Professional Education


  • Bachelor of Science, University of Rochester (2010)
  • Doctor of Philosophy, University of Wisconsin Madison (2016)

Stanford Advisors


All Publications


  • Nanomaterials for in vivo imaging of mechanical forces and electrical fields NATURE REVIEWS MATERIALS Mehlenbacher, R. D., Kolbl, R., Lay, A., Dionne, J. A. 2018; 3 (2)
  • Bright, Mechanosensitive Upconversion with Cubic-Phase Heteroepitaxial Core-Shell Nanoparticles. Nano letters Lay, A., Siefe, C., Fischer, S., Mehlenbacher, R. D., Ke, F., Mao, W. L., Alivisatos, A. P., Goodman, M. B., Dionne, J. A. 2018

    Abstract

    Lanthanide-doped nanoparticles are an emerging class of optical sensors, exhibiting sharp emission peaks, high signal-to-noise ratio, photostability, and a ratiometric color response to stress. The same centrosymmetric crystal field environment that allows for high mechanosensitivity in the cubic-phase (α), however, contributes to low upconversion quantum yield (UCQY). In this work, we engineer brighter mechanosensitive upconverters using a core-shell geometry. Sub-25 nm α-NaYF4:Yb,Er cores are shelled with an optically inert surface passivation layer of ∼4.5 nm thickness. Using different shell materials, including NaGdF4, NaYF4, and NaLuF4, we study how compressive to tensile strain influences the nanoparticles' imaging and sensing properties. All core-shell nanoparticles exhibit enhanced UCQY, up to 0.14% at 150 W/cm2, which rivals the efficiency of unshelled hexagonal-phase (β) nanoparticles. Additionally, strain at the core-shell interface can tune mechanosensitivity. In particular, the compressive Gd shell results in the largest color response from yellow-green to orange or, quantitatively, a change in the red to green ratio of 12.2 ± 1.2% per GPa. For all samples, the ratiometric readouts are consistent over three pressure cycles from ambient to 5 GPa. Therefore, heteroepitaxial shelling significantly improves signal brightness without compromising the core's mechano-sensing capabilities and further, promotes core-shell cubic-phase nanoparticles as upcoming in vivo and in situ optical sensors.

    View details for DOI 10.1021/acs.nanolett.8b01535

    View details for PubMedID 29927609

  • Upconverting Nanoparticles as Optical Sensors of Nano- to Micro-Newton Forces NANO LETTERS Lay, A., Wang, D. S., Wisser, M. D., Mehlenbacher, R. D., Lin, Y., Goodman, M. B., Mao, W. L., Dionne, J. A. 2017; 17 (7): 4172–77

    Abstract

    Mechanical forces affect a myriad of processes, from bone growth to material fracture to touch-responsive robotics. While nano- to micro-Newton forces are prevalent at the microscopic scale, few methods have the nanoscopic size and signal stability to measure them in vivo or in situ. Here, we develop an optical force-sensing platform based on sub-25 nm NaYF4 nanoparticles (NPs) doped with Yb3+, Er3+, and Mn2+. The lanthanides Yb3+ and Er3+ enable both photoluminescence and upconversion, while the energetically coupled d-metal Mn2+ adds force tunability through its crystal field sensitivity. Using a diamond anvil cell to exert up to 3.5 GPa pressure or ∼10 μN force per particle, we track stress-induced spectral responses. The red (660 nm) to green (520, 540 nm) emission ratio varies linearly with pressure, yielding an observed color change from orange to red for α-NaYF4 and from yellow-green to green for d-metal optimized β-NaYF4 when illuminated in the near infrared. Consistent readouts are recorded over multiple pressure cycles and hours of illumination. With the nanoscopic size, a dynamic range of 100 nN to 10 μN, and photostability, these nanoparticles lay the foundation for visualizing dynamic mechanical processes, such as stress propagation in materials and force signaling in organisms.

