Age-related macular degeneration (AMD), a retinal degenerative disease that results in a continuous degeneration of photoreceptors in the retina, which eventually leads to complete blindness. One approach to combat AMD is through the use of artificially implantable photodetectors that are physically placed on the retina. Interestingly, 2D materials such as photosensitive and semiconducting molybdenum disulfide (MoS2) and electrically conducting graphene have recently received tremendous promise due to their unique photonic and optoelectronic properties and their potential in various types of micro and nano-devices. In this study, a highly biocompatible 2D graphene-MoS2 photodetectors on a flexible polyimide substrate were designed, fabricated using inkjet printing to form photosensitive pixels and tested as a function of photo intensity and strain. The inkjet printed 2D heterostructure devices were photoresponsive and the photocurrent scaled proportionally with the incident light intensity, exhibiting a photoresponsivity R ∼ 0.30 A/W at room temperature. The strain-dependent measurements of photocurrent with bending showed a photocurrent of Iph ∼ 1.16 μA with strain levels for curvature up to ∼ 0.262 cm-1. Inkjet printed graphene and MoS2 inks were also characterized using techniques such as Raman Spectroscopy, Photoluminescence (PL) and Scanning Electron Microscopy (SEM).

The presence of a direct optical bandgap in the transition metal dichalcogenide (TMD) layers leads to promising applications in optoelectronic devices such as phototransistors and photodetectors. These devices are commonly fabricated using few-layer and monolayer MoS2 sheets obtained using mechanical exfoliation or chemical vapor deposition techniques. The hybrid structure of quantum dots (QDs) and 2D materials has been investigated to provide outstanding properties for various applications. Herein we report the fabrication of a hybrid QDs/MoS2 photodetector consisting of graphene quantum dots (GQDs) and multilayer MoS2 sheets. The hybrid GQDs and MoS2 films are characterized by atomic force microscopy (AFM); additionally, the I-V characteristics are measured by two-point probe station.

Chemically exfoliated two-dimensional (2D) materials have shown promise in a variety of applications such as thin film transistors and photovoltaic devices. Here, we present a scalable method for preparing black phosphorus membranes via direct liquid-phase exfoliation of the bulk crystal in organic solvents. The black phosphorus sheets are characterized by Raman and Photoluminescence (PL) spectroscopy, and optical microscopy. The properties of these chemically exfoliated black phosphorus was compared to mechanically exfoliated nanomembranes. The liquid-phase exfoliation nonetheless yields the potential for large-area scalability. Our results highlight the important aspects of forming solution dispersions of 2D black phosphorus which can subsequently be the stepping stone for future work on the ink-jet printing of such dispersions over a wide range of substrates.

Two-dimensional layered materials, materials with weak out-of-plane van der Waals bonding and strong in-plane covalent bonding, have attracted special attention in recent years since the isolation and characterization of monolayer graphite, the graphene. The electrical bandgap in Transition Metal Di-Chalcogenides (TMDCs), non-existent in graphene, make them a good alternative family of materials for novel electronic and optoelectronic applications. 2H- MoS2, one of the most stable TMDCs, has been extensively studied, including the synthesis methods, and its potential applications in photodetection. The chemical vapor deposition (CVD) synthesis method has increased its potential over the years. The advantages of this method are scalability compared to micromechanical exfoliation, common process used in research laboratories, and the maintenance of the quality and intrinsic properties of the material compared to the liquid exfoliation methods. In this work, we synthesized high quality pristine 2H-MoS2 via atmospheric pressure chemical vapor deposition (APCVD) by vapor phase reaction of MoO3 and S powder precursors. The samples were characterized via Raman and photoluminescence (PL) spectroscopy and compared to mechanically exfoliated MoS2 crystal by measuring the full-width half maxima (FWHM) of monolayer and few-layer mesoscopic flakes. In addition, the CVD synthesized single and few-layered MoS2 domains were transferred to different substrates using a high yield process, including a flexible substrate, preserving the quality of the material. Finally, and mechanically exfoliated MoS2 two-terminal photodetector was designed, fabricated, and measured. Demonstrating thus the capability of heterostructure fabrication and the quality of our synthesis and device fabrication process.

In this work, a new method is presented to synthesize graphene-C60 hybrid materials using an electrophoretic deposition technique to study the graphene-C60 interactions. Electronic measurements of the structure were conducted before and after the attachment of C60 clusters at different applied voltages on graphene devices. The assembled clusters of C60 on mechanically exfoliated graphene were investigated using Raman Spectroscopy and Scanning Electron Microscopy (SEM), which reveal a uniform morphology of C60 on graphene. The results indicate that graphene-C60 hybrids are excellent electron accepting/charge transporting materials which can provide an effective route to facilitate the application of these hybrids in electronic or opto-electronic device platforms.

