Anupama B. Kaul

The ability to detect the flow of gases is critical in a number of applications such as environmental monitoring and safety, agricultural plants, automation, and the healthcare sector. Conventional metal oxide-based gas sensors typically exhibit poor stability at high operating temperatures, while polymers degrade in the presence of moisture. Two-dimensional (2D) materials such as graphene serve as promising candidates for such studies because of their large surface-to-volume ratio, ease of surface functionalization, and ability to be integrated within a three-dimensional (3D) device architecture. In particular, solution-processed graphene, nucleated into thin films via inkjet printing, offers a favorable template for gas adsorption through the largely porous microstructure characteristic of such films compared to, for example, vapor-deposited pristine and dense metal films. In this work, we report on the synthesis, fabrication, and characterization of our two-terminal contacted graphene-based device. The active channel region for the device under test (DUT), comprising loosely oriented, multilayer graphene platelets, was characterized using scanning electron microscopy, Raman spectroscopy, atomic force microscopy, X-ray diffraction, and four-point electrical probing for resistivity determination. Current modulations in the DUT revealed a strong, dynamic response to the flow of semi-inert gases, such as N2 and CO2, within a controlled ambient environment in a vacuum chamber. The cyclical, time-dependent behavior of the current, observed with each ON and OFF gas flow pulse, was also highly reversible given that the adsorption dynamics were largely governed by physisorption processes. The sensitivity of the response also increased with the gas flow rate and as the bias voltage decreased, validating the device's operational features. The results presented in this work on inkjet-printed, solution-processed graphene films suggest that their porous, network-like microstructure is ideally suited for monitoring such physical and environmental parameters useful for a wide-variety of applications.

Engineered defects in multilayer 2D materials can be embedded beneath the sample surface, providing enhanced stability of their optical response. In multilayer transition-metal dichalcogenides (TMDs) (MoS2 and MoSe2), we study defect-assisted emission involving chalcogen-vacancy-related states, where strain and anisotropic thermal or mechanical perturbations are used to modify carrier lifetimes and level alignment in indirect-gap systems. Polarization-dependent emission associated with these defect-related transitions is observed. In layered group-III chalcogenides, particularly GaSe and InSe, defects are introduced through rapid thermal quenching and UV laser irradiation. We report photoluminescence (PL) tunability, emission enhancement, polarization effects, and spatially resolved PL lifetime distributions associated with free excitons, strain-bound excitons, and defect-localized excitons. In mechanically strained GaSe, bound excitons exhibit longer PL lifetimes than free excitons, correlating with enhanced emission intensity, while such polarization effects are absent under thermally induced strain. Spatial variations in PL lifetime consistent with strain-induced exciton energy gradients are observed and discussed in the context of exciton redistribution in strained regions. At low temperatures, thermal activation energies for exciton escape from defect-localized states in GaSe and InSe are extracted, and spectrally sharp emission features in GaSe are characterized. Finally, UV laser writing is used to create grating patterns with spatially modulated defect densities in GaSe, enabling controlled modification of local optical emission.

Metal halide perovskite solar cells (PSCs) have attained an impressive power conversion efficiency (PCE) of over 26% to date, in a period that is comparatively shorter compared to other solid-state semiconducting photovoltaic (PV) technologies. Some of the first reports on cell performance showed a PCE of ~ 3.8% in 2009, and issues of device stability remain topics of current exploration for many within the PV community. While the stability of the solar-cell stack is intimately tied to the fundamental qualities of the intrinsic perovskite absorber layer itself, the absorbers interaction with adjacent films within the solar cell stack are equally important as external factors influencing stability. In this work, we discuss the integration of the perovskite absorber with solution exfoliated inorganic tungsten diselenide (WSe₂) as an alternative to the conventional hole transport layer (HTL), Spiro-OMeTAD, where the latter is hydrophilic. The characterization of the solution exfoliated WSe₂ layer is first discussed with its heterostructures involving graphene ink-jet printed on flexible substrates, and subsequently, the integration of solution exfoliated WSe₂ as an alternative HTL to examine moisture stability of the PSC is then illustrated.