    View details for DOI 10.1021/acs.nanolett.7b00963

    View details for Web of Science ID 000405643300025

    View details for PubMedID 28608687

  • Broadband 2D electronic spectrometer using white light and pulse shaping: noise and signal evaluation at 1 and 100 kHz OPTICS EXPRESS Kearns, N. M., Mehlenbacher, R. D., Jones, A. C., Zanni, M. T. 2017; 25 (7): 7869-7883

    Abstract

    We have developed a broad bandwidth two-dimensional electronic spectrometer that operates shot-to-shot at repetition rates up to 100 kHz using an acousto-optic pulse shaper. It is called a two-dimensional white-light (2D-WL) spectrometer because the input is white-light supercontinuum. Methods for 100 kHz data collection are studied to understand how laser noise is incorporated into 2D spectra during measurement. At 100 kHz, shot-to-shot scanning of the delays and phases of the pulses in the pulse sequence produces a 2D spectrum 13-times faster and with the same signal-to-noise as using mechanical stages and a chopper. Comparing 100 to 1 kHz repetition rates, data acquisition time is decreased by a factor of 200, which is beyond the improvement expected by the repetition rates alone due to reduction in 1/f noise. These improvements arise because shot-to-shot readout and modulation of the pulse train at 100 kHz enables the electronic coherences to be measured faster than the decay in correlation between laser intensities. Using white light supercontinuum for the pump and probe pulses produces high signal-to-noise spectra on samples with optical densities <0.1 within a few minutes of averaging and an instrument response time of <46 fs thereby demonstrating that that simple broadband continuum sources, although weak, are sufficient to create high quality 2D spectra with >200 nm bandwidth.

    View details for DOI 10.1364/OE.25.007869

    View details for Web of Science ID 000398536000065

    View details for PubMedID 28380905

  • Polarization-Controlled Two-Dimensional White-Light Spectroscopy of Semiconducting Carbon Nanotube Thin Films JOURNAL OF PHYSICAL CHEMISTRY C Mehlenbacher, R. D., McDonough, T. J., Kearns, N. M., Shea, M. J., Joo, Y., Gopalan, P., Arnold, M. S., Zanni, M. T. 2016; 120 (30): 17069-17080
  • Ultrafast Exciton Hopping Observed in Bare Semiconducting Carbon Nanotube Thin Films with Two-Dimensional White-Light Spectroscopy JOURNAL OF PHYSICAL CHEMISTRY LETTERS Mehlenbacher, R. D., Wang, J., Kearns, N. M., Shea, M. J., Flach, J. T., McDonough, T. J., Wu, M., Arnold, M. S., Zanni, M. T. 2016; 7 (11): 2024–31

    Abstract

    We observe ultrafast energy transfer between bare carbon nanotubes in a thin film using two-dimensional (2D) white-light spectroscopy. Using aqueous two-phase separation, semiconducting carbon nanotubes are purified from their metallic counterparts and condensed into a 10 nm thin film with no residual surfactant. Cross peak intensities put the time scale for energy transfer at <60 fs, and 2D anisotropy measurements determine that energy transfer is most efficient between parallel nanotubes, thus favoring directional energy flow. Lifetimes are about 300 fs. Thus, these results are in sharp contrast to thin films prepared from nanotubes that are wrapped by polymers, which exhibit picosecond energy transfer and randomize the direction of energy flow. Ultrafast energy flow and directionality are exciting properties for next-generation photovoltaics, photodetectors, and other devices.

    View details for DOI 10.1021/acs.jpclett.6b00650

    View details for Web of Science ID 000377239200015

    View details for PubMedID 27182690

  • Energy transfer pathways in semiconducting carbon nanotubes revealed using two-dimensional white-light spectroscopy NATURE COMMUNICATIONS Mehlenbacher, R. D., McDonough, T. J., Grechko, M., Wu, M., Arnold, M. S., Zanni, M. T. 2015; 6: 6732

