Showing papers by "Michael S. Arnold published in 2022"

Energy cascades in donor-acceptor exciton-polaritons observed by ultrafast two-dimensional white-light spectroscopy

Abstract: Exciton-polaritons are hybrid states formed when molecular excitons are strongly coupled to photons trapped in an optical cavity. These systems exhibit many interesting, but not fully understood, phenomena. Here, we utilize ultrafast two-dimensional white-light spectroscopy to study donor-acceptor microcavities made from two different layers of semiconducting carbon nanotubes. We observe the delayed growth of a cross peak between the upper- and lower-polariton bands that is oftentimes obscured by Rabi contraction. We simulate the spectra and use Redfield theory to learn that energy cascades down a manifold of new electronic states created by intermolecular coupling and the two distinct bandgaps of the donor and acceptor. Energy most effectively enters the manifold when light-matter coupling is commensurate with the energy distribution of the manifold, contributing to long-range energy transfer. Our results broaden the understanding of energy transfer dynamics in exciton-polariton systems and provide evidence that long-range energy transfer benefits from moderately-coupled cavities.

Physics and applications of nanotubes

Abstract: Nanotubes have been pursued aggressively over the last three decades. Significant progress has been made in the selective growth and post-synthetic sorting of highly monodisperse carbon nanotubes, in understanding their physics, and in assembling and integrating them into high-performance devices. These discoveries have led to promising applications in areas such as high-performance CMOS, high-speed RF, thin-film transistors, flexible electronics, thermoelectrics, sensors, and optoelectronics. The rapid development of modern information technology depends on the exploitation of new and novel materials, and nanotubes have emerged as promising candidates for the post-Moore's Law era. This Special Topic on Physics and Applications of Nanotubes provides a valuable forum where researchers studying the fundamentals of nanotubes can share their most recent and novel findings.

Freestanding epitaxial SrTiO3 nanomembranes via remote epitaxy using hybrid molecular beam epitaxy

Abstract: The epitaxial growth of functional oxides using a substrate with a graphene layer is a highly desirable method for improving structural quality and obtaining freestanding epitaxial nanomembranes for scientific study, applications, and economical reuse of substrates. However, the aggressive oxidizing conditions typically used in growing epitaxial oxides can damage graphene. Here, we demonstrate the successful use of hybrid molecular beam epitaxy for SrTiO3 growth that does not require an independent oxygen source, thus avoiding graphene damage. This approach produces epitaxial films with self-regulating cation stoichiometry. Furthermore, the film (46-nm-thick SrTiO3) can be exfoliated and transferred to foreign substrates. These results open the door to future studies of previously unattainable freestanding oxide nanomembranes grown in an adsorption-controlled manner by hybrid molecular beam epitaxy. This approach has potentially important implications for the commercial application of perovskite oxides in flexible electronics and as a dielectric in van der Waals thin-film electronics.

Graphene nanoribbons initiated from molecularly derived seeds

Abstract: Abstract Semiconducting graphene nanoribbons are promising materials for nanoelectronics but are held back by synthesis challenges. Here we report that molecular-scale carbon seeds can be exploited to initiate the chemical vapor deposition (CVD) synthesis of graphene to generate one-dimensional graphene nanoribbons narrower than 5 nm when coupled with growth phenomena that selectively extend seeds along a single direction. This concept is demonstrated by subliming graphene-like polycyclic aromatic hydrocarbon molecules onto a Ge(001) catalyst surface and then anisotropically evolving size-controlled nanoribbons from the seeds along $$\left\langle 110\right\rangle$$ 110 of Ge(001) via CH 4 CVD. Armchair nanoribbons with mean normalized standard deviation as small as 11% (3 times smaller than nanoribbons nucleated without seeds), aspect ratio as large as 30, and width as narrow as 2.6 nm (tunable via CH 4 exposure time) are realized. Two populations of nanoribbons are compared in field-effect transistors (FETs), with off-current differing by 150 times because of the nanoribbons’ different widths.

Controlling the Balance between Remote, Pinhole, and van der Waals Epitaxy of Heusler Films on Graphene/Sapphire.

