研究目的
To review recent advances in ultrafast time-resolved scanning tunneling microscopy (STM) techniques that combine high spatial resolution with high temporal resolution for studying carrier, spin, and molecular dynamics at the atomic scale.
研究成果
The review concludes that ultrafast time-resolved STM techniques, such as all-electric pump-probe STM, SPPX-STM, and THz-STM, successfully combine atomic-scale spatial resolution with femtosecond to nanosecond temporal resolution. These methods enable the study of various ultrafast dynamics, including carrier recombination, spin relaxation, and molecular vibrations. Challenges remain in minimizing thermal effects, improving signal-to-noise ratios, and extending applications to complex materials. Future directions include combining STM with atomic force microscopy, X-rays, STM-induced luminescence, and achieving attosecond resolution through field emission.
研究不足
Limitations include the intrinsic bandwidth of electronics restricting temporal resolution to nanoseconds in electric methods, thermal effects from laser illumination interfering with measurements, complexity in data interpretation due to nonlinear responses, small signal-to-noise ratios requiring high-repetition lasers or modulation, pulse broadening in UHV environments limiting time resolution, and potential alteration of sample dynamics by tunneling electrons.
1：Experimental Design and Method Selection:
The review discusses various methods to achieve time-resolved STM, including high-speed STM, atom-tracking STM, I(t) curve measurements, photoconductively gated STM (PG-STM), junction mixing STM (JM-STM), all-electric pump-probe STM, shaken pulse-pair-excited STM (SPPX-STM), and THz-STM. These methods involve coupling STM with pulsed electric waves, THz, near-infrared, and visible lasers using pump-probe techniques to overcome the temporal limitations of conventional STM.
2：Sample Selection and Data Sources:
Samples include semiconductors (e.g., GaAs, AlGaAs, LT-GaAs), magnetic atoms (e.g., Fe on Cu2N/Cu(100)), nanodots (e.g., InAs on GaAs), silicon surfaces (e.g., Si(111)-(7x7)), and single molecules (e.g., pentacene on surfaces). Data are acquired through STM imaging and time-resolved current measurements.
3：List of Experimental Equipment and Materials:
Equipment includes STM systems, ultrafast lasers (e.g., Ti:Sapphire laser), THz generators (e.g., ZnTe crystal), photoconductive switches, pulse pickers, arbitrary wave generators (AWG), delay generators, lock-in amplifiers, and various electronic components for pulse generation and modulation. Materials involve tips (e.g., Pt/Ir tips), samples as mentioned, and transmission lines for pulse propagation.
4：Experimental Procedures and Operational Workflow:
Procedures involve illuminating samples or tips with pump and probe pulses (electric or optical) with controlled delay times, measuring tunneling current changes, using modulation techniques (e.g., mechanical shaker for delay modulation), and demodulating signals with lock-in techniques. For THz-STM, THz pulses are focused on the tip to modulate the junction voltage, and rectified currents are measured.
5：Data Analysis Methods:
Data analysis includes exponential fitting of decay curves (e.g., for spin relaxation times), Fourier transform for vibrational frequencies, and comparison with theoretical models. Signal processing uses lock-in detection and integration methods to extract dynamic information from averaged current measurements.
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