricate top-gate transistors on the three-layer molybdenum disulfide (MoS2) with three, two, and one layers in the source and drain regions using atomic layer etching (ALE). In the presence of ALE, the device at zero gate voltage can exhibit high and low levels of drain current under the forward and reverse gate bias, respectively. The hysteresis loop on the transfer curve of transistor indicates that two distinct charge states exist in the device within a range of gate bias. A long retention time of the charge is observed. Unlike conventional semiconductor memories with transistors and capacitors, the two-dimensional (2D) material itself plays two parts in the current conduction and charge storage. The persistent charge storage and memory operation of the multilayer MoS2transistors with thicknesses of a few atomic layer will further expand the device application of 2D materials with reduced linewidths.

Revealing the interlayer van der Waals coupling of bi-layer and tri-layer MoS2 using terahertz coherent phonon spectroscopy.

In this research, we applied THz coherent phonon spectroscopy to optically probe the vibrational modes of the epitaxially-grown bi-layer and tri-layer MoS2 on sapphire substrate. The layers' THz vibration is displacively stimulated and temporally retrieved by near-UV femtosecond laser pulses, revealing Raman-active and Raman-inactive modes in one measurement. With the complete breathing modes revealed, here we extend the linear chain model by considering the elastic contact with the substrate and vdWs coupling of the next nearest MoS2 layer to analyze the effective spring constants. We further considered the intralayer stiffness as a correction term to acquire the actual interlayer vdWs coupling. Our THz phonon spectroscopy results indicate the interlayer spring constants of 9.03 × 1019 N/m3 and 9.86 × 1019 N/m3 for bi-layer and tri-layer respectively. The extended model further suggests that a non-negligible substrate mechanical coupling and next nearest neighbor vdWs coupling of 1.48 × 1019 N/m3 and 1.04 × 1019 N/m3 have to be considered.

The influence of contact metals on epitaxially grown molybdenum disulfide for electrical and optical device applications.

Bottom-gate transistors with mono-layer MoS2channels and polycrystalline antimonene source/drain contact electrodes deposited at 75 °C are fabricated. Significant performance enhancement of field-effect mobility 11.80 cm2V-1·s-1and >106ON/OFF ratio are observed for the device. Increasing photocurrents are also observed for the MoS2transistor under light irradiation, which is attributed to the reduced carrier recombination at the metal/2D material interfaces. The results have demonstrated that besides the matching of work function values with the 2D material channel, the crystallinity of the contact electrodes is the other important parameter for the Ohmic contact formation of 2D material devices.

Layered Graphene Growth Directly on Sapphire Substrates for Applications.

Layer-by-layer graphene growth is demonstrated by repeating CVD growth cycles directly on sapphire substrates. Improved field-effect mobility values are observed for the bottom-gate transistors fabricated by using the bilayer graphene channel, which indicates an improved crystallinity is obtained after the second CVD growth cycle. Despite the poor wettability of copper on graphene surfaces, graphene may act as a thin and effective diffusion barrier for copper atoms. The low resistivity values of thin copper films deposited on thin monolayer MoS2/monolayer graphene heterostructures have demonstrated its potential to replace current thick liner/barrier stacks in back-end interconnects. The unique van der Waals epitaxy growth mode will be helpful for both homo- and heteroepitaxy on 2D material surfaces.

Nanometer-thick copper films with low resistivity grown on 2D material surfaces.

Thin Copper (Cu) films (15 nm) are deposited on different 2D material surfaces through e-beam deposition. With the assist of van der Waals epitaxy growth mode on 2D material surfaces, preferential planar growth is observed for Cu films on both MoS2 and WSe2 surfaces at room temperature, which will induce a polycrystalline and continuous Cu film formation. Relative low resistivity values 6.07 (MoS2) and 6.66 (WSe2) µΩ-cm are observed for the thin Cu films. At higher growth temperature 200 °C, Cu diffusion into the MoS2 layers is observed while the non-sulfur 2D material WSe2 can prevent Cu diffusion at the same growth temperature. By further increasing the deposition rates, a record-low resistivity value 4.62 µΩ-cm for thin Cu films is observed for the sample grown on the WSe2 surface. The low resistivity values and the continuous Cu films suggest a good wettability of Cu films on 2D material surfaces. The thin body nature, the capability to prevent Cu diffusion and the unique van der Waals epitaxy growth mode of 2D materials will make non-sulfur 2D materials such as WSe2 a promising candidate to replace the liner/barrier stack in interconnects with reducing linewidths.

