Experiment for cryogenic large-aperture intensity mapping: instrument design

The experiment for cryogenic large-aperture intensity mapping (EXCLAIM) is a balloon-borne telescope designed to survey star formation in windows from the present to z  =  3.5. During this time, the rate of star formation dropped dramatically, while dark matter continued to cluster. EXCLAIM maps the redshifted emission of singly ionized carbon lines and carbon monoxide using intensity mapping, which permits a blind and complete survey of emitting gas through statistics of cumulative brightness fluctuations. EXCLAIM achieves high sensitivity using a cryogenic telescope coupled to six integrated spectrometers employing kinetic inductance detectors covering 420 to 540 GHz with spectral resolving power R  =  512 and angular resolution ≈4  arc min. The spectral resolving power and cryogenic telescope allow the survey to access dark windows in the spectrum of emission from the upper atmosphere. EXCLAIM will survey 305  deg2 in the Sloan Digital Sky Survey Stripe 82 field from a conventional balloon flight in 2023. EXCLAIM will also map several galactic fields to study carbon monoxide and neutral carbon emission as tracers of molecular gas. We summarize the design phase of the mission

Strain and interfacial engineering of 2D materials

Deformable device technology with advanced functionalities spanning from wearable/personal electronics to flexible bio-implantable sensors, which require both static performance stability and dynamic sensitivity, has been ever-evolving with the substantial development in the field of two-dimensional (2D) materials. 2D materials are atomically thin, layered crystalline materials that possess strong intralayer (in-plane) covalent bond and weak interlayer (out-of-plane) van der Waals interaction resulting in extraordinary properties including high mechanical strength (in-plane) and extremely low bending stiffness (out-of-plane). Distinct from 3D bulk materials, the reduced dimensionality and the spatial confinement of 2D materials result in unique characteristics such as excellent electrical transport behavior, strong light-matter interaction, and vanishing flexural rigidity at the ultimate thickness limit, which will permit the achievement of next-generation flexible electronics beyond the capability of conventional semiconductor technology. Therefore, 2D materials have demanded substantial attention from the scientific community for both fundamental quantum physics/mechanics and device-level applications. Because of the exceptional mechanical strength and intrinsic flexibility, however, 2D materials can be strongly affected by external influences. Mechanical deformations, especially out-of-plane deformation, are commonly observed with 2D materials-based systems in the forms of intrinsic corrugation, wrinkles, delaminated buckles or crumples, and other non-linear complex deformations in an uncontrolled manner, which have been considered as inevitable topological defects. Furthermore, due to weak interlayer interactions of 2D materials via van der Waals interactions with surrounding environments, structural instability of delamination at the interfaces between 2D materials and substrate surfaces can occur. These mechanical instabilities are often considered as parameters deteriorating the mechanical integrity and functional performances of 2D materials-based systems. Contrary to such conventional wisdom, I have sought ways to leverage mechanical instabilities of 2D materials to advance functionalities of 2D materials-based systems including electrical sustainability, excitonic characteristics, energy generation, and novel optoelectronic phenomena by manipulating interfacial characteristics (interfacial engineering) and mechanical deformation (strain-engineering) of the constituent 2D material layers. First, I discussed a readily adaptable approach of inserting 2D materials to thin-film metal based flexible electrodes (2D-interfacial engineering) where we achieved several orders-of-magnitude enhanced strain resilient electrical functionality (which we termed ‘electrical ductility’) by manipulating mechanical fracture behaviors of thin-film metals from rapid straight cracking to progressive tortuous cracking via a buckle-guided fracture mechanism induced by 2D materials at the interface. I demonstrated that our 2D-interfacial engineering is not limited to a certain combination of metals and 2D materials, which can be incorporated into the existing flexible/wearable electronics applications. Next, I demonstrated how the wrinkling deformation of artificially stacked 2D materials via strain engineering affects the optical characteristics of interlayer excitons in heterobilayer system where the effect of mechanical strain remains relatively uncharacterized. We observed highly strain-tunable interlayer excitons with non-monotonic photoluminescence characteristics in MoS2/WSe2 heterobilayer. I further provided an insight on the competition between in-plane strain and out-of-plane interlayer coupling effects on the photoluminescence characteristics, which can be an additional tuning knob to manipulate excitonic behaviors in 2D-multilayered systems. As an extension of the strain effect on the heterobilayer system, I explored how crumpling deformation of 2D materials affects both mechanical (the effective stiffness) and electromechanical (piezoelectricity) properties. Our results suggest that the effective elastic modulus can be reduced for the controlled crumpled heterostructure where the effective modulus decreased as the aspect ratio of formed crumples decreased. I further demonstrated that the crumpled MoS2/graphene heterostructures can be utilized as an effective active layer for mechanical-to-electrical energy conversion under both instantaneous and continuous strain-driven modes of stretching, bending, acoustic and mechanical vibrations. Finally, I introduced a strain-control platform to create freestanding wrinkled structures of monolayer 2D materials for the first time, offering exciting opportunities to further investigate the fundamental strain-tunability of various materials properties in 2D materials. In particular, I showed the spatial modulation of photo-induced force (near-field dipole-dipole interactions) in freestanding wrinkled 2D materials, which we attributed to a combination of the in-plane strain-induced piezoelectric effect and the out-of-plane strain gradient-induced flexoelectric effect. In conclusion, I demonstrated that controlling the mechanical deformation of 2D materials permits emergent functionalities ranging from the nanoscale to macroscale including electrical ductility, controllable excitonic behaviors, and even energy generation. Furthermore, since the deformation of 2D materials can directly manipulate bond length and angle in the atomic lattice, deformation can tune the electronic structure and interfacial characteristics. Thus, strain and interfacial engineering can serve as effective strategies to explore advanced functionalities of 2D materials. I believe our approaches to manipulate interfacial characteristics and deformation of 2D materials-based systems, as well as in a freestanding form, contribute to the larger research community by opening up exciting opportunities to study fundamental strain physics and to advance multifunctional deformable device technologies including robust flexible electronics, novel excitonic sensors, and self-powering wearable/implantable devices for health and structural monitoring.LimitedAuthor requested closed access (OA after 2yrs) in Vireo ETD syste