In this work, we demonstrate optical and electrical transport properties of chemically exfoliated WS2 in cyclohexanone/ terpineol solvent using different sonication times. High electrical conductivity of WS2 nanodispersions was observed when appropriate amount of voltage was applied indicating their semi-conductive behavior. Surface morphology of WS2 nanodispersions sonicated at different times were studied using optical microscopy. Optical bandgap of WS2 nanodispersions were determined from optical absorbance spectrum. Inkjet printing was used to demonstrate uniform distribution of WS2 nanosheets and their precise and large scale printability. These dispersions indicate the potential of WS2 in various optoelectronic and semiconducting device applications.

In this paper, we have developed composites with Poly-methyl methacrylate (PMMA) as the matrix material, while transition metal dichalcogenides (TMDCs), MoS2 and WS2 and graphite served as the filler materials. The PMMA was chosen as the matrix material due to its low-cost, wide availability, as well as its promising mechanical and optical properties for enabling opto-electro-mechanical sensing devices. The amount of filler material used ranged from 100 mg/ml up to 400 mg/ml. With the aid of designed fixtures we related the electrical properties of the PMMA-based composite sensors to the degree of strain or deformation. Additionally, a nanoindenter was used to measure the modulus of elasticity, with values as low as 2 GPa and as high as 20 GPa for the graphite composites, and hardness values which ranged from 0.1 GPa to ∼ 1.6 GPa.

Two dimensional (2D) nanomaterials such as graphene and transition-metal dichalcogenides (TMDCs) have attracted tremendous attention over recent years due to their unique properties and potential for numerous applications. Given the wide range of compositions of 2D-layered materials that have emerged in recent years, it is not surprising that they offer a rich spectrum of properties, ranging from metallic, insulating, superconducting to semiconducting. Here we report on the solution-based production of 2D layered material flakes, in particular graphene and MoS2 where the materials are chemically exfoliated in organic solvents which can then be ink jet printed using a commercially available material printer, for printed electronics applications.

We studied nonlinear absorption characteristics of exfoliated Molybdenum disulphide (MoS2) dispersion in 1-Methyl-2-pyrrolidinone and demonstrate a dual absorption characteristic at 532nm nanosecond pulsed laser wavelength. A number of recent reports demonstrate a saturable absorption in MoS2 and other 2D materials at low fluences and a deviation from this saturable absorption at higher fluence using open aperture Z scan (OZ scan) technique. It has been suggested that this deviation at higher fluences is due to nonlinear optical scattering. We have recently developed a new technique which combines OZ scan and photoacoustic Z-scan (PAZ-scan). It can measure photoacoustic and optical transmission signals simultaneously. The data obtained from both signals are employed to find nonlinear absorption parameters in non-linear optical materials. Our results reveal that non-linear scattering is not the cause of deviation of 2D materials from saturable absorption at higher fluences. We propose that the optical limiting behavior at higher fluence in these 2D materials is dominated by free carrier absorption.

2D materials have shown to be the next step in semiconductor use and device manufacturing that can allow us to reduce the size of most electronics. One of the novel ways to obtain 2D materials is through liquid exfoliation, in which these materials can be obtained by dispersing the smallest possible particles in different solvents. Once obtained, the solutions can be used to manufacture devices via different processes, one of which is inkjet printing. This process relies in selecting “jettable” fluids, which need to have the necessary combination of viscosity and surface energy or “wettability”. In this work we have modified the viscosities and surface energies of five solvents: IPA (Isopropanol), NMP (N-methyl – 2 pyrrolidone), DMA (Dimethylacetamide), DMF (Dimethylformamide) and a mixture of Cyclohexanone / Terpineol 7:3. We have found an avenue to tailor the viscosity of these solvents though the addition of Ethyl Cellulose (EC), where the viscosity has been increased by up to 15 times at an EC concentration of 6%. For inkjet printing, ideally a viscosity of 4 – 10 cP is recommended, which we have been able to achieve with all of the solvents studied. It has been found that the different solvents present different susceptibilities to the EC addition, with DMA and DMF being the least sensitive to the EC addition. We have also studied the change in the drop dynamics and interactions of the 2D solutions with the substrate. Through this analysis we have found solvents that appear to be attractive for inkjet printing of MoS2 and graphite.

