Session E01: Advances in Scanned Probe Microscopy II

Resonant coupling of nano-resonator coatings measured by near-field scanning photocurrent microscopy

Dielectric arrays of nano-resonators as antireflection coatings is a potential candidate for improving efficiency of photovoltaic cells. Currently, the evaluation and optimization of the coatings relies on calculations and macro-/mesoscale experiments that are not fully consistent. We demonstrate how collective optical resonances within a monolayer of nano-resonators improve efficiency of solar cells using near-field scanning photocurrent microscopy. At wavelengths corresponding to the excitation of whispering gallery mode, we detect strong photocurrent enhancement over each nano-resonator. Combining nanoscale photocurrent imaging and macroscale experiments and calculations, we show that the collective optical couplings including whispering gallery modes and thin-film interference improve the photocurrent of solar cells by more than 20 %. We also examine the properties of multiple layers of nano-resonators by the near-field scanning photocurrent microscopy to evaluate the role of possible manufacturing imperfections.   

Nonlinear Optical Defect Detection

Improving the performance and downsizing the carrier channels in semiconductor and electro-optics devices has faced the obstacle of power dissipation. New materials like III-V with higher carrier velocity for these channels are attractive as they can operate at lower voltage and avoid the energy and information loss. Growing these materials such as GaAs or InP over mismatch Si substrate leave us with variety of defects including threading dislocations which act as sink or scattering points for electrons. For the first time a fast and noninvasive nonlinear optical technique has been used to map the localization of the light by these defects which act as effective cavity for scattering light. We demonstrate how in nonlinear regime, presence of dislocation defects can be revealed in sub-micron hotspot looking domains collected by nearfield scanning optical microscope (NSOM) system. Films over different substrate with variety of dislocation defects density has been studied and comparison has been done with invasive technique such as cross STEM imaging.
  

Nanoscale Imaging of Heterogeneity in Local Chemical Environments

The nanoscale morphology of molecular materials and the associated relationships between structure, inter- and intra-molecular coupling, and dynamics define material properties and macroscopic response. Vibrational spectroscopy can be used as a structurally sensitive probe of local chemical environment. However, the limited spatial resolution of conventional far-field spectroscopy results in ensemble averaged measurements of inhomogeneous samples. We use IR scattering scanning near-field optical microscopy (s-SNOM) with a tunable femtosecond laser source to probe local chemical environments on the natural length scales of ordering. Broadband IR s-SNOM spectroscopy measures the molecular response of a small homogeneous sub-ensemble within a 20 nm sample volume with sensitivity of a few vibrational oscillators approaching the single molecule limit. We image and spectroscopically resolve the vibrational response of characteristic reporter modes of molecules as a measure of intra- and intermolecular coupling. We demonstrate spatio-spectral imaging in polytetrafluoroethylene films, porphyrin nano-crystals, and self-assembled monolayers of 4-nitrobenzenethiol with few-wavenumber spectral resolution and few-nm spatial resolution and relate this to molecular disorder and crystallinity.   

Sub-molecular Control of Photo-assisted Activation of a Single C-H Bond

We demonstrate the bond-selected and photo-assisted activation of a single C-H bond in individual azulene molecules adsorbed on a Ag(110) surface using a scanning tunneling microscope combined with a femtosecond laser. Under laser irradiation, the electrons in the STM tip can be photo-excited and tunnel to when irradiated by laser, and dissociate individual C-H bonds of a molecule in the STM junction. The threshold bias to break the bond decreased from 2.5 V to 1.0 V under 820 nm (1.5 eV) laser illumination. The C-H bond to be activated can be chosen by positioning the tip over that bond in the molecule with sub-Angstrom resolution. The inelastic tunneling probe (itProbe) images provide structural identification of the intact molecule and the reaction products. The demonstration of a chemical reaction induced by photo-assisted tunneling electrons opens a new avenue to probe the photochemistry of single adsorbed molecules with sub-molecular resolution.   

Single Molecule Conformational Change Driven by Overtone Absorption in a Laser-coupled Scanning Tunneling Microscope

Excitation of vibrational overtones in molecules has been shown to selectively drive molecular transformations, including conformational change, isomerization, bond rotation, and bond dissociation. As such, interest in overtone chemistry has grown in different fields, such as atmospheric chemistry and photocatalysis, to better understand how photon absorption affects molecular reactivity. In this talk, I will discuss our recent advances in using a laser-coupled scanning tunneling microscope (STM) to probe the conformational change of single molecules adsorbed on a metal substrate. The STM is used to monitor the change in conformation of individual molecules while the photon wavelength is varied within the visible to near-IR range. In our studies, the kinetics and equilibrium constant of the reversible conformational change can be directly determined. By using the topographic capabilities of the STM, we can also directly measure how this conformational change is affected by the heterogeneity of the molecule’s local environment.   

