Toward Monolayered Solar Cells: Luminescence Properties and Light Soaking in TMDs.

The optical and photonic characteristics of monolayer transition metal dichalcogenides (TMDs) play a pivotal role in their functionality as solar cell materials, light-emitting diodes (LEDs), and other electro-optical applications. In this study, we reveal the impact of prolonged illumination on the luminescence properties and Raman spectra of monolayered MoS2 and WS2─a process known as "light soaking". We find a light-induced transition from the physisorption to the chemisorption of ambient O2 and H2O molecules. In parallel, we observe the activation and passivation of defect sites in the samples (depending on their initial defect density), which is attributed to the adsorbed ambient molecules and the resulting light-driven interactions with defect sites. Thus, we can control the active defect density of monolayered TMDs and shed light on the fundamental mechanisms underlying their luminescence properties. Therefore, this work clarifies the source of changes to the luminescence properties of TMDs and opens the path toward their integration into advanced applications that may be affected by light soaking, such as solar cells and energy devices.

Exceptionally large fracture strength and stretchability of 2D ReS2 and ReSe2.

Two-dimensional rhenium disulfide (ReS2) and rhenium diselenide (ReSe2) have gained popularity due to their outstanding optoelectronic properties. However, their mechanical behavior has not been investigated experimentally and many of their mechanical parameters are still unexplored. Here we conducted atomic force microscopy (AFM) indentation experiments and extracted their Young's moduli and found that it is thickness-independent. In addition, we found that both materials are capable of sustaining large pretension. Importantly, fracture tests showed that these materials exhibit exceptionally large fracture strength (32.9 ± 2.4 GPa and 27.7 ± 3.9 GPa for ReS2 and ReSe2, respectively) and stretchability (up to 24.2% for ReS2 and 23.0% for ReSe2). Therefore, this study shows the superior mechanical properties of ReS2 and ReSe2. Thus, it will open the path for their future integration into advanced applications that will benefit from their outstanding mechanical durability and attractive optoelectronic properties, such as flexible photodetectors, stretchable photonic devices, and strain-engineered electronics.

Self-Sensing WS2 Nanotube Torsional Resonators.

We demonstrate self-sensing tungsten disulfide nanotube (WS2 NT) torsional resonators. These resonators exhibit all-electrical self-sensing operation with electrostatic excitation and piezoresistive motion detection. We show that the torsional motion of the WS2 NT resonators results in a change of the nanotube electrical resistance, with the most significant change around their mechanical resonance, where the amplitude of torsional vibrations is maximal. Atomic force microscopy analysis revealed the torsional and bending stiffness of the WS2 NTs, which we used for modeling the behavior of the WS2 NT devices. In addition, the solution of the electrostatic boundary value problem shows how the spatial potential and electrostatic field lines around the device impact its capacitance. The results uncover the coupling between the electrical and mechanical behaviors of WS2 and emphasize their potential to operate as key components in functional devices, such as nanosensors and radio frequency devices.

Freestanding Laser-Induced Graphene Ultrasensitive Resonative Viral Sensors.

Early and reliable detection of an infectious viral disease is critical to accurately monitor outbreaks and to provide individuals and health care professionals the opportunity to treat patients at the early stages of a disease. The accuracy of such information is essential to define appropriate actions to protect the population and to reduce the likelihood of a possible pandemic. Here, we show the fabrication of freestanding laser-induced graphene (FLIG) flakes that are highly sensitive sensors for high-fidelity viral detection. As a case study, we show the detection of SARS-CoV-2 spike proteins. FLIG flakes are nonembedded porous graphene foams ca. 30 µm thick that are generated using laser irradiation of polyimide and can be fabricated in seconds at a low cost. Larger pieces of FLIG were cut forming a cantilever, used as suspended resonators, and characterized for their electromechanics behavior. Thermomechanical analysis showed FLIG stiffness comparable to other porous materials such as boron nitride foam, and electrostatic excitation showed amplification of the vibrations at frequencies in the range of several kilo-hertz. We developed a protocol for aqueous biological sensing by characterizing the wetting dynamic response of the sensor in buffer solution and in water, and devices functionalized with COVID-19 antibodies specifically detected SARS-CoV-2 spike protein binding, while not detecting other viruses such as MS2. The FLIG sensors showed a clear mass-dependent frequency response shift of ∼1 Hz/pg, and low nanomolar concentrations could be detected. Ultimately, the sensors demonstrated an outstanding limit of detection of 2.63 pg, which is equivalent to as few as ∼5000 SARS-CoV-2 viruses. Thus, the FLIG platform technology can be utilized to develop portable and highly accurate sensors, including biological applications where the fast and reliable protein or infectious particle detection is critical.