    Abstract

    Thin film networks of highly purified semiconducting carbon nanotubes (CNTs) are being explored for energy harvesting and optoelectronic devices because of their exceptional transport and optical properties. The nanotubes in these films are in close contact, which permits energy to flow through the films, although the pathways and mechanisms for energy transfer are largely unknown. Here we use a broadband continuum to collect femtosecond two-dimensional white-light spectra. The continuum spans 500 to 1,300 nm, resolving energy transfer between all combinations of bandgap (S1) and higher (S2) transitions. We observe ultrafast energy redistribution on the S2 states, non-Förster energy transfer on the S1 states and anti-correlated energy levels. The two-dimensional spectra reveal competing pathways for energy transfer, with S2 excitons taking routes depending on the bandgap separation, whereas S1 excitons relax independent of the bandgap. These observations provide a basis for understanding and ultimately controlling the photophysics of energy flow in CNT-based devices.

    View details for DOI 10.1038/ncomms7732

    View details for Web of Science ID 000353701900001

    View details for PubMedID 25865487

  • Experimental Measurement of the Binding Configuration and Coverage of Chirality-Sorting Polyfluorenes on Carbon Nanotubes JOURNAL OF PHYSICAL CHEMISTRY LETTERS Shea, M. J., Mehlenbacher, R. D., Zanni, M. T., Arnold, M. S. 2014; 5 (21): 3742–49

    Abstract

    Poly(9,9-dioctylfluorene-2,7-diyl) (PFO) exhibits exceptional (n,m) chirality and electronic-type selectivity for near-armchair semiconducting carbon nanotubes. To better understand and control the factors governing this behavior, we experimentally determine the surface coverage and binding configuration of PFO on nanotubes in solution using photoluminescence energy transfer and anisotropy measurements. The coverage increases with PFO concentration in solution, following Langmuir-isotherm adsorption behavior with cooperativity. The equilibrium binding constant (PFO concentration in solution at half coverage), KA, depends on (n,m) and is 1.16 ± 0.30, 0.93 ± 0.12, and 1.13 ± 0.26 mg mL(-1) for the highly selected (7,5), (8,6), and (8,7) species, respectively, and the corresponding PFO wrapping angle at low coverage is 12, 17, and 14 ± 2°, respectively. In contrast, the inferred KA for metallic nanotubes is nearly an order of magnitude greater, indicating that the semiconducting selectivity increases with decreasing PFO concentration. This understanding will quantitatively guide future experimental and computational efforts on electronic type-sorting carbon nanotubes.

    View details for DOI 10.1021/jz5017813

    View details for Web of Science ID 000344579500048

    View details for PubMedID 26278744

  • Diffusion-Assisted Photoexcitation Transfer in Coupled Semiconducting Carbon Nanotube Thin Films ACS NANO Grechko, M., Ye, Y., Mehlenbacher, R. D., McDonough, T. J., Wu, M., Jacobberger, R. M., Arnold, M. S., Zanni, M. T. 2014; 8 (6): 5383–94

    Abstract

    We utilize femtosecond transient absorption spectroscopy to study dynamics of photoexcitation migration in films of semiconducting single-wall carbon nanotubes. Films of nanotubes in close contact enable energy migration such as needed in photovoltaic and electroluminescent devices. Two types of films composed of nanotube fibers are utilized in this study: densely packed and very porous. By comparing exciton kinetics in these films, we characterize excitation transfer between carbon nanotubes inside fibers versus between fibers. We find that intrafiber transfer takes place in both types of films, whereas interfiber transfer is greatly suppressed in the porous one. Using films with different nanotube composition, we are able to test several models of exciton transfer. The data are inconsistent with models that rely on through-space interfiber energy transfer. A model that fits the experimental results postulates that interfiber transfer occurs only at intersections between fibers, and the excitons reach the intersections by diffusing along the long-axis of the tubes. We find that time constants for the inter- and intrafiber transfers are 0.2-0.4 and 7 ps, respectively. In total, hopping between fibers accounts for about 60% of all exciton downhill transfer prior to 4 ps in the dense film. The results are discussed with regards to transmission electron micrographs of the films. This study provides a rigorous analysis of the photophysics in this new class of promising materials for photovoltaics and other technologies.