Abstract: Remote epitaxy is promising for the synthesis of lattice-mismatched materials, exfoliation of membranes, and reuse of expensive substrates. However, clear experimental evidence of a remote mechanism remains elusive. Alternative mechanisms such as pinhole-seeded epitaxy or van der Waals epitaxy can often explain the resulting films. Here, we show that growth of the Heusler compound GdPtSb on clean graphene/sapphire produces a 30° rotated (R30) superstructure that cannot be explained by pinhole epitaxy. With decreasing temperature, the fraction of this R30 domain increases, compared to the direct epitaxial R0 domain, which can be explained by a competition between remote versus pinhole epitaxy. Careful graphene/substrate annealing and consideration of the relative lattice mismatches are required to obtain epitaxy to the underlying substrate across a series of other Heusler films, including LaPtSb and GdAuGe. The R30 superstructure provides a possible experimental fingerprint of remote epitaxy, since it is inconsistent with the leading alternative mechanisms.

Population of Subradiant States in Carbon Nanotube Microcavities in the Ultrastrong Light–Matter Coupling Regime

Abstract: Strong light–matter coupling results in eigenstates called polaritons which share the properties of both light and matter and provide a useful way to engineer electronic energies and behaviors. In this work, we study nearly monochiral (6,5) semiconducting carbon nanotubes (CNTs) in a Fabry–Pérot microcavity. Light–matter coupling leads to the formation of three bands of bright polariton states (upper, middle, and lower─resulting from coupling to the bright S11 CNT exciton and the X1 phonon sideband of the K-momentum dark exciton state). The structure also supports many exciton-like subradiant states at the bright S11 and X1 energies. Here, ultrafast transient reflection spectroscopy is used to study the dynamics and spectral signatures of excited subradiant-state polariton populations and the pathways by which they are populated. After a pump pulse, the excited subradiant-state population is revealed by (i) spectral signatures with relaxation times (∼5 ps) similar to those of CNT S11 band gap excitons outside of the cavity and (ii) a Rabi contraction of the lower polariton energy, whose magnitude quantifies the excited subradiant-state population. Data show that, following the excitation of the upper polariton (UP), the excited subradiant-state population is maximized at a sample position with a detuning of 118 meV, light–matter coupling of 336 meV, and UP transition energy of 1.52 eV. The excited subradiant-state population is reduced for other detunings. The X1 Hopfield coefficient of the UP also peaks at the same energy, revealing UP to X1 scattering as a potentially efficient relaxation pathway. These results will be important for understanding and controlling energy relaxation and transport in future CNT polariton devices.

The Development of Energy-Recovery Linacs

Abstract: Energy-recovery linacs (ERLs) have been emphasised by the recent (2020) update of the European Strategy for Particle Physics as one of the most promising technologies for the accelerator base of future high-energy physics. The current paper has been written as a base document to support and specify details of the recently published European roadmap for the development of energy-recovery linacs. The paper summarises the previous achievements on ERLs and the status of the field and its basic technology items. The main possible future contributions and applications of ERLs to particle and nuclear physics as well as industrial developments are presented. The paper includes a vision for the further future, beyond 2030, as well as a comparative data base for the main existing and forthcoming ERL facilities. A series of continuous innovations, such as on intense electron sources or high-quality superconducting cavity technology, will massively contribute to the development of accelerator physics at large. Industrial applications are potentially revolutionary and may carry the development of ERLs much further, establishing another shining example of the impact of particle physics on society and its technical foundation with a special view on sustaining nature.

High transconductance and current density in field effect transistors using arrays of bundled semiconducting carbon nanotubes

Abstract: We examine if the bundling of semiconducting carbon nanotubes (CNTs) can increase the transconductance and on-state current density of field effect transistors (FETs) made from arrays of aligned, polymer-wrapped CNTs. Arrays with packing density ranging from 20 to 50 bundles μm−1 are created via tangential flow interfacial self-assembly, and the transconductance and saturated on-state current density of FETs with either (i) strong ionic gel gates or (ii) weak 15 nm SiO2 back gates are measured vs the degree of bundling. Both transconductance and on-state current significantly increase as median bundle height increases from 2 to 4 nm, but only when the strongly coupled ionic gel gate is used. Such devices tested at −0.6 V drain voltage achieve transconductance as high as 50 μS per bundle and 2 mS μm−1 and on-state current as high as 1.7 mA μm−1. At low drain voltages, the off-current also increases with bundling, but on/off ratios of ∼105 are still possible if the largest (95th percentile) bundles in an array are limited to ∼5 nm in size. Radio frequency devices with strong, wraparound dielectric gates may benefit from increased device performance by using moderately bundled as opposed to individualized CNTs in arrays.

A simple simulation-derived descriptor for the deposition of polymer-wrapped carbon nanotubes on functionalized substrates.