Dihydrolevoglucosenone, commonly known as Cyrene, is a renewable and fully biodegradable cellulose-waste derived, environmentally friendly solvent, presenting a non-toxic alternative to N-methyl-2-pyrrolidone (NMP). Currently, solution-based processing of graphene and other similar van der Waals solids favor toxic solvents such as NMP, limiting their use for biosensing. However, with the use of Cyrene, bio-compatible printable devices are possible, and studies have already demonstrated its use in temperature and other biosensing methods through screen-printing. Screen-printing unfortunately often requires masks that constrain the minimum acquirable feature size to be above hundreds of centimeters and wastes material, adding to process complexity and cost. Conversely, inkjet-printing is an attractive alternative for the maskless patterning of hierarchically assembled structures, with micron length scales attainable. Graphene's high conductivity positions it ideally for long-wear sensors such as dry electrodes or respiration monitors. Here, we demonstrate the potential of Cyrene-based graphene inks through few-layer inkjet printing on flexible substrates for the first time, to produce non-toxic conductors toward a strain-mediated mechanism for biosensing, used to detect bodily motion for wearable electronics. The challenges overcome in this study include engineering ink chemistry and printing parameters such that Cyrene's relatively high viscosity compared to typical inkjet solvents, still allows for droplet ejection in a conventional material printer, yielding well-resolved clean line-edges in contrast to other solvents that exhibit diffuse line-edges possibly from stray droplets and ink-splashing. Temperature-dependent transport measurements on the inkjet-printed Cyrene-based graphene films showed the conductivity to be largely temperature-invariant but at lower temperatures below 100 K, conductivity decreased, likely as a result of increased inter-membrane separation arising from thermal contraction. Additionally, temperature-dependent Raman spectroscopy showed the red-shift in the G-band, 2D-band and D-band peaks, as temperature increased. By validating flexion motion detection of the proximal interphalangeal joint demonstrated in this study, our work is the first of its kind to successfully additively manufacture inkjet-printed Cyrene-based graphene strain sensors on flexible substrates for bio-sensing and wearables.

In this work, we report on defect generation in multilayer GaSe through hydrostatic pressure quenching and UV laser irradiation. The Raman line width from the UV 266 nm irradiated sample is much wider than that in pressure-quenched GaSe, corresponding to a wider defect energy distribution range in the former sample than the latter. After quenching from 11.2 GPa, three photoluminescence (PL) peaks from defect states are observed at 657, 681, and 695 nm at a low temperature of 93 K. Defect-related peaks at 649, 694, 750, and 774 nm also appear in low-temperature PL spectra after UV laser irradiation, with a nonmonotonous intensity dependence on irradiation duration. There are common features in defects produced by these two methods: the PL peaks with the lowest energy are sharp, and their PL intensities increase linearly with the excitation laser power and saturate above a certain excitation laser power. These two features are similar to those in defects for single-photon emission (SPE) in other 2D materials at even lower temperatures. Fluorescence lifetime imaging shows distinguished short (2.3 ns) and long (75.6 nm) lifetimes of the 695 nm PL line in pressure-quenched GaSe. The density functional theory predicts defect energy levels related to Se vacancy.

Strong light–matter interactions in two-dimensional layered materials (2D materials) have attracted the interest of researchers from interdisciplinary fields for more than a decade now. A unique phenomenon in some 2D materials is their large exciton binding energies (BEs), increasing the likelihood of exciton survival at room temperature. It is this large BE that mediates the intense light–matter interactions of many of the 2D materials, particularly in their monolayer limit, where the interplay of excitonic phenomena poses a wealth of opportunities for high-performance optoelectronics and quantum photonics. Within quantum photonics, quantum information science (QIS) is growing rapidly, where photons are a promising platform for information processing due to their low-noise properties, excellent modal control, and long-distance propagation. A central element for QIS applications is a single photon emitter (SPE) source, where an ideal on-demand SPE emits exactly one photon at a time into a given spatiotemporal mode. Recently, 2D materials have shown practical appeal for QIS which is directly driven from their unique layered crystalline structure. This structural attribute of 2D materials facilitates their integration with optical elements more easily than the SPEs in conventional three-dimensional solid state materials, such as diamond and SiC. In this review article, we will discuss recent advances made with 2D materials towards their use as quantum emitters, where the SPE emission properties maybe modulated deterministically. The use of unique scanning tunneling microscopy tools for the in-situ generation and characterization of defects is presented, along with theoretical first-principles frameworks and machine learning approaches to model the structure-property relationship of exciton–defect interactions within the lattice towards SPEs. Given the rapid progress made in this area, the SPEs in 2D materials are emerging as promising sources of nonclassical light emitters, well-poised to advance quantum photonics in the future.