Spatial Tuning of Light–Matter Interaction via Strain-Gradient-Induced Polarization in Freestanding Wrinkled 2D Materials

To date, controlled deformation of two-dimensional (2D) materials has been extensively demonstrated with substrate-supported structures. However, interfacial effects arising from these supporting materials may suppress or alter the unique behavior of the deformed 2D materials. To address interfacial effects, we report, for the first time, the formation of a micrometer-scale freestanding wrinkled structure of 2D material without any encapsulation layers where we observed the enhanced light–matter interactions with a spatial modulation. Freestanding wrinkled monolayer WSe2 exhibited about a 330% enhancement relative to supported wrinkled WSe2 quantified through photoinduced force microscopy. Spatial modulation and enhancement of light interaction in the freestanding wrinkled structures are attributed to the enhanced strain-gradient effect (i.e., out-of-plane polarization) enabled by removing the constraining support and proximate dielectrics. Our findings offer an additional degree of freedom to modulate the out-of-plane polarization and enhance the out-of-plane light–matter interaction in 2D materials

EXCLAIM: the EXperiment for cryogenic large-aperture intensity mapping

The EXperiment for Cryogenic Large-Aperture Intensity Mapping (EXCLAIM) will constrain star formation over cosmic time by carrying out a blind and complete census of redshifted carbon monoxide (CO) and ionized carbon ([CII]) emission in cross-correlation with galaxy survey data in redshift windows from the present to z=3.5 with a fully cryogenic, balloon-borne telescope. EXCLAIM will carry out extragalactic and Galactic surveys in a conventional balloon flight planned for 2023. EXCLAIM will be the first instrument to deploy µ-Spec silicon integrated spectrometers with a spectral resolving power R=512 covering 420-540 GHz. We summarize the design, science goals, and status of EXCLAIM