Ultrafast optical pump-probe scanning tunneling microscopy

Since the invention of scanning tunneling microscopy (STM), the addition of high time-resolution to STM has been one of the most challenging issues, and various time-resolved STMs have been considered [1]. The most successful approach among them is to combine STM with electric and optical pump-probe (OPP) techniques [2-5]. In OPP-STM which we have been developing, the sample surface below STM tip is excited by a train of pulse pairs, similarly to the case of the OPP method, and tunneling current is measured as a function of delay time. In addition to the use of absorption bleaching mechanism, a new technique is to use THz pulses. Although it is difficult to apply a high bias voltage between the STM tip and sample in general, the tip-enhanced electric field obtained by ultrashort THz pulse enables it, and taking a snapshot of ultrafast dynamics becomes possible [6,7]. Control of the carrier envelope phases in pump and probe pulses paves the way for the development of new time-resolved analyses. Details will be discussed at the conference with recent results and the prospects for future researches.

[1] Terada, Y.; Yoshida, S.; Takeuchi, O.; and Shigekawa, H., J. Phys. Condens. Matter., 22, 264008-264015.
(2010) and references therein.
[2] Loth, S.; Etzkorn, M.; Lutz, C.P.; Eigler, D.M; and Heinrich, A.J., Science, 329, 1628-1630 (2010).
[3] Terada, Y.; Yoshida, S.; Takeuchi, O.; and Shigekawa, H., Nature Photonics, 4, 869-874 (2010).
[4] Cocker, T. L.; and Hegmann, F. A. et al., Nature Photonics, 7, 620-625 (2013).
[5] Yoshida, S.; and Shigekawa, H. et al., Nature Nanotechnology, 9, 588-593 (2014).
[6] Yoshioka, K.; Katayama, I.; Minami, Y.; Kitajima, M.; Yoshida, S.; Shigekawa, H.; and Takeda, J., Nature
, 10, 762-765 (2016).
[7] Cocker T.; Peller D.; Yu P.; Repp J.; and Huber R., Nature 539, 263-267 (2016).   

Detection and Manipulation of the Hyperfine Interaction of Individual Atoms using Scanning Tunneling Microscopy

Taking advantage of nuclear spins for electronic structure analysis, magnetic resonance imaging and quantum devices hinges on the knowledge and control of their surrounding electronic configuration. Thus far, nuclear spin properties of single adatoms could not be studied individually using scanning probe techniques. Here, we measure and control the hyperfine interaction of individual atoms on magnesium oxide (MgO) by using spin-polarized scanning tunneling microscopy in combination with electron spin resonance[1]. This allows to extract properties of the nuclear spin, for example to distinguish different isotopes, as well as information on the local electronic structure of single atoms and nanostructures.
[1] Baumann et al., Science 350, 6259 (2015).   

Ultrafast AFM: sub-femtosecond time resolution at the nanometer scale

Advancing the time resolution of AFM has been a primary pursuit of multiple research groups. This would enable the measurment of photocarriers, ion mobility in battery cathodes or molecular motions with nm spatial resolution. As we have recently demonstrated, time resolution in force measurments is not limited by the (slow) mechanical time constants of force transducing cntilevers. It is limited by the smallest measurable energy [Appl. Phys. Lett. 110 (2017) 053111].

We experimentally demonstrated this by achieving picosecond time resolution with nc-AFM and ultrafast laser pulses in low temperature grown GaAs. In this presentation we will discuss our most recent advances, enabling sub-fs time resolution. Specifically, we developed an autocorrelation measurement for ultrashort laser pulses by force detection. A non-linear crystal is used to generate an electric field which follows the intensity of the impinging ultrashort laser pulse; we directly trace the emitted electric field with attosecond temporal, and nanometer spatial, resolution using our nc-AFM setup. As such, we are able to demonstrate that the lower limit of time resolution in AFM is solely given by the minimal time delay achievable by the optical setup and the thermal noise of the nc-AFM.   

A combined atomic force- and tunneling microscopy system at 10mK temperature

We present the implementation of atomic force microscopy (AFM) with a self-sensing quartz tuning fork (qPlus) sensor in our ultra-low temperature scanning tunneling microscopy (STM) system. The system operates in an ultra-high vacuum (UHV) environment inside a dilution refrigerator (DR) with a base temperature of 10mK and magnetic fields up to 15T^[1]. Radio frequency (RF) filtering of all signal lines entering the UHV chamber and improved home built RF powder filters at low temperatures were implemented to produce an improved energy resolution in tunneling spectroscopy, i.e. a lower effective electron temperature. Low noise preamplifiers for the sensor deflection^[2] and the STM current signal^[3] were implemented at the 4K stage within the DR. This allows for reduced Johnson noise of the amplifier feedback resistors and a relatively short distance (1.2m) between amplifier and the mK stage and magnet where the sensor is operating. As cable capacity between sensor and amplifier adds noise, a special cable with a capacity of merely 30pF/m has been designed and implemented.