Abstract: Controlling the deposition of polymer-wrapped single-walled carbon nanotubes (s-CNTs) onto functionalized substrates can enable the fabrication of s-CNT arrays for semiconductor devices. In this work, we utilize classical atomistic molecular dynamics (MD) simulations to show that a simple descriptor of solvent structure near silica substrates functionalized by a wide variety of self-assembled monolayers (SAMs) can predict trends in the deposition of s-CNTs from toluene. Free energy calculations and experiments indicate that those SAMs that lead to maximum disruption of solvent structure promote deposition to the greatest extent. These findings are consistent with deposition being driven by solvent-mediated interactions that arise from SAM-solvent interactions, rather than direct s-CNT-SAM interactions, and will permit the rapid computational exploration of potential substrate designs for controlling s-CNT deposition and alignment.

COPD inhaler therapy: A path to success.

Abstract: Keys to therapeutic success include choosing the right device and drug regimen, providing rigorous patient education, and reducing environmental exposures.

Growth and Properties of Graphene and Graphene Nanoribbons on Ge

Abstract: The synthesis of graphene directly on Ge and on Ge deposited on Si provides a scalable route toward integrating graphene onto conventional semiconductors. This presentation will first survey the growth modes of graphene on Ge(001), Ge(011), Ge(111), Ge(112), Ge(001)-6°, and Ge(001)-9° via chemical vapor deposition (CVD), reporting on the effect of Ge surface orientation on graphene island formation and shape, strain in large-area graphene films, and the nanofaceting of the Ge below graphene.[1] We will then focus on the anisotropic growth of semiconducting graphene nanoribbons on Ge(001) and Ge(001)-9° and of nominally single crystal graphene on Ge(110). On Ge(001), we have discovered how to drive graphene crystal growth with a large shape anisotropy through control of kinetics.[2-6] This discovery enables the direct synthesis of narrow, armchair, semiconducting nanoribbons. The ribbons are self-orienting, self-defining, and have smooth edges. The ribbons exhibit excellent transport properties (e.g., high on-state conductance and on/off ratio at room temperature) and provide a promising pathway towards the direct integration of high-performance nanocarbon electronics onto conventional semiconductor wafer platforms. On Ge(001), nominally single crystal graphene has been reported in limited cases; however, conflicting studies have evidenced polycrystallinity. Here, the factors affecting the mosaicity of graphene on Ge(110) will be elucidated using low energy electron diffraction and microscopy data.[7] [1] R. M. Jacobberger, D. E. Savage, X. Zheng, P. Sookchoo, R. R. Delgado, M. G. Lagally, M. S. Arnold, SUBMITTED (2022). [2] R. M. Jacobberger, M. S. Arnold, et al., Direct Oriented Growth of Armchair Graphene Nanoribbons on Germanium, NATURE COMMUNICATIONS, 6, 8006 (2015). [3] B. Kiraly, M. S. Arnold, M. C. Hersam, N. P. Guisinger et al., Sub-5 nm, Globally Aligned Graphene Nanoribbons on Ge (001), APPLIED PHYSICS LETTERS, 108, 213101 (2016). [4] A. J. Way, R. M. Jacobberger, M. S. Arnold, Seed-Initiated Anisotropic Growth of Unidirectional Armchair Graphene Nanoribbon Arrays on Germanium, NANO LETTERS, 18, 898 (2018). [5] V. Saraswat, Y. Yamamoto, H.J. Kim, R.M. Jacobberger, K.R. Jinkins, A.J. Way, N.P. Guisinger, M.S. Arnold Synthesis of armchair graphene nanoribbons on germanium-on-silicon, THE JOURNAL OF PHYSICAL CHEMISTRY C 123 (30), 18445-18454 (2019). [6] A. J. Way, R. M. Jacobberger, N. P. Guisinger, V. Saraswat, X. Zheng, A. Suresh, J. H. Dwyer, P. Gopalan, Michael S. Arnold, SUBMITTED (2022). [7] R. M. Jacobberger, Z. Miao, T. Yu, V. Saraswat, M. G. Lagally, M. S. Altman, M. S. Arnold, SUBMITTED (2022).

(Invited) Hybridization of Dark Excitons, Bright Excitons, and Photons in an Ultrastrongly Coupled Carbon Nanotube Microcavity and the Importance of Sub-Radiant Polariton States during Relaxation

Abstract: This talk will present on strong light-matter interaction in optical microcavities incorporating dense semiconducting carbon nanotubes, characterizing hybridization of the bright and dark exciton states of nanotubes, mediated by a cavity photon plus exciton-phonon coupling. This talk will also present on the relaxation of optically excited, bright polariton modes into sub-radiant dark polaritons.