    View details for DOI 10.1021/nn4041798

    View details for Web of Science ID 000338089200004

    View details for PubMedID 24806792

  • Photoexcitation Dynamics of Coupled Semiconducting Carbon Nanotube Thin Films NANO LETTERS Mehlenbacher, R. D., Wu, M., Grechko, M., Laaser, J. E., Arnold, M. S., Zanni, M. T. 2013; 13 (4): 1495–1501

    Abstract

    Carbon nanotubes are a promising means of capturing photons for use in solar cell devices. We time-resolved the photoexcitation dynamics of coupled, bandgap-selected, semiconducting carbon nanotubes in thin films tailored for photovoltaics. Using transient absorption spectroscopy and anisotropy measurements, we found that the photoexcitation evolves by two mechanisms with a fast and long-range component followed by a slow and short-range component. Within 300 fs of optical excitation, 20% of nanotubes transfer their photoexcitation over 5-10 nm into nearby nanotube fibers. After 3 ps, 70% of the photoexcitation resides on the smallest bandgap nanotubes. After this ultrafast process, the photoexcitation continues to transfer on a ~10 ps time scale but to predominantly aligned tubes. Ultimately the photoexcitation hops twice on average between fibers. These results are important for understanding the flow of energy and charge in coupled nanotube materials and light-harvesting devices.

    View details for DOI 10.1021/nl304591w

    View details for Web of Science ID 000317549300022

    View details for PubMedID 23464618

  • Probing the Charge Transfer Reaction Coordinate of 4-(Dimethylamino)benzonitrile with Femtosecond Stimulated Raman Spectroscopy JOURNAL OF PHYSICAL CHEMISTRY B Rhinehart, J. M., Mehlenbacher, R. D., McCamant, D. 2010; 114 (45): 14646–56

    Abstract

    Femtosecond stimulated Raman spectroscopy (FSRS) and femtosecond transient absorption have been used to probe the photoinduced charge transfer (CT) dynamics of 4-(dimethylamino)benzonitrile in methanol and n-hexane. Through a combined analysis of temporal changes in the Raman modes and transient absorption kinetics, a more complete picture of the reaction coordinate of the intramolecular charge transfer process has been established. FSRS spectra of the phenyl C═C stretching mode (Wilson mode 8a) at 1607 cm(-1), which shifts to 1581 cm(-1) in the CT state, and transient absorption measurements ranging from 360 to 700 nm support internal conversion from the locally excited to the charge transfer state in 4-5 ps and then a subsequent vibrational relaxation within the CT state manifold on a 6-8 ps time scale. Dramatic shifting and narrowing of the 1581 cm(-1) quinoidal C═C stretch (ν(8a)) on the ∼7 ps time scale indicates that the quinoidal distortion is an important probe of the CT reaction dynamics. The cause of the spectral shifts is determined by comparing the observed shifts in the vibrational spectrum to anharmonic couplings computed for the benzonitrile radical anion by density functional theory (DFT) and with quantitative theoretical models of the solvent induced vibrational peak shifts. The DFT calculations indicate that the 10 cm(-1) downshift of the C═C stretch is most likely attributable to significant vibrational excitation in nontotally symmetric modes that are strongly anharmonically coupled to the C═C stretch.

    View details for DOI 10.1021/jp1023982

    View details for Web of Science ID 000284018000065

    View details for PubMedID 20568804

  • Theoretical analysis of anharmonic coupling and cascading Raman signals observed with femtosecond stimulated Raman spectroscopy JOURNAL OF CHEMICAL PHYSICS Mehlenbacher, R. D., Lyons, B., Wilson, K. C., Du, Y., McCamant, D. W. 2009; 131 (24): 244512