In this work, we have explored the prospects of MoS2 and WS2, both of which are semiconducting 2D materials, for potential composite applications. In order to form 2D materials composites we have to first disperse them chemically in solution. MoS2 and WS2 powders were oversaturated in N-Methyl-2-pyrrolidone (NMP) solution at 37.5 mg/mL and sonicated at room temperature (RT) for sonication times ranging from 30 minutes to close to 24 hours. After solution processing, the samples with the 2D flakes were transferred to an Isopropyl Alcohol (IPA) bath for particle size distribution analysis. We have observed significant changes in particle size distribution spanning two orders of magnitude as a function of the sonication conditions. Specifically, the observed changes in particle size distribution for MoS2 and WS2 powders ranged from 44 microns down to 0.409 microns, and 148 microns down to 0.409, respectively, as compared to the untreated materials. Structural analysis was conducted using the SEM and X-Ray diffraction. The structural analysis using the SEM revealed morphological signatures between the two materials, where the MoS2 flakes had a randomly oriented distribution with occasional triangular flakes. In the case of the WS2, regardless of the sonication conditions, the WS2 flakes seemed to have a characteristic 120° angular distribution at the vertices, representing a rhombus with concave edges. The XRD analysis showed a minute shift in the characteristic peaks that maybe due to strain-induced effects as a result of the solution processing. Optical characterization of the materials was also conducted using Raman Spectroscopy to validate the average layer number resulting from the solution dispersions and the spatial and compositional uniformity of the two material samples.

For optoelectronic application, two-dimensional materials such as molybdenum disulfide (MoS2) are very promising candidate with their interesting electronic and optical properties. The layered structure of these materials makes them amenable to mechanical exfoliation to form scalable 2D atomic crystals. For width range of applications, liquid phase exfoliation using sonication and centrifugation in appropriate solvent is needed. This simple and scalable technique gives high quality of exfoliation of 2D materials without chemical reactions. In this paper, we report an example of the optical and electronic characterizations on MoS2 synthesized by liquid exfoliation in specific solvent.

Since the isolation of graphene, a monolayer of sp2-bonded carbon atoms arranged in a hexagonal lattice, two-dimensional (2D) layered materials have attracted a great deal of attention due to their outstanding mechanical, optical and electronic properties. The research areas of interest for these new materials include exploring their novel properties, developing scalable approaches to synthesize these materials, and integrating them into a new generation of nanodevices. The utilization of 2D materials in devices has many advantages, which includes scaled materials to the limit of atomic-scale membranes, and the potential to form device structures on flexible and transparent substrates, among others. Transition metal dichalcogenides (TMDs) monolayers in particular, have received increasing attention in recent years, especially molybdenum disulfide (MoS2), which is one of the most well explored semiconducting materials in the 2D materials system. In this work we present the synthesis of MoS2 using chemical vapor deposition (CVD), where we have varied the synthesis parameters and compared the structure and quality of the CVD synthesized MoS2. At the same time, we have compared the characteristics with those obtained for mechanically exfoliated flakes from the bulk MoS2 crystal. The MoS2 quality has been analyzed using Raman spectroscopy.

Transition metal dichalcogenides (TMDs) are emerging among the potential alternatives to graphene. The monolayer of TMDs can easily be exfoliated mechanically and their electronic properties can also be tuned by controlling the number of layers. TMDs possess an advantage over graphene by controlling band gap magnitude appropriate for the electronic and optoelectronic applications. Here we show, mechanically exfoliated TMDs such as NbSe2 and MoTe2 exhibit metallic and fluctuating conductance behavior respectively. Metallic conduction in NbSe2 was investigated under atmospheric conditions and compered with vacuum conditions. Furthermore, NbSe2 resistance was measured at low temperature up to 5.6 K. The above electronic investigations clearly demonstrate ohmic and fluctuating conduction in NbSe2 and MoTe2 respectively which could be applicable for electronic and optoelectronic devices.

Graphene's layered structure has opened new prospects for the exploration of properties of other monolayer-thick two-dimensional (2D) layered crystals. The emergence of these inorganic 2D atomic crystals beyond graphene promises a diverse spectrum of properties. For example, hexagonal-boron nitride (h-BN), a layered material closest in structure to graphene is an insulator, while niobium selenide (NbSe2), a transition metal dichalcogenide, is metallic, and monolayers of other transition metal dichalcogenides such as molybdenum disulfide (MoS2) and tungsten disulfide (WS2) are direct band gap semiconductors. The rich spectrum of properties exhibited by these 2D layered material systems can potentially be engineered on-demand and creates exciting prospects for using such systems in device applications ranging from electronics, photonics, energy harvesting, flexible electronics, transparent electrodes, and sensing. A review of the structure, properties, and the emerging device applications of these materials is presented in this paper. While the layered structure of these materials makes them amenable to mechanical exfoliation for quickly unveiling their novel properties and for fabricating proof-of-concept devices, an overview of the synthesis routes that can potentially enable scalable avenues for forming these 2D atomic crystals is also discussed.