Free-Standing Epitaxial SrTiO$_3$ Nanomembranes via Remote Epitaxy using Hybrid Molecular Beam Epitaxy

Abstract: 1Department of Chemical Engineering and Materials Science, University of Minnesota – Twin Cities, Minneapolis, MN 55455, USA 2Energy and Environment Directorate, Pacific Northwest National Laboratory, Richland, Washington 99352, USA 3Department of Electrical and Computer Engineering, University of Minnesota – Twin Cities, Minneapolis, MN 55455, USA 4Department of Materials Science and Engineering, University of Wisconsin – Madison, Madison, WI 53706, USA 5Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, Washington 99352, USA 6Department of Physics, University of Washington, Seattle, Washington 98195, USA 7Physical and Computational Sciences Directorate, Pacific Northwest National Laboratory, Richland, Washington 99352, USA

CVD Synthesis of Graphene Nanomesh on Ge(001)

Abstract: Bottom-up synthesis of functional graphene nanostructures on industry-compatible semiconductors such as Si and Ge is the key to unlocking the tremendous potential of graphene in electronic applications. Herein, we demonstrate the growth of graphene nanomesh on Ge using chemical vapor deposition – a technique used widely in the semiconductor industry. Graphene nanomesh is a promsing material in semiconductor electronics because of its ability to simultaneously achieve a large drive current and on/off ratio – one of the key challenges in graphene nanoribbon electronics[1]. However, in order to realize a nanomesh with desired and tunable electrical characteristics – an accurate control over the nanoribbon width, presence of atomically smooth edges, and nanoribbon placement is desired. In the literature, several techniques have been demonstrated to fabricate graphene nanomesh such as top-down lithography[2], bottom-up polymerization[3], and molecule-assisted CVD[4]; yet, a combination of accurate placement, tunable widths and atomically smooth edges has been challenging to achieve. We attempt to overcome this bottleneck by initiating CVD nanoribbon synthesis from deterministically placed arrays of graphene nanoseeds on Ge. We are able to tune the nanoribbon widths by changing the starting seed size and array pitch. The nanoribbons evolved from these nanoseeds fuse into each other when grown long enough – forming a nanomesh. These nanomeshes have atomically smooth edges and exhibit semiconducting characteristics. We also perform extensive simulations to identify the seed-sizes and pitches and demonstrate that this technique allows us to synthesize nanomeshes of desired architecture and electrical characteristics and thus can be used as channel materials in high performance / RF applications as well as BEOL interconnects. References Saraswat et al. ACS Nano, 15,3674-3708 (2021) Bai et al. Nature Nanotechnology, 5, 190–194 (2010) Moreno et al. Science, 360, 199-203 (2018) Kim et al. ACS Applied Materials & Interfaces, 13, 28593-28599 (2021)

Observation of Polariton-Assisted Long-Range Energy Transfer in Semiconducting Carbon Nanotube Microcavity by 2D White-Light Spectroscopy

Abstract: We resolve a sub-picosecond long-range energy transfer pathway in a carbon nanotube microcavity using 2D white-light spectroscopy. This pathway is primarily mediated by polaritonic states, highlighting the role of light-matter coupling in controlling the photophysics.

Arrays of Bundled Semiconducting Carbon Nanotubes for High Transconductance Field Effect Transistors

Abstract: Forming aligned arrays from bundled (rather than individualized) semiconducting carbon nanotubes can beneficially increase the density of nanotubes in the channel of field effect transistors (FETs). Here, we prepare aligned arrays of bundles at a density of 20 – 50 bundles μm-1 via tangential flow interfacial self-assembly and show that bundles can increase the transconductance of FETs when a strong gate is used. When encapsulated with an ion gel gate, the transconductance and saturated on-current increases as the diameter of the bundles increases. Devices tested at -0.6 V drain voltage using an ion gel gate achieve transconductance as high as 50 μS per bundle and 2000 μS μm-1 and on-state current as high as 1.7 mA μm-1. At low drain voltages the off-current also increases with bundling, but on/off ratios of ~105 are possible if the bundle size is limited to ~ 5 nm. Radio frequency devices with very strong gates may benefit from the increased device performance by using moderately bundled semiconducting carbon nanotube arrays. Figure Caption: Panel (a) has transfer curves from 4 devices each with ~42 CNTs-bundles μm-1 but with different median bundle heights. Devices with larger bundles have higher transconductance and saturated on-currents. Panel (b) shows the transconductance per bundle vs the median bundle height and shows the positive correlation between bundle size (height) and transconductance per bundle. Panel (c) is a rendering of a large bundle formed from polymer wrapped CNTs. Figure 1