Mechanical Properties and a Brittle-to-Ductile Fracture Transition in 3D Boron Nitride Foams.

We uncover the fracture characteristics of a boron nitride foam (BNF): a highly promising nanomaterial with a large band gap, superelastic behavior, and high surface area. By applying tension tests to BNF samples and characterizing them using image-processing tools and detailed scanning and transmission electron microscopies, we demonstrate a transition from brittle to a ductile fracture. Complementary mechanical analyses revealed that constraints originating from the synthesis process induce significant prestresses in the BNF and that wall thickness variations explain the fracture transition. We also show that BNF has a nearly zero Poisson's ratio and a high (>200 MPa) shear strength and that it absorbs a significant amount of energy before the fracture occurs. Thus, our findings shed light on the fundamental microscopic-scale mechanics of BNF, paving the way toward its integration into advanced applications, such as wearable electronics and energy absorbers.

Heat transfer of graphene foams and carbon nanotube forests under forced convection.

The effective dissipation of heat from electronic devices is essential to enable their long-term operation and their further miniaturization. Graphene foams (GF) and carbon nanotube (CNT) forests are promising materials for thermal applications, including heat dissipation, due to their excellent thermal conduction and low thermal interface resistance. Here, we study the heat transfer characteristics of these two materials under forced convection. We applied controlled airflow to heated samples of GF and CNT forests while recording their temperature using infrared micro-thermography. Then, we analyzed the samples using finite-element simulations in conjunction with a genetic optimization algorithm, and we extracted their heat fluxes in both the horizontal and vertical directions. We found that boundary layers have a profound impact on the heat transfer characteristics of our samples, as they reduce the heat transfer in the horizontal direction. The heat transfer in the vertical direction, on the other hand, is dominated by the material conduction and is much higher than the horizontal heat transfer. Accordingly, we uncover the fundamental thermal behavior of GF and CNT forests, paving the way toward their successful integration into thermal applications, including cooling devices.

Outstanding stretchability and thickness-dependent mechanical properties of 2D HfS2, HfSe2, and hafnium oxide.

We experimentally determine the elastic properties of 2D HfS2 and HfSe2 - two emerging nano-materials whose moderate energy bandgap and dielectric oxidized layer make them highly attractive for functional electronic and optoelectronic systems. We found that the average Young's moduli of HfS2 and HfSe2 nano-drumheads are relatively low (45.3 ± 3.7 GPa for a 12.2 nm thick HfS2 and 39.3 ± 8.9 GPa for a 13.4 nm thick HfSe2) and depend on the thickness of the nano-drumhead (increasing with thickness for HfS2 and decreasing for HfSe2). Moreover, both materials demonstrate outstanding stretchability (fracture strength and maximal strain of 5.7 ± 0.4 GPa and 12.2-14.3%, respectively, for HfS2; fracture strength and maximal strain of 4.5 ± 1.4 GPa and 14.0-20.9%, respectively, for HfSe2), which far exceeds the stretchability of other 2D materials and of polymers that are commonly used in flexible electronic applications. Finally, we describe the controlled oxidation of HfSe2 using a relatively simple laser treatment, which increased the Young's moduli of the thin oxidized layers to 182.6 ± 54.3 GPa. The extraordinary elastic properties of HfS2 and HfSe2, together with their excellent electrical and optoelectrical properties, make these 2D materials highly attractive for use in strain engineering and in various stretchable electronic and optoelectronic applications, such as wearable devices.

Height and morphology dependent heat dissipation of vertically aligned carbon nanotubes.