[1] Song et al., RSI 81, 121101 (2010); doi: 10.1063/1.3520482
[2] Huber and Giessibl, RSI 88, 073702 (2017); doi: 10.1063/1.4993737
[3] adapted from le Sueur and Joyez, RSI 77, 123701 (2006); doi: 10.1063/1.2400024   

Test of Alternative Electrodynamic Theory of Superconductivity by using Ultra Low Temperature Scanning Probe Spectroscopy

One of the main assumptions of London’s theory of superconductivity is the screening of an electric field inside a conventional superconductor on a short length scale, on the order of the Thomas-Fermi length. According to the alternative electrodynamic theory of conventional superconductors proposed by Hirsch[1], the electric field inside the superconductor should have a much larger decay length, similar to London penetration of the magnetic field. To test this hypothesis, we measured the force vs distance curves between a metal probe tip and superconducting Al surface. We correlated the force curve with the superconducting tunneling spectra, in and out of the superconducting state of the Al sample. The measurements were performed in a mK STM system, which has been recently upgraded to include a quartz tuning fork sensor for AFM measurement capability. The Al film was grown by MBE onto graphene/SiC substrate and transferred in the mK STM in ultra-high vacuum with a base temperature of 10 mK. The Al film was brought into the normal state with the application of a magnetic field above its critical field limit. We describe the results and compare to similar experiments performed on Nb surface.[2]

[1] Hirsch, PRB, 69, 214515 (2004)
[2] Peronio and Giessibl, PRB, 94, 094503 (2016)   

Magnetic Resonance Force Microscopy at milliKelvin Temperatures

Magnetic Resonance Force Microscopy (MRFM) combines the high spatial resolution of scanning probe microscopy with magnetic resonance techniques to measure the magnetic properties of samples using sub-attoNewton tip-sample forces. In contrast to conventional MRFM setups, our MRFM makes use of a SQUID based detection to measure the motion of the magnetically tipped cantilever, and a superconducting NbTiN transmission line to apply the spin-manipulating radio-frequency magnetic field. These innovations significantly reduce the heating of both the cantilever and the sample, and lower the operating temperature of the MRFM setup to only tens of milliKelvins. This feat was demonstrated by measuring the Korringa relation in copper down to 42 mK. The low operating temperatures open up the possibility to study a variety of condensed matter systems, such as LAO-STO, topological insulators, and iron-doped palladium. Furthermore, we have developed a theory describing the spin-induced dissipation of magnetic resonators, allowing us to experimentally determine the density and relaxation time of the dangling bonds on the surfaces of SiO2 and diamond, and to measure the density of electron spins associated to the N-centers in diamond.   

Hybrid System of Atomic Force Microscopy and Optical Spectroscopy: Photo-induced Force Microscopy

Photo-induced force microscopy(PiFM) is a variant of the multifrequency atomic force microscopy that measures the force between the sample and a tip induced by a laser. The PiFM technique uses a mechanical resonance of the probe to amplify the photo-induced force, enabling the detection of forces in the (sub-) picoNewton range under ambient conditions. Furthermore, by measuring a gradient of the force through detection at side-band frequencies background contributions are suppressed and sensitive measurements of the photo-induced gradient force can be made. When a femtosecond light source is used to illuminate the tip-sample junction, it is possible to record the nonlinear optical response of nanoscopic objects with PiFM. In this talk, we introduce the principles of PiFM and present recent results in ultrafast spectroscopy at the nanoscale that are enabled by force detection.   

Development of Nonlinear Scanning Microwave Microscopy for 3D Tomographic Imaging at the Nanoscale

Although the potential of scanning microwave microscopy (SMM) to image subsurface-structures has been demonstrated, a 3D tomographic imaging technique with SMM has not been achieved to date. A modified STM that utilizes the harmonics from the nonlinear interactions between the probe and the sample has been proposed. In the nonlinear scanning microwave microscope, two microwave sources are introduced. One with frequency f₁, is excited at the scanning tip, and the other with frequency f₂, is generated by an antenna below the sample stage which is simultaneously modulated at its mechanical resonant frequency Ω. The frequencies of the sources are chosen such that Ω= |f₁ - f₂|. Intermodulation of the two excitation signals is expected in the presence of nonlinearities in the probe-sample interface. The heterodyne detection of the low-frequency harmonics of the beat signal Ω in the reflection spectrum of STM can be easily attained on a multi-frequency lock-in amplifier with improved S/N. The limited spatial extents of the higher harmonics of Ω from intermodulation ensure the desired depth resolution. Lastly, simulations to relate the detected parameters with the material properties to achieve quantitative imaging are also reported.