Optical Mode Tuning of Monolayer Tungsten Diselenide (WSe2) by Integrating with One-Dimensional Photonic Crystal through Exciton-Photon Coupling.

Two-dimensional materials, such as transition metal dichalogenides (TMDs), are emerging materials for optoelectronic applications due to their exceptional light-matter interaction characteristics. At room temperature, the coupling of excitons in monolayer TMDs with light opens up promising possibilities for realistic electronics. Controlling light-matter interactions could open up new possibilities for a variety of applications, and it could become a primary focus for mainstream nanophotonics. In this paper, we show how coupling can be achieved between excitons in the tungsten diselenide (WSe2) monolayer with band-edge resonance of one-dimensional (1-D) photonic crystal at room temperature. We achieved a Rabi splitting of 25.0 meV for the coupled system, indicating that the excitons in WSe2 and photons in 1-D photonic crystal were coupled successfully. In addition to this, controlling circularly polarized (CP) states of light is also important for the development of various applications in displays, quantum communications, polarization-tunable photon source, etc. TMDs are excellent chiroptical materials for CP photon emitters because of their intrinsic circular polarized light emissions. In this paper, we also demonstrate that integration between the TMDs and photonic crystal could help to manipulate the circular dichroism and hence the CP light emissions by enhancing the light-mater interaction. The degree of polarization of WSe2 was significantly enhanced through the coupling between excitons in WSe2 and the PhC resonant cavity mode. This coupled system could be used as a platform for manipulating polarized light states, which might be useful in optical information technology, chip-scale biosensing and various opto-valleytronic devices based on 2-D materials.

Charge Storage of Isolated Monolayer Molybdenum Disulfide in Epitaxially Grown MoS2/Graphene Heterostructures for Memory Device Applications.

We epitaxially grew bilayer molybdenum disulfide (MoS2) on monolayer graphene by sulfurizing a molybdenum-trioxide film (MoO3) which was deposited with thermal evaporation. The Hall mobilities of graphene before and after the growth of MoS2 are similar, indicating that the underlying 2D layer was little affected during the deposition and sulfurization. Through the atomic-layer etching, the topmost layer of MoS2 is isolated from the source and drain electrodes. The top-gate transistor with the isolated monolayer MoS2 on top of the graphene channel exhibits hysteresis of drain current as the gate voltage varies. This may be due to the weak tunneling through 2D layers bonded by the van der Waals force in the absence of an external electric field. The long retention time of the device features robust charge storage around the isolated MoS2 layer. The one-transistor-zero-capacitor memory module based on this thin heterostructure of 2D materials can be advantageous for applications in dynamic random access memories with reduced thickness.

Enhancing Raman signals from bacteria using dielectrophoretic force between conductive lensed fiber and black silicon.

An osmium-coated lensed fiber (OLF) probe combined with a silver-coated black silicon (SBS) substrate was used to generate a dielectrophoretic (DEP) force that traps bacteria and enables Raman signal detection from bacteria. The lensed fiber coated with a 2-nm osmium layer was used as an electrode for the DEP force and also as a lens to excite Raman signals. The black silicon coated with a 150-nm silver layer was used both as the surface-enhanced Raman scattering (SERS) substrate and the counter electrode. The enhanced Raman signal was collected by the same OLF probe and further analyzed with a spectrometer. For Raman measurements, a drop of bacterial suspension was placed between the OLF probe and the SBS substrate. By controlling the frequency of an AC voltage on the OLF probe and SBS substrate, a DEP force at 1 MHz concentrated bacteria on the SBS surface and removed the unbound micro-objects in the solution at 1 kHz. A bacteria concentration of 6 × 104 CFU/mL (colony forming units per mL) could be identified in less than 15 min, using a volume of only 1 µL, by recording the variation of the Raman peak at 740 cm-1.

Ultrafast Exciton Dynamics in Scalable Monolayer MoS2 Synthesized by Metal Sulfurization.