We demonstrate one-dimensional (1D) and two-dimensional (2D) resonant nanoelectromechanical systems (NEMS) derived from nano carbon materials, where the resonance frequency and the quality (Q) factor of the devices are measured experimentally using ultrasensitive optical interferometry. The 1D nano carbon resonators are formed using carbon nanofibers (CNFs) which are synthesized using a plasma-enhanced chemical vapor deposition (PECVD) process, while the 2D nanocarbon resonators are based on CVD grown graphene. The CNFs are prototyped into few-μm-long cantilever-shaped 1D resonators, where the resonance frequency and Qs are extracted from measurements of the undriven thermomechanical noise spectrum. The thermomechanical noise measurements yield resonances in the ∼3–15 MHz range, with Q of ∼200–800. Significant changes in resonance characteristics are observed due to electron beam induced amorphous carbon deposition on the CNFs, which suggests that 1D CNF resonators have strong prospects for ultrasensitive mass detection. We also present NEMS resonators based on 2D graphene nanomembranes, which exhibit robust undriven thermomechanical resonances for the extraction of ultrasmall strain levels.

Carbon-based nanostructures have been the center of intense research and development for more than two decades now. Of these materials, graphene, a two-dimensional (2D) layered material system, has had a significant impact on science and technology in recent years after it was experimentally isolated in single layers in 2004. The recent emergence of other classes of 2D layered systems beyond graphene has added yet more exciting and new dimensions for research and exploration given their diverse and rich spectrum of properties. For example, h-BN a layered material closest in structure to graphene, is an insulator, while NbSe, a transition metal dichalcogenide is metallic and monolayers of other transition metal di-chalcogenides such as MoS2 are direct band-gap semiconductors. The rich variety of properties that 2D layered material systems offer can potentially be engineered on-demand, and creates exciting prospects for their device and technological applications ranging from electronics, sensing, photonics, energy harvesting and flexible electronics in the near future.

Optical absorption efficiency, an important metric for sensing, radiometric and energy harvesting applications, has been studied theoretically and experimentally in porous, ordered nanostructures, including multi-walled- (MW) carbon nanotubes (CNTs) and single-walled- (SW) CNTs. We have characterized the absorption efficiencies in the 350 nm -7000 nm wavelength range of vertically aligned MWCNT arrays with high site densities synthesized directly on metallic substrates using a plasma-enhanced (PE)- chemical vapor deposition (CVD) process. Our ultra-thin absorbers exhibit a reflectance as low as ∼ 0.02 % (100 X lower than the benchmark). Such high efficiency absorbers are particularly attractive for radiometry, as well as energy harnessing applications. This work increases the portfolio of materials that can be integrated with such absorbers due to the potential for reduced synthesis temperatures arising from a plasma process. Optical modeling calculations were conducted that enabled a determination of the extinction coefficient in the films.

Developing energy efficient electronics or green electronics is an area that is largely driven by the performance limitations of scaled Si-based CMOS due to the exceptionally high power dissipation and high leakage currents arising in such devices at nanoscale dimensions. It is clear now that Si-based CMOS has been stretched over the past several decades to the point that further miniaturization will make such simple size scaling non-sustainable in the future. New materials and technologies are thus vigorously being explored beyond Si, in order to overcome performance limitations from ultra-miniaturized Si-CMOS. Among these materials, carbon-based nanostructures such as graphene and carbon nanotubes are being considered as viable alternatives to Si-CMOS to enable energy efficient green electronics. Novel architectures for enabling low-power, energy-efficient computation are currently being explored, which include tunneling field-effect-transistors (TFETs), as well as nano-electro-mechanical-systems (NEMS) due to their abrupt ON/OFF transitions, low OFF state currents and high speed operation. In this paper, an overview of carbon nanomaterials is presented and the role they play in enabling energy efficient TFETs and NEMS is also highlighted. Finally, the emergence of a new class of 2D systems beyond graphene is discussed such as MoS2, which may open up new avenues for exploration and enabling applications in electronics.

We report on the material characterization of carbon nanofibers (CNFs) which are assembled into a three-dimensional (3D) configuration for making new nanoelectromechanical systems (NEMS). High-resolution scanning electron microscopy (SEM) and x-ray electron dispersive spectroscopy (XEDS) are employed to decipher the morphology and chemical compositions of the CNFs at various locations along individual CNFs grown on silicon (Si) and refractory nitride (NbTiN) substrates, respectively. The measured characteristics suggest interesting properties of the CNF bodies and their capping catalyst nanoparticles, and growth mechanisms on the two substrates. Laser irradiation on the CNFs seems to cause thermal oxidation and melting of catalyst nanoparticles. The structural morphology and chemical compositions of the CNFs revealed in this study should aid in the applications of the CNFs to nanoelectronics and NEMS.