The continuous miniaturization of electronic devices substantially increases their power density, and consequently, requires effective cooling of these components. Vertically aligned carbon nanotubes (VA-CNTs) constitute one of the most promising materials for use as a high-end heat dissipation element due to their high thermal conductivity and large surface area. However, the lack of a clear understanding of the heat transfer mechanisms of VA-CNTs has so far impeded their large-scale use as cooling elements. Our infrared micro-thermography analysis revealed that the heat dissipation of VA-CNTs is determined mainly by their height, such that the heat dissipation behavior of tall samples was dominated by convection from the carbon nanotube (CNT) sidewalls. The mechanism of heat transfer in short VA-CNTs, in contrast, was determined by their morphology. Short VA-CNTs with highly organized CNT formations or with low thermal conductance exhibited convective heat dissipation similar to that of tall VA-CNTs, while other short VA-CNTs exhibited heat transfer dominated by conduction along the CNTs. This study provides important guidelines regarding the parameters that can be changed to optimize the performances of VA-CNTs in thermal applications. These applications include cooling elements in electronic devices, where convection is required, or thermal interface materials, where conduction is required.

Elastic properties and breaking strengths of GaS, GaSe and GaTe nanosheets.

Gallium sulphide (GaS), gallium selenide (GaSe), and gallium telluride (GaTe), belonging to the group-III monochalcogenide family, have shown promising optoelectronic performance over graphene and monolayer molybdenum disulphide (MoS2). However, to date, the mechanical properties of these materials have not been investigated, which hinders their utilisation in flexible electronics and optomechanics. Here, we characterize the elastic properties and breaking strengths of suspended two-dimensional (2D) nanosheets of GaS, GaSe, and GaTe, using atomic force microscopy. The 2D Young's modulus values of ∼10 nm thick GaS, GaSe, and GaTe were found to be 1732 ± 154 N m-1, 819 ± 127 N m-1, and 246 ± 160 N m-1, respectively, corresponding to the three-dimensional (3D) Young's modulus values of 173 ± 15 GPa, 81.9 ± 12.7 GPa, and 24.6 ± 16 GPa, respectively. The pre-tension values of these nanosheets were estimated to be 0.34 ± 0.12 N m-1, 0.14 ± 0.04 N m-1, and 0.15 ± 0.03 N m-1 for GaS, GaSe, and GaTe, respectively. GaS nanosheets exhibited the highest Young's modulus (173 GPa) among these nanosheets, which is comparable to that of WS2 and WSe2. A failure characteristic study over these group-III monochalcogenides revealed that these materials can withstand stresses of up to 8 GPa and a maximal strain of 7% before breaking. Altogether, our findings indicate that GaS, GaSe, and GaTe are attractive candidates for use in stretchable electronic applications and in future optomechanical devices.

High-frequency electromechanical resonators based on thin GaTe.

Gallium telluride (GaTe) is a layered material, which exhibits a direct bandgap (∼1.65 eV) regardless of its thickness and therefore holds great potential for integration as a core element in stretchable optomechanical and optoelectronic devices. Here, we characterize and demonstrate the elastic properties and electromechanical resonators of suspended thin GaTe nanodrums. We used atomic force microscopy to extract the Young's modulus of GaTe (average value ∼39 GPa) and to predict the resonance frequencies of suspended GaTe nanodrums of various geometries. Electromechanical resonators fabricated from suspended GaTe revealed fundamental resonance frequencies in the range of 10-25 MHz, which closely match predicted values. Therefore, this study paves the way for creating a new generation of GaTe based nanoelectromechanical devices with a direct bandgap vibrating element, which can serve as optomechanical sensors and actuators.

Torsional Resonators Based on Inorganic Nanotubes.

We study for the first time the resonant torsional behaviors of inorganic nanotubes, specifically tungsten disulfide (WS2) and boron nitride (BN) nanotubes, and compare them to that of carbon nanotubes. We have found WS2 nanotubes to have the highest quality factor (Q) and torsional resonance frequency, followed by BN nanotubes and carbon nanotubes. Dynamic and static torsional spring constants of the various nanotubes were found to be different, especially in the case of WS2, possibly due to a velocity-dependent intershell friction. These results indicate that inorganic nanotubes are promising building blocks for high-Q nanoelectromechanical systems (NEMS).