    Abstract

    We present a classical theoretical treatment of a two-dimensional Raman spectroscopy based on the initiation of vibrational coherence with an impulsive Raman pump and subsequent probing by two-pulse femtosecond stimulated Raman spectroscopy (FSRS). The classical model offers an intuitive picture of the molecular dynamics initiated by each laser pulse and the generation of the signal field traveling along the probe wave vector. Previous reports have assigned the observed FSRS signals to anharmonic coupling between the impulsively driven vibration and the higher-frequency vibration observed with FSRS. However, we show that the observed signals are not due to anharmonic coupling, which is shown to be a fifth-order coherent Raman process, but instead due to cascades of coherent Raman signals. Specifically, the observed vibrational sidebands are generated by parallel cascades in which a coherent anti-Stokes or Stokes Raman spectroscopy (i.e., CARS or CSRS) field generated by the coherent coupling of the impulsive pump and the Raman pump pulses participates in a third-order FSRS transition. Additional sequential cascades are discussed that will give rise to cascade artifacts at the fundamental FSRS frequencies. It is shown that the intended fifth-order FSRS signals, generated by an anharmonic coupling mechanism, will produce signals of approximately 10(-4) DeltaOD (change in the optical density). The cascading signals, however, will produce stimulated Raman signal of approximately 10(-2) DeltaOD, as has been observed experimentally. Experiments probing deuterochloroform find significant sidebands of the CCl(3) bend, which has an E type symmetry, shifted from the A(1) type C-D and C-Cl stretching modes, despite the fact that third-order anharmonic coupling between these modes is forbidden by symmetry. Experiments probing a 50:50 mixture of chloroform and d-chloroform find equivalent intensity signals of low-frequency CDCl(3) modes as sidebands shifted from both the C-D stretch of CDCl(3) and the C-H stretch of CHCl(3). Such intermolecular sidebands are allowed in the cascade mechanism, but are expected to be extremely small in the fifth-order frequency modulation mechanism. Each of these observations indicates that the observed signals are due to cascading third-order Raman signals.

    View details for DOI 10.1063/1.3276684

    View details for Web of Science ID 000273217000042

    View details for PubMedID 20059084

  • Two-dimensional femtosecond stimulated Raman spectroscopy: Observation of cascading Raman signals in acetonitrile JOURNAL OF CHEMICAL PHYSICS Wilson, K. C., Lyons, B., Mehlenbacher, R., Sabatini, R., McCamant, D. W. 2009; 131 (21): 214502

    Abstract

    A new methodology for two-dimensional Raman spectroscopy-termed two-dimensional femtosecond stimulated Raman spectroscopy (2D-FSRS)-is presented and experimental results for acetonitrile are discussed. 2D-FSRS can potentially observe molecular anharmonicity by measuring the modulation of the frequency of a probed Raman mode, at frequency omega(hi), by the coherent motion of an impulsively driven mode, at frequency omega(low). In acetonitrile, the signal is generated by driving the CCN bend (379 cm(-1)) and CC stretch (920 cm(-1)) into coherence via impulsive stimulated Raman scattering and subsequently probing the stimulated Raman spectrum of the CC stretch, the CN stretch (2250 cm(-1)) and the CH stretch (2942 cm(-1)). The resultant signal can be generated by two alternative mechanisms: a fifth-order Raman process that would directly probe anharmonic coupling between the two modes, or a third-order cascade in which a third-order coherent Raman process produces a field that goes on to participate in a third-order stimulated Raman transition. The third-order cascade is shown to dominate the 2D-FSRS spectrum as determined by comparison with the predicted magnitude of the two signals, the 2D spectrum of a mixed isotope experiment, and the concentration dependence of the signal. In acetonitrile, theoretical calculations of the vibrational anharmonicity indicate that the third-order cascade signal should be 10(4) times larger than the fifth-order Raman signal. 2D-FSRS signals are observed between acetonitrile's CCN bend, of E symmetry, and several different A(1) modes but are forbidden by symmetry in the fifth-order pathway. A 2D-FSRS spectrum of a 50:50 mixture of acetonitrile and d(3)-acetonitrile shows equivalent intensity for intramolecular coupling peaks and intermolecular coupling peaks, indicating that the observed signal cannot be probing molecular anharmonicity. Finally, the magnitudes of the 2D-FSRS peaks are observed to be proportional to the square of the number density, supporting the cascade mechanism.

    View details for DOI 10.1063/1.3263909

    View details for Web of Science ID 000272494300018

    View details for PubMedID 19968346