UltraFast STM

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Ultrafast STM is an exciting new research direction that combines the atomic resolution of scanning tunneling microscopy (STM) with the sub-picosecond time resolution of fast optics. This is a new research area and so there is a wealth of physical systems to explore. The Crommie group is actively pursuing this research topic in collaboration with the Feng Wang group. Our emphasis is on exploring electronic, structural, and magnetic excitations in molecular systems and 2D materials (Fig. 1)

Combining Spatial Resolution with Time Resolution
Fig. 1: The combination of ultra-high spatial resolution with ultra-fast time resolution opens many new opportunities for exploring dynamical processes in (a) molecular nanostructures and (b) 2D materials.

One difficulty in obtaining ultrafast information from an STM is the low bandwidth of tunnel current detection (which is typically limited to audio frequencies). One way around this limitation is to illuminate the STM tunnel junction area with fast optical pulses in a pump-probe configuration. If two closely-spaced pulses are incident on the STM then the transient STM tunnel current induced by the second pulse will depend on the delay time between the two pulses. By measuring the integrated STM tunnel current (i.e., the DC current) as a function of the pulse delay time it is possible to extract fast dynamical information at the site of the tip. The time resolution is then set by the width of the pulses and the delay between them (Fig. 2). This and related techniques have been used by STM practitioners to explore dynamical behavior in a variety of different physical systems.1-3

Dynamical resolution in scanned probe microscopy
Fig. 2: Dynamical resolution in scanned probe microscopy comes from performing pump-probe measurements using fast optical pulses. For example, the tunnel current induced by the second of two fast pulses will depend on the delay time between them. Even though an STM has poor bandwidth, dynamical information can be obtained by integrating the tunnel current induced by a train of such pulses with fixed delay time. The integrated tunnel current as a function of delay time can then yield dynamical information with sub-picosecond time resolution.

Our ultrafast STM system integrates pump/probe optics with a cryogenic ultrahigh vacuum STM. The laser source includes a mode-locked Ti:sapphire oscillator and a broadly tunable optical parametric oscillator capable of generating independently tunable output pulses (Fig. 3).

Schematic representation of ultrafast STM
Fig. 3: Schematic representation of ultrafast STM shows scheme for generating fast optical pulses at the site of the STM tunnel junction with a controlled delay time between them.

The Crommie and Wang groups have previously collaborated to develop a new technique for performing infrared (IR) spectroscopy on molecules using the tip of an STM (IRSTM).4,5 This technique involves illuminating an adsorbate-decorated surface with a frequency-tunable IR laser and measuring changes in tunnel current as the laser frequency is swept. When the laser is on-resonance with a molecular mode then the molecule absorbs optical energy and the molecule/surface expands. STM tunnel current is a very sensitive detector of this type of expansion and so can measure molecular IR resonances with high resolution (Figs. 4a, b). We implemented IRSTM by integrating a homemade tunable mode-hop-free IR laser with a cryogenic UHV STM (Fig. 4c). An IRSTM spectrum of tetramantane diamondoid molecules on Au(111) is shown in Fig. 4d. Six molecular vibrational modes are clearly observed (black curve). Comparison of these modes to a conventional STM inelastic tunneling spectrum (IETS) (blue curve) shows that IRSTM has an energy resolution that is better than IETS by a factor of ~30 at this temperature (T = 13 K).4

Infrared Spectroscopy of Molecular Adsorbates via STM
Fig. 4: IRSTM is a technique developed by Crommie and Wang to perform infrared (IR) spectroscopy using the tip of an STM. To accomplish this a frequency-tunable IR laser (c) is used to illuminate the STM tunnel junction (a). When the laser is on-resonance (b) with a molecular vibration then the molecule and surface will absorb photons and slightly expand. This small expansion is detectable by the STM. This technique was used to detect the vibrational resonance of diamondoid molecules on Au(111), as shown in (d). IRSTM (black curve in (d)) is seen to have an energy resolution that is ~30 times better than conventional IESTM (blue curve in (d)) at T=13K.

Analysis of IRSTM spectra using DFT-based first principles simulations provides insight into how molecule-molecule and molecule-substrate interactions affect molecular modes observed by IRSTM (Fig. 5). Comparison of theoretical calculations to our experimental IRSTM spectra shows that environmental interactions cause some vibrational modes of adsorbed molecules to significantly red-shift relative to isolated gas phase molecules (theory performed by the Cohen and Louie groups).5

IRSTM Theory vs Experiment
Fig. 5: First principles simulations using DFT were developed to analyze IRSTM spectra. Comparison of theory (red curve) and experiment (green curve) reveal the degree to which molecule-molecule and molecule-surface interactions modify the vibrational spectra of molecular nanostructures on surfaces. (IRSTM spectroscopy: Crommie and Wang groups; theory: Cohen and Louie groups).

References

1) Y. Terada, S. Yoshida, O. Takeuchi & H. Shigekawa,"Real-space imaging of transient carrier dynamics by nanoscale pump–probe microscopy", Nature Photonics 4, 869 (2010).

2) T. L. Cocker, D. Peller, P. Yu, J. Repp & R. Huber,"Tracking the ultrafast motion of a single molecule by femtosecond orbital imaging", Nature 539, 263 (2016).

3) Shaowei Li, Siyu Chen, Jie Li, Ruqian Wu, and W. Ho, "Joint Space-Time Coherent Vibration Driven Conformational Transitions in a Single Molecule", Phys. Rev. Lett., 119, 176002 (2017).

4) I. V. Pechenezhskiy, X. Hong, G. D. Nguyen, J. E. P. Dahl, R. M. K. Carlson, F. Wang & M. F. Crommie,"Infrared Spectroscopy of Molecular Submonolayers on Surfaces by Infrared Scanning Tunneling Microscopy: Tetramantane on Au(111)", Physical Review Letters 111, 126101 (2013).

5) Y. Sakai, G.D. Nguyen, R.B. Capaz, S. Coh, I.V. Pechenezhskiy, X. Hong, F. Wang, M.F. Crommie, S. Saito, S.G. Louie, and M.L. Cohen. Intermolecular interactions and substrate effects for an adamantane monolayer on a Au(111) surface. Phys. Rev. B 88, 235407 (2013).