Excitons in monolayer transition metal dichalcogenides (TMDs) have exceptionally large binding energies and dominate the optical properties of materials. Exploring the relaxation behavior of excitons is crucial for understanding the fundamental physics as well as the performance of TMD-based optoelectronic devices. However, ultrafast carrier dynamics is sensitive to the structural defects and surface conditions of TMDs, depending on the growth or transfer process. Here, we utilized pump-probe transient absorption (TA) spectroscopy with a white-light probe to investigate the dynamics of excitons in monolayer MoS2 synthesized by the metal sulfurization method. The sulfurization method was used for the fabrication of large-scale, continuous, and uniform thin films with a controllable number of layers. The excitation dynamics of the wafer-size monolayer MoS2 is found to be comparable to that of monolayer MoS2 flakes grown by chemical vapor deposition (CVD). The dominant processes of carrier relaxation in the monolayer MoS2 are exciton-exciton annihilation (hundreds of femtoseconds), the trapping of the excitons by surface states (a few picoseconds), and interband carrier-phonon scattering (tens of picoseconds). Moreover, the induced absorption due to mid-gap defects, which is often observed for samples fabricated by growth methods, such as CVD, is not observed for our continuous and uniform monolayer films. Understanding the charge carrier dynamics of the exciton in the scalable and uniform monolayer MoS2 can provide physical insights that are valuable in the design and development of complex 2D devices.

Luminescence enhancement and dual-color emission of stacked mono-layer 2D materials.

With additional precursor soaking, a thin Al2O3 dielectric layer can be grown on mono-layer MoS2 by using atomic layer deposition (ALD). Similar optical characteristics are observed before and after ALD growth for the mono-layer MoS2, which indicates that minor damage to the thin 2D material film is introduced during the growth procedure. With the thin separation layer, luminescence enhancement and dual-color emission are observed by transferring MoS2 and WS2 mono-layer 2D materials to 5 nm Al2O3/mono-layer MoS2 samples, respectively. The results demonstrate that with careful treatment of the interfaces of 2D crystals with other materials, different stacked structures can be established.

Combination of an Axicon Fiber Tip and a Camera Device into a Sensitive Refractive Index Sensor.

An axicon fiber tip combined with a camera device is developed to sensitively detect refractive indexes in solutions. The transparent axicon tips were made by etching optical fibers through a wet end-etching method at room temperature. When the axicon fiber tip was immersed in various refractive index media, the angular spectrum of the emitted light from the axicon fiber tip was changed. Using a low numerical aperture lens to collect the directly transmitted light, a high intensity sensitivity was achieved when the tip cone angle was about 35 to 40 degrees. We combined the axicon fiber tip with a laser diode and a smartphone into a portable refractometer. The front camera of the smartphone was used to collect the light emitted from the axicon fiber tip. By analyzing the selected area of the captured images, the refractive index can be distinguished for various solutions. The refractive index sensitivity was up to 56,000%/RIU, and the detection limit was 1.79 × 10-5 RIU. By measuring the refractive index change via the axicon fiber tip, the concentration of different mediums can be sensitively detected. The detection limits of the measurement for sucrose solutions, saline solutions, and diluted wine were 8.86 × 10-3 °Bx, 0.12‱, and 0.35%, respectively.

Enhancing Surface Sensitivity of Nanostructure-Based Aluminum Sensors Using Capped Dielectric Layers.

The studies of nanostructure-based aluminum sensors have attracted huge attention because aluminum is a more cost-effective plasmonic material. However, the intrinsic properties of the aluminum metal, having a large imaginary part of the dielectric function and a longer electromagnetic field decay length and problems of poor long-term chemical stability, limit the surface-sensing capability and applicability of nanostructures. We propose the combination of capped aluminum nanoslits and a thin-capped dielectric layer to overcome these limitations. We show that the dielectric layer can positively enhance the wavelength sensitivities of the Wood's anomaly-dominant resonance and asymmetric Fano resonance in capped aluminum nanoslits. The maximum improvement can be reached by a factor of 3.5. Besides, there is an optimal layer thickness for the surface sensitivity because of the trade-off relationship between the refractive index sensitivity and decay length. We attribute the enhanced surface sensitivity to a reduced evanescent length, which is confirmed by the finite difference time-domain calculations. The protein-protein interaction experiments verify the high-surface sensitivity of the structures, and a limit of quantification (LOQ) of 1 pg/mL anti-bovine serum albumin is achieved. Such low-cost, highly sensitive aluminum-based nanostructures can benefit